JPH072010B2 - Power supply circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power supply circuit suitable for use in OA equipment and the like,
In particular, the present invention relates to a switching regulator type power supply circuit that generates a plurality of power supply voltages.

[Prior Art] In order to stabilize the power supply voltage, a power supply circuit based on a switching regulator system is known.
This is because the current in the primary winding of the transformer is turned on and off by the switching means, and a voltage having a magnitude corresponding to the pulse width of the switching pulse for driving this switching means is induced in the secondary winding. The induced voltage is rectified and smoothed to obtain a DC power supply voltage.

By the way, in such a power supply circuit, due to the increase in the number of devices that require the supply of a power supply voltage due to the increase in the size of the equipment used and the increase in the scale, the increase in output, the increase in output, and the increase in accuracy have been achieved. However, for this type of power supply circuit used in equipment with strict specifications for output load fluctuations, it is necessary to provide a control system for each output. As a control method for stabilizing the output, a method using a three-terminal regu- lator or a tipper has been conventionally known, but there is a problem that the power loss increases when the load is large.

On the other hand, a power supply circuit called a master-slave system is known in which each output is generated using a separate switching regulator, and the switching frequency that turns on and off the current flowing in the primary winding of each transformer is synchronized. I am letting you.

FIG. 4 is a block diagram showing an example of such a conventional power supply circuit.

In the figure, when the DC voltage V ₀ is applied between the input terminals 1 and 2 (however, the input terminal 2 side is connected to the ground line), the input DC voltage V ₀ is divided by the resistors 3 and 4. The voltage starts charging the capacitor 5. The charging voltage of the capacitor 5 is applied as a power supply voltage _Vcc to the switching regulator control circuits 6a and 6b formed into an IC (integrated circuit). When the charging of the capacitor 5 progresses and the power supply voltage V _cc exceeds a predetermined value, the switching regulator control circuit 6
The a and 6b start operating and output a switching pulse. The switching pulse from the switching regulator control circuit 6a is supplied to the gate of the FET 9a. Also, this FET9a
The input DC voltage V ₀ is applied to the drain of the input terminals 1 and 2 via the primary winding 13a of the transformer 12a, and the source of the FET 9a is the ground terminal GND of the switching regulator control circuit 6a. It is connected to the input terminal 2. The switching pulse from the switching regulator control circuit 6b is supplied to the gate of the FET 9b. The input DC voltage V ₀ is applied to the drain of the FET 9b from the input terminal 1 through the primary winding 13b of the transformer 12b, and the source of the FET 9b is connected to the input terminal together with the ground terminal GND of the switching regulator control circuit 6b. The FET 9a connected to 2 is turned on / off by the switching pulse from the switching regulator control circuit 6a. Transformer 12
A current flows through the primary winding 13a of the a when the FET 9a is on, and no current flows when the FET 9a is off. When a current flows through the primary winding 13a, energy is stored in the transformer 12a, and when the current is cut off by the FET 9a in the primary winding 13a, the stored energy causes the reset circuit 15a to be reset from the primary winding 13a. Current flows to the transformer 12a.
Is reset. Next, when the FET 9a is turned on and a current flows through the primary winding 13a, energy is stored in the transformer 12a. The induced voltage on the secondary winding edge 14a is the rectifying / smoothing circuit 19a.
The capacitor 20a suppresses the ripple voltage, and the output terminals 22a and 23a output the voltage V ₁ .

Similarly in the transformer 12b, when the FET 9b is turned on and off, a voltage having an amplitude corresponding to the amount of energy accumulated in the secondary winding 14b is induced, and the rectification / smoothing circuit 19b is generated.
After being rectified and smoothed by, the ripple voltage is suppressed by the capacitor 20b and output as the voltage V ₂ from the output terminals 22b and 23b.

The amount of energy stored in the transformer 12a depends on the pulse width of the switching pulse output from the switching regulator control circuit 6a, and thus the output voltage V obtained between the output terminals 22a and 23a. The value of ₁ corresponds to the pulse width of this switching pulse. Similarly, the value of the output voltage V ₂ obtained between the output terminals 22b and 23b also corresponds to the pulse width of the switching pulse output from the switching regulator control circuit 6b.

The same control voltage is set to the terminals F of the switching regulator control circuits 6a and 6b.
The frequencies of the switching pulses output from now on are synchronized.

