JP2894444B1 - Power circuit - Google Patents

Abstract

The present invention relates to a power supply circuit that outputs a stabilized output according to a reference voltage that is externally selected or set, and it is possible to easily improve a response speed and the like in a protection operation at the time of abnormality. It is an object to provide a simple power supply circuit. A transformer for inputting an input voltage to a primary side, switching means for switching a current path on a primary side of the transformer, a rectifier for rectifying and smoothing an electromotive force generated on a secondary side of the transformer, Current detecting means for detecting a current flowing in the power supply circuit, comprising: a stabilizing means for stabilizing an output of the rectifier according to a predetermined reference voltage via a switching means; Multiplying means for outputting a result of multiplication with a voltage; and when the multiplication result output from the multiplying means reaches a predetermined power limit value, an error between the multiplication result and the power limit value is negatively fed back to the output of the rectifier. And limiting means for restricting

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for outputting a stabilized voltage according to a reference voltage selected or set from outside.

[0002]

2. Description of the Related Art FIG. 9 shows an example of a conventional power supply circuit. (Configuration of Conventional Power Supply Circuit 700) The configuration of the conventional power supply circuit 700 will be described below with reference to FIG. First, the (+) output terminal of the rectifier circuit 1 is connected in series with the “primary winding of the transformer 2” and “between the drain and source of the switching FET 3”, and the (−) output terminal of the rectifier circuit 1 is connected. Connected to terminal.

Due to the switching operation of the switching FET 3, an induced electromotive force is generated on the secondary side of the transformer 2. The induced electromotive force is rectified and smoothed via the rectifier circuit 4 and then supplied from the load-side terminal 5 to an external power-supplied device. On the other hand, the load side terminal 5 is connected to the input terminal of the reciprocal amplifier 8. In addition, the output voltage of the load side terminal 5 is passed through a voltage dividing circuit composed of voltage dividing resistors R1 and R2.
The voltage is applied to the (+) input terminal of the output voltage setting error amplifier 7.

A reference voltage V output from a reference voltage section 9 is applied to the (−) side input terminal of the output voltage setting error amplifier 7.
r is given. The reference voltage unit 9 is a circuit that selectively outputs a plurality of types of reference voltages Vr according to the setting states of switches 9a to 9c set from outside.

On the other hand, the output terminal of the reciprocal amplifier 8 is connected to the (−) side input terminal of the overcurrent setting error amplifier 10.
A voltage drop (−Io × Rs) generated at both ends of the current monitoring resistor Rs in the rectifier circuit 4 is supplied to the (+) side input terminal of the overcurrent setting error amplifier 10.
The signal is inverted and input via. Each output terminal of the output voltage setting error amplifier 7 and the overcurrent setting error amplifier 10 is individually connected to two input terminals of the MAX circuit 11. The MAX circuit 11 is a circuit that selectively outputs a higher input voltage among input voltages of two input terminals.

The output terminal of the MAX circuit 11 is connected to the gate electrode of the switching FET 3 via the pulse width modulator 12. The inside of the pulse width modulator 12 includes, for example, an integration circuit that integrates an input voltage, and a modulation circuit that generates a pulse whose duty ratio or frequency is modulated according to the output of the integration circuit. Next, a specific circuit configuration of the reciprocal amplifier 8 will be described.

FIG. 10 is a diagram showing a circuit example of the reciprocal amplifier 8. In FIG. 10, an input voltage Vin input to an input terminal of the reciprocal amplifier 8 is applied to one end of a resistor Ra after passing through a voltage buffer or the like (not shown). The other end of the resistor Ra is connected to the (−) side input terminal of the OP amplifier 81. The (-) side input terminal is connected to an offset voltage source Vof via a resistor Rb.

The output terminal of the OP amplifier 81 and (-)
The feedback resistor Rf and the function generator 82
Are connected. This function generator 82
Is composed of, for example, a combination of a series switching circuit or a parallel switching circuit using a zener diode. On the other hand, the (+) side input terminal of the OP amplifier 81 is connected to the ground potential via a resistor Rc for canceling an input offset.

In the reciprocal amplifier 8 having such a circuit configuration,
Depending on the magnitude of the input voltage Vin, conduction / non-conduction of the Zener diode in the function generator 82 is sequentially switched, and the amplification factor of the output voltage changes. As a result, the input / output characteristics of the reciprocal amplifier 8 change as shown in FIG.
The characteristic is bent at 1 and P2. This inflection point P
By adjusting the positions of P1 and P2 and the amplification factors before and after the inflection points P1 and P2, the input / output characteristics of the reciprocal amplifier 8 become characteristics approximated by a broken line to the reciprocal function.

Next, the operation of the above-described power supply circuit 700 will be described separately for a normal operation and an abnormal operation. (Normal Operation of Power Supply Circuit 700) Under normal conditions, a current within a normal range flows through the resistor Rs inside the rectifier circuit 4. In this state, the output voltage of the inverting amplifier circuit 10a, ie, the “voltage drop across the resistor Rs” is lower than the output voltage of the reciprocal amplifier 8.

Therefore, in the overcurrent setting error amplifier 10, a (-) potential difference is constantly applied between the input terminals,
The saturated voltage is always output from the output terminal to the (-) side.
Since such a saturation voltage on the (−) side is input to one input terminal of the MAX circuit 11, the MAX circuit 11 preferentially passes the output voltage on the output voltage setting error amplifier 7 side.

By the operation of the MAX circuit 11,
A first negative feedback loop including the output voltage setting error amplifier 7, the pulse width modulator 12, and the switching FET 3 is configured. The first negative feedback loop controls the pulse width of the switching operation of the switching FET 3 in a direction to reduce the error amplification output output from the output voltage setting error amplifier 7. At this time, between the input terminals of the output voltage setting error amplifier 7 is maintained in an imaginary short state.

Here, assuming that the reference voltage of the reference voltage section 9 is Vr and the voltage dividing resistors are R1 and R2, the output voltage Vo of the load terminal 5 is stable at the voltage value indicated by the right side of the following equation (1). Be transformed into Vo = Vr × (R1 + R2) / R2 (1) The value of the output voltage Vo output at this time is changed by operating the switch states of the switches 9a to 9c of the reference voltage unit 9. By doing so, it is possible to make appropriate changes.

