JP7633904B2 - Switching Power Supply - Google Patents

Description

The present invention relates to a multiple-output switching power supply with multiple outputs, and more specifically to a switching power supply that suppresses losses in the constant voltage circuit of a non-voltage-controlled output circuit.

Patent document 1 describes an invention in which a multi-output switching power supply has one output configured as a voltage-controlled output circuit and the other outputs configured as non-voltage-controlled output circuits, and in which power is supplied from the voltage-controlled output circuit to the non-voltage-controlled output circuit in order to stabilize the output voltage of the non-voltage-controlled output circuit.
The invention described in Patent Document 1 is a switching power supply that, when the load on a non-voltage-controlled output circuit including a constant voltage circuit increases and the operating voltage of the non-voltage-controlled output circuit decreases, supplies power from the voltage-controlled output circuit to increase the voltage of the non-voltage-controlled output circuit, thereby stabilizing the input voltage of the constant voltage circuit.

The invention described in Patent Document 1 has the effect of stabilizing the input voltage of the constant voltage circuit, but it cannot reduce losses in the constant voltage circuit.

FIG. 6 is a circuit diagram of a switching power supply having the same function as the conventional switching power supply shown in FIG. 3 of Patent Document 1.
This switching power supply 50 includes a voltage-controlled output circuit 57 that rectifies and smoothes the voltage induced in a winding N2 on the secondary side of the transformer T and controls a controlled output voltage Vo1 using an output voltage feedback (FB) circuit 53 or the like, and a non-voltage-controlled output circuit 58 that rectifies and smoothes the voltage induced in another secondary winding N3 and constant-voltages it using a regulator element (constant voltage circuit) to generate a non-controlled output voltage Vo2.

In this switching power supply 50, no constant voltage adjustment circuit is connected between the voltage-controlled output circuit 57 and the non-voltage-controlled output circuit 58. When the output of the voltage-controlled output circuit 57 is heavily loaded and the output current increases, the voltage of winding N3 also increases accordingly. However, even if the input voltage of the regulator element of the non-voltage-controlled output circuit 58 increases, the non-controlled output voltage is kept constant due to the function of the regulator element.

JP 2002-272108 A

However, in the switching power supply 50 shown in FIG. 6, as the load on the voltage-controlled output circuit 57 increases, the input voltage of the regulator element of the non-voltage-controlled output circuit 58 increases, resulting in increased losses. In addition, the regulator element generates heat, causing an increase in temperature, resulting in a further decrease in efficiency.

The present invention solves the above problems by providing a switching power supply configured as follows:

That is, the device is provided with a transformer having a primary winding and multiple secondary windings, a switching circuit connected to the primary winding of the transformer, a voltage-controlled output circuit connected to one of the multiple secondary windings, and a non-voltage-controlled output circuit composed of an input voltage switching circuit and a constant voltage circuit connected to the other secondary winding, in which a switching voltage obtained by turning on/off the input DC voltage with the switching circuit is applied to the primary winding of the transformer, and the voltage induced in the multiple secondary windings is supplied to the load as a controlled output voltage from the voltage-controlled output circuit, while being supplied to the load as a non-controlled output voltage via the constant voltage circuit connected to the input voltage switching circuit. In the switching power supply, the secondary winding of the transformer connected to the input voltage switching circuit has multiple winding output terminals that generate different voltages, the voltage-controlled output circuit converts the voltage induced in the secondary winding of the transformer into DC and supplies the stabilized voltage to the load as a controlled output voltage by an output voltage feedback circuit and a control circuit, and the input voltage switching circuit receives the voltage induced in the multiple winding output terminals, detects the temperature of the constant voltage circuit with a temperature sensor, switches to one of the voltages induced in the multiple winding output terminals, converts the switched voltage into DC, and supplies it to the constant voltage circuit.

With this configuration, even if the load on the voltage-controlled output circuit increases and the loss in the constant voltage circuit increases, the temperature sensor detects the temperature of the constant voltage circuit and switches the input voltage of the input voltage switching circuit, thereby reducing the loss in the constant voltage circuit.

