JP7318528B2 - Power supply circuit and electronic equipment with power supply circuit - Google Patents

Description

TECHNICAL FIELD The present application relates to a power supply circuit and an electronic device including the power supply circuit.

Conventionally, in a DC power supply that converts an AC voltage input from an AC commercial power supply into a DC voltage and outputs it, when the AC commercial power supply is cut off, the electric charge of the electrolytic capacitor included in the DC power supply is reduced to a safe state. Techniques for discharging are known.

Also disclosed is a technique for discharging the electric charge of an electrolytic capacitor using a discharge resistor in a control unit such as a power supply IC (Integrated Circuit) when the reference voltage of the auxiliary winding drops below a predetermined value (for example, , see Patent Document 1).

However, in the technique disclosed in Patent Document 1, when the auxiliary winding is wound forward or the like, the reference voltage of the auxiliary winding may drop when the voltage across the terminals of the electrolytic capacitor is large (eg, 300 Vdc). be. If the resistance value and rated power of the discharge resistor are set so as to discharge such a large terminal voltage, the control unit will become large, and the heat dissipation of the control unit must be considered, resulting in an increase in cost. There is

An object of the present invention is to discharge the charge of an electrolytic capacitor without increasing the cost.

A power supply circuit according to an aspect of the present invention includes an electrolytic capacitor that smoothes an AC voltage input from an AC commercial power source, a transformer to which the DC voltage smoothed by the electrolytic capacitor is applied, and a transformer connected to the transformer. , a switching element that switches between on and off, and a control section that controls the switching element, wherein the control section cuts off the AC commercial power supply and the voltage between the terminals of the electrolytic capacitor drops below a predetermined value. In this case, the electric charge of the electrolytic capacitor is discharged by setting the gate voltage of the switching element so that a predetermined impedance exists between the drain terminal and the source terminal of the switching element .

ADVANTAGE OF THE INVENTION According to this invention, the electric charge of an electrolytic capacitor can be discharged, without increasing a cost.

1 is a diagram showing a configuration example of a power supply circuit according to a first embodiment; FIG. 4 is a timing chart showing an operation example of the power supply circuit according to the first embodiment; FIG. 5 is a diagram showing an example of Vgs-Id characteristics of the switching element according to the first embodiment; It is a figure which shows the structure of the power supply circuit which concerns on a comparative example. 4 is a timing chart showing the operation of a power supply circuit according to a comparative example; FIG. 4 is a timing chart of an operation example of the power supply circuit according to the first modified example, in which (a) is a diagram of AC voltage, (b) is a diagram of voltage across electrolytic capacitor terminals, and (c) is a diagram of power supply IC output voltage. . FIG. 10 is a diagram showing an example of Vgs-Id characteristics of a switching element according to a first modified example; FIG. 9 is a timing chart of an operation example of the power supply circuit according to the second modification, where (a) is a diagram of AC voltage, (b) is a diagram of voltage between terminals of an electrolytic capacitor, and (c) is a diagram of power supply IC output voltage. . FIG. 10 is a timing chart of an operation example of a power supply circuit according to a third modification, in which (a) is a diagram of an AC voltage, (b) is a diagram of a voltage between terminals of an electrolytic capacitor, and (c) is a diagram of a power supply IC output voltage. . It is a figure which shows the structural example of the power supply circuit which concerns on 2nd Embodiment. FIG. 4 is a timing chart of an operation example of the power supply circuit according to the second embodiment, (a) is a diagram of AC voltage, (b) is a diagram of voltage across electrolytic capacitor terminals, (c) is a diagram of power supply IC output voltage, ( d) is a diagram of the Vcc terminal voltage of the power supply IC, and (e) is a diagram of the open/close state of the relay contact. 4 is a timing chart of the operation of the power supply circuit according to the comparative example, in which (a) is a diagram of AC voltage, (b) is a diagram of terminal voltage of an electrolytic capacitor, and (c) is a diagram of power supply IC output voltage. FIG. 11 is a diagram illustrating a configuration example of an image forming apparatus according to a third embodiment;

DETAILED DESCRIPTION OF THE INVENTION Embodiments for carrying out the invention will be described below with reference to the drawings. In each drawing, the same components may be denoted by the same reference numerals, and redundant description may be omitted.

A power supply circuit according to an embodiment is a DC power supply circuit that converts an AC voltage input from an AC commercial power supply into a DC voltage and outputs the DC voltage. Further, in the embodiment, an electrolytic capacitor that smoothes an AC voltage input from an AC commercial power supply, a transformer to which the AC voltage smoothed by the electrolytic capacitor is applied, and a transformer connected to the transformer to switch on or off a switching element; and a controller for controlling the switching element, the controller for controlling the switching element when the AC commercial power supply is interrupted and the voltage across the terminals of the electrolytic capacitor drops below a predetermined value. By setting to the state of , the charge of the electrolytic capacitor is discharged.

The switching element is an element attached externally to the control unit, and was originally designed with heat dissipation in mind. There is no need to consider the heat dissipation of the heat dissipation of the control unit. This allows the electrolytic capacitor to be discharged without increasing cost.