(Protective operation when power supply circuit 700 is abnormal) When an abnormal situation such as a short circuit accident occurs on the power-supplied device side, an excessive current flows through resistor Rs inside rectifier circuit 4. In such an abnormal state, the output voltage of the inverting amplifier circuit 10a, ie, the “voltage drop across the resistor Rs” exceeds the output voltage of the reciprocal amplifier 8. Then, the output voltage of the overcurrent setting error amplifier 10 temporarily rises to near the (+) side saturation voltage. As a result, the MAX circuit 11 preferentially passes the output voltage of the overcurrent setting error amplifier 10 side.

By the operation of the MAX circuit 11,
A second negative feedback loop including the overcurrent setting error amplifier 10, the pulse width modulator 12, and the switching FET 3 is temporarily configured. By the second negative feedback loop, the switching operation of the switching FET 3 is pulse width controlled in a direction to reduce the error amplification output output from the overcurrent setting error amplifier 10. At this time, the imaginary short state is maintained between the input terminals of the overcurrent setting error amplifier 10.

Here, the current flowing through the resistor Rs is Io, the output voltage of the load terminal 5 is Vo, and the reciprocal amplifier 8
Is the output voltage of (K / Vo), the power value (Vo × I
o) is maintained (restricted) to the value indicated by the right side of the following equation (2). (Vo × Io) = K / Rs (2) By such an operation, when an excessive current flows through the resistor Rs, the power in the power supply circuit 700 is satisfied so as to satisfy the expression (2). (Vo × Io) is limited. As a result, the power supply circuit 700 and the power-supplied device can be safely protected.

[0017]

The reciprocal amplifier 8 described above is a circuit in which the switching time and the parasitic capacitance of the zener diode cannot be neglected, so that the frequency characteristics in a high frequency range are easily restricted and a phase delay is liable to occur. . Since such a reciprocal amplifier 8 is disposed in the second negative feedback loop formed at the time of abnormality, a phase delay or the like is likely to occur in the negative feedback operation at the time of abnormality. Therefore, there is a problem that the phase characteristics of the reciprocal amplifier 8 have a serious adverse effect when improving the control performance (response speed, followability, stability, etc.) of the protection operation of the power supply circuit 700.

Further, in the reciprocal amplifier 8, since a semiconductor element such as a zener diode is used without compensating for temperature characteristics, the output of the reciprocal amplifier 8 fluctuates according to a temperature change. Further, since the reciprocal amplifier 8 has a characteristic approximated by a broken line, the output accuracy of the reciprocal amplifier 8 is low. For these reasons, there is a problem that it cannot be used in a power supply circuit that requires a highly accurate protection operation.

Furthermore, since the reciprocal amplifier 8 is a circuit that operates when an abnormality occurs, it is difficult to accurately estimate the magnitude of the input voltage. For this reason, the input dynamic range of the reciprocal amplifier 8 needs to be kept sufficiently wide as necessary. In such a wide input dynamic range,
In order to obtain highly accurate reciprocal characteristics, it is necessary to take measures such as increasing the number of inflection points shown in FIG. As a result, there is a problem that the circuit of the reciprocal amplifier 8 becomes complicated and large.

In order to solve the above-mentioned problems, the invention according to claims 1 to 4 solves the above-mentioned problems caused by the reciprocal amplifier 8 while facilitating the improvement of the control performance of the protection operation at the time of abnormality. It is an object to provide a power supply circuit.

[0021]

[Means for Solving the Problems]

According to a first aspect of the present invention, there is provided a transformer in which an input voltage is input to a primary side, switching means for switching a current path on the primary side of the transformer, and a secondary side of the transformer. A rectifier that rectifies and smoothes the electromotive force generated on the secondary side of the transformer by the switching operation of the switching means and outputs the rectified and smoothed output; and controls the switching operation of the switching means to stabilize the output of the rectifier according to a predetermined reference voltage. Current detecting means for detecting a current flowing in the power supply circuit, and a multiplying means for outputting a result of multiplication of the detected current detected by the current detecting means and a predetermined reference voltage. When the multiplication result output from the multiplication means reaches a predetermined power limit value, the error between the multiplication result and the power limit value is negatively fed back to the output of the rectifier. Characterized in that a limiting means limits for.

Usually, in the power supply circuit according to the first aspect,
The stabilizing means controls the switching means to stabilize the output of the rectifier according to the reference voltage. In such a normal state, power supply to the outside is smoothly performed within the range of the absolute rating of the power supply circuit. On the other hand, when an abnormal state such as a short circuit accident occurs, an excessive current flows in the power supply circuit as the output current increases. As a result, the value of the detection current detected by the current detection means rapidly increases, and the “multiplication result corresponding to the product of the detection current and the reference voltage” output from the multiplication means rapidly increases. When the multiplication result exceeds a predetermined power limit value, the limiting unit starts an operation of negatively feeding back an error between the multiplication result and the power limit value.

By such a negative feedback operation, the power supply circuit is controlled in a direction to reduce the error between the multiplication result and the power limit value, and the output of the rectifier is limited. The reference voltage appearing in the expression of the multiplication result keeps a constant value even during the abnormal state. Therefore, the output limitation of the rectifier described above is a current limiting operation in a direction in which the value of the detected current approaches a value obtained by dividing the power limit value by the reference voltage.

For example, in a power supply circuit with a power limit value of 200 W, when the reference voltage is set to 5 V and the value of the detected current suddenly increases to 40 A in the event of an abnormality, the negative feedback operation by the limiting means is started, and the detected current is reduced. Current limiting is performed in a direction approaching 40A. Similarly, in a power supply circuit having a power limit value of 200 W, a reference voltage of 10 V
When the value of the detection current suddenly increases to 20 A at the time of an abnormality, the negative feedback operation is started by the limiting means, and the current is limited in a direction to approach the detection current to 20 A.

As described above, the protection operation for limiting the detection current to the “appropriate current value inversely proportional to the reference voltage” is performed. As a result, the amount of power supplied to the outside is limited to an appropriate range, and the power supply circuit and the power-supplied device are reliably protected. In the power supply circuit according to the first aspect, it is also possible to operate the limiting unit during normal power supply by setting such as lowering the power limit value. In such a setting, a power supply circuit is configured to automatically switch between constant voltage output and constant current output so that the multiplication result does not exceed the power limit value. In particular, such a power supply circuit becomes a power supply circuit suitable for applications in which the output voltage is limited to a predetermined limit value or less when a load is opened or the like while operating as a constant current source.

