Switching power supply

Abstract

To provide a switching power supply that switches output voltage, and that executes an overvoltage protection operation while preventing the influence of extraneous noise without providing an overvoltage detection circuit on a secondary side of a transformer.SOLUTION: A switching power supply comprises: a transformer T1 that has primary winding, secondary winding, and auxiliary winding; a switching circuit that is connected with the primary winding of the transformer; a control circuit that controls the switching circuit; an output voltage switching circuit that switches a voltage induced on the secondary winding and outputs it as output voltage; and a feedback circuit, and the switching power supply drives the control circuit with the voltage induced on the auxiliary winding. The switching power supply comprises a VCC switching circuit that switches a voltage to be applied to a power supply voltage terminal of the control circuit based on the voltage induced on the auxiliary winding. When the output voltage rises and an abnormality occurs, the VCC switching circuit switches the voltage to be applied to the power supply voltage terminal of the control circuit to a voltage equal to or more than a threshold for executing the overvoltage protection operation in the control circuit.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a protection circuit for a switching power supply with a switchable output voltage, and more particularly to a switching power supply that detects an overvoltage of the output voltage and stops the output.

Background Art

[0002] Conventionally, as one type of switching power supply, it includes a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) as a switching element for intermittently flowing a current through a primary coil of a transformer and a control circuit for on / off controlling the switching element. By flowing a current through the primary coil, a current induced in the secondary coil is rectified and smoothed by a diode and a capacitor and then output.

[0003] Some of this type of switching power supply is provided with a protection function that notifies the primary control circuit of an abnormality on the secondary side and stops the switching operation when an excessive output current flows due to a short circuit of the load or the secondary side circuit is disconnected, resulting in an overvoltage. For example, Patent Document 1 describes a switching power supply device configured to transmit an abnormality in a secondary circuit of a transformer to a primary control circuit via a photocoupler.

[0004] The switching power supply device described in Patent Document 1, as shown in Figure 6, is a power supply that applies a DC power supply voltage Vin as an input voltage, switches on / off using a switching element, rectifies and smooths on the secondary side, and outputs an output voltage Vout. A constant voltage control circuit controls the output voltage on the secondary side to be constant, transmits a feedback signal corresponding to the output voltage on the secondary side to the control circuit on the primary side via a photocoupler, and has an abnormality detection circuit on the secondary side that turns off the feedback signal when an abnormality occurs in the secondary side circuit. Furthermore, it also has a function that stops the switching operation by increasing the auxiliary winding voltage of the transformer when an overvoltage occurs on the secondary side. In other words, the invention described in Patent Document 1 is an invention that stops the operation of a switching power supply control IC by using a feedback signal and an auxiliary winding voltage in combination. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-58166 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The invention described in Patent Document 1 above has the following problem. Specifically, this switching power supply rectifies and smooths the auxiliary winding voltage of the transformer and uses it as the power supply for the control circuit of the switching circuit, while simultaneously dividing the same auxiliary winding voltage with a resistor and inputting the resulting voltage drop to the external terminal (DMG terminal) of the control circuit as a voltage for detecting abnormalities in the switching circuit. In this way, when the same auxiliary winding voltage is used to generate the power supply voltage of the control circuit and when the voltage is dropped to detect abnormalities, there is a risk that the power supply voltage of the control circuit may exceed the rated voltage depending on the detected abnormal voltage.

[0007] In particular, when designing the number of transformer turns for a switching power supply that has the function of switching between a high voltage (e.g., 24V) and a low voltage (e.g., 6V), the number of transformer turns is designed based on the lower output voltage in order to supply the power supply voltage (VCC) of the switching circuit's control circuit (switching power supply control IC) from the transformer's auxiliary winding. However, in such cases, when the output voltage is high, the power supply voltage VCC of the switching power supply control IC often exceeds its maximum rating, requiring a step-down regulator to reduce the auxiliary winding voltage of the transformer before supplying it to the switching power supply control IC. On the other hand, when a step-down regulator is used, the overvoltage protection of the power supply voltage VCC terminal provided by the switching power supply control IC does not function. As an alternative to the overvoltage protection function of the VCC terminal, it is necessary to implement an overvoltage detection circuit consisting of a Zener diode and a photocoupler on the secondary side of the transformer, which has resulted in increased costs and limitations on the implementation location. [Means for solving the problem]