Here, the smoothing of the AC voltage means that, in the AC voltage that is continuously input over time, if the AC voltage value input at a predetermined time deviates from the AC voltage value that was input immediately before, , to bring the AC voltage value closer to the AC voltage value that was input immediately before, so that there is no overall divergence in the AC voltage value.

Embodiments will be described below by taking as an example a switching power supply circuit that stabilizes an output by controlling switching between an ON state and an OFF state of a switching element using a feedback circuit.

[First embodiment]
<Configuration example of power supply circuit 1>
First, the configuration of the power supply circuit 1 according to the first embodiment will be described. FIG. 1 is a diagram showing an example of the configuration of a power supply circuit 1. As shown in FIG. As shown in FIG. 1, the power supply circuit 1 includes a rectifying circuit DB11, electrolytic capacitors C11, C12, C13, a switching element Q10, a transformer T10, rectifying diodes D11, D12, resistors R11, R12, and a capacitor C15. , a power supply IC (Integrated Circuit) 12, a snubber circuit Z2, and a feedback circuit Z3.

An AC input terminal of a rectifier circuit DB11 having a diode bridge configuration is connected to an AC commercial power supply 2 via a filter, and an AC voltage input from the AC commercial power supply 2 is full-wave rectified by the rectifier circuit DB11 and output. be. An electrolytic capacitor C11 is connected between the rectification output positive terminal and the rectification output negative terminal of the rectification circuit DB11. As a result, a DC voltage obtained by rectifying and smoothing the AC commercial power supply 2 with the rectifying circuit DB11 and the electrolytic capacitor C11 is obtained.

A primary winding P1 of a transformer T10, a switching element Q10, and a resistor R11 are connected in series between the positive terminal and the negative terminal of the electrolytic capacitor C11. A DC voltage rectified and smoothed by the rectifier circuit DB11 and the electrolytic capacitor C11 is applied to the transformer T10.

The switching element Q10 is composed of an N-type power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and has a drain terminal 101, a source terminal 102, and a gate terminal 103.

The drain terminal 101 is connected to the primary winding P1 of the transformer T10, the source terminal 102 is connected to the resistor R11, and the gate terminal 103 is connected to the drive output (DRIVE) terminal of the power supply IC12.

A power supply IC 12, which is an example of a control unit, is a circuit for performing switching control to oscillate (on or off) the switching element Q10. The power supply IC 12 includes a START terminal 121 as a starting voltage input terminal of the power supply IC 12, a Vcc terminal 122 as a power supply voltage input terminal, an AC terminal 123, and a ground (GND) terminal. Here, the START terminal is an example of a first terminal, and the AC terminal is an example of a second terminal.

The positive terminal of the electrolytic capacitor C11 is connected to the START terminal 121, which is the starting voltage input terminal of the power supply IC 12, and the negative terminal of the electrolytic capacitor C11 is connected to the GND terminal. Based on the input voltage from the electrolytic capacitor C11 via the START terminal 121, the power supply IC 12 can detect a drop in the voltage across the terminals of the electrolytic capacitor C11.

The DC voltage obtained by rectifying and smoothing the AC commercial power supply 2 by the rectifying circuit DB11 and the electrolytic capacitor C11 is applied to the transformer T10 during the OFF period by the ON/OFF operation of the switching element Q10 connected via the primary winding P1 of the transformer T10. Output to the next winding S1.

An electrolytic capacitor C12 is connected between both terminals of the secondary winding S1 of the transformer T10 via a rectifier diode D11. A pulse voltage induced in the secondary winding S1 of the transformer T10 is rectified and smoothed by a secondary rectifying/smoothing circuit composed of a rectifying diode D11 and an electrolytic capacitor C12, and is connected to a positive output terminal OUT+ and a ground output terminal OUT-. It is supplied as a DC output voltage to a load (not shown) connected between . The line connected to the positive output terminal OUT+ becomes the power supply line, and the line connected to the ground output terminal OUT− becomes the GND line.

An electrolytic capacitor C13 is connected between both terminals of the auxiliary winding P2 of the transformer T10 via a rectifier diode D12 and a resistor R12, and a connection point between the rectifier diode D12 and the electrolytic capacitor C13 is connected to the Vcc terminal 122. there is As a result, the voltage generated in the auxiliary winding P2 is rectified and smoothed by the rectifier diode D12 and the electrolytic capacitor C13, and then supplied to the Vcc terminal 122 of the power IC 12 and used as a control circuit power source for the power IC 12.

Also, the power supply IC 12 can detect interruption of the AC commercial power supply 2 via the AC terminal 123 . When the AC voltage input from the AC commercial power supply 2 is cut off, the voltage across the terminals of the electrolytic capacitor C11 drops, and the output voltage of the positive output terminal OUT+ cannot be obtained normally, or the switching element Q10 is damaged. Sometimes. The AC terminal 123 is provided to detect interruption of the AC commercial power supply 2 and prevent such malfunction and damage.

The snubber circuit Z2 is a protection circuit that is connected between the primary winding P1 of the transformer T10 and absorbs a transient high voltage that occurs when the switching element Q10 is cut off. The snubber circuit Z2 includes a diode D13, a capacitor C16, and a resistor R13.

