JP5588305B2 - Power supply - Google Patents

Description

  The present invention relates to a noise reduction technique for a switching power supply device.

Conventionally, a switching power supply device is known as a highly efficient power supply device. The switching power supply device includes a transistor as a switching element and a control IC for turning on / off the transistor, and the control IC turns on / off the transistor with a duty ratio corresponding to the input voltage (PWM: Pulse Width Modulation). Thus, by controlling the switching operation of the transistor, a predetermined output voltage is generated with high efficiency.
As this type of switching power supply device, an insulating switching power supply device is known in which a primary side and a secondary side are insulated by using, for example, a transformer as a voltage converter. The isolated switching power supply is highly efficient, has excellent insulation, and can easily configure a multi-output power supply by providing multiple secondary windings in the transformer. It is suitably used for the purpose of converting to.

  By the way, in the switching power supply device, since the control IC turns on / off the transistor at high speed, it is known that high frequency noise corresponding to the switching frequency of the transistor occurs. More specifically, in a switching power supply device, when a transistor is turned on, a high-frequency oscillation voltage is generated due to current flowing from the secondary side of the transformer to the primary side through the coupling capacitance of the transformer. When is turned off, a surge current is generated on the primary side. These high-frequency oscillating voltages and surge currents contain a large amount of alternating current components whose frequencies correspond to the switching frequency, and cause the above-mentioned noise such as line noise and radiation noise.

  As a noise countermeasure technique, a technique of slowing the transistor switching speed is widely adopted. However, if the switching speed is slowed, the switching loss also increases, leading to an increase in transistor size and an increase in current consumption. As another method for preventing noise, there is a case where a snubber circuit is provided in parallel with a rectifier diode provided on the secondary side of the transformer to suppress the generation of high-frequency oscillation voltage. However, depending on the capacity of the snubber circuit, the current flowing into the transistor increases, leading to an increase in the size of the transistor.

  Therefore, in recent years, as a countermeasure against noise, in a switching power supply device configured to input the switching frequency of the transistor to the input terminal of the control IC, a device that applies a voltage that varies with a periodic function of time to the input terminal is provided. There has been proposed a method of periodically changing the switching frequency in conjunction with the change in the voltage applied to the input terminal. According to this method, since the switching frequency fluctuates periodically, the spectrum of the switching noise is diffused, thereby reducing the noise level (see, for example, Patent Document 1 and Patent Document 2).

JP 2009-273215 A JP 2008-5682 A

However, in the conventional technique, a device that applies a voltage that changes with a periodic function of time is realized by an electric circuit having a complicated circuit configuration, resulting in high cost and complicated mounting.
The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a power supply device capable of suppressing the noise level with a simple circuit configuration.

In order to achieve the above object, the present invention provides a switching element connected to a primary winding of a transformer, and switching of the switching element so that an output voltage on a secondary side of the transformer becomes a predetermined voltage level. And a control circuit that controls the operation, a frequency setting circuit that generates a frequency regulation voltage that regulates a switching frequency of the switching element and inputs the voltage to the control circuit, and an oscillation signal whose voltage continuously changes A fluctuation signal generation circuit that fluctuates the frequency regulation voltage that is input to the frequency setting circuit and generated by the frequency setting circuit, and the fluctuation signal generation circuit includes a Schmitt trigger as a generation source of the oscillation signal. RC in which the output and input of the inverter are connected via a resistor and a capacitor is connected to the input of the Schmitt trigger inverter. An oscillation circuit or multiple stages of inverters are connected in series, and the output of one of these inverters and the input of the first stage inverter are connected via a resistor, and the output of any other inverter and the first stage An RC oscillation circuit having a capacitor connected to the input of the inverter, and a current flowing from the primary winding through the switching element is equal to or less than a predetermined value, between the RC oscillation circuit and the frequency setting circuit. It is characterized by including a shut-off circuit for shutting off the power.

According to the present invention, the generation source of the oscillation signal for changing the frequency regulation voltage that regulates the switching frequency of the power supply apparatus is an RC composed of a Schmitt trigger inverter or a plurality of stages of inverters, resistors, and capacitors. Since the oscillation circuit is configured, the oscillation signal can be generated with a simpler circuit. Further, the operation of suppressing the noise level by changing the switching frequency due to the change of the frequency regulation voltage is performed when the current flowing from the primary winding through the switching element is large and exceeds a predetermined value, that is, when the noise is large. It can be limited.

  According to the present invention, in the power supply apparatus, the Schmitt trigger inverter or the inverter is configured by a CMOS logic gate.

  According to the present invention, current consumption can be suppressed and high-speed driving can be achieved as compared with a Schmitt trigger (hysteresis comparator) type inverter using an operational amplifier (operational amplifier) as a comparator.

  Further, the present invention provides the above-described power supply device, wherein the RC oscillation circuit is configured by connecting the resistor and the capacitor to a one-chip inverter logic IC incorporating the Schmitt trigger inverter, or a plurality of stages of the inverters. The RC oscillation circuit is configured by connecting the resistor and the capacitor to a built-in one-chip inverter logic IC.