[0008] The present invention solves the above problems and is a switching power supply configured as follows. In other words, a switching power supply comprising a transformer having a primary winding, a secondary winding, and an auxiliary winding; a switching circuit connected to the primary winding of the transformer; a control circuit for controlling the switching circuit; a DC conversion circuit for converting the voltage induced in the secondary winding to DC; an output voltage switching circuit for switching the voltage converted to DC by the DC conversion circuit using an output resistor and outputting it as an output voltage; and a feedback circuit for feeding back a signal corresponding to the output voltage to the control circuit, wherein the voltage induced in the auxiliary winding is applied to the power supply voltage terminal of the control circuit to drive the control circuit, and the control circuit controls the switching circuit based on a signal corresponding to the output voltage from the feedback circuit, further comprising a VCC switching circuit for switching the voltage applied to the power supply voltage terminal of the control circuit based on the voltage induced in the auxiliary winding, wherein when the output voltage rises and an abnormality occurs, the VCC switching circuit switches the voltage applied to the power supply voltage terminal of the control circuit to a voltage above a threshold that enables the control circuit to perform overvoltage protection operation.

[0009] With this configuration, overvoltage protection is performed by switching the voltage applied to the power supply voltage terminal (VCC terminal) of the control circuit to a voltage above a threshold voltage using the VCC switching circuit. Therefore, it is possible to detect overvoltage on the secondary side and stop the switching operation to protect the circuit without providing an overvoltage detection circuit on the secondary side of the transformer. Furthermore, because it utilizes the overvoltage protection function of the power supply voltage terminal of the control circuit, it is possible to suppress the effects of external noise compared to conventional configurations that use an external terminal (DMG terminal) of the control circuit to detect abnormalities and stop the switching operation.

[0010] Furthermore, the present invention is characterized in that the auxiliary winding has a first winding with a high induced voltage and a second winding with a low induced voltage, and the VCC switching circuit has a regulator circuit, and the voltage induced in the first winding is stepped down by the regulator circuit and supplied to the control circuit, while the voltage induced in the second winding is used to switch the output voltage of the regulator circuit, thereby switching the voltage applied to the power supply voltage terminal of the control circuit. This configuration allows switching operations to be stopped in the event of an overvoltage without using external terminals of the control circuit.

[0011] Furthermore, the regulator circuit comprises a step-down regulator element and a plurality of Zener diodes connected in series with the control terminal of the step-down regulator element, and is characterized by switching the output voltage of the regulator circuit by short-circuiting at least one of the plurality of Zener diodes in accordance with the voltage induced in the second winding. This configuration allows the output voltage of the regulator circuit to be switched using a simple setup based on the voltage value of the induced voltage of the auxiliary winding.

[0012] Furthermore, it is preferable that the VCC switching circuit has a switch connected in parallel to at least one of the multiple Zener diodes, and that the switch is turned on when the voltage induced in the second winding is less than a predetermined value, and turned off when the voltage induced in the second winding is equal to or greater than the predetermined value. [Effects of the Invention]

[0013] According to the present invention, it is possible to detect overvoltage on the secondary side of the transformer and stop the switching operation to protect the circuit without providing an overvoltage detection circuit on the secondary side of the transformer. Furthermore, since the output is stopped using the power supply voltage terminal of the control circuit, the influence of external noise can be suppressed compared to a configuration that stops the output using an external terminal for abnormality detection separate from the power supply voltage terminal. [Brief explanation of the drawing]

[0014] [Figure 1] This is a configuration diagram of a switching power supply according to an embodiment of the present invention. [Figure 2] This is a circuit diagram of an output voltage switching circuit. [Figure 3] This is a circuit diagram of the VCC switching circuit. [Figure 4] This is a diagram illustrating the operation of the VCC switching circuit. [Figure 5]It is a diagram for explaining the temperature coefficient of the Zener voltage. [Figure 6] It is a configuration diagram of a switching power supply having an overvoltage protection function of a conventional circuit.