(Claim 2) The invention according to claim 2 is characterized in that a transformer whose input voltage is inputted to the primary side and one of the transformers
A switching means for switching the current path on the secondary side, a rectifier connected to the secondary side of the transformer, for rectifying and smoothing the electromotive force generated on the secondary side of the transformer by the switching operation of the switching means, and outputting the rectified output; By controlling the switching operation, the output of the rectifier
In a power supply circuit including a stabilizing means for stabilizing according to a predetermined reference voltage, a current detecting means for detecting a current flowing in the power supply circuit, a detection current detected by the current detecting means, an output voltage of a rectifier, When the multiplication result output from the multiplication means reaches a predetermined power limit value, the error between the multiplication result and the power limit value is negatively fed back, and the output of the rectifier is reduced. And limiting means for limiting.

Usually, in the power supply circuit according to claim 2,
The stabilizing means controls the switching means to stabilize the output of the rectifier according to the reference voltage. In such a normal state, power supply to the outside is smoothly performed within the range of the absolute rating of the power supply circuit. On the other hand, when an abnormal state such as a short circuit accident occurs, an excessive current flows in the power supply circuit as the output current increases. As a result, the value of the detection current detected by the current detection means rapidly increases, and the “multiplication result corresponding to the product of the detection current and the output voltage” output from the multiplication means rapidly increases. When the multiplication result exceeds a predetermined power limit value, the limiting unit starts an operation of negatively feeding back an error between the multiplication result and the power limit value.

By such a negative feedback operation, the power supply circuit is controlled in a direction to reduce the error between the multiplication result and the power limit value, and the output of the rectifier is limited. For example, in the case of a power supply circuit having a power limit value of 200 W, the multiplication result is 2 at the time of abnormality.
When the current value rapidly increases to 00 W, the negative feedback operation by the limiting unit is started, and the power is limited in a direction in which the multiplication result of the detection current and the output voltage approaches 200 W.

Since the power supply to the outside is limited with such power limitation, it is possible to reliably protect the power supply circuit and the power-supplied device. As described above, the protection operation for causing the multiplication result to follow the power limit value is performed.
As a result, the amount of power supply to the outside is limited to an appropriate range,
The power supply circuit and the power-supplied device are reliably protected.

In the power supply circuit according to the second aspect of the present invention, the limiting means can be operated at the time of normal power supply by setting such as lowering the power limit value. In such a setting, make sure that the multiplication result does not exceed the power limit value.
A power supply circuit that automatically switches between constant voltage output and constant power output is configured. In particular, such a power supply circuit serves as a power supply circuit suitable for use in which the output voltage is limited to a predetermined limit value or less when a load is opened or the like while operating as a constant power source.

(Claim 3) The invention according to claim 3 is characterized in that a transformer whose input voltage is inputted to the primary side and one of the transformers
A switching means for switching the current path on the secondary side, a rectifier connected to the secondary side of the transformer, for rectifying and smoothing the electromotive force generated on the secondary side of the transformer by the switching operation of the switching means, and outputting the rectified output; By controlling the switching operation, the output of the rectifier
In a power supply circuit having a stabilizing means for stabilizing according to a predetermined reference voltage, a current detecting means for detecting a current flowing in the power supply circuit, and a predetermined reference voltage and a rectifier output voltage in accordance with an external operation. Switching means for extracting one of the above, a multiplication means for outputting a multiplication result of a detection current detected by the current detection means and a switching output of the switching means, and a multiplication result output from the multiplication means for determining a predetermined power. When the limit value is reached, limiting means for negatively feeding back the error between the multiplication result and the power limit value to limit the output of the rectifier is provided.

With such a configuration, in the power supply circuit according to the third aspect, by switching the switching means, one of the following can be selected as the multiplication result. (A) Multiplication result∝detection current × reference voltage (B) Multiplication result∝detection current × output voltage When the multiplication result is selected, the same protection operation as the power supply circuit according to claim 1 can be performed. In such a case, in an abnormal state, the current is limited in such a manner that the detected current approaches a constant current value.

On the other hand, when the multiplication result of (B) is selected,
The same protection operation as the power supply circuit according to the second aspect can be performed. In such a case, in an abnormal state, power limitation is performed such that the result of multiplication of the detected current and the output voltage approaches the power limitation value. Therefore, these two types of protection operations can be appropriately selected in one power supply circuit.

In the power supply circuit according to the third aspect of the present invention, the limiting means can be operated during normal power supply by setting such as lowering the power limit value. By operating the switching means in the power supply circuit of such a setting, "the power supply operation in which the constant voltage output and the constant power output are automatically switched over at the power limit value" and "the power supply operation at the power limit value" The power supply operation in which the voltage output and the constant current output are automatically switched can be appropriately selected.

(Claim 4) The invention according to claim 4 is characterized in that a transformer in which a human-powered voltage is input to the primary side and one of the transformers
A switching means for switching the current path on the secondary side, a rectifier connected to the secondary side of the transformer, for rectifying and smoothing the electromotive force generated on the secondary side of the transformer by the switching operation of the switching means, and outputting the rectified output; By controlling the switching operation, the output of the rectifier
In a power supply circuit having a stabilizing means for stabilizing according to a predetermined reference voltage, a current detecting means for detecting a current flowing in the power supply circuit, and a reciprocal means for outputting a value corresponding to a reciprocal of the predetermined reference voltage And limiting means for limiting the output of the rectifier by negatively feeding back the error between the detected current and the output value of the reciprocal means when the detection current detected by the current detecting means reaches the output value of the reciprocal means. It is characterized by having.

Usually, in the power supply circuit according to claim 4,
The stabilizing means controls the switching means to stabilize the output of the rectifier according to the reference voltage. In such a normal state, power supply to the outside is smoothly performed within the range of the absolute rating of the power supply circuit. On the other hand, when an abnormal state such as a short circuit accident occurs, an excessive current flows in the power supply circuit as the output current increases. As a result, the value of the detection current detected by the current detection means increases rapidly. When the detected current exceeds the output value from the reciprocal means, the limiting means starts an operation of negatively feeding back the error between the detected current and the output value of the reciprocal means.

By such a negative feedback operation, the power supply circuit is controlled so that the detected current approaches the output value of the reciprocal means, and the output of the rectifier is limited. As a result, the amount of power supplied to the outside is limited to an appropriate range, and the power supply circuit and the power-supplied device are reliably protected.