The anode of the diode D13 is connected to the connection point between the drain terminal of the switching element Q10 and the primary winding P1 of the transformer T10. A capacitor C16 and a resistor R13 are connected in parallel between the cathode of the diode D13 and the connection point between the positive terminal of the capacitor C16 and the primary winding P1 of the transformer T10.

The feedback circuit Z3 includes photocouplers PC11 and PC12, an error amplifier EA, resistors R14, R15, R16 and R17, and a capacitor C17. Between the power supply line and the GND line, a resistor R14, a light-emitting element (light-emitting diode) of the photocoupler PC12, and an error amplifier EA are connected in series, and a resistor R14 in which a resistor R15 is connected in series and a photodiode are connected in series. It is connected in parallel with the light emitting element of the coupler PC12.

A voltage dividing resistor R16 and a resistor R17 are connected in series between the power supply line and the GND line, and the connection point between the resistor R16 and the resistor R17 is connected to the control terminal a of the error amplifier EA. there is Furthermore, a capacitor C17 is connected between the connection point between the resistors R16 and R17 and the connection point between the light-emitting element of the photocoupler PC12 and the error amplifier EA. As a result, the output voltage output between the positive output terminal OUT+ and the ground output terminal OUT- is divided by the resistors R16 and R17, and the divided output voltage is input to the control terminal a of the error amplifier EA. be.

The divided output voltage is compared with a reference voltage (not shown) built in the error amplifier EA. It is fed back to the light-receiving element (light-receiving transistor).

The light-receiving side element of the photocoupler PC11 is connected in parallel with the capacitor C15 between the feedback signal input (FB) terminal of the power supply IC12 and the negative terminal of the electrolytic capacitor C11. is entered in

A connection point between the source terminal of the switching element Q10 and the resistor R11 is connected to an overcurrent detection (OCP) terminal of the power supply IC 12, and the drain current flowing through the switching element Q10 is detected as a voltage signal by the resistor R11. The detected voltage signal is input to the OCP terminal of the power IC 12 .

<Operation example of power supply circuit 1>
Next, operation of the power supply circuit 1 will be described with reference to FIG. FIG. 2 is a timing chart showing an example of the operation of the power supply circuit 1. (a) is the AC voltage input from the AC commercial power supply 2, (b) is the voltage across the terminals of the electrolytic capacitor C11, and (c) is the power supply voltage. The output voltage of the DRIVE terminal of IC12 is shown respectively.

When an AC voltage is input from the AC commercial power supply 2, the voltage across the terminals of the electrolytic capacitor C11 maintains a high state, and the output voltage of the DRIVE terminal maintains a constant DC voltage at the positive output terminal OUT+ in FIG. The output voltage has a PWM (Pulse Width Modulation) waveform like this.

On the other hand, when the AC voltage input from the AC commercial power supply 2 is cut off, the voltage across the terminals of the electrolytic capacitor C11 begins to gradually decrease. When the power supply IC 12 detects through the START terminal 121 that the voltage between the terminals of the electrolytic capacitor C11 has dropped below a predetermined value and through the AC terminal 123 detects that the AC commercial power supply 2 is cut off, the drain of the switching element Q10 is detected. The voltage (gate voltage) of the gate terminal 103 of the switching element Q10 is set through the DRIVE terminal so that the terminal 101 and the source terminal 102 have a predetermined impedance.

As a result, the impedance between the drain terminal 101 and the source terminal 102 causes the START terminal 121 to discharge until the voltage of the START terminal 121 becomes approximately 0 V, thereby discharging the electric charge of the electrolytic capacitor C11. An impedance between the drain terminal 101 and the source terminal is, for example, 100 kΩ. A state in which the voltage of the gate terminal 103 is set so that the impedance between the drain terminal 101 and the source terminal of the switching element Q10 is 100 kΩ is an example of the "predetermined state."

Even if the voltage between the terminals of the electrolytic capacitor C11 drops and, for example, the impedance between the drain terminal 101 and the source terminal 102 must discharge a voltage of 300 V, the switching element Q10 is originally designed with heat dissipation taken into consideration. Therefore, there is no need to newly consider heat dissipation.

A more specific example will be described. When the voltage between the terminals of the electrolytic capacitor C11 drops and the impedance between the drain terminal 101 and the source terminal 102 must discharge a voltage of 300 V, and the impedance between the drain terminal 101 and the source terminal 102 is 100 kΩ, Based on the following formula, the maximum power consumption E is 0.9 W.
E = {300 (V) x 300 (V)}/100 (kΩ) = 0.9 (W)
However, how long the maximum power of 0.9 W continues to be consumed depends on the capacitance of the electrolytic capacitor C11.

Here, FIG. 3 is a diagram showing an example of the Vgs-Id characteristics of the switching element Q10. The horizontal axis of FIG. 3 indicates the voltage Vgs between the gate terminal 103 and the source terminal 102 of the switching element Q10, and the vertical axis indicates the current Id of the drain terminal 101. As shown in FIG.

A current Id of the drain terminal 101 when discharging a voltage of 300 V with an impedance of 100 kΩ between the drain terminal 101 and the source terminal 102 is 3 mA based on the following equation.
Id = 300 (V)/100 (kΩ) = 3 (mA)
The voltage of the gate terminal 103 that causes the current Id of the drain terminal 101 to be 3 mA is 1.05 V as shown in FIG.