Embodiments for Carrying Out the Invention

[0015] The switching power supply according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a switching power supply according to an embodiment of the present invention. This switching power supply 1 includes a switching circuit 2 that inputs a DC power supply voltage Vin and turns it on / off for voltage conversion, a transformer T1 having a primary winding NP, a secondary winding NS, and an auxiliary winding Nsub to which the converted switching voltage is supplied, a rectification / smoothing circuit (DC conversion circuit) 7 composed of a diode D11 and a smoothing capacitor C11 that converts the switching voltage induced in the secondary winding NS into a DC voltage, an output voltage switching circuit 3 that switches and outputs the output voltage Vout rectified by the DC conversion circuit 7, a control circuit 4 that controls the switching circuit 2, a VCC (power supply voltage for control circuit) switching circuit 5 connected to the auxiliary winding Nsub that supplies a power supply voltage to the control circuit 4, and a feedback (FB) circuit 6 composed of a photocoupler PC1 and connected to the output voltage switching circuit 3 so that the output voltage Vout becomes a predetermined voltage. A feedback signal is connected from the FB circuit 6 to the control circuit 4.

[0016] The control circuit 4 includes a switching power supply control IC (not shown) having an OUT terminal that outputs a signal for driving the switching circuit 2 and a FB terminal that inputs a signal from the FB circuit 6 on the secondary side. It also has a power supply voltage VCC terminal supplied from the VCC switching circuit 5. Further, the switching power supply control IC of the control circuit 4 has an overvoltage protection operation function that stops the control of the switching circuit when the power supply voltage VCC becomes an overvoltage. Note that the control circuit 4 is a well-known circuit that outputs a drive signal from the OUT terminal to the switching circuit 2 so that the output voltage Vout becomes a predetermined voltage value and controls the on / off cycle of the switching circuit 2. Also, the switching circuit 2 is a well-known circuit composed of a MOSFET (switching element) that turns on and off a DC voltage and a resistor.

[0017] The auxiliary winding Nsub has two winding output terminals (Nsub1 and Nsub2) with different induced voltages, and the two winding output terminals are connected to the VCC switching circuit 5. The output of the VCC switching circuit 5 is supplied to the control circuit 4 as the power supply voltage VCC.

[0018] Figure 2 is a circuit diagram of the output voltage switching circuit. One end of the series resistance circuit of the shunt regulator IC11 and the resistors R11 and R12 connected in series to its cathode is connected, and the other end of the series resistance circuit is connected to the output voltage + side. The anode of the shunt regulator IC11 is connected to the output voltage - side. The reference terminal of the shunt regulator IC11 is connected to the connection point of the resistors R14 and R15. The other end of the resistor R14 is connected to the output voltage + side, and the other end of the resistor R15 is connected to the output voltage - side. A resistor R13 and a capacitor C12 connected in series are connected to the connection point of the resistors R14 and R15 and the cathode of the shunt regulator IC11.

[0019] One end of a resistor R16 is connected to the connection point of the resistors R14 and R15. The other end of the resistor R16 is connected to the collector of a transistor (NPN type) Q11, and the emitter of the transistor Q11 is connected to the output voltage - side. When the transistor Q11 is turned on, the resistors R15 and R16 are connected in parallel. An external voltage switching signal terminal is connected to the base of transistor Q11. When this voltage switching signal is input, the resistance value of resistor R15 switches to the combined resistance value of resistors R15 and R16 connected in parallel. Since the combined resistance value of resistors R15 and R16 is smaller than that of resistor R15, the voltage at the reference (Ref) terminal of shunt regulator IC11 drops momentarily. When the reference terminal voltage drops, the cathode current of shunt regulator IC11 increases, the feedback circuit 6 works, and the output voltage rises, and eventually the voltage at the reference terminal stabilizes at approximately the same voltage as before the switching. In this way, when a voltage switching signal is input, the output voltage switches to a higher voltage. Note that resistors R14, R15, and R16 correspond to the "output resistors" of this invention.

[0020] Furthermore, the diode of the photocoupler PC1 of the FB circuit 6 shown in Figure 1 is connected in parallel to resistor R12, and the current value flowing through the shunt regulator IC11 is transmitted to the primary control circuit 4 as a feedback (FB) signal via the photocoupler PC1. This FB signal is used by the control circuit 4 and the switching circuit 2 to control the output voltage so that it remains constant.