  According to the present invention, a generation source of an oscillation signal for changing a frequency regulation voltage can be simply configured by simply connecting a resistor and a capacitor to a one-chip inverter logic IC.

The present invention, in the power supply, the shutoff circuit, the inserted and the RC oscillator circuit between said frequency setting circuit, while current flowing through the switching element is less than the predetermined value, it turns off P A channel MOSFET is provided, and a voltage is applied to the drain side of the P-channel MOSFET on the drain side of the P-channel MOSFET to maintain a higher potential than the source side while a voltage is applied to the drain side of the P-channel MOSFET while the current flowing through the switching element is below a predetermined value. And a potential holding circuit that applies the voltage to the capacitor of the RC oscillation circuit connected to the drain to hold the charging voltage constant and stop the oscillation.

  According to the present invention, since the RC oscillation circuit and the frequency setting circuit are cut off by the P-channel MOSFET, it is possible to prevent a contact surge that occurs at the time of connection as compared with a configuration in which the RC relay is cut off by a mechanical relay or the like. Further, since the drain side is held at a higher potential than the source side while the P-channel MOSFET is turned off, it is possible to prevent a backflow through which a current flows through a parasitic diode in the P-channel MOSFET, and to improve insulation. Further, since the charging voltage of the capacitor of the RC oscillation circuit is held constant by the high potential on the drain side, the oscillation of the RC oscillation circuit is stopped, and thereby the RC oscillation circuit and the frequency setting circuit are shut off. Wasteful current consumption can be suppressed.

According to the present invention, the generation source of the oscillation signal for changing the frequency regulation voltage that regulates the switching frequency of the power supply apparatus is an RC composed of a Schmitt trigger inverter or a plurality of stages of inverters, resistors, and capacitors. Since the oscillation circuit is configured, the oscillation signal can be generated with a simpler circuit.
Further, in the present invention, the Schmitt trigger inverter or the inverter is composed of a CMOS logic gate, so that current consumption can be suppressed and high-speed driving can be achieved as compared with a Schmitt trigger type inverter using an operational amplifier as a comparator.
In the present invention, the RC oscillation circuit is configured by connecting the resistor and the capacitor to a one-chip inverter logic IC including the Schmitt trigger inverter, or a one-chip inverter including a plurality of stages of inverters. By connecting the resistor and the capacitor to the inverter logic IC to configure the RC oscillation circuit, it is possible to change the frequency regulation voltage only by connecting the resistor and the capacitor to the one-chip inverter logic IC. The generation source of the oscillation signal can be configured easily.
In the present invention, a cutoff circuit that cuts off the output from the RC oscillation circuit to the frequency setting circuit while the current flowing from the primary winding through the switching element is not more than a predetermined value is provided. The operation of suppressing the noise level by changing the switching frequency due to the fluctuation can be limited to when the current flowing from the primary winding through the switching element is large and exceeds a predetermined value, that is, when the noise is large.
In the present invention, the cutoff circuit includes a P-channel MOSFET that is inserted between the RC oscillation circuit and the frequency setting circuit, and is turned off while a current flowing through the switching element is equal to or less than a predetermined value. On the drain side of the P-channel MOSFET, while the current flowing through the switching element is equal to or less than a predetermined value, a voltage is applied to the drain side of the P-channel MOSFET to maintain a higher potential than the source side, and the voltage is applied to the drain. A potential holding circuit that applies a voltage to the capacitor of the connected RC oscillation circuit to hold the charging voltage constant and stops oscillation may be provided. According to this configuration, the RC oscillation circuit and the frequency setting circuit are blocked by the P-channel MOSFET, and therefore, contact surge that occurs at the time of connection can be prevented as compared with the configuration of blocking by a mechanical relay or the like. Further, while the P-channel MOSFET is off, the drain side is held at a higher potential than the source side, so that a so-called reverse flow in which a current flows through a parasitic diode in the P-channel MOSFET can be prevented, and insulation can be improved. it can. Furthermore, since the oscillation of the RC oscillation circuit is stopped while the P-channel MOSFET is off, useless current consumption while the RC oscillation circuit and the frequency setting circuit are cut off can be suppressed.

1 is a circuit diagram of an insulating switching power supply device according to a first embodiment of the present invention. FIG. 3 is a circuit diagram of a frequency setting circuit and a fluctuation signal generation circuit. It is a signal waveform diagram showing the oscillation operation of the RC oscillation circuit. It is a wave form diagram which shows operation of a switching power supply device, (A) shows the case where a switching frequency is fixed constant, (B) shows the case where switching frequency is made low and fixed to a fixed value, (C) is The case where the switching frequency is varied by the output of the variation signal generation circuit is shown. It is a figure which shows the measurement result of the noise which generate | occur | produces with the switching operation | movement of a switching power supply device, (A) shows the case where a switching frequency is fixed, (B) is an output of the signal generation circuit for a fluctuation | variation. It shows the case where it is changed by. It is a circuit diagram of the switching power supply device concerning a 2nd embodiment of the present invention. It is a signal waveform diagram showing the oscillation operation of the RC oscillation circuit. It is a circuit diagram of the switching power supply device concerning a 3rd embodiment of the present invention. It is a signal waveform diagram showing the oscillation operation of the RC oscillation circuit. It is a circuit diagram of the RC oscillation circuit which concerns on the modification of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<First Embodiment>
FIG. 1 is a circuit diagram of an insulating switching power supply device 1 according to the present embodiment.
As shown in FIG. 1, the switching power supply device 1 is a separately-excited (flyback type) switching power supply (DC-DC converter), and has a flyback transformer 10 on the primary side of the flyback transformer 10. The inputted input voltage Vin is converted into a plurality of output voltages Vout having a predetermined voltage level, and supplied to a plurality of load circuits 11 connected to the secondary side.