Therefore, by setting the voltage of the gate terminal 103 to 1.05 V through the DRIVE terminal, the power supply IC 12 has an impedance of 100 kΩ between the drain terminal 101 and the source terminal 102 .

<Action and effect of power supply circuit 1>
Next, functions and effects of the power supply circuit 1 will be described. Prior to this description, the configuration and operation of a power supply circuit 1' according to a comparative example will be described.

FIG. 4 is a diagram showing the configuration of the power supply circuit 1'. Components having the same functions as those in FIG. 1 are given the same part numbers as in FIG. 1, and overlapping descriptions are omitted here.

As shown in FIG. 4, the power supply circuit 1' includes a power supply IC 12'. A discharge resistor 124 is provided inside the power supply IC 12' for discharging the electric charge of the electrolytic capacitor C11 when the reference voltage of the auxiliary winding P2 drops below a predetermined value.

Next, FIG. 5 is a timing chart showing the operation of the power supply circuit 1'.

When an AC voltage is input from the AC commercial power supply 2, the voltage across the terminals of the electrolytic capacitor C11 maintains a high state, and the output voltage of the DRIVE terminal maintains a constant DC voltage at the positive output terminal OUT+ in FIG. The output voltage has a PWM waveform like this.

On the other hand, when the AC voltage input from the AC commercial power supply 2 is cut off, the voltage across the terminals of the electrolytic capacitor C11 begins to gradually decrease. When the power supply IC 12 detects that the voltage between the terminals of the electrolytic capacitor C11 drops below a predetermined value through the START terminal 121, it turns off the output of the DRIVE terminal, turns on the conduction to the discharge resistor 124, and turns on the electrolysis. Discharge until the voltage of the capacitor C11 reaches approximately 0V.

However, in the configuration of the power supply circuit 1′, even if the voltage across the terminals of the electrolytic capacitor C11 drops, for example, if the voltage of the AC commercial power supply 2 is 230 Vac, the voltage across the terminals during normal operation is about 320 Vdc (=230 Vac× √2). Therefore, when the voltage between the terminals drops, the discharge resistor 124 inside the power supply IC 12 may have to discharge the voltage of 300V.

Further, if the electrostatic capacity of the electrolytic capacitor C11 is large, it takes a long time to cause the discharge resistor 124 to discharge. In order to deal with these problems, it is preferable to increase the rated power of the discharge resistor 124 inside the power supply IC 12 and cool the amount of heat generated by the discharge resistor 124 . However, the size of the power supply IC 12 may be increased in order to cool the power supply IC 12, or a dedicated heat sink may be required to improve the heat dissipation of the power supply IC 12, resulting in an increase in cost.

A more specific example will be described. For example, when the discharge resistor 124 inside the power supply IC 12 must discharge a voltage of 300 V, and the resistance constant of the discharge resistor 124 is 100 kΩ, the maximum power consumption E of the discharge resistor 124 is 0.00 V according to the following equation. 9W.
E = {300 (V) x 300 (V)}/100 (kΩ) = 0.9 (W)
Therefore, it is required to set the rated power of the discharge resistor 124 corresponding to a maximum of 0.9 W and to improve the heat dissipation.

Note that the maximum power consumption of 0.9 W is only at the initial stage of discharge, and the power consumption at the discharge resistor 124 gradually decreases as the voltage across the terminals of the electrolytic capacitor C11 decreases. This decreasing curve varies depending on the magnitude of the capacitance of the electrolytic capacitor C11.

In contrast to such a comparative example, in the power supply circuit 1 according to the present embodiment, the power supply IC 12 detects a drop in the voltage across the terminals of the electrolytic capacitor C11 to a predetermined value or less via the START terminal 121, , the voltage of the gate terminal 103 of the switching element Q10 is set via the DRIVE terminal so that a predetermined impedance exists between the drain terminal 101 and the source terminal 102 of the switching element Q10. do. As a result, the impedance between the drain terminal 101 and the source terminal 102 causes the START terminal 121 to discharge until the voltage of the START terminal 121 becomes approximately 0 V, thereby discharging the electric charge of the electrolytic capacitor C11.

The switching element Q10 is an element attached externally to the power supply IC 12, and is originally designed with heat dissipation taken into consideration. It is possible to eliminate the need to consider an increase in the size of the power supply IC 12 and the heat dissipation of the power supply IC 12 . Therefore, the charge of the electrolytic capacitor can be discharged without increasing the cost.

In addition, in the example described above, an example in which the power supply IC 12 is provided as a control unit for controlling the switching element Q10 has been shown, but the present invention is not limited to this. If the switching element Q10 can be set to a predetermined state when the AC commercial power supply 2 is cut off and the voltage across the terminals of the electrolytic capacitor C11 drops below a predetermined value, an electric circuit or the like other than the power supply IC 12 can be used. can also

Moreover, various modifications are possible for the control method of the power supply circuit 1 . Various modifications will be described below.

(First modification)
FIG. 6 is a timing chart showing an operation example of the power supply circuit 1 according to the first modification, where (a) is the AC voltage input from the AC commercial power supply 2, (b) is the voltage across the terminals of the electrolytic capacitor C11, (c) shows the output voltage of the DRIVE terminal of the power supply IC 12, respectively.