The constant voltage circuit is also characterized in that it is composed of a regulator element, and the temperature sensor is a thermistor that is disposed in the vicinity of the regulator element.
With this configuration, the temperature of the regulator element of the constant voltage circuit is converted into a resistance value by the thermistor to switch the voltage induced in the input voltage switching circuit, thereby reducing loss in the regulator element.

The input voltage switching circuit is also characterized in that a temperature sensor that detects the temperature of the regulator element of the constant voltage circuit is an NTC thermistor, and when the resistance value of the thermistor is equal to or greater than a predetermined value, the input voltage switching circuit switches to an output terminal having a higher induced voltage among the multiple winding output terminals to supply the voltage to the constant voltage circuit, and when the resistance value of the thermistor is less than the predetermined value, the input voltage switching circuit switches to an output terminal having a lower induced voltage, thereby suppressing a rise in temperature of the constant voltage circuit.
With this configuration, when the resistance value of the NTC thermistor falls below a predetermined value, the input voltage of the constant voltage circuit can be switched to the output terminal with a lower induced voltage.

Furthermore, the input voltage switching circuit is a circuit in which an output terminal having a high induced voltage among the multiple winding output terminals is connected to one terminal of a current path of a voltage switch element that turns the induced voltage on and off, the other terminal of the current path of the voltage switch element is connected to a first diode, an output terminal having a low induced voltage among the multiple winding output terminals is connected to a second diode, and the second diode and the first diode are OR-connected and connected to the input terminal of the constant voltage circuit, and the input voltage switching circuit controls a control terminal of the voltage switch element in accordance with a resistance value of a thermistor, and is characterized in that when the resistance value of the thermistor is equal to or greater than a predetermined value, the voltage switch element is turned on, and when the resistance value of the thermistor is less than the predetermined value, the voltage switch element is turned off.
With this configuration, when the temperature of the regulator element rises, the voltage switch element turns off the output terminal with a high induced voltage and switches to the output terminal with a low induced voltage, thereby suppressing loss in the constant voltage circuit.

The input voltage switching circuit is also characterized in that a temperature sensor that detects the temperature of the regulator element is a PTC thermistor, and when the resistance value of the thermistor is less than a predetermined value, the input voltage switching circuit switches to the output terminal with the higher induced voltage among the multiple winding output terminals to supply the voltage to the constant voltage circuit, and when the resistance value of the thermistor is equal to or greater than the predetermined value, the input voltage switching circuit switches to the output terminal with the lower induced voltage to suppress a temperature rise in the constant voltage circuit.
With this configuration, even if the temperature sensor is a PTC thermistor, the induced voltage can be switched by the input voltage switching circuit, thereby suppressing loss in the constant voltage circuit.

By configuring as described above, the switching power supply according to the present invention can reduce power loss by lowering the input voltage of the constant voltage circuit by switching the voltage of the secondary winding even if the load of the voltage-controlled output circuit increases and the input voltage of the constant voltage circuit of the non-voltage-controlled output circuit rises.

1 is a circuit diagram of a switching power supply according to an embodiment of the present invention. FIG. 2 is a circuit diagram of an input voltage switching circuit. FIG. 4 is an explanatory diagram of the operation of the input voltage switching circuit. 5 is a diagram showing the relationship between the temperature of a regulator element and the resistance value of a thermistor. 1 is a part of a circuit diagram in which a PTC thermistor is used to detect the temperature of a constant voltage circuit. FIG. 1 is a circuit diagram of a conventional switching power supply described in Patent Document 1.