  More specifically, on the primary side of the flyback transformer 10, a transistor circuit 13 as a switching element connected to the primary winding 12 of the flyback transformer 10 to turn on / off current, A control IC 14 that controls the switching operation and a smoothing capacitor 15 that smoothes the input voltage Vin to the primary side are provided. A power MOSFET is used for the transistor circuit 13 to increase the switching speed and increase the conversion efficiency. In addition, a surge absorption circuit 35 for removing a surge current generated in accordance with the switching operation is provided between the collector of the transistor circuit 13 and the input line of the input voltage Vin, and the transistor circuit 13 is configured to prevent the surge circuit from the input voltage Vin. Protected.

The control IC 14 supplies a drive signal from the output terminal 14A to the gate terminal 13A of the transistor circuit 13 to perform a switching operation. The control IC 14 performs PWM control by turning on / off the transistor circuit 13 with a duty ratio corresponding to the input voltage Vin. The switching operation of the transistor circuit 13 is controlled, and a predetermined output voltage Vout is generated on the secondary side of the flyback transformer 10. The primary input voltage Vin is input to the power input terminal 14B of the control IC 14, and the control IC 14 operates using the input voltage Vin as a power source. The power input terminal 14B is provided with a backup power source capacitor 17 that is charged as the input voltage Vin is supplied. When generation of the output voltage Vout is stopped, the power input terminal 14B is controlled by the electric power stored in the capacitor 17. The IC 14 is configured to be operable.
Further, the control IC 14 generates a drive signal Ks (see FIG. 4) using the input voltage Vin as a voltage source, and supplies it to the gate terminal 13A of the transistor circuit 13.

  On the secondary side of the flyback transformer 10, a plurality of secondary windings 18 are provided for realizing multiple outputs. A diode 19 and a smoothing capacitor 20 are connected to each secondary winding 18. When a current change occurs in the primary winding 12 with the switching operation of the transistor circuit 13, an electromotive force is induced in each of the secondary windings 18 with the current change, and the diode 19 and the smoothing A predetermined output voltage Vout is generated by rectification and smoothing by the capacitor 20, and this output voltage Vout is supplied to the load circuit 11.

  The switching power supply device 1 has a configuration for feedback control of the output voltage Vout. In other words, the primary side of the flyback transformer 10 is provided with a detection circuit 22 that outputs a detection signal having a voltage level corresponding to the output voltage Vout. The detection circuit 22 rectifies the electromotive force of the tertiary winding 21 in which an electromotive force corresponding to the electric power of the secondary winding 18 is induced by the current change of the secondary winding 18, A diode 23 and a smoothing capacitor 24 for smoothing and converting to a direct current, a noise removing capacitor 25, and a load resistor 26, and a voltage corresponding to the voltage level of the output voltage Vout (hereinafter, “ A detection signal of “detection signal voltage Vs” is output.

A feedback adjustment circuit 30 and an error voltage adjustment circuit 31 are provided on the primary side of the switching power supply device 1, and a detection signal of the detection circuit 22 is input to each of them.
The feedback adjustment circuit 30 generates an adjustment signal for adjusting the duty ratio of the PWM control according to the detection signal so that the output voltage Vout is maintained at a constant voltage level, and outputs the adjustment signal to the control IC 14. When the control IC 14 dynamically varies the duty ratio of the PWM control based on the adjustment value of the feedback adjustment circuit 30, a fluctuation in the input voltage Vin on the primary side or a load fluctuation on the secondary side occurs. However, the output voltage Vout is kept constant.
The error voltage adjustment circuit 31 detects whether the voltage level of the output voltage Vout is within a predetermined range, and outputs the detection result to the control IC 14. When the voltage level of the output voltage Vout is out of the predetermined range, the control IC 14 promptly stops the switching operation of the transistor circuit 13 to indicate that some abnormality has occurred.

In addition, a soft start circuit 33, a starting voltage adjustment circuit 34, a frequency setting circuit 37, and a fluctuation signal generation circuit 40 are provided on the primary side of the switching power supply device 1.
The soft start circuit 33 gradually increases the pulse width of the drive signal Ks to the transistor circuit 13 until the output voltage Vout is stabilized by feedback control by the control IC 14 in order to prevent an inrush current at the start. This is a circuit that controls the control IC 14 to increase the output voltage Vout substantially linearly (so-called soft start).
The startup voltage adjustment circuit 34 is a circuit that adjusts the startup voltage at which the control IC 14 starts operating when the input voltage Vin is input. That is, the voltage range of the drive signal Ks of the transistor circuit 13 includes a voltage range in which the operation of the transistor circuit 13 is possible but the ON resistance is high and the loss is increased. The start-up voltage adjustment circuit 34 adjusts the start-up voltage so that the control IC 14 does not operate during the voltage range of the input voltage Vin, and suppresses the driving of the transistor circuit 13 in this voltage range.