As shown in FIG. 6, when the input of the AC commercial power supply 2 is cut off, the voltage across the terminals of the electrolytic capacitor C11 drops. When the power supply IC 12 detects a drop in the voltage across the terminals of the electrolytic capacitor C11 through the START terminal 121, and detects interruption of the AC commercial power supply 2 through the AC terminal 123, the drain of the switching element Q10 is detected through the DRIVE terminal. The voltage of the gate terminal 103 is set so that the terminal 101 and the source terminal 102 have a predetermined impedance.

Further, in this modification, the predetermined impedance is changed according to the magnitude of the voltage across the terminals of the electrolytic capacitor C11.

Specifically, as shown in FIG. 6B, immediately after it is detected that the voltage across the terminals of the electrolytic capacitor C11 has dropped below a predetermined value, the voltage across the terminals of the electrolytic capacitor C11 is high. and the source terminal 102, the voltage of the gate terminal 103 is set low via the DRIVE terminal (FIG. 6(c)).

As the voltage across the terminals of the electrolytic capacitor C11 gradually decreases, the voltage at the gate terminal 103 is gradually increased through the DRIVE terminal so that the impedance between the drain terminal 101 and the source terminal 102 is small. set as

In this manner, the voltage of the START terminal 121 of the power supply IC 12 is discharged until the voltage of the START terminal 121 becomes approximately 0 V, and the electric charge of the electrolytic capacitor C11 is discharged. By changing the impedance between the drain terminal 101 and the source terminal 102 according to the voltage across the terminals of the electrolytic capacitor C11, the voltage at the gate terminal 103 can be suppressed. power consumption can be suppressed.

A specific example will be described. When the voltage between the terminals of the electrolytic capacitor C11 drops and the impedance between the drain terminal 101 and the source terminal 102 must discharge a voltage of 300 V, and the impedance between the drain terminal 101 and the source terminal 102 is 1 MΩ, Based on the following formula, the maximum power consumption E is 0.09W.
E = {300 (V) x 300 (V)}/1 (MΩ) = 0.09 (W)
In addition, when a voltage of 300 V must be discharged with the impedance between the drain terminal 101 and the source terminal 102, and the impedance between the drain terminal 101 and the source terminal 102 is 10 kΩ, the power consumption E is maximum based on the following formula. It becomes 0.09 W, which is the same as the case described above.
E={30(V)×30(V)}/10(kΩ)=0.09(W)
Here, FIG. 7 is a diagram showing an example of the Vgs-Id characteristics of the switching element Q10 according to this modification. Since the view of the figure is the same as that of FIG. 3, redundant description will be omitted here.

In the above example, a voltage of 300 V is discharged with an impedance of 1 MΩ between the drain terminal 101 and the source terminal 102. In this case, the current Id of the drain terminal 101 is 3 mA based on the following equation.
Id = 300 (V)/1 (MΩ) = 0.3 (mA)
The voltage of the gate terminal 103 is 0.8V as shown in FIG. 7 so that the current Id of the drain terminal 101 is 0.3 mA when the electrolytic capacitor C11 is 300V. Therefore, by setting the voltage of the gate terminal 103 to 0.8 V through the DRIVE terminal, the power supply IC 12 has an impedance of 1 MΩ between the drain terminal 101 and the source terminal 102 .

On the other hand, the current Id of the drain terminal 101 when discharging a voltage of 30 V with an impedance of 10 kΩ between the drain terminal 101 and the source terminal 102 is 0.3 mA based on the following equation.
Id=30(V)/10(kΩ)=0.3(mA)
When the electrolytic capacitor C11 is 300V, the voltage of the gate terminal 103 is 1.3V or more as shown in FIG. 7 so that the current Id of the drain terminal 101 is 0.3mA. Therefore, by setting the voltage of the gate terminal 103 to 1.3 V or higher through the DRIVE terminal of the power supply IC 12, the impedance between the drain terminal 101 and the source terminal 102 becomes 10 kΩ.

By changing the impedance between at least the drain terminal 101 and the source terminal 102 in the range of 10 kΩ to 1 MΩ in this way, the power consumption E becomes 0.09 W at the maximum, which is higher than the example shown in the first embodiment. Power can be reduced to 1/10.

(Second modification)
Next, FIG. 8 is a timing chart showing an operation example of the power supply circuit 1 according to the second modification, where (a) is the AC voltage input from the AC commercial power supply 2, and (b) is the voltage between the terminals of the electrolytic capacitor C11. voltage, and (c) the output voltage of the DRIVE terminal of the power supply IC 12, respectively.

As shown in FIG. 8, when the input of the AC commercial power supply 2 is cut off, the voltage across the terminals of the electrolytic capacitor C11 drops. When the power supply IC 12 detects through the START terminal 121 that the voltage across the terminals of the electrolytic capacitor C11 has dropped below a predetermined value, and through the AC terminal 123 detects that the AC commercial power supply 2 has been cut off, the power supply IC 12 detects through the DRIVE terminal. to output a PWM waveform.