A switching power supply according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram of a switching power supply according to an embodiment of the present invention.
This switching power supply 1 includes a switching circuit 5 that rectifies an AC voltage AC by a diode bridge DB, smoothes it by a capacitor C10, and turns the DC voltage on and off to convert the voltage, a transformer T having a primary winding N1 to which the converted switching voltage is supplied and three secondary windings, namely a first secondary winding N2, a second secondary winding N3, and a third winding N4, a voltage-controlled output circuit 7 that converts the switching voltage induced in the first secondary winding N2 to a DC voltage by a DC conversion circuit (rectifier/smoothing circuit) consisting of a diode D10 and a smoothing capacitor C11, a control circuit 4 that controls the switching circuit 5, an output voltage feedback (FB) circuit 3 that feeds back the controlled output voltage of the voltage-controlled output circuit 7 to the control circuit 4 via a photocoupler PC, and an input voltage switching circuit 2 and a constant voltage circuit 6 to which a plurality of voltages induced in the second secondary winding N3 are input. An output voltage feedback (FB) circuit 3 and a control circuit 4 control a switching circuit 5 so that the controlled output voltage of a voltage controlled output circuit 7 becomes a predetermined voltage.
Furthermore, the induced voltage of the third winding N4 is rectified and smoothed by a diode D11, a resistor R10 and a capacitor C14, and is supplied as a power supply VCC to the control circuit 4.

The second secondary winding N3 has a plurality of winding output terminals which generate different voltages, and the plurality of winding output terminals are connected to the input voltage switching circuit 2. A constant voltage circuit 6 is connected to the output terminal (voltage Vo2′) of the input voltage switching circuit 2.
Capacitors C12 and C13 are connected between the input side (Vo2' side) and the output side (Vo2 side) of the constant voltage circuit 6 and GND, respectively.
The input voltage switching circuit 2 and the constant voltage circuit 6 constitute a non-voltage controlled output circuit 8 .

The constant voltage circuit 6 is configured with a regulator element 9 that keeps the output voltage constant. In this embodiment, a three-terminal regulator is used as the regulator element 9, but a shunt regulator or a Zener diode can also be used as the regulator element 9.
The output voltage FB circuit 3 is a well-known circuit that divides the voltage using resistors connected in series between the controlled output voltage terminals, inputs the divided voltage to the reference (Ref) terminal of the shunt regulator, converts the change in the voltage-controlled output voltage into a current flowing through the photocoupler PC, and outputs it to the feedback (FB) terminal of the control circuit 4. The control circuit 4 is also a well-known circuit that outputs from the OUT terminal to the switching circuit 5 and controls the on/off cycle of the switching circuit 5 so that the controlled output voltage Vo1 of the voltage-controlled output circuit 7 becomes a predetermined voltage value.
The switching circuit 5 is made up of a switching element that turns on/off the DC voltage and a resistor.

FIG. 2 is a circuit diagram of the input voltage switching circuit 2.
The input voltage switching circuit 2 is composed of a voltage switching element Q1, transistors Q2 to Q4, diodes D1 to D3, capacitors C1 to C3, resistors R1 to R8, and a thermistor TH1. The thermistor TH1, transistors Q2 and Q3, capacitor C2, resistors R5 to R8
constitutes the thermistor circuit 21.
In this embodiment, the voltage switch element Q1 is a P-channel MOS-FET, the transistors Q2 and Q3 are NPN types, and the transistor Q4 is a PNP type.

The winding output terminal NS1 (hereinafter referred to as the "winding end terminal") at the end of the secondary winding N3 of the transformer T is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to the capacitor C3, one end of the capacitor C1, one end of the resistor R1, and the source of the voltage switch element Q1 (one terminal of the current path). The other end of the capacitor C3 is connected to GND, and the other ends of the capacitor C1 and the resistor R1 are connected together and are connected to the gate of the voltage switch element Q1 (corresponding to the "control terminal" in this invention) and one end of the resistor R2. The drain of the voltage switch element Q1 (the other terminal of the current path) is connected to the anode of the diode D2 (first diode).

A winding output terminal NS2 (hereinafter referred to as the "center tap") from the middle of the secondary winding N3 is connected to the anode of a diode D3 (second diode), the cathode of the diode D3 is OR-connected with the cathode of the diode D2, and is connected to an output voltage Vo2' terminal. The output voltage Vo2' terminal is connected to a constant voltage element 9.
The start of the secondary winding is connected to GND, and the voltage at the end terminal is higher than the voltage at the center tap.