The frequency setting circuit 37 generates a frequency defining voltage V1 that defines a basic frequency (reference frequency) of a switching frequency used for PWM control of the transistor circuit 13 by the control IC 14, and inputs the frequency defining voltage V1 to the input terminal 14C of the control IC 14. The control IC 14 incorporates a carrier wave generator 36 that generates a carrier wave having a carrier frequency corresponding to the voltage level of the frequency regulation voltage V1, and adjusts the duty ratio of the carrier wave input from the feedback adjustment circuit 30. A drive signal Ks for the transistor circuit 13 is generated by adjusting based on the signal.
The fluctuation signal generation circuit 40 is a circuit that generates an oscillation signal V2 whose voltage continuously changes and inputs the oscillation signal V2 to the frequency setting circuit 37, and changes the frequency regulation voltage V1 generated by the frequency setting circuit 37.
When the frequency regulation voltage V1 is varied by the variation signal generation circuit 40, the switching frequency spectrum is spread by continuously varying the switching frequency of the transistor circuit 13 in conjunction with the variation of the frequency regulation voltage V1. As a result, the noise level is reduced.

FIG. 2 is a circuit diagram of the frequency setting circuit 37 and the fluctuation signal generation circuit 40.
The frequency setting circuit 37 includes a charging / discharging capacitor 51, and a charging / discharging circuit that outputs the charging voltage of the capacitor 51 as a frequency regulation voltage V1, and the charging voltage of the capacitor 51 is a fluctuation signal. It is configured to vary depending on the oscillation signal V2 of the generation circuit 40.
That is, a current limiting resistor 52 is connected in series to the charging / discharging capacitor 51, and the power supply voltage Vref is applied from the constant voltage source via the resistor 52, and the capacitor 51 is connected by the power supply voltage Vref. Charged. The charging voltage of the capacitor 51 defines the fundamental frequency (reference frequency) of the switching frequency, and the charging voltage of the capacitor 51 is output as the frequency defining voltage V1 from the connection point P1 between the capacitor 51 and the resistor 52.
The output of the fluctuation signal generation circuit 40 is connected to the high potential side of the charge / discharge capacitor 51 via a resistor 53. Therefore, when the potential of the output terminal P2 fluctuates due to the output of the oscillation signal V2 of the fluctuation signal generation circuit 40, the charging voltage of the capacitor 51, that is, the frequency regulation voltage V1 fluctuates in conjunction with this fluctuation.

The fluctuation signal generation circuit 40 is a circuit including a self-excited RC oscillation circuit 41 configured by using a one-chip inverter logic IC as a generation source of the oscillation signal V2.
More specifically, the RC oscillation circuit 41 includes a Schmitt trigger type inverter logic IC 43, a resistor 44 provided between the output terminal P3 and the input terminal P4 of the Schmitt trigger type inverter logic IC 43, and a Schmitt trigger type inverter logic IC 43. And a charge / discharge capacitor 45 connected to the input terminal P4.
The Schmitt trigger inverter logic IC 43 is an IC that incorporates a Schmitt trigger inverter circuit 42 and is packaged in one chip. Further, the Schmitt trigger inverter circuit 42 is a logic gate circuit of an inverter logic in which the output state changes with hysteresis with respect to the change of the input potential because the threshold for the input potential is different between the high level and the low level. .

FIG. 3 is a signal waveform diagram showing the oscillation operation of the RC oscillation circuit 41.
In the RC oscillation circuit 41, when a power supply voltage (not shown) is applied to the Schmitt trigger type inverter logic IC 43 (t = 0), the charging voltage of the capacitor 45, that is, the input of the Schmitt trigger type inverter logic IC 43. Since the voltage V4 at the terminal P4 is “zero” (= Low level), the voltage V3 at the output terminal P3 of the Schmitt trigger inverter logic IC 43 is at the High level (= Va> 0). Since the voltage V3 at the output terminal P3 is applied to the capacitor 45 through the resistor 44 and charged, the voltage V4 at the input terminal P4 gradually increases.

  When the voltage V4 reaches the high level threshold VH (t = t1), the voltage V3 at the output terminal P3 of the Schmitt trigger type inverter logic IC 43 is inverted to the low level (V = 0), and the capacitor 45 is changed from the charged state. Switch to the discharge state. When the voltage V4 at the input terminal P4 gradually decreases and reaches the low level threshold VL (t = t2), the voltage V3 at the output terminal P3 of the Schmitt trigger inverter logic IC 43 is inverted again to the high level, The capacitor 45 is charged.

  That is, in the RC oscillation circuit 41, the Schmitt trigger type inverter logic IC 43 functions as a switch circuit that switches charge / discharge of a charge / discharge circuit formed by connecting a resistor 44 and a capacitor 45 in series at a predetermined cycle. Thus, the voltage V4 at the input terminal P4 continuously changes in a triangular wave (sawtooth wave) shape, and an oscillation signal is generated by this voltage fluctuation.