Incidentally, a reference voltage is input to the reciprocal means. This reference voltage is a voltage that changes only when the setting is changed, and is usually a constant value. Therefore, the output value of the reciprocal means is usually a constant value. Therefore, even if a phase delay occurs in the reciprocal means, it does not affect the phase characteristics of the negative feedback loop at all. For this reason, it is possible to easily improve the control performance of the protection operation (response characteristics, follow-up property, stability, etc. of the current limiting operation) ignoring the phase delay of the reciprocal means.

The input dynamic range of the reciprocal means only needs to cover a reference voltage which may be input. Therefore, it is not necessary to secure a wide input dynamic range as in the conventional example (FIG. 9), and the circuit configuration of the reciprocal means can be simplified. In addition, when the number of reference voltage options is limited, such as for 9 V, 12 V, and 15 V, the circuit configuration of the reciprocal means can be further simplified. That is, an interlock switch that interlocks with the reference voltage changeover switch may be provided, and a “voltage corresponding to a reciprocal value of each reference voltage” may be output via the interlock switch. In this case, unlike the above-described conventional example (FIG. 9), an error due to a polygonal line approximation or the like, and an output fluctuation due to a temperature do not occur.

[0040]

Embodiments of the present invention will be described below with reference to the drawings. <First Embodiment> The first embodiment is an embodiment corresponding to the first aspect of the present invention. FIG. 1 is a diagram illustrating a configuration of a power supply circuit 100 according to the first embodiment.

In FIG. 1, the (+) side output terminal of the rectifier circuit 1 is connected in series with the “primary winding of the transformer 2” and “between the drain and source of the switching FET 3”. Connected to the (-) side output terminal.

When the pulsating current output from the rectifier circuit 1 is intermittently switched by the switching FET 3, an induced electromotive force is generated on the secondary side of the transformer 2. The induced electromotive force is rectified and smoothed through the rectifier circuit 4 and then supplied from the load-side terminal 5 to an external power-supplied device. On the other hand, the load side terminal 5 is connected to the voltage dividing resistors R1, R2
Output voltage setting error amplifier 7 via a voltage dividing circuit comprising
(+) Side input terminal. The reference voltage Vr output from the reference voltage unit 9 is applied to the (−) side input terminal of the output voltage setting error amplifier 7. The reference voltage unit 9 is a circuit that selectively outputs a plurality of types of reference voltages Vr according to the setting states of switches 9a to 9c set from outside.

In addition, the reference voltage Vr of the reference voltage section 9 is
It is applied to the input terminal X2 of the analog multiplier 21. An input terminal X1 of the analog multiplier 21 has a voltage drop (I) generated across the current monitoring resistor Rs in the rectifier circuit 4.
oxRs) is input via the inverting amplifier circuit 10a. The output terminal of the analog multiplier 21 is connected to the (+) side input terminal of the overcurrent setting error amplifier 22. The power limiting voltage K1 is supplied to the (−) side input terminal of the overcurrent setting error amplifier 22 from the constant voltage circuit including the zener diode 23.

The output terminals of the output voltage setting error amplifier 7 and the overcurrent setting error amplifier 22 are connected to two input terminals of the MAX circuit 20, respectively. This M
The AX circuit 20 is a circuit that selects and outputs a higher input voltage from the input voltages of the two input terminals. This MAX
The output terminal of the circuit 20 is connected via a pulse width modulator 24 to
It is connected to the gate electrode of the switching FET 3. The inside of the pulse width modulator 24 includes, for example, an integration circuit that integrates an input voltage, and a modulation circuit that modulates a duty ratio or a frequency of an output pulse according to an output of the integration circuit.

As for the correspondence between the invention described in claim 1 and the first embodiment, the transformer corresponds to the transformer 2, the switching means corresponds to the switching FET 3, and the rectifier corresponds to the rectifier circuit 4. The stabilizing means corresponds to the pulse width modulator 24 and the output voltage setting error amplifier 7, the current detecting means corresponds to the current monitoring resistor Rs and the inverting amplifier circuit 10a, and the multiplying means corresponds to the analog multiplier 2.
The limiting means corresponds to the overcurrent setting error amplifier 22 and the MAX circuit 20.

Next, the operation of the power supply circuit 100 will be described.
The operation in the normal state and the operation in the abnormal state will be described separately. (Normal Operation of Power Supply Circuit 100) Under normal conditions, the current I within the normal range is applied to the current monitoring resistor Rs inside the rectifier circuit 4.
o flows. The inverting amplifying circuit 10a inverts the voltage drop (−Rs × Io) generated at both ends of the resistor Rs, and obtains a voltage (Rs × Is) proportional to the current Io flowing through the power supply circuit 100.
Io).

The analog multiplier 21 calculates the voltage (Rs ×
Io) is multiplied by the reference voltage Vr output from the reference voltage unit 9 to output a multiplication result S as described below. Multiplication result S = Rs × Io × Vr (3) The overcurrent setting error amplifier 22 amplifies and outputs an error voltage between the multiplication result S and the power limiting voltage K1. By the way, in the normal operation, the multiplication result S is equal to the power limiting voltage K1.
From the overcurrent setting error amplifier 22,
A saturated voltage is output to the (−) side.

The saturation voltage on the (-) side is equal to the MAX circuit 2
0 is input to one input terminal of the MAX circuit 20.
Allows the output voltage on the side of the output voltage setting error amplifier 7 to pass preferentially. Due to such an operation of the MAX circuit 20,
A first negative feedback loop including the output voltage setting error amplifier 7, the pulse width modulator 24, and the switching FET 3 is configured.

The switching operation of the switching FET 3 is controlled by the first negative feedback loop so as to reduce the error amplification output output from the output voltage setting error amplifier 7. At this time, between the input terminals of the output voltage setting error amplifier 7 is maintained in an imaginary short state.
As a result, the output voltage Vo of the load terminal 5 becomes the following (4)
It is stabilized at the voltage value indicated by the right side of the equation.

Vo = Vr × (R1 + R2) / R2 (4) (Protection operation at the time of abnormality of the power supply circuit 100) On the other hand, when an abnormal situation such as a short circuit accident occurs in the external power-supplied device, the current monitor An excessive current flows through the resistor Rs. In such an abnormal state, the multiplication result S of the analog multiplier 21 temporarily exceeds the power limit voltage K1.