The other end of the resistor R2 is connected to the collector of the transistor Q2, and the emitter of the transistor Q2 is connected to GND. The base of the transistor Q2 is connected to one end of a resistor R3, the collector of the transistor Q3, and one end of a resistor R4, and the other end of the resistor R4 is connected to the output voltage Vo1 terminal.
The emitter of the transistor Q3 is connected to GND, and the base of the transistor Q3 is connected to one end of a capacitor C2 and to a connection point of a series connection of resistors R6 and R5. The other end of the capacitor C2 and the other end of the resistor R5 are connected to GND, and the other end of the resistor R6 is connected to the collector of the transistor Q4.

The emitter of the transistor Q4 is connected to one end of a resistor R8 and to an output voltage Vo2 terminal. The other end of the resistor R8 is connected to the base of the transistor Q4 and one end of the thermistor TH1. The other end of the thermistor TH1 is connected to one end of a resistor R7, and the other end of the resistor R7 is connected to GND.
In this embodiment, the thermistor TH1 is an NTC (Negative Temperature Coefficient) thermistor that exhibits a negative characteristic change in resistance. The thermistor TH1 is disposed near the terminal of the regulator element 9 and detects heat generation from the regulator element 9.

The operation of the non-voltage controlled output circuit 8 constituted by the input voltage switching circuit 2 and the constant voltage circuit 6 as described above will be described with reference to FIGS.
When the controlled output voltage Vo1 is not being output, the voltage at point A of the transistor Q2 shown in FIG. 2 becomes the GND potential (reference potential), so that the transistor Q2 is turned off, no current flows through the resistor R2, and the gate-source voltage of the voltage switch element Q1 becomes 0 V, so that the voltage switch element Q1 is turned off.

When the controlled output voltage Vo1 is output, the voltage (point A) divided by resistors R4 and R3 is applied to the base of transistor Q2, turning on transistor Q2. Then, the voltage divided by resistors R1 and R2 is applied between the gate and source of voltage switch element Q1, turning on voltage switch element Q1.
The diodes D2 and D3 are OR-connected, and the voltage of the output terminal NS1 is higher than that of the output terminal NS2, so that the voltage of the output terminal NS1 is output as it is to the constant voltage circuit 6 as the voltage Vo2'.

As the load current of the voltage-controlled output circuit 7 increases, the induced voltage of the secondary winding N3 also increases, and the input voltage Vo2' of the constant voltage circuit 6 supplied from the output terminal NS1 and the voltage switch element Q1 also increases. This causes the loss of the constant voltage circuit 6 to increase, and the amount of heat generated also increases.

Thermistor TH1 located immediately adjacent to the terminal of regulator element 9 of constant voltage circuit 6 detects heat generation from regulator element 9, and when the resistance value of thermistor TH1 decreases, the base voltage of transistor Q4 decreases due to voltage division by resistor R7, thermistor TH1, and resistor R8, turning transistor Q4 on. When transistor Q4 turns on, the voltage divided by resistors R5 and R6 increases, the base voltage of transistor Q3 increases, and transistor Q3 turns on.
When the transistor Q3 is turned on, the base-emitter voltage of the transistor Q2 becomes 0 V, so that the transistor Q2 is turned off and the voltage switch element Q1 is turned off.

When the voltage switch element Q1 is turned off, the output of the output terminal NS1 with the high induced voltage is turned off, so that the induced voltage of the output terminal NS2 with the low induced voltage is output as is, and the input voltage Vo2' of the constant voltage circuit 6 is switched to be supplied from the induced voltage of the NS2 winding.
Since the NS2 winding terminal voltage is lower than the NS1 winding terminal voltage, the loss in the regulator element 9 is reduced, and the amount of heat generated is also reduced. This allows the heat sink for dissipating the heat of the regulator element 9 to be made smaller.

In addition, a capacitor C2 is inserted between the base and emitter of transistor Q3. This capacitor C2 creates a time delay so that the on or off state of transistor Q3 can be maintained to some extent even when the load current of voltage control output circuit 7 increases and transistor Q3 turns on, or when the load current of voltage control output circuit 7 decreases and transistor Q3 turns off.