The voltage V4 (that is, the oscillation signal) at the input terminal P4 of the Schmitt trigger type inverter logic IC 43 is taken out as an oscillation signal V2 at the output terminal P2 of the fluctuation signal generation circuit 40. The voltage of the oscillation signal V2 is the frequency setting circuit 37. The charge / discharge capacitor 51 is applied. Therefore, the charging voltage of the capacitor 51, that is, the frequency regulation voltage V1 varies in conjunction with the voltage variation of the oscillation signal V2 of the variation signal generation circuit 40.
Specifically, as shown in FIG. 3, as the voltage of the oscillation signal V2 of the fluctuation signal generation circuit 40 increases, the capacitor 51 is charged, so that the frequency regulation voltage V1 gradually increases. On the contrary, the frequency defining voltage V1 gradually decreases as the capacitor 51 is discharged as the voltage of the oscillation signal V2 of the fluctuation signal generating circuit 40 decreases.
As the frequency regulation voltage V1 varies in this way, the switching frequency generated by the control IC 14 varies.

The fluctuation period of the switching frequency depends on the frequency of the oscillation signal that varies the frequency defining voltage V1, and the frequency of the oscillation signal is defined by the resistance value of the resistor 44 of the RC oscillation circuit 41 and the capacitance of the capacitor 45. It is adjusted by changing these values.
The fluctuation range of the switching frequency depends on the fluctuation range of the frequency regulation voltage V1. In the present embodiment, the frequency setting circuit 37 includes a charge / discharge circuit including a charge / discharge capacitor 51, and outputs a charge voltage of the capacitor 51 as a frequency regulation voltage V 1, and also outputs a fluctuation signal to the capacitor 51. Since the charging voltage is varied by applying the oscillation signal V2 from the generation circuit 40, as a result, the variation width of the frequency regulation voltage V1 is the value of each element of the frequency setting circuit 37 and the variation signal generation circuit 40. Further, it can be finely adjusted by changing each value.

FIG. 4 is a diagram illustrating a waveform indicating the operation of the switching power supply device 1 together with a comparative example. 4A shows the case where the switching frequency is fixed, FIG. 4B shows the case where the switching frequency is lowered and fixed to a constant value, and FIG. 4C shows the switching frequency. A case where the variation is generated by the output of the variation signal generation circuit 40 is shown.
A drive signal Ks that changes at the switching frequency is supplied to the transistor circuit 13 and the transistor circuit 13 performs a switching operation. As a result, the output voltage on the secondary side is synchronized with the drive signal Ks as shown in FIG. Vout is generated. In addition, a surge voltage Vk is generated on the primary side as the drive signal Ks is switched on / off.

When the switching frequency is lowered and the switching speed is lowered, as shown by an arrow α in FIG. 4 (B), the surge voltage Vk changes relatively slowly when the drive signal Ks is switched on / off. , The noise level is reduced. However, if the switching speed is slowed, the switching loss also increases, leading to an increase in the size of the transistor circuit 13 and an increase in current consumption.
On the other hand, even when the switching frequency is changed by the output of the fluctuation signal generation circuit 40, the surge voltage Vk is generated when the drive signal Ks is switched on / off as indicated by an arrow β in FIG. It turns out that the noise level is reduced without lowering the switching speed.

5A and 5B are diagrams showing measurement results of noise generated along with the switching operation of the switching power supply apparatus 1. FIG. 5A shows a case where the switching frequency is fixed, and FIG. 5B shows switching. The case where the frequency is varied by the output of the variation signal generation circuit 40 is shown.
As shown in this figure, it can be seen that by varying the switching frequency, the noise level D is suppressed as compared with the case where the switching frequency is fixed.

As described above, the fluctuation signal generation circuit 40 that varies the switching frequency includes the Schmitt trigger inverter logic IC 43 in the RC oscillation circuit 41 that is the generation source of the oscillation signal V2.
In the present embodiment, the Schmitt trigger type inverter logic IC 43 uses a Schmitt trigger inverter circuit 42 as a plurality of CMOS logic gates that are a combination of a pair of P-channel MOS transistors and N-channel MOS transistors in a complementary manner. A so-called CMOS type IC is used. As a result, current consumption can be suppressed and high-speed driving can be achieved as compared with a Schmitt trigger (hysteresis comparator) type inverter using an operational amplifier (operational amplifier) as a comparator.

  Further, by configuring the RC oscillation circuit 41 using such a one-chip Schmitt-trigger inverter logic IC 43, only the resistor 44 and the capacitor 45 are connected to the one-chip Schmitt-trigger inverter logic IC 43. Thus, the generation source of the oscillation signal can be configured easily.

  As described above, according to the present embodiment, the generation source of the oscillation signal for changing the frequency defining voltage V1 that defines the switching frequency of the switching power supply device 1 is the Schmitt trigger type inverter logic IC 43, the resistor 44, and Since the RC oscillator circuit 41 is configured with the capacitor 45, the oscillation signal can be generated with a simple circuit having a small number of components.

  Further, according to the present embodiment, the switching power supply device 1 is configured to use a CMOS type IC for the Schmitt trigger type inverter logic IC 43. Therefore, the current consumption is suppressed as compared with a Schmitt trigger type inverter using an operational amplifier as a comparator. High-speed driving is possible.