In this modification, the voltage of the gate terminal 103 in the ON state of the switching element Q10 in this PWM waveform is set lower than a predetermined voltage, the PWM control period is set longer than the predetermined period, and the duty ratio of the PWM control pulse signal is set to a predetermined value. Make it smaller than the duty ratio. As an example, the predetermined voltage is the voltage before the voltage across the terminals of the electrolytic capacitor C11 drops below the predetermined value. The predetermined cycle is the PWM control cycle before the voltage across the terminals of the electrolytic capacitor C11 drops below a specified value, and the specified duty ratio is the duty ratio before the voltage across the terminals of the electrolytic capacitor C11 drops below a specified value. is. Note that the duty ratio is the time ratio of the ON state time to the OFF state time.

By controlling in this manner, the voltage of the START terminal 121 of the power supply IC 12 is discharged to approximately 0 V, and the electric charge of the electrolytic capacitor C11 is discharged.

Since the voltage is modulated by the PWM waveform and output, it is possible to suppress the power consumption of the switching element Q10 when discharging the voltage across the terminals of the electrolytic capacitor C11.

(Third modification)
Next, FIG. 9 is a timing chart showing an operation example of the power supply circuit 1 according to the third modification, (a) is the AC voltage input from the AC commercial power supply 2, and (b) is the voltage between the terminals of the electrolytic capacitor C11. voltage, and (c) the output voltage of the DRIVE terminal of the power supply IC 12, respectively.

As shown in FIG. 9, when the input of the AC commercial power supply 2 is cut off, the voltage across the terminals of the electrolytic capacitor C11 drops. When the power supply IC 12 detects through the START terminal 121 that the voltage across the terminals of the electrolytic capacitor C11 has dropped below a predetermined value, and through the AC terminal 123 detects that the AC commercial power supply 2 has been cut off, the power supply IC 12 detects through the DRIVE terminal. to output a PWM waveform.

Further, in this modification, the PWM control cycle is changed according to the voltage across the terminals of the electrolytic capacitor C11. Specifically, as shown in FIG. 9, when the voltage across the terminals of the electrolytic capacitor C11 is large, the PWM control cycle is set to 15 μs (frequency: 65 kHz), and when the voltage across the terminals of the electrolytic capacitor C11 is small, PWM control The period is shortened to 10 μs (frequency: 100 kHz).

The power consumption of the switching element Q10 is suppressed when the voltage across the terminals of the electrolytic capacitor C11 is discharged because the voltage is modulated with a PWM waveform and the PWM control cycle is changed according to the voltage across the terminals of the electrolytic capacitor C11. can do.

In the above example, the control period is set to 15 μs and 10 μs. do not have.

[Second embodiment]
Next, the power supply circuit 1a according to the second embodiment will be described.

FIG. 10 is a diagram showing an example of the configuration of the power supply circuit 1a. As shown in FIG. 10, the power supply circuit 1a includes a relay RY10 between the positive terminal of the electrolytic capacitor C11 and the Vcc terminal 122 of the power supply IC 12, and a resistor R17 connected in series with the relay RY10. . Relay RY10 is a normally closed type relay whose contacts are opened while power is being supplied to auxiliary winding P2 of transformer T10.

FIG. 11 is a timing chart showing an operation example of the power supply circuit 1a. (a) is the AC voltage input from the AC commercial power supply 2, (b) is the voltage across the terminals of the electrolytic capacitor C11, and (c) is the power supply IC 12. (d) shows the voltage of the Vcc terminal 122 of the power supply IC 12, and (e) shows the open/close state of the contact of the relay RY10.

A hatched region in FIG. 11 indicates a time region in which the relay RY10 and the resistor R17 act. As shown in FIG. 11B, after the electrolytic capacitor C11 is discharged and the voltage across the terminals reaches approximately 0 V, the voltage across the terminals may rise again due to the recurrence voltage of the electrolytic capacitor C11.

In this embodiment, when the voltage of the electrolytic capacitor C11 drops and the power supplied to the auxiliary winding P2 of the transformer T10 drops, the relay RY10 is turned off and the contacts are closed. This connects the positive terminal of the electrolytic capacitor C11 to the Vcc terminal of the power supply IC12.

When the positive voltage of the electrolytic capacitor C11 is large, the power supply IC 12 receives power from the electrolytic capacitor C11 and outputs a voltage from the DRIVE terminal so that the voltage of the electrolytic capacitor C11 does not rise above a predetermined voltage. do. As a result, an impedance is provided between the drain terminal 101 and the source terminal 102 of the switching element Q10 to discharge the electric charge of the electrolytic capacitor C11 so that no electric charge remains in the electrolytic capacitor C11.

Here, as a comparative example, the operation of the power supply circuit 1'' which does not include the relay RY10 between the positive terminal of the electrolytic capacitor C11 and the Vcc terminal 122 of the power supply IC 12 and the resistor R17 connected in series with the relay RY10. explain.

FIG. 12 is a timing chart of an operation example of the power supply circuit 1'' according to the comparative example, where (a) is the AC voltage input from the AC commercial power supply 2, (b) is the voltage between the terminals of the electrolytic capacitor C11, ( c) shows the output voltage of the DRIVE terminal of the power supply IC 12, respectively.