FIG. 3 is a diagram illustrating the operation of the input voltage switching circuit 2 in a state in which the voltage controlled output circuit 7 of the switching power supply 1 starts output, switches from a light load to a heavy load, and then returns to the light load.
Fig. 3 is composed of one graph (a) and seven waveforms (b) to (h). The top graph (a) is a diagram showing the transition of the temperature of the regulator element 9 of the constant voltage circuit 6 when the load changes from the start of output to a light load, a heavy load, and then back to a light load. The solid line (B) shown in the figure shows the temperature of the regulator element of the conventional switching power supply shown in Fig. 6, and the curved solid line (A) shows the temperature of the regulator element 9 of the constant voltage circuit 6 of the present invention. It shows that when the voltage-controlled output circuit 7 goes into a heavy load and when it returns from a heavy load to a light load, the temperature of the regulator element of the conventional switching power supply and the switching power supply of the present invention is different.

The four waveforms (b) to (e) show the on/off states of the transistors Q4 to Q1 when the temperature of the regulator element 9 of the constant voltage circuit 6 reaches a predetermined temperature and the resistance value of thermistor TH1 reaches a predetermined value.
The NS1 winding voltage (f) and the NS2 winding voltage (g) indicate that the induced winding voltage changes due to light and heavy loads on the voltage-controlled output circuit 7, and the Vo2' voltage (h) is a voltage that is switched by the function of the input voltage switching circuit 2 and output to the constant voltage circuit 6.

That is, the temperature of the regulator element 9 of the constant voltage circuit 6 changes depending on the load of the voltage-controlled output circuit 7. The predetermined value of thermistor TH1 is the resistance value of thermistor TH1 when the regulator element 9 of the constant voltage circuit 6 reaches a predetermined temperature, and the resistance value of the thermistor circuit 21 is selected so that transistor Q4 switches between off and on when thermistor TH1 reaches the predetermined value. Even if the load of the voltage-controlled output circuit 7 continues to be heavy, the temperature (A) of the regulator element 9 of the constant voltage circuit 6 drops because the input voltage of the constant voltage circuit 6 has switched to a low voltage.

3(f) to (h) (the period transitioning from heavy load to light load) shows that when the temperature of constant voltage circuit 6 rises due to a heavy load on voltage controlled output circuit 7, and then the load returns to light again and the temperature of regulator element 9 of constant voltage circuit 6 drops to a predetermined temperature, the resistance value of thermistor TH1 exceeds a predetermined value, transistor Q4 switches from on to off again, and output voltage VO2' also switches.
4 is a diagram showing the relationship between the temperature of the regulator element and the resistance value of the thermistor. The horizontal axis is temperature and the vertical axis is resistance value, and shows the characteristics of an NTC thermistor, in which the resistance value decreases as the temperature increases. The resistance value of thermistor TH1 when the temperature of regulator element 9 of constant voltage circuit 6 reaches a predetermined temperature is set to a predetermined value.
Furthermore, the PTC thermistor will be described later, but FIG. 4 also shows the relationship between temperature and resistance value and the predetermined values when a PTC thermistor is used.

Next, the setting of the number of turns of the second secondary winding N3, and the calculated values of the induced voltage and the loss of the regulator element 9 are shown in Table 1.

Here, the winding voltage of NS1 and NS2 is 1V per turn, and the number of turns of the NS1 terminal is 2 turns in addition to the NS2 terminal. Also, when the voltage-controlled output circuit 7 is under heavy load, the voltage of the second secondary winding N3 is 3V higher than when it is under light load. Furthermore, the output voltage of the regulator element 9 is 16V, and the output current is 1A, and the loss of the regulator element 9 is calculated. The loss of the regulator element 9 is calculated by (Vo2'-Vo2) x Iout.