A voltage is induced in the auxiliary winding 16 of the transformer 12a in the same manner as above. This voltage is rectified by the rectifying / smoothing circuit 17.
It is smoothed and further charged by the capacitor 5. As a result, the switching pulse generating circuits 6a and 6b are brought into a stable operating state by further increasing the power supply voltage.

The reset circuits 15a and 15b perform reset so that the transformers 12a and 12b are not saturated, respectively.
Absorbs spike noise generated when T9a and 9b are turned on.

[Problems to be Solved by the Invention] By the way, the above-mentioned prior art has the following problems.

First, in the two switching regulator control circuits 6a and 6b, the charging voltage of the same capacitor 5 is the power supply voltage V _cc.
Therefore, at the time of starting when the input DC voltage V ₀ is applied, a large load is placed on the starting circuit including the resistors 3 and 4 and the capacitor 5 for starting the switching regulator control circuits 6a and 6b. Therefore, a large current i must be applied.

Secondly, at startup, the power supply voltage is applied simultaneously to the load connected to the output terminals 22a and 23a and the load connected to the output terminals 22b and 23b, so that the input inrush current becomes excessive and affects the device. In addition to the large influence, the burden on the main transistor and other semiconductor elements also becomes heavy, and the life of these elements becomes short.

Thirdly, although not shown in FIG. 4, the switching regulator control circuits 6a and 6b are provided with an overcurrent protection circuit, and the primary winding 13a of the transformer 12a and the primary winding of the transformer 12a are provided. When an excessive current flows through 13b, these switching regulator control circuits 6a, 6b stop operating. These overcurrent protection circuits consist of a switching regulator control circuit 6
When the power supply voltage V _cc of a and 6b decreases, the protection function is released (that is, restored), and the switching regulator control circuits 6a and 6b become operable.

Therefore, now, the switching regulator control circuit 6a
However, when the operation is stopped and the switching pulse is not output due to the operation of the overcurrent protection circuit, the voltage is not induced from the auxiliary winding 16. Therefore, the capacitor 5 is discharged toward the switching regulator 6b, the charging voltage of the capacitor 5 drops, and the switching regulator 6b also turns off. On the other hand, when the charging voltage of the capacitor 5 is lowered, the overcurrent protection circuit of the switching regulator 6a is restored, and the switching regulators 6a and 6b are activated as the capacitor 5 is recharged by the current i. However, the overcurrent protection circuit of the switching regulator control circuit 6a operates again, and the same operation is repeated.

This enables the switching regulator control circuits 6a and 6b.
Will be activated and deactivated repeatedly, and output terminal 22a,
The load connected to 23a and the load connected to the output terminals 22b and 23b are repeatedly turned on and off, resulting in malfunction, and an excessive input rush power supply is generated each time the power is turned on. There is also a risk of damaging elements such as transistors.

An object of the present invention is to solve the above problems, reduce the load on the starting circuit and the input rush current at the time of starting, and prevent the malfunction of the overcurrent protection circuit during operation. To provide.

[Means for Solving the Problems] In order to achieve the above object, the present invention uses a charging voltage of a capacitor forming a starting circuit as a power supply voltage of a first switching regulator control circuit, and a second method. Of the switching regulator control circuit, a voltage induced by an auxiliary winding and rectified / smoothed by a rectifying / smoothing circuit is used, and the anode is the rectifying / smoothing circuit side. A diode is provided between the capacitor and the capacitor.

[Operation] The input DC voltage is applied and the capacitor starts charging,
When this charging voltage becomes equal to or higher than a predetermined value, first, the first switching regulator control circuit starts operating. As a result, a voltage starts to be induced in the auxiliary winding. When the rectifying / smoothing circuit starts to output a voltage due to the induced voltage, the second switching regulator control circuit starts operating. The output voltage of the rectifying / smoothing circuit also charges the capacitor via the diode, increasing the charging voltage. As a result, the first and second switching regulator control circuits operate stably with their power supply voltages sufficiently high.

The diode prevents the charging voltage of the capacitor from being applied to the power supply terminal of the second switching regulator control circuit when starting. Therefore, the operation start timings of the first and second switching regulator control circuits are shifted as described above, and the starting power supply and input inrush current required by the starting circuit can be reduced.

Further, when the overcurrent protection circuit of the first switching regulator control circuit operates, the second switching regulator control circuit also stops operating, but the discharge of the capacitor is blocked by the diode and the first switching regulator control circuit is stopped. Since the power supply voltage of the switching regulator control circuit is maintained, the overcurrent protection circuit does not recover and the operation stop state continues.