As shown in FIG. 12, in the comparative example, after the electrolytic capacitor C11 is discharged, electric charges may remain in the electrolytic capacitor C11 due to the recovery voltage. This residual charge may cause problems such as sparks from the operator's body or from the electrolytic capacitor C11 during work such as PSU (Power Supply Unit) replacement.

According to this embodiment, it is possible to prevent an excessive increase in the recovery voltage of the electrolytic capacitor C11 after discharge, and to prevent charge from remaining in the electrolytic capacitor C11. As a result, it is possible to prevent troubles such as sparks from the operator's body or from the electrolytic capacitor C11.

Any of the control methods shown in the first embodiment and the first to third modifications may be used as the control method for the voltage output from the DRIVE terminal by the power supply IC 12. FIG.

Further, in this embodiment, an example in which the power supply circuit 1a includes the relay RY10 is shown, but the present invention is not limited to this. It is also possible to use a semiconductor or a switch component so that the auxiliary winding P2 of the transformer T10 is disconnected while power is being supplied.

[Third embodiment]
Next, an image forming apparatus 100 including the power supply circuit 1 described above will be described as an example of an electronic device according to the third embodiment. FIG. 13 is a diagram showing an example of the configuration of the image forming apparatus 100. As shown in FIG.

The image forming apparatus 100 combines functions such as a copy function, a FAX function, a print function, a scanner function, and a function of saving and distributing an input image (a document read by the scanner function, or an image input by the printer function or FAX function). It is a multifunctional machine called MFP (Multi Functional Peripheral/Printer).

The image forming apparatus 100 can also communicate with an external device such as a PC (Personal Computer), and can perform operations according to instructions received from the external device. In the embodiment, the “image” processed by the image forming apparatus 100 includes not only image data but also data that does not contain image data, that is, data that is only text information.

The image forming apparatus 100 adheres toner to an electrostatic latent image written by selectively exposing the surface of a charged photoreceptor, and transfers the adhered toner to a recording medium such as paper. This is an electrophotographic image forming apparatus.

As shown in FIG. 9, the image forming apparatus 100 includes an operation panel 10, a start switch 20, a controller 3, a reading section 4, an engine control section 5, a printer unit 6, and paper feed cassettes 7A and 7B. , a transport unit 8 and a power supply circuit 1 .

An operation panel 10, which is an operation unit, receives various inputs corresponding to user operations, and also receives various types of information (for example, information indicating received operations, information indicating the operation status of the image forming apparatus 100, information indicating the operation status of the image forming apparatus 100, (such as information indicating the setting status of The operation panel 10 is configured by, for example, a liquid crystal display device (LCD: Liquid Crystal Display) equipped with a touch panel function, but is not limited to this. For example, an organic EL (Electro-Luminescence) display device equipped with a touch panel function may be used. Furthermore, in addition to or instead of this, an operation unit such as hardware keys and a display unit such as a lamp may be provided.

Start switch 20 starts image forming apparatus 100 when pressed by a user while image forming apparatus 100 is powered off. When the user presses the button while the image forming apparatus 100 is activated, that is, when the power is on, the image forming apparatus is turned off. As described above, start switch 20 may turn on/off image forming apparatus 100 by being pressed by the user, but is not limited to this. good too.

The controller 3 comprehensively controls the image forming apparatus 100 . As an example, the image forming apparatus 100 is caused to perform an operation according to an operation or information received by the operation panel 10 . As another example, the image forming apparatus 100 is caused to execute an instruction received by the image forming apparatus 100 from an external device such as a PC. Further, as another example, when a specific condition is detected, for example, when pressing of the start switch 20 is detected, and as another example, when an abnormality occurring in the image forming apparatus 100 is detected, a predetermined condition is detected. The image forming apparatus 100 is made to perform an operation.

A specific example of the controller 3 is a controller board on which a circuit for overall control of the image forming apparatus 100 is mounted. As an example, this circuit is equipped with a CPU (Central Processing Unit), ROM (Read Only Memory), and RAM (Random Access Memory). The image forming apparatus 100 is controlled by executing a program stored in the .

The reading unit 4 has an ADF (Auto Document Feeder) 41 and a scanner unit 42 . The ADF 41 sequentially conveys the originals placed on the ADF 41 to optically generate read image data. The scanner unit 42 fixes a document on a transparent document platen and optically reads the fixed document to generate image data.

The engine control section 5 generates control signals for controlling the printer unit 6 and the transport unit 8 based on the image data generated by the reading section 4 . A specific example of the engine control unit 5 is a circuit board for generating control signals based on image data.

The printer unit 6 serving as an image forming section is charged based on image data read by a photosensitive drum 61 as a photosensitive member, a charging member 62 for charging the outer surface of the photosensitive drum 61 , and the reading section 4 . A writing unit 63 that exposes the photoreceptor drum 61 to light and writes an electrostatic latent image on the photoreceptor, a developing member 64 that develops the written latent image with toner, and a recording medium that forms a toner image are conveyed. and a fixing unit 66 for fixing the toner on the recording medium to form a toner image on the recording medium.

Paper feed cassettes 7A and 7B accommodate recording media before image formation. In FIG. 1, two paper feed cassettes are provided as an example, and recording media of different sizes are accommodated in each, but the number may be one or three or more.