  In addition, according to the present embodiment, the RC oscillation circuit 41 is configured by connecting the resistor 44 and the capacitor 45 to the one-chip Schmitt-trigger inverter logic IC 43 including the Schmitt-trigger inverter circuit 42. With only the resistor 44 and the capacitor 45, an oscillation signal generating source for changing the frequency regulation voltage V1 can be easily configured.

Second Embodiment
In 1st Embodiment mentioned above, the switching power supply device 1 of the structure which always fluctuates a switching frequency was illustrated. On the other hand, in the present embodiment, a switching power supply device 100 configured to change the switching frequency only when the noise is relatively large will be described.

FIG. 6 is a circuit diagram of the switching power supply device 100 according to the present embodiment. In addition, in the same figure, about the member demonstrated in 1st Embodiment, the same code | symbol is attached | subjected and the description is abbreviate | omitted.
As shown in this figure, the fluctuation signal generation circuit 140 of the switching power supply apparatus 100 is provided with a cutoff circuit 160 in addition to the RC oscillation circuit 41.
This cut-off circuit 160 cuts off the input of the oscillation signal from the RC oscillation circuit 41 to the frequency setting circuit 37 while the current flowing from the primary winding 12 through the transistor circuit 13 is below a predetermined value. 161, a current detection circuit 162, and a determination circuit 163.

The connection switch circuit 161 is a circuit that is provided between the RC oscillation circuit 41 and the frequency setting circuit 37 and turns on / off the input from the RC oscillation circuit 41 to the frequency setting circuit 37. The current detection circuit 162 is a circuit that detects a current flowing from the primary winding 12 through the transistor circuit 13 and outputs the current to the determination circuit 163. The determination circuit 163 is a circuit that controls on / off of the connection switch circuit 161 in accordance with the current value detected by the current detection circuit 162, and turns off the connection switch circuit 161 while the current value is equal to or less than a predetermined value. Thus, the input of the oscillation signal from the RC oscillation circuit 41 to the frequency setting circuit 37 is cut off.
This predetermined value is set corresponding to a state in which the current flowing from the primary winding 12 through the transistor circuit 13 is small and the noise level is sufficiently low without changing the switching frequency.

  Therefore, while the current flowing through the transistor circuit 13 is small and the noise level is low, the input from the RC oscillation circuit 41 to the frequency setting circuit 37 is cut off, so that the frequency defining voltage V1 is kept constant and the switching frequency is Fixed. When the current flowing through the transistor circuit 13 increases and the noise level increases, an oscillation signal is input from the RC oscillation circuit 41 to the frequency setting circuit 37, whereby the frequency regulation voltage V1 varies and the switching frequency varies. As a result, the noise level is suppressed.

  As described above, according to the present embodiment, the operation of suppressing the noise level by changing the switching frequency is limited to the case where the current flowing from the primary winding 12 through the transistor circuit 13 is large and the noise level becomes large. Can do.

<Third Embodiment>
In the second embodiment described above, the configuration in which the output of the oscillation signal is cut off by inserting the connection switch circuit 161 between the RC oscillation circuit 41 and the frequency setting circuit 37 has been described. In this configuration, when the connection switch circuit 161 is off (cut-off state), it is desirable to ensure high insulation and keep the frequency regulation voltage constant. For this reason, it is conceivable to use a mechanical relay for the connection switch circuit 161. However, when the mechanical relay is turned on (at the time of connection), a contact surge may occur, and the surrounding elements may be destroyed.

In contrast, by using a P-channel MOSFET for the connection switch circuit 161, it is possible to perform on / off switching while suppressing a contact surge at the time of connection.
However, a parasitic diode in which a current flows from the source to the drain exists between the source and the drain of the P-channel MOSFET. For this reason, even when the P-channel MOSFET is turned off and the RC oscillation circuit 41 and the frequency setting circuit 37 are disconnected, the charging voltage (frequency regulation voltage V1) of the capacitor 51 of the frequency setting circuit 37 is When the voltage is higher than the charging voltage of the capacitor 45 of the RC oscillation circuit 41, the RC oscillation circuit 41 and the frequency setting circuit 37 are connected through a parasitic diode of a P-channel MOSFET. As a result, for example, as shown in FIG. 7, even when the RC oscillation circuit 41 and the frequency setting circuit 37 are cut off, the frequency regulation voltage V <b> 1 fluctuates in conjunction with the oscillation operation of the RC oscillation circuit 41. The switching frequency will not be fixed.

  Therefore, the switching power supply device 200 of the present embodiment increases the insulation of the P-channel MOSFET by maintaining the drain side of the P-channel MOSFET at a higher potential than the source side while the P-channel MOSFET is off. It is said.