When the overcurrent protection circuit of the second switching regulator control circuit is activated, the output voltage of the rectifying / smoothing circuit is not affected, so the second switching regulator control circuit is in a deactivated state. Continue.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a power supply circuit according to the present invention. 1, 2 are input terminals, 3, 4 are resistors, 5 are capacitors, 6a, 6b are switching regulator control circuits, 7a, 7b
Is a delay circuit, 8a and 8b are gate limiting circuits, 9a and 9b are FETs (field effect transistors), 10a and 10b are buffer circuits, 11a and 11b are overcurrent detection circuits, 12a and 12b are transformers, and 13a and 13b are primary Windings, 14a and 14b are secondary windings, 15a and 15b are reset circuits, 16 is an auxiliary winding, 17 is a rectifying / smoothing circuit, 18 is a diode, 19a,
19b is a rectifying / smoothing circuit, 20a, 20b are capacitors, 21a, 21b
Is a constant voltage circuit, and 22a, 22b, 23a and 23b are output terminals.

In the figure, when the DC voltage V ₀ is applied between the input terminals 1 and 2 (however, the input terminal 2 side is connected to the ground line), the input DC voltage V ₀ by the resistors 3 and 4 is divided. Charging is started in the capacitor 5 by the piezoelectric voltage. The rising voltage of the charging voltage of the capacitor 5 is delayed by the delay circuit 7a and is applied as a power supply voltage _Vcc to the switching regulator control circuit 6a formed into an IC (integrated circuit). When the charging of the capacitor 5 progresses and the power supply voltage V _cc exceeds a predetermined value, the switching regulator control circuit 6a starts operating and the output terminal
Output a switching pulse from OUT. This switching pulse is supplied to the gate of the FET 9a via the gate limiting circuit 8a. The input DC voltage V ₀ is applied to the drain of the FET 9a from the input terminals 1 and 2 via the primary winding 13a of the transformer 12a, and the source of the FET 9a is the ground terminal of the switching regulator control circuit 6a. Connected to GND.

The FET 9a is turned on / off by a switching pulse from the switching regulator control circuit 6a. As a result, the current is turned on and off in the primary winding 13a of the transistor 12, and the output terminal 22a,
A DC voltage V ₁ is obtained during 23a.

A voltage is induced in the auxiliary winding 16 of the transformer 12a in the same manner as above. This voltage is rectified by the rectifying / smoothing circuit 17.
It is smoothed and further charged by the capacitor 5 via the diode 18. This enables the switching pulse generation circuit
6a is in a stable operating state as the power supply voltage is further increased. In addition, the output voltage of the rectifying / smoothing circuit 17 is delayed by the delay circuit 7b, and then is output as the power supply voltage _Vcc to the IC.
It is applied to the converted switching regulator control circuit 6b.

The switching regulator control circuit 6b is connected to the power supply voltage V _cc.
Rises and becomes a predetermined value or more, the operation is started, a switching pulse is output from the output terminal OUT, and is supplied to the gate of the FET 9b via the gate limiting circuit 8b. This FET
The input DC voltage V ₀ is applied to the drain of 9b from the input terminal 1 through the primary winding 13b of the transformer 12b, and the source of the FET 9b is the ground terminal GN of the switching regulator control circuit 6b.
Connected to D. As a result, the FET 9b is turned on / off according to the switching pulse from the switching regulator control circuit 6b. Therefore, the voltage V ₂ is output from the output terminals 22b and 23b as in the prior art shown in FIG.

The amount of energy stored in the transformer 12a depends on the pulse width of the switching pulse output from the switching regulator control circuit 6a, and thus the output voltage V obtained between the output terminals 22a and 23a. The value of ₁ corresponds to the pulse width of this switching pulse. Similarly, the value of the output voltage V ₂ obtained between the output terminals 22b and 23b also corresponds to the pulse width of the switching pulse output from the switching regulator control circuit 6b.

The constant voltage circuit 21a amplitude compares the output voltage of the rectifying / smoothing circuit 19a with a preset reference voltage, and generates a difference voltage between them. This difference voltage is supplied to the terminal FB of the switching regulator control circuit 6a. The switching regulator control circuit 6a changes the pulse width of the switching pulse according to the difference voltage. Thereby, the output terminal 22a,
The output voltage V ₁ obtained during 23a is maintained at a predetermined constant value. Similarly, the switching regulator control circuit 6b
The differential voltage is supplied from the constant voltage circuit 21b to the terminal FB, and the pulse width of the switching pulse is changed according to the differential voltage. As a result, the output voltage V ₂ obtained between the output terminals 22b and 23b is held at a predetermined constant value.