According to the present invention, even if the voltage controlled output circuit 7 is heavily loaded, the loss in the regulator element 9 of the constant voltage circuit 6 is 4.0 W, which is an improvement of 2.0 W compared to 6.0 W in the conventional case.
The predetermined temperature of the regulator element 9 is lower than the temperature at a loss of 4.0 W in the case of a heavy load as shown in Fig. 3(a) and higher than the temperature at a loss of 3.0 W in the case of a light load. When the load of the voltage-controlled output circuit 7 shifts from a heavy load to a light load, the Vo2' voltage temporarily becomes 17 V as shown in Fig. 3(h), and the output voltage Vo2 drops, but this is temporary and the effect on the load is small. In other words, it is desirable to connect a load that will not be affected by a drop in output voltage of about 2 V.

The thermistor TH1 used in the input voltage switching circuit 2 shown in FIG. 2 is an NTC thermistor whose resistance value decreases as the temperature increases, but a PTC (Positive Temperature Coefficient) thermistor whose resistance value increases with increasing temperature may also be used.
Fig. 5 is a part of a circuit diagram in which a PTC thermistor is used to detect the temperature of a regulator element. This circuit replaces the thermistor circuit 21 in the dotted line portion including an NTC thermistor shown in Fig. 2. In this case, the circuit is composed of a transistor Q3, a capacitor C4, a resistor R9 and a PTC thermistor TH2, and the number of parts is smaller than when an NTC thermistor TH1 is used.
In this circuit, when the temperature of regulator element 9 rises to a predetermined temperature, the resistance value of PTC thermistor TH2 also rises to a predetermined value, which causes the base voltage of transistor Q3 to rise and turn on transistor Q3. Once transistor Q3 is on, the rest of the explanation is omitted because it is the same as for the circuit using NTC thermistor TH1.

Explanation of operation using this PTC thermistor TH2 is omitted here, as it can be explained in the same way using Figure 3, which uses an NTC thermistor. However, although a diagram showing the on/off of transistor Q4 is unnecessary, the temperature change of regulator element 9 of constant voltage circuit 6 is the same. The specified value of thermistor TH2 can be found as the resistance value of thermistor TH2 corresponding to the specified temperature of regulator element 9, as shown in Figure 4.

The thermistor is placed near the terminal of the regulator element 9 of the constant voltage circuit 6, but it may also be attached and fixed near the terminal of the regulator element 9 of the constant voltage circuit 6, or it may be attached and fixed to the main body of the regulator element 9 of the constant voltage circuit 6 with an adhesive or the like.
In this embodiment, the secondary windings are three, the first secondary winding N2, the second secondary winding N3, and the third winding N4, but the secondary windings may be four or more. In this case, a voltage-controlled output circuit is connected to one of the secondary windings, and non-voltage-controlled output circuits are connected to the other secondary windings, and the present invention can be applied to at least one of the non-voltage-controlled output circuits connected to the secondary windings.

1, 50... switching power supply, 2... input voltage switching circuit, 3, 53... output voltage feedback (FB) circuit, 4, 54... control circuit, 5, 55... switching circuit, 6... constant voltage circuit, 7, 57... voltage controlled output circuit, 8, 58... non-voltage controlled output circuit, 9... regulator element, 21, 22... thermistor circuit.

Claims (2)