FIG. 8 is a circuit diagram of the switching power supply apparatus 200 according to the present embodiment. In the figure, members described in the first and second embodiments are denoted by the same reference numerals, and description thereof is omitted.
As shown in this figure, the fluctuation signal generation circuit 240 of the switching power supply apparatus 200 includes an RC oscillation circuit 41 and a cutoff circuit 260. The cutoff circuit 260 includes a connection switch circuit 261, a current detection circuit 262, and the like. The determination circuit 263 and the pull-up connection switch circuit 264 are provided. Among these, the connection switch circuit 261, the current detection circuit 262, and the determination circuit 263 correspond to the connection switch circuit 161, the current detection circuit 162, and the determination circuit 163 described in the second embodiment, respectively.

The connection switch circuit 261 is a switch circuit including a P-channel MOSFET 277 and a resistor 278 provided between the RC oscillation circuit 41 and the frequency setting circuit 37. An output terminal of the determination circuit 263 is connected to the gate of the P-channel MOSFET 277, and the on / off of the P-channel MOSFET 277 is switched according to the output signal of the determination circuit 263.
The current detection circuit 262 includes a current detection resistor 271 connected in series to the source of the transistor circuit 13, and a voltage generated in the current detection resistor 271 by the current flowing through the transistor circuit 13 is input to the determination circuit 263 as the detection voltage V5. Yes. In addition to the current detection resistor 271, the current detection circuit 262 includes an overcurrent prevention resistor 272 provided in parallel with the current detection resistor 271 and connected to the ground, and a filter capacitor 273. Yes. Note that a current sensor can be used for the current detection circuit 262.

  The determination circuit 263 compares the determination voltage V6 that defines a threshold value for blocking the RC oscillation circuit 41 and the frequency setting circuit 37 with the detection voltage V5 of the current detection circuit 262, and the detection voltage V5 is the determination voltage. A comparator 244 that outputs a determination voltage V6 for turning on the P-channel MOSFET 277 is provided when the voltage is lower than V6, that is, when the current flowing through the transistor circuit 13 is small. The determination voltage V6 is generated by dividing the resistances 275 and 276 connected in series, and can be adjusted by changing the resistance values.

The pull-up connection switch circuit 264 operates in synchronization with on / off of the P-channel MOSFET 277 of the connection switch circuit 261, and keeps the drain potential higher than the source while the P-channel MOSFET 277 is off.
More specifically, the pull-up connection switch circuit 264 includes an N-channel MOSFET 279 that is provided between the power supply voltage Vcc that is higher than the frequency regulation voltage V1 and the output terminal P2 of the RC oscillation circuit 41, and is turned on / off. The switch circuit includes a resistor 280, and the output terminal of the determination circuit 263 is connected to the gate terminal of the N-channel MOSFET 279.

That is, ON / OFF of the pull-up connection switch circuit 264 and the connection switch circuit 261 is opposite to each other. While the connection switch circuit 261 is OFF, the pull-up connection switch circuit 264 is ON and the power supply voltage Vcc is applied to the output terminal P2 of the RC oscillation circuit 41.
As a result, while the P-channel MOSFET 277 of the connection switch circuit 261 is off, the drain potential is maintained at the power supply voltage Vcc and is held at a higher potential than the source. Therefore, when the P-channel MOSFET 277 is off. High insulation can be provided without current flowing.

Further, the pull-up connection switch circuit 264 is turned on and the power supply voltage Vcc is applied to the output terminal P2 of the RC oscillation circuit 41, so that the power supply voltage Vcc is also applied to the capacitor 45 of the RC oscillation circuit 41 and the charging voltage is increased. Thus, the oscillation of the RC oscillation circuit 41 is stopped.
That is, while the current flowing through the transistor circuit 13 is small, the oscillation operation of the RC oscillation circuit 41 is stopped as shown in FIG. 9A, so that useless current consumption is suppressed. At this time, the drain side of the P-channel MOSFET 277 of the connection switch circuit 261 (that is, the voltage at the output terminal P2 of the RC oscillation circuit 41 (= the voltage of the oscillation signal V2)) is maintained at a higher voltage than the source side. By doing so, the frequency regulation voltage V1 is kept constant.
When the current flowing through the transistor circuit 13 increases, the pull-up connection switch circuit 264 is turned off and the connection switch circuit 261 is turned on. As a result, as shown in FIG. 9B, the RC oscillation circuit 41 starts an oscillation operation and outputs the oscillation signal V2 to the output terminal P2, whereby the voltage at the output terminal P2 fluctuates. As the frequency regulation voltage V1 fluctuates with voltage fluctuation, the switching frequency fluctuates and the noise level is reduced.

  As described above, according to the present embodiment, while the current flowing through the transistor circuit 13 is equal to or lower than a predetermined value (current value with a low noise level), the potential on the drain side of the P-channel MOSFET 277 of the connection switch circuit 261 is changed from the source side. In addition, a pull-up connection switch circuit 264 that maintains a high potential and holds the charge voltage of the capacitor 45 of the RC oscillation circuit 41 connected to the drain constant to stop oscillation is provided.

  According to this configuration, the RC oscillation circuit 41 and the frequency setting circuit 37 are disconnected by the P-channel MOSFET 277, so that contact surge that occurs at the time of connection can be prevented compared to the configuration in which the mechanical relay or the like is used. Further, while the P-channel MOSFET 277 is off, the drain side is held at a higher potential than the source side, so that a so-called reverse flow in which a current flows through a parasitic diode in the P-channel MOSFET 277 can be prevented, and insulation can be improved. it can. Furthermore, since the oscillation operation of the RC oscillation circuit 41 is stopped while the RC channel circuit 277 is interrupted between the RC oscillation circuit 41 and the frequency setting circuit 37, wasteful current consumption can be suppressed.