The overcurrent detection circuit 11a detects the magnitude of the current flowing through the FET 9a. Now, the load (not shown) contacted between the output terminals 22a and 23a is abnormal, and the primary winding 13a, F of the transformer 12a is
When an overcurrent flows through the ET9a, the overcurrent detection circuit 11a outputs an overcurrent detection signal to output the switching regulator control circuit.
Supply to terminal 6a OCP. The switching regulator control circuit 6a has an overcurrent protection circuit, and when the overcurrent detection signal is supplied, the switching pulse width is narrowed sufficiently or the generation of the switching pulse is stopped. As a result, the output voltage V ₁ obtained between the output terminals 22a and 23a becomes almost zero or equal to zero. Further, since the output voltage of the rectifying / smoothing circuit 17 becomes almost zero or zero, the switching regulator control circuit 6b stops its operation. Therefore, the output voltage V ₂ obtained between the output terminals 22b and 23b becomes zero.

As a result, it is possible to simultaneously prohibit the supply of the power supply voltage to the load connected between the output terminals 22a and 23a and the load connected to the output terminals 22b and 23b.

Further, the overcurrent detection circuit 11b detects the current flowing through the FET 9b. If the load connected between the output terminals 22b and 23b becomes abnormal and the current flowing in the primary winding 13b of the transformer 12b and the FET 9b becomes an overcurrent, the overcurrent detection circuit 11b detects this and detects an overcurrent detection signal. To the terminal OCP of the switching regulator control circuit 6b. The switching regulator control circuit 6b has an overcurrent protection circuit, and when the overcurrent detection current is supplied, the pulse width of the switching pulse is sufficiently narrowed or the generation of the switching pulse is stopped. Therefore, the voltage V ₂ from the output terminals 22b and 23b to the load
Will not be supplied. However, in this case, the switching regulator control circuit 6a is operating normally, and the load connected between the output terminals 22a and 23a has a voltage V _{1 of a} predetermined value.
Is supplied as the power supply voltage.

As described above, when the input DC voltage V ₀ is input to the input terminals 1 and 2, at the time of startup, the switching regulator control circuit 6a
After the start of operation, the switching regulator control circuit
6b starts operating. As a result, after the power supply voltage V ₁ is applied to the load connected to the output terminals 22a and 23a, the power supply voltage V ₂ is applied to the load connected to the output terminals 22b and 23b. The power supply voltage is not applied to the load at the same time. Therefore, the input inrush current at the time of startup does not become excessive.

In this way, the diode 18 performs its function so that the operation start timings of the switching regulator control circuits 6a and 6b can be shifted. That is,
The capacitor 5 is charged together with the input DC voltage V ₀ from the input terminals 1 and 2. The charging voltage causes the diode 18 to be reverse biased, and the switching regulator control circuit 6b receives the power supply voltage. V _cc is not applied. If the switching regulator control circuit 6a operates and the output voltage of the rectifying / smoothing circuit 17 does not rise sufficiently,
The switching regulator control circuit 6b does not operate.

In addition, since the current i flowing through the resistor 3 is not sufficiently large and the change of the charging voltage of the capacitor 5 is slow, and even if the switching regulator control circuit 6a starts to operate with such charging, the power source voltage V _cc Even when the rising speed is slow and the unstable state continues, once the switching regulator control circuit 6a starts operating, the voltage is output from the rectifying / smoothing circuit 17 to turn on the diode 18, and the rectifying / smoothing circuit is activated.
The current supplied from 17 through the diode 18 causes the capacitor 5 to be rapidly charged to a sufficiently large voltage. Therefore, the power consumption of the resistor 3 can be reduced, and
Immediately after startup, the operation of the switching regulator control circuit 6a can be stabilized.