a transformer having a primary winding and a plurality of secondary windings, a switching circuit connected to the primary winding of the transformer, and a voltage controlled output circuit connected to one of the plurality of secondary windings;
a switching power supply comprising an input voltage switching circuit connected to another secondary winding and a non-voltage controlled output circuit composed of a constant voltage circuit, wherein a switching voltage obtained by turning on/off an input DC voltage by the switching circuit is applied to a primary winding of the transformer, and a voltage induced in the plurality of secondary windings is supplied to a load from the voltage controlled output circuit as a controlled output voltage, while being supplied to the load as a non-controlled output voltage via the constant voltage circuit connected to the input voltage switching circuit;
a secondary winding of the transformer connected to the input voltage switching circuit has a plurality of winding output terminals each having a different generated voltage;
the voltage controlled output circuit converts the voltage induced in the secondary winding of the transformer into a direct current and supplies the stabilized voltage by an output voltage feedback circuit and a control circuit to a load as the controlled output voltage;
the input voltage switching circuit receives the voltages induced in the plurality of winding output terminals, detects the temperature of the constant voltage circuit by an NTC thermistor, switches to one of the voltages induced in the plurality of winding output terminals, converts the switched voltage into a direct current, and supplies the direct current to the constant voltage circuit;
the constant voltage circuit is configured by a regulator element, the NTC thermistor is disposed in the vicinity of the regulator element to detect a temperature of the regulator element,
the input voltage switching circuit switches to an output terminal having a higher induced voltage among the plurality of winding output terminals when a resistance value of the NTC thermistor is equal to or higher than a predetermined value, and switches to the output terminal having a lower induced voltage to suppress a temperature rise of the constant voltage circuit when the resistance value of the NTC thermistor is less than a predetermined value, the output terminal having a higher induced voltage among the plurality of winding output terminals is connected to one terminal of a current path of a voltage switch element that turns on/off the induced voltage, the other terminal of the current path of the voltage switch element is connected to a first diode, the output terminal having a lower induced voltage among the plurality of winding output terminals is connected to a second diode, the second diode and the first diode are OR-connected to an input terminal of the constant voltage circuit, and the circuit controls a control terminal of the voltage switch element in accordance with the resistance value of the NTC thermistor,
a switching power supply which turns on the voltage switch element when the resistance value of the NTC thermistor is equal to or greater than a predetermined value, and turns off the voltage switch element when the resistance value of the NTC thermistor is less than the predetermined value . a transformer having a primary winding and a plurality of secondary windings, a switching circuit connected to the primary winding of the transformer, and a voltage controlled output circuit connected to one of the plurality of secondary windings;
a switching power supply comprising an input voltage switching circuit connected to another secondary winding and a non-voltage controlled output circuit composed of a constant voltage circuit, wherein a switching voltage obtained by turning on/off an input DC voltage by the switching circuit is applied to a primary winding of the transformer, and a voltage induced in the plurality of secondary windings is supplied to a load from the voltage controlled output circuit as a controlled output voltage, while being supplied to the load as a non-controlled output voltage via the constant voltage circuit connected to the input voltage switching circuit;
a secondary winding of the transformer connected to the input voltage switching circuit has a plurality of winding output terminals each having a different generated voltage;
the voltage controlled output circuit converts the voltage induced in the secondary winding of the transformer into a direct current and supplies the stabilized voltage by an output voltage feedback circuit and a control circuit to a load as the controlled output voltage;
the input voltage switching circuit receives the voltages induced in the plurality of winding output terminals, detects the temperature of the constant voltage circuit using a PTC thermistor, switches to one of the voltages induced in the plurality of winding output terminals, converts the switched voltage into a direct current, and supplies the direct current to the constant voltage circuit;
the constant voltage circuit is configured by a regulator element, the PTC thermistor is disposed in the vicinity of the regulator element to detect a temperature of the regulator element,
the input voltage switching circuit switches to the output terminal with a higher induced voltage among the plurality of winding output terminals when the resistance value of the PTC thermistor is less than a predetermined value, and switches to the output terminal with a lower induced voltage to suppress a temperature rise in the constant voltage circuit when the resistance value of the PTC thermistor is equal to or greater than a predetermined value, the output terminal with a higher induced voltage among the plurality of winding output terminals is connected to one terminal of a current path of a voltage switch element that turns on/off the induced voltage, the other terminal of the current path of the voltage switch element is connected to a first diode, the output terminal with a lower induced voltage among the plurality of winding output terminals is connected to a second diode, the second diode and the first diode are OR-connected to an input terminal of the constant voltage circuit, and the circuit controls a control terminal of the voltage switch element in accordance with the resistance value of the PTC thermistor,
a switching power supply which turns on the voltage switch element when the resistance value of the PTC thermistor is less than a predetermined value, and turns off the voltage switch element when the resistance value of the PTC thermistor is equal to or greater than the predetermined value .

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