  The first to third embodiments described above merely illustrate one aspect of the present invention, and can be arbitrarily modified and applied without departing from the spirit of the present invention.

For example, in each of the embodiments described above, the RC oscillation circuit 41 is configured using the Schmitt trigger inverter circuit 42. However, the present invention is not limited to this, and an RC oscillation circuit may be configured by including a plurality of stages of inverter circuits.
FIG. 10 is a circuit diagram of an RC oscillation circuit according to this modification. FIG. 10A shows a case where the RC oscillation circuit 141 is configured by a two-stage inverter circuit, and FIG. A case where the RC oscillation circuit 241 is configured by an inverter circuit is shown.

As shown in the figure, in the RC oscillation circuits 141 and 240 according to this modification, a plurality of stages of inverter circuits 142 are connected in series, and the output of any one of these inverter circuits 142 is output. And the input of the first-stage inverter circuit 142 are connected via a resistor 144, and a charge / discharge capacitor 145 is connected between the output of one of the other inverter circuits 142 and the input of the first-stage inverter circuit 142. Configured. In the figure, a resistor 146 is for current limiting.
In the RC oscillation circuits 141 and 240 having such a configuration, the charging voltage of the charging / discharging capacitor 145 fluctuates due to the oscillation operation, and this charging voltage is input to the frequency setting circuit 37 as the oscillation signal V2.
In addition, one-chip inverter logic ICs 143 and 243 can be used for the plurality of stages of inverter circuits 142 connected in series, respectively, whereby a circuit configuration including a one-chip IC, a resistor 144, and a capacitor 145. Thus, the oscillation signal V2 for changing the frequency regulation voltage V1 can be easily generated.

  Further, for example, in each of the above-described embodiments, the flyback type switching power supply device 1, 100, 200 is illustrated as the power supply device. It may be a power supply.

1 Switching power supply (power supply)
10 Flyback transformer (transformer)
12 Primary winding 13 Transistor circuit (switching element)
14 Control IC (control circuit)
18 Secondary winding 37 Frequency setting circuit 36 Carrier wave generator 40, 140, 240 Fluctuating signal generation circuit 41, 141, 241 RC oscillation circuit 42 Schmitt trigger inverter circuit 44, 144 Resistor 45, 145 Capacitor 43 Schmitt trigger type inverter Logic IC
142 Inverter circuit 143 Inverter logic IC
160, 260 Cutoff circuit 161, 261 Connection switch circuit 162, 262 Current detection circuit 163, 264 Judgment circuit 264 Pull-up connection switch circuit (high potential holding circuit)
277 P-channel MOSFET
V1 Frequency regulation voltage V2 Oscillation signal V5 Detection voltage V6 Judgment voltage

Claims (4)

A power supply apparatus comprising: a switching element connected to a primary winding of a transformer; and a control circuit that controls a switching operation of the switching element so that an output voltage on a secondary side of the transformer becomes a predetermined voltage level. In
A frequency setting circuit that generates a frequency defining voltage that defines a switching frequency of the switching element and inputs the voltage to the control circuit;
An oscillation signal whose voltage continuously changes is input to the frequency setting circuit, and a fluctuation signal generation circuit that varies the frequency regulation voltage generated by the frequency setting circuit;
With
The variation signal generation circuit is a generation source of the oscillation signal,
An RC oscillation circuit in which the output and input of a Schmitt trigger inverter are connected via a resistor, and a capacitor is connected to the input of the Schmitt trigger inverter, or
Connect multiple stages of inverters in series, connect the output of one of these inverters and the input of the first stage via a resistor, and connect the output of one of the other inverters to the input of the first stage includes an RC oscillator circuit connected to the capacitor between,
A power supply apparatus comprising: a cut-off circuit that cuts off between the RC oscillation circuit and the frequency setting circuit while a current flowing from the primary winding through the switching element is equal to or less than a predetermined value .   The power supply apparatus according to claim 1, wherein the Schmitt trigger inverter or the inverter is configured by a CMOS logic gate. The RC oscillation circuit is configured by connecting the resistor and the capacitor to a one-chip inverter logic IC including the Schmitt trigger inverter, or
3. The power supply device according to claim 1, wherein the RC oscillation circuit is configured by connecting the resistor and the capacitor to a one-chip inverter logic IC including a plurality of stages of inverters. The cutoff circuit includes a P-channel MOSFET that is inserted between the RC oscillation circuit and the frequency setting circuit, and is turned off while a current flowing through the switching element is equal to or less than a predetermined value.
On the drain side of the P-channel MOSFET during current flowing through the switching element is less than the predetermined value, with keeping a high potential than the source side by applying a voltage to the drain side of the P-channel MOSFET, the voltage The power supply according to any one of claims 1 to 4, further comprising a potential holding circuit that applies a voltage to the capacitor of the RC oscillation circuit connected to a drain to hold the charging voltage constant and stop the oscillation. apparatus.