Then, the output voltage of the overcurrent setting error amplifier 22 instantaneously rises to near the (+) side saturation voltage. As a result, the MAX circuit 20 preferentially passes the output voltage of the overcurrent setting error amplifier 22 side. By the operation of the MAX circuit 20, the analog multiplier 21, the overcurrent setting error amplifier 22, and the pulse width modulator 24
And a second negative feedback loop composed of the switching FET 3.

By the second negative feedback loop, the switching operation of the switching FET 3 is controlled in a direction to reduce the error amplification output output from the overcurrent setting error amplifier 22. At this time, between the input terminals of the overcurrent setting error amplifier 22 is maintained in an imaginary short state as shown by the following equation (5). K1 = Rs × Io × Vr (5) Due to this imaginary short state, the current Io flowing through the current monitoring resistor Rs is maintained (limited) at the current value indicated by the right side of the following equation (6).

Io = K1 / (Rs × Vr) (6) FIG. 2 is a view for explaining such a current limiting operation. As shown in FIG. 2, the multiplication result S is the power limiting voltage K1.
If the current Io and the output voltage Vo are
It changes along the trajectory indicated by the dotted arrow in FIG. If the short-circuit accident of the power-supplied device is not solved, the output voltage Vo of the power supply circuit 100 rapidly decreases due to the above-described current limitation.

At this time, since a voltage difference in the (-) direction is applied between the input terminals of the output voltage setting error amplifier 7,
The output voltage setting error amplifier 7 outputs a saturated voltage to the (−) side. Therefore, the MAX circuit 20 continuously passes the output of the overcurrent setting error amplifier 22 side.
As a result, once the second negative feedback loop is formed, the current limiting operation is safely continued unless a short circuit accident or the like on the power-supplied device is solved.

On the other hand, when the short-circuit accident of the power-supplied device is resolved, the power-supplied device in the non-short-circuited state receives the current I shown in the equation (6).
o, the output voltage Vo of the power supply circuit 100
Rises. At this time, since a voltage difference in the (+) direction is applied between the input terminals of the output voltage setting error amplifier 7,
The output voltage setting error amplifier 7 outputs a saturated voltage to the (+) side.

Therefore, the output of the output voltage setting error amplifier 7 passes from the MAX circuit 20, and the first negative feedback loop is formed again. As a result, the operation automatically returns to the normal operation. As described above, in the first embodiment,
When the multiplication result S reaches the power limiting voltage K1, current limiting is performed to reliably protect the power supply circuit 100 and the power-supplied device.

In the first embodiment, when a short-circuit accident on the power-supplied device side is solved, the normal power-supply operation can be automatically restored. Further, in the first embodiment, the analog amplifier 21 is provided instead of the reciprocal amplifier 8 in the conventional example (FIG. 9). As such an analog multiplier 21, for example, a circuit that can operate at high speed, such as a balanced modulator or a Gilbert amplifier, can be used. Therefore, there is no element causing a large phase delay in the second negative feedback loop, and it is possible to easily improve the control performance of the protection operation (response characteristics of current limiting, followability, stability, etc.).

In addition, in the analog multiplier 21,
Unlike the reciprocal amplifier 8, temperature compensation is possible. Also, there is no circuit element such as a broken line approximation. Therefore, a power supply circuit that realizes a highly accurate protection operation can be configured. In the above-described first embodiment, the second negative feedback loop is formed only when an abnormality occurs, but the present invention is not limited to this. For example, by setting change such as lowering the value of the power limiting voltage K1, it is possible to configure the second negative feedback loop in a normal state. In this case, it is possible to configure a power supply circuit that automatically switches between the voltage stabilizing operation and the current stabilizing operation so that the multiplication result S does not exceed the power limiting voltage K1. In particular, such a power supply circuit can be applied to an application in which the output voltage is limited by Vo when the output voltage increases due to an open load or the like, while normally functioning as a constant current source.

Next, another embodiment will be described. <Second Embodiment> A second embodiment is an embodiment corresponding to the second aspect of the present invention. FIG. 3 is a diagram illustrating a configuration of a power supply circuit 200 according to the second embodiment. The feature of the configuration in the second embodiment is that the load-side terminal 5
Is connected to the input terminal X2 of the analog multiplier 21, and
The point is that the power limiting voltage K2 is applied to the (−) side input terminal of the overcurrent setting error amplifier 22.

As for other constituent requirements, the first
Since the configuration requirements are the same as those of the embodiment (FIG. 1), the same reference numerals are given and shown in FIG. 3, and the description is omitted here. Regarding the correspondence between the invention described in claim 2 and the second embodiment, the transformer corresponds to the transformer 2;
The switching means corresponds to the switching FET3,
The rectifier corresponds to the rectifier circuit 4, the stabilizing means corresponds to the pulse width modulator 24 and the output voltage setting error amplifier 7, the current detecting means corresponds to the current monitoring resistor Rs and the inverting amplifier circuit 10a, and the multiplying means corresponds to The limiting means corresponds to the analog multiplier 21 and the overcurrent setting error amplifier 22 and MAX
It corresponds to the circuit 20.

Hereinafter, the operation of the power supply circuit 200 will be described separately for a normal operation and an abnormal operation. (Normal Operation of the Power Supply Circuit 200) Under normal conditions, the current Ir within the normal range is applied to the current monitoring resistor Rs inside the rectifier circuit 4.
o flows. The inverting amplifying circuit 10a inverts the voltage drop (−Rs × Io) generated at both ends of the resistor Rs, and generates a voltage (Rs ×
Io).

The analog multiplier 21 calculates the voltage (Rs ×
Io) is multiplied by the output voltage Vo, and the following multiplication result S is output. Multiplication result S = Rs × Io × Vo (7) The overcurrent setting error amplifier 22 amplifies and outputs an error voltage between the multiplication result S and the power limiting voltage K2. By the way, in the normal operation, the multiplication result S is the power limiting voltage K2
From the overcurrent setting error amplifier 22,
A saturated voltage is output to the (−) side.

The saturation voltage on the (−) side is MAX
Since the signal is input to one input terminal of the circuit 20, the MAX circuit 20 preferentially passes the output voltage on the output voltage setting error amplifier 7 side.

As a result, a first negative feedback loop comprising the output voltage setting error amplifier 7, the pulse width modulator 24 and the switching FET 3 is formed, and the output voltage Vo
Is stabilized at a voltage value indicated by the right side of the following equation (8). Vo = Vr × (R1 + R2) / R2 (8) (Protection operation at the time of abnormality of the power supply circuit 200) On the other hand, if an abnormal situation such as a short circuit accident occurs on the external power-supplied device side, the current monitoring resistor Rs Excessive current flows in In such an abnormal state, the multiplication result S of the analog multiplier 21 temporarily exceeds the power limit voltage K2.