Further, when the switching regulator control circuit 6a stops its operation due to the operation of the overcurrent protection circuit, no voltage is output from the rectification / smoothing circuit 17, and the diode 18 is reverse biased by the charging voltage of the capacitor 5. Then, the switching regulator control circuit 6b also stops operating. Therefore, due to the action of this diode 18, the capacitor 5
Discharge of the switching regulator control circuit 6b is prohibited. Therefore, the switching regulator control circuit 6a
The high power supply voltage V _cc is continuously applied to the overcurrent protection circuit 6a of the switching regulator control circuit 6a.
As long as this is activated, the switching regulator control circuit 6b will continue to deactivate. The overcurrent protection circuits of the switching regulator control circuits 6a, 6b are restored when the power supply voltage V _cc drops, and therefore, in order to return the switching regulator control circuits 6a, 6b to the operable state, ,
It suffices to temporarily stop the supply of the input DC voltage V ₀ .

By the way, if the diode 18 is not provided, when the overcurrent protection circuit of the switching regulator control circuit 6a operates,
The following malfunctions occur. That is, when the overcurrent protection circuit operates, no voltage is output from the rectifying / smoothing circuit 17, and thus the instantaneous switching regulator control circuit 6b stops operating, but the charging voltage of the capacitor 5 causes the power supply voltage to rise. V _cc rises and starts operating again. However, the switching regulator control circuit 6b consumes the electric power accumulated in the capacitor 5 when the operation is started, so that the charging voltage of the capacitor 5 decreases. As a result, in the switching regulator control circuit 6a, the power supply voltage V _cc is lowered, and the overcurrent protection circuit is restored. However, when the capacitor 5 is charged thereafter and the charging voltage becomes sufficiently high, the switching regulator control circuit
6a starts operating and the rectifying / smoothing circuit 17 outputs a voltage. At the same time, the overcurrent detection circuit 11a causes the primary windings 13a, F
The overcurrent of the ET 9a is detected, and the switching regulator control circuit 6a is deactivated by the activation of the overcurrent protection circuit. In this way, the switching regulator control circuits 6a and 6b are repeatedly operated and stopped. The diode 18 prevents the malfunction voltage by preventing the charging voltage of the capacitor 5 from being applied to the switching regulator control circuit 6b.

Delay circuits 7a, 7b are used the output terminal 22a, output voltages V ₁ and the output terminal 22b at 23a, the time difference between generation timing and these generation timing of the output voltage V ₂ at 23b for Motaraseru. FIG. 2 shows the generation timings of these output voltages V ₁ and V ₂ , where t ₀ is the input DC voltage V ₀ , t ₁
Indicates the generation start time of the output voltage V ₁ , and t ₂ indicates the generation start time of the output voltage V ₂ . Then, t ₀ ~t between ₁ is determined by the delay time of the charging time and the delay circuit 7a of the capacitor 5, t ₁
The time difference Δt between t _{2 and} t ₂ is determined by the delay time of the delay circuit 7b.

FIG. 3 is a circuit diagram showing a concrete example of the delay circuits 7a and 7b in FIG. 1, in which 24 is a power supply terminal, 25 to 28 are resistors,
29 is a capacitor, 30 is a zener diode, 31 is a transistor, 32 is a ground terminal, and 33 is an output terminal.

In the figure, the power supply terminal 24 is charged with the charging voltage (in the case of the delay circuit 7a) of the capacitor 5 of FIG.
The output voltage of 17 is applied, and the ground terminal 32 is the terminal GN of the switching regulator control circuit 6a or 6b in FIG.
Connected to D, the output terminal 33 is also connected to the power supply terminal _Vcc of the switching regulator control circuit 6a or 6b.

When the voltage V _s is applied to the power supply terminal 24, the capacitor 29 is charged by the voltage divided by the resistors 25 and 27 of the applied voltage V _s . The zener diode 30 is off and the transistor 31 is also off until the charging voltage of the capacitor 29 reaches the zener voltage V _z of the zener diode 30. Therefore, the potential of the output terminal 33 is equal to the potential of the ground terminal 32.

When the charging voltage of the capacitor 29 becomes equal to or higher than the zener voltage V _z , the zener diode 30 turns on, the charging voltage of the capacitor 29 is applied to the base of the transistor 31, and the transistor 31 turns on. Thus the applied voltage V _s of the power supply terminal 24
Is divided by the resistors 26 and 28 and output from the output terminal 33.

As described above, the voltage from the output terminal 33 to the power supply terminal 24
The zener voltage V _z at the capacitor 29 is higher than that when V _s is applied.
The voltage is output with a delay of the charging time up to. This delay time can be arbitrarily set according to the resistance values of the resistors 25 and 27 and the capacitance value of the capacitor 29.