A transport unit 8 as a paper feed transport section has various rollers, and transports recording media accommodated in the paper feed cassette 7A and the paper feed cassette 7B to the printer unit 6 . An arrow C in FIG. 9 indicates the conveying direction of the recording medium.

The power supply circuit 1 is a power supply circuit that converts an input voltage into a desired output voltage and supplies power to each component of the image forming apparatus 100 .

Here, the flow of image formation in the image forming apparatus 100 will be described using the copy mode as an example. First, by operating a function switching key or the like on the operation panel 10, the user can sequentially switch and select the copy function, the printer function, and the facsimile function of the image forming apparatus 100, and operate each function. becomes. When the copy function is selected, the mode is the copy mode, when the printer function is selected, the printer mode is selected, and when the facsimile function is selected, the facsimile mode is selected.

In the copy mode, the image information of each document to be copied is read by the reading section 4 and image data is generated.

The outer peripheral surface of the photoreceptor drum 61 is uniformly charged by the charging member 62 in the dark and then exposed to light emitted from the writing unit 63 (indicated by the dotted arrow A in FIG. 1). An electrostatic latent image is formed on the outer peripheral surface of the photosensitive drum 61 . An arrow B indicates the direction of rotation of the photosensitive drum 61 .

The developing member 64 visualizes this electrostatic latent image with toner. Thereby, a toner image is formed on the photosensitive drum 61 . The toner image formed on the photosensitive drum 61 is transferred onto the recording medium on the conveying belt 65 . Then, the fixing unit 66 heats and melts the toner of the toner image on the recording medium with a heater to fix the toner image on the recording medium, and ejects the recording medium from the image forming apparatus 100 .

Although the case where the printer unit 6 forms an image by a monochrome electrophotographic method has been described, a color electrophotographic method, an inkjet method, or the like may be used, and the image forming method is not limited to these.

Further, the operation panel 10 described above may be controlled by the controller 3 or may be controlled by having a control circuit 7 for controlling the operation panel 10 separately from the controller 3 . In that case, the control circuit of the controller 3 and the control circuit of the operation panel 10 are connected so as to be able to communicate with each other, and the controller 3 controls the entire image forming apparatus 100 including the operation panel 10 .

Note that the controller 3, the engine control unit 5, the printer unit 6, the paper feed cassettes 7A and 7B, and the transport unit 8 are provided inside the exterior of the image forming apparatus 100, but in FIG. showing.

In this manner, the image forming apparatus 100 including the power supply circuit 1 can be configured. Note that the image forming apparatus 100 can also include a power supply circuit 1a.

Although the embodiments have been described above, the present invention is not limited to the specifically disclosed embodiments described above, and various modifications and changes are possible without departing from the scope of the claims. .

1 power supply circuit 12 power supply IC (an example of a control unit)
121 START terminal (an example of the first terminal)
122 Vcc terminal 123 AC terminal (example of second terminal)
2 AC commercial power supply 100 Image forming apparatus (an example of electronic equipment)
101 drain terminal 102 source terminal 103 gate terminal C11 electrolytic capacitor T10 transformer Q10 switching element

Japanese Patent No. 6155586

Claims (9)

an electrolytic capacitor for smoothing an AC voltage input from an AC commercial power supply;
a transformer to which a DC voltage smoothed by the electrolytic capacitor is applied;
a switching element connected to the transformer and switched on or off;
A control unit that controls the switching element,
When the AC commercial power supply is interrupted and the voltage between the terminals of the electrolytic capacitor drops below a predetermined value, the control unit controls the switching element so that a predetermined impedance exists between the drain terminal and the source terminal of the switching element. A power supply circuit that discharges the electric charge of the electrolytic capacitor by setting the gate voltage of the switching element . 2. The power supply circuit according to claim 1, wherein said control unit is a power supply IC (Integrated Circuit). The control unit comprises a first terminal for detecting that the voltage across the terminals of the electrolytic capacitor has dropped below the predetermined value, and a second terminal for detecting interruption of the AC commercial power supply. 3. The power supply circuit according to item 1 or 2. 4. The power supply circuit according to claim 1, wherein said predetermined impedance is an impedance of 10 kΩ to 1 MΩ. 5. The power supply circuit according to any one of claims 1 to 4 , wherein said control unit changes said gate voltage according to a voltage between terminals of said electrolytic capacitor. 6. The controller according to any one of claims 1 to 5 , wherein the control unit performs PWM (Pulse Width Modulation) control on the gate voltage, lowers the gate voltage in an ON state below a predetermined voltage, and makes a PWM control period longer than the predetermined period. 2. The power supply circuit according to item 1. 7. The power supply circuit according to claim 6 , wherein said control unit changes said PWM control period according to the voltage between terminals of said electrolytic capacitor. The control unit connects the positive terminal of the electrolytic capacitor to the power supply voltage input terminal of the control unit when the power supply to the auxiliary winding included in the transformer is interrupted. 8. The power supply circuit according to any one of claims 1 to 7 , wherein the electric charge is discharged. An electronic device comprising the power supply circuit according to any one of claims 1 to 8 .

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