Then, the output voltage of the overcurrent setting error amplifier 22 instantaneously rises to near the (+) side saturation voltage. As a result, the MAX circuit 20 preferentially passes the output voltage of the overcurrent setting error amplifier 22 side. as a result,
A second negative feedback loop including the overcurrent setting error amplifier 22, the pulse width modulator 24, and the switching FET 3 is formed, and the power value (Io × Vo) is maintained at the value indicated by the right side of the following equation (9) ( Restricted).

(Io × Vo) = K2 / Rs (9) FIG. 4 is a diagram for explaining the operation of such power limitation. As shown in FIG. 4, the multiplication result S is the power limiting voltage K2
If the current Io and the output voltage Vo are
It changes along the trajectory of the inverse proportional curve in FIG. If the short-circuit accident of the power-supplied device is not resolved, the current Io does not decrease.
The output voltage Vo of 0 rapidly decreases.

At this time, since a voltage difference in the (-) direction is applied between the input terminals of the output voltage setting error amplifier 7,
The output voltage setting error amplifier 7 outputs a saturated voltage to the (−) side. Therefore, the MAX circuit 20 continuously passes the output of the overcurrent setting error amplifier 22 side.
As a result, once the second negative feedback loop is formed, the power limiting operation safely continues unless a short circuit accident or the like on the power-supplied device is solved.

On the other hand, when the short-circuit accident of the power-supplied device is resolved, the power value shown in the equation (9) is supplied to the non-short-circuited power-supplied device.
o rises. At this time, since a voltage difference in the (+) direction is applied between the input terminals of the output voltage setting error amplifier 7, a saturated voltage is output from the output voltage setting error amplifier 7 to the (+) side. .

Therefore, the output of the output voltage setting error amplifier 7 passes from the MAX circuit 20, and the first negative feedback loop is formed again. As a result, the operation automatically returns to the normal operation. As described above, in the second embodiment,
When the multiplication result S reaches the power limiting voltage K2, power limiting is performed to reliably protect the power supply circuit 200 and the power-supplied device.

In the second embodiment, when a short-circuit accident on the power-supplied device side is solved, the normal power-supply operation can be automatically restored. Further, in the second embodiment, the analog multiplier 21 is provided instead of the reciprocal amplifier 8 in the conventional example (FIG. 9). As such an analog multiplier 21, for example, a circuit that can operate at high speed, such as a balanced modulator or a Gilbert amplifier, can be used. Therefore, there is no element causing a large phase delay in the second negative feedback loop, and it is possible to easily improve the control performance of the protection operation (response characteristics of power limitation, followability, stability, etc.).

In addition, in the analog multiplier 21,
Unlike the reciprocal amplifier 8, temperature compensation is possible. Further, there is no circuit element such as a broken line approximation. Therefore, a power supply circuit that realizes a highly accurate protection operation can be configured. In the above-described second embodiment, the second negative feedback loop is formed only at the time of abnormality, but is not limited to this. For example, it is possible to configure the second negative feedback loop in a normal state by changing the setting such as lowering the value of the power limiting voltage K2. In this case, it is possible to configure a power supply circuit that automatically switches between the voltage stabilizing operation and the power stabilizing operation so that the multiplication result S does not exceed the power limiting voltage K2. In particular, such a power supply circuit is suitable for applications in which the output voltage is limited by Vo when the output voltage increases due to an open load or the like, while normally functioning as a constant power source. Incidentally, as an application of the power supply circuit that enables such a constant power output, for example, there is a lamp power supply, a heater power supply, and the like.

Next, another embodiment will be described. Third Embodiment A third embodiment is an embodiment corresponding to the third aspect of the present invention. FIG. 5 is a diagram illustrating a configuration of a power supply circuit 300 according to the third embodiment. The features of the configuration according to the third embodiment are as follows.

(1) The reference voltage Vr of the reference voltage section 9 is connected to one input contact (Ref in FIG. 5) of the switch 31. (2) The intermediate node between the resistors R1 and R2 is connected to the other input contact (S in FIG. 5) of the switch 31. (3) The switching output of the switch 31 is connected to the input terminal X2 of the analog multiplier 21.

As for other constituent requirements, the first
Since the configuration requirements are the same as those of the embodiment (FIG. 1), the same reference numerals are given and shown in FIG. 5, and the description thereof will be omitted. In the power supply circuit 300 having such a configuration, when the switch 31 is switched to the Ref side by an external operation, a circuit similar to that of the first embodiment is configured. Therefore, in such a setting state of the switch 31, the current limiting operation can be performed at the time of abnormality.

On the other hand, when the switch 31 is switched to the S side by an external operation, a circuit substantially similar to that of the second embodiment can be formed. Therefore, in such a setting state of the switch 31, the power limiting operation can be performed at the time of abnormality. Thus, in the third embodiment,
By switching the switch 31, the protection operation according to the first embodiment and the protection operation according to the second embodiment can be appropriately switched and selected in accordance with a use situation or the like.

In the third embodiment described above,
By performing setting changes such as lowering the power limit value K1, it is possible to configure a second negative feedback loop during normal power supply. In the power supply circuit having such a setting, the operation of the switch 31 causes “the multiplication result S to be the power limit value K1
Automatically switching between the constant voltage output and the constant power output so as not to exceed the power limit value ”and“ the operation for automatically switching between the constant voltage output and the constant current output so that the multiplication result S does not exceed the power limit value K1 ”. Can be appropriately selected.

Next, another embodiment will be described. <Fourth Embodiment> A fourth embodiment is an embodiment corresponding to the fourth aspect of the present invention. FIG. 6 is a diagram illustrating a configuration of a power supply circuit 400 according to the fourth embodiment. The feature of the configuration in the fourth embodiment is that the reference voltage unit 9
Is connected to the input terminal of the reciprocal amplifier 8.

The other components are the same as those of the conventional example (FIG. 9), and therefore, are denoted by the same reference numerals and are shown in FIG. 6, and the description thereof will be omitted. In addition,
Regarding the correspondence between the invention described in claim 4 and the fourth embodiment, the transformer corresponds to the transformer 2, the switching means corresponds to the switching FET 3, the rectifier corresponds to the rectifier circuit 4, and the stabilization means. Corresponds to the pulse width modulator 12 and the output voltage setting error amplifier 7, the current detecting means corresponds to the current monitoring resistor Rs and the inverting amplifier circuit 10a, the reciprocal means corresponds to the reciprocal amplifier 8, and the limiting means corresponds to overcurrent. It corresponds to the setting error amplifier 10 and the MAX circuit 11.