When this embodiment is used in a printer, the output voltage V ₁ can be used as a power supply voltage for a control system such as a digital circuit, and the output voltage V ₂ can be used as a power supply voltage for a motor or a thermal head. According to this, when the control system is abnormal, the power of the motor and the thermal head, etc. is turned off at the same time with the control system, and when the motor, the thermal head, etc. are abnormal, these power sources are turned off, and the printer is inadvertent. Accidents can be prevented. Also, after the power supply voltage of the control system is stabilized, the power supply voltage is applied to the motor and thermal head, so the control system does not operate correctly even when the power is turned on, and the motor and thermal head do not malfunction. .

In FIG. 1, the buffer circuits 10a and 10b connected in parallel to the FETs 9a and 9b respectively absorb spike noise generated when the FETs 9a and 9b are switched from on to off. Further, the gate control circuits 8a and 8b limit the currents flowing in the FETs 9a and 9b, respectively, thereby simplifying the on / off rising and falling of the FETs 9a and 9b, and the switching regulator control circuits 6a and 6b.
The output current is regulated by the power supply voltage _Vcc of b.

Further, either one of the delay circuits 7a and 7b may be provided, or these may be omitted.

[Effects of the Invention] As described above, according to the present invention, it is possible to shift the timing of applying the power supply voltage to different loads at the time of startup, so that the input inrush current can be reduced and the large current can be reduced. It is possible to prevent breakage of electrical parts and deterioration of characteristics due to the above, and to reduce the drive current of the starter circuit, thereby reducing power consumption and using elements with low durability.

In addition, it is possible to provide a time difference in the application timing of the power supply voltage to each load, and it is also possible to prevent malfunction of each load at startup.

Further, the overcurrent protection circuit can also be operated in stages, and their malfunctions can be prevented.

[Brief description of drawings]

1 is a configuration diagram showing an embodiment of a power supply circuit according to the present invention, FIG. 2 is a diagram showing the operation of the delay circuit in FIG. 1,
FIG. 3 is a circuit diagram showing a specific example of this delay circuit, and FIG. 4 is a configuration diagram of a power supply circuit according to a conventional example. 5 ... Capacitor, 6a, 6b ... Switching regulator control circuit, 7a, 7b ... Delay circuit, 9a, 9b ... Field effect transistor, 11a, 11b ... Overcurrent detection circuit, 12a, 12b.
Transformers, 13a, 13b ... Primary winding, 14a, 14b ... Secondary winding, 16 ... Auxiliary winding, 17 ... Rectifying / smoothing circuit, 18 ... Diode.

Claims (3)

[Claims] 1. A first transformer having a primary winding, a secondary winding, and an auxiliary winding to which an input DC voltage is applied, a capacitor charged by a divided voltage of the input DC voltage, and the capacitor. A first switching regulator control circuit using the charging voltage of the power supply voltage as a power supply voltage and a current flowing in the primary winding in response to a switching pulse output from the first switching regulator control circuit. , Turn off first
Transistor, a rectifying / smoothing circuit for rectifying / smoothing the voltage induced in the auxiliary winding by turning on / off the current flowing in the primary winding, and the rectifying / smoothing circuit using the rectifying / smoothing circuit side as an anode. A diode connected between the smoothing circuit and the capacitor, and a second switching regulator control circuit using the output voltage of the rectifying / smoothing circuit as a power supply voltage,
A second transformer having a primary winding and a secondary winding to which the input DC voltage is applied, and the second transformer according to a switching pulse output from the second switching regulator control circuit. A second transistor for turning on and off a current flowing through the primary winding of
The power supply circuit is configured so that different power supply voltages can be obtained by rectifying and smoothing the voltages induced in the secondary windings of the transformer. 2. The voltage output terminal of the rectifying / smoothing circuit and the second switching regulator between the capacitor and the power supply terminal of the first switching regulator control circuit according to claim 1. A power supply circuit, wherein a delay circuit is provided in at least one of the power supply terminal of the control circuit. 3. The first and the second according to claim 1 or 2,
To detect the overcurrent flowing in the primary winding of each transformer
First and second overcurrent detection circuits are provided, and the first and second switching regulator control circuits are deactivated at the same time when the first overcurrent detection circuit detects the overcurrent. A power supply circuit characterized in that the second switching regulator control circuit is deactivated at the same time when the detection circuit detects overcurrent.