In the power supply circuit 400 having such a configuration, the output voltage (Rs × Io) of the inverting amplifier circuit 10 a rapidly increases and exceeds the output voltage of the reciprocal amplifier 8 in an abnormal state such as a short circuit accident. Then, a second negative feedback loop including the overcurrent setting error amplifier 10, the pulse width modulator 12, and the switching FET 3 is formed, and a current limiting operation for limiting the current Io to a constant current is performed. Therefore,
The power supply circuit 400 and the power-supplied device can be reliably protected.

Further, in the fourth embodiment, since a time-invariant reference voltage is input to the reciprocal amplifier 8, even if a phase delay occurs in the reciprocal amplifier 8, the phase characteristic of the second negative feedback loop is affected. Will never give. for that reason,
By ignoring the phase delay of the reciprocal amplifier 8 at all, it is possible to easily improve the control performance of the protection operation (response characteristics, follow-up property, stability, etc. of the current limiting operation).

Further, the input dynamic range of the reciprocal means only needs to cover a reference voltage which may be input. Therefore, there is no need to secure a wide input dynamic range as in the conventional example (FIG. 9), and the circuit configuration of the reciprocal amplifier 8 can be simplified. In the first to fourth embodiments, the reference voltage unit 9 is
Although a circuit for discretely switching the value of the reference voltage using the Zener diode and the switches 9a to 9c has been described, the present invention is not limited to this. For example, as shown in FIG. 7A, a circuit for continuously switching the value of the reference voltage using a zener diode and a variable resistor is provided in a reference voltage unit 9.
You may use as. Further, as shown in FIG. 7B, a voltage variable circuit using a shunt regulator may be used as the reference voltage unit 9. Further, FIG.
As shown in (1), the reference voltage unit 9 may be configured by connecting an external reference voltage source via a connector.

In the first to fourth embodiments described above,
As the current detecting means, a circuit including the current monitoring resistor Rs and the inverting amplifier circuit 10a is used, but the present invention is not limited to this. For example, the circuit shown in FIG. 8 may be used as the current detecting means. In this circuit,
A current transformer 51 is arranged on the current path A or B shown in the fourth embodiment (FIGS. 1, 3, 5, and 6). An electromotive force is generated at an output terminal of the current transformer 51 in accordance with a current flowing through the power supply circuit 400. Current detection can be performed by peak detection of this electromotive force via the rectifier 52 and the peak detector 53. Note that the current detection may be performed by directly connecting the peak detector 53 to the output terminal of the current transformer 51.

Further, in the above-described first to fourth embodiments, the power supply circuit having a constant voltage output for stabilizing the output voltage of the rectifier circuit 4 in a normal state has been described. However, the present invention is not limited to this. For example, the present invention can be applied to a constant current output power supply circuit that stabilizes the output current of the rectifier circuit 4. Further, the present invention can also be applied to a power supply circuit having a constant power output for stabilizing the output power of the rectifier circuit 4.

In the first to fourth embodiments described above,
Although the protection operation is performed via the pulse width modulators 24 and 12, the present invention is not limited to this. In general,
Circuits for performing the protection operation are pulse width modulators 24 and 12
And may be provided separately. For example, a voltage drop type limiting circuit is provided on the primary side or the secondary side of the transformer 2, and the voltage drop of the limiting circuit is controlled via a second negative feedback loop, so that the output of the rectifier circuit 4 is controlled. Restrictions may be made.

[0085]

According to the first aspect of the present invention, when the multiplication result of the multiplication means reaches a predetermined power limit value in an abnormal state such as a short circuit accident, a negative feedback operation is performed by the restriction means. Is done. By this negative feedback operation, current limitation is performed in a direction such that the value of the detection current approaches the inversely proportional value of the reference voltage. As a result, the power supply circuit and the power-supplied device can be reliably protected.

In the power supply circuit according to the first aspect,
It does not have the reciprocal amplifier 8 in the conventional example (FIG. 9), but is provided with multiplication means instead. As such a multiplying means, for example, a circuit that can operate at high speed, such as a balanced modulator or a Gilbert amplifier, can be used. Therefore,
There is no element that causes a large phase delay in the negative feedback loop at the time of abnormality, and the control performance of the protection operation (response characteristics, followability, stability, etc. of the current limiting operation) can be easily improved.

Further, in the known multiplication circuit, unlike the reciprocal amplifier (FIG. 10), temperature characteristic compensation can be performed. Also, there is no circuit element such as a broken line approximation. Therefore, it is possible to configure a power supply circuit that realizes a highly accurate protection operation. In the power supply circuit according to the first aspect, it is also possible to operate the limiting unit during normal power supply by setting such as lowering the power limit value. With such a setting, a power supply circuit that automatically switches between constant voltage output and constant current output can be configured so that the multiplication result does not exceed the power limit value.

(Claim 2) In the invention according to claim 2,
When the multiplication result of the multiplication means reaches a predetermined power limit value in an abnormal state such as a short circuit accident, a negative feedback operation is performed by the restriction means. With this negative feedback operation, power limitation is performed in such a direction that the multiplication result of the detection current and the output voltage approaches the power limitation value. As a result, the power supply circuit and the power-supplied device can be reliably protected.

In the power supply circuit according to the second aspect,
It does not have the reciprocal amplifier 8 in the conventional example (FIG. 9), but is provided with multiplication means instead. As such a multiplying means, for example, a circuit that can operate at high speed, such as a balanced modulator or a Gilbert amplifier, can be used. Therefore,
The element that causes a large phase delay in the negative feedback loop at the time of abnormality is eliminated, and the control performance of the protection operation (response characteristics, followability, stability, etc. of the power limiting operation) can be easily improved.

Further, in the known multiplying circuit, different from the reciprocal amplifier, it is possible to perform temperature compensation. Also, there is no circuit element such as a broken line approximation. Therefore, it is possible to configure a power supply circuit that realizes a highly accurate protection operation.
In the power supply circuit according to the second aspect, it is also possible to operate the limiting means during normal power supply by setting such as lowering the power limit value. With such a setting, it is possible to configure a power supply circuit that automatically switches between constant voltage output and constant power output so that the multiplication result does not exceed the power limit value.

(Claim 3) In the invention according to claim 3,
By switching the switching means, the protection operation according to the first aspect or the protection operation according to the second aspect can be appropriately selected in accordance with a use situation or the like. In the power supply circuit according to the third aspect of the present invention, the limiting means can be operated during normal power supply by setting such as lowering the power limit value. In the power supply circuit with such a setting, by operating the switching means, “the operation of automatically switching between the constant voltage output and the constant power output so that the multiplication result does not exceed the power limit value” and “the multiplication result becomes the power Automatically switching between the constant voltage output and the constant current output so as not to exceed the limit value ".

(Claim 4) In the invention according to claim 4,
When the detected current reaches the output value of the reciprocal means in an abnormal state such as a short circuit accident, the limiting means performs a negative feedback operation. By this negative feedback operation, current limitation is performed in a direction such that the value of the detection current approaches the inversely proportional value of the reference voltage. As a result, the power supply circuit and the power-supplied device can be reliably protected.

In the power supply circuit according to the fourth aspect,
A time-invariant reference voltage is input to the reciprocal means. Therefore, even if a phase delay occurs in the reciprocal means, it does not affect the phase characteristics of the negative feedback loop at all. Therefore, it is possible to easily improve the control performance of the protection operation (response characteristics, follow-up characteristics, stability, etc. of the current limiting operation) ignoring any phase delay of the reciprocal means.

Further, the input dynamic range of the reciprocal means only needs to cover a reference voltage which may be input. Therefore, it is not necessary to secure a wide input dynamic range as in the conventional example (FIG. 9), and the circuit configuration of the reciprocal means can be simplified.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a circuit configuration of a first embodiment.

FIG. 2 is a diagram illustrating a protection operation according to the first embodiment.

FIG. 3 is a diagram illustrating a circuit configuration of a second embodiment.

FIG. 4 is a diagram illustrating a protection operation according to a second embodiment.

FIG. 5 is a diagram illustrating a circuit configuration of a third embodiment.

FIG. 6 is a diagram illustrating a circuit configuration of a fourth embodiment.

FIG. 7 is a diagram illustrating a circuit example of a reference voltage unit 9;

FIG. 8 is a diagram showing a circuit configuration of a current detection unit.

FIG. 9 is a diagram showing a configuration of a conventional power supply circuit.

FIG. 10 is a diagram illustrating a circuit example of a reciprocal amplifier 8;

FIG. 11 is a diagram showing input / output characteristics of a reciprocal amplifier.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rectifier circuit 2 Transformer 3 Switching FET 4 Rectifier circuit 5 Load side terminal 6a, 6b Resistance 7 Output voltage setting error amplifier 8 Reciprocal amplifier 9 Reference voltage section 10 Overcurrent setting error amplifier 10a Inverting amplifier circuit 11 MAX circuit 12 Pulse width modulation 20 MAX circuit 21 Analog multiplier 22 Overcurrent setting error amplifier 23 Zener diode 24 Pulse width modulator 31 Switch 51 Current transformer 52 Rectifier 53 Peak detector 100 Power supply circuit 200 Power supply circuit 300 Power supply circuit 400 Power supply circuit 700 Conventional power supply circuit Rs Current monitor resistance Vr Reference voltage

Claims (4)

(57) [Claims] A transformer for inputting an input voltage to a primary side; a switching unit for switching a current path on a primary side of the transformer; a switching operation of the switching unit connected to a secondary side of the transformer. A rectifier for rectifying and smoothing and outputting an electromotive force generated on the secondary side of the transformer, and controlling a switching operation of the switching means;
A power supply circuit including a stabilizing unit that stabilizes an output of the rectifier according to a predetermined reference voltage; a current detection unit that detects a current flowing in the power supply circuit; and a current detection unit that detects the current. A multiplying unit that outputs a multiplication result of the detection current and the predetermined reference voltage; and a multiplication result output from the multiplying unit reaches a predetermined power limit value. And a limiting means for limiting the output of the rectifier by negatively feeding back the error of the rectifier. 2. A transformer for inputting an input voltage to a primary side, switching means for switching a current path on a primary side of the transformer, and a switching operation of the switching means connected to a secondary side of the transformer. A rectifier for rectifying and smoothing and outputting an electromotive force generated on the secondary side of the transformer, and controlling a switching operation of the switching means;
A power supply circuit including a stabilizing unit that stabilizes an output of the rectifier according to a predetermined reference voltage; a current detection unit that detects a current flowing in the power supply circuit; and a current detection unit that detects the current. A multiplying unit that outputs a multiplication result of the detection current and the output voltage of the rectifier; and a multiplication result output from the multiplication unit reaches a predetermined power limit value. And a limiting means for limiting the output of the rectifier by negatively feeding back the error of the rectifier. 3. A transformer for inputting an input voltage to a primary side, switching means for switching a current path on a primary side of the transformer, and a switching operation of the switching means connected to a secondary side of the transformer. A rectifier for rectifying and smoothing and outputting an electromotive force generated on the secondary side of the transformer, and controlling a switching operation of the switching means;
A power supply circuit comprising: a stabilizing means for stabilizing an output of the rectifier in accordance with a predetermined reference voltage; a current detecting means for detecting a current flowing in the power supply circuit; and Switching means for extracting one of the reference voltage and the output voltage of the rectifier, multiplication means for outputting a multiplication result of a detection current detected by the current detection means and a switching output of the switching means, When the output multiplication result reaches a predetermined power limit value, a negative means for negatively feeding back an error between the multiplication result and the power limit value to limit the output of the rectifier is provided. Characteristic power supply circuit. 4. A transformer to which a manual voltage is input to a primary side, switching means for switching a current path on a primary side of the transformer, and a switching operation of the switching means connected to a secondary side of the transformer. A rectifier for rectifying and smoothing and outputting an electromotive force generated on the secondary side of the transformer, and controlling a switching operation of the switching means;
A power supply circuit comprising: a stabilizing unit that stabilizes an output of the rectifier according to a predetermined reference voltage; a current detection unit that detects a current flowing in the power supply circuit; and a reciprocal of the predetermined reference voltage. Reciprocal means for outputting a corresponding value, When the detection current detected by the current detection means reaches the output value of the reciprocal means, the error between the detected current and the output value of the reciprocal means is negatively fed back. And a limiting means for limiting the output of the rectifier.