[0021] Figure 3 is a circuit diagram of the VCC switching circuit (voltage switching circuit for control circuits). The VCC switching circuit 5 consists of transistors Q2 and Q3, diodes D1 to D6, capacitors C4 and C5, and resistors R4 to R8. Transistor Q3 is an integrated circuit composed of two digital transistors with built-in resistors, and diodes D2 to D4 are Zener diodes. Transistors Q2 and Q3 are NPN type transistors. The step-down regulator element of this invention is transistor Q2, which corresponds to the "regulator circuit" in this invention.

[0022] The winding output terminal Nsub1 at the end of the auxiliary winding Nsub of transformer T1 (hereinafter referred to as the "first winding" in this invention) is connected to the anode of diode D5, and the cathode of diode D5 is connected to one end of capacitor C5, one end of resistor R5, the collector of transistor Q2, and the cathode of diode D1. The other end of capacitor C5 is connected to GND, and the other end of resistor R5 is connected to the base of transistor Q2 (the "control terminal" of the step-down regulator element in this invention corresponds to the "base" of transistor Q2). The emitter of transistor Q2 is connected to the anode of diode D1 and one end of resistor R4, and is output as the power supply voltage VCC. The other end of resistor R4 is connected to the base of transistor Q2.

[0023] The base of transistor Q2 is connected to the cathode of Zener diode D2, and Zener diodes D2 and D3 are connected in series. The anode of Zener diode D3 is connected to GND. Transistor Q2 functions as a step-down regulator, outputting a voltage obtained by adding the base-emitter voltage of transistor Q2 to the sum of the Zener voltages of Zener diodes D2 and D3. This voltage is then output as the power supply voltage VCC of the control circuit 4.

[0024] The winding output terminal Nsub2 (hereinafter referred to as the "second winding" in this invention) from the middle of the auxiliary winding Nsub is connected to the anode of diode D6, the cathode of diode D6 is connected to the cathode of Zener diode D4, one end of resistor R7 and capacitor C4, and the other ends of resistor R7 and capacitor C4 are connected to GND. The anode of Zener diode D4 is connected to one end of resistor R6, and the other end of resistor R6 is connected to the base of one of the digital transistors Q3 (the right-hand side of Q3 in Figure 3) via an internal resistor. The collector of one of the digital transistors Q3 is connected to the base of the other transistor (the left-hand side of Q3 in Figure 3) via an internal resistor, and is connected to one end of resistor R8. The other end of resistor R8 is connected to the emitter of transistor Q2, i.e., the power supply voltage VCC.

[0025] The bases of both the first and second transistors of digital transistor Q3 are connected to GND via resistors built into the digital transistor, and their emitters are also connected to GND. Furthermore, the collector of the other transistor of digital transistor Q3 is connected to the connection point of Zener diodes D2 and D3. When the other transistor of digital transistor Q3 is turned on, the Zener diode D3 becomes short-circuited.

[0026] Here, the other transistor of digital transistor Q3 (to the left of Q3 in Figure 3) corresponds to a "switch" connected in parallel to Zener diode D3 in this invention. When this switch is turned on, Zener diode D3 is short-circuited, and when the switch is turned off, it functions as Zener diode D3. Also, the starting point of the auxiliary winding Nsub is connected to GND. Furthermore, the induced voltage of the first winding (Nsub1) is higher than the induced voltage of the second winding (Nsub2).

[0027] In the VCC switching circuit 5 configured as described above, when the output of the auxiliary winding is in a normal state (no abnormality occurs), of the two Zener diodes D2 and D3 connected to the base of transistor Q2, Zener diode D3 is short-circuited, and the power supply voltage VCC is output at a low voltage.

[0028] On the other hand, when the voltage of the primary winding becomes overvoltage and the output voltage of the secondary winding becomes an abnormal voltage value, the induced voltage of the second winding Nsub2 rises and exceeds a predetermined value (in this invention, the induced voltage of the second winding Nsub2 at which the power supply voltage VCC switches). At this point, the other transistor of digital transistor Q3 (to the left of Q3) turns off, and the Zener diode D3 connected to the base of transistor Q2 begins to function as a Zener diode. As a result, the base voltage of transistor Q2 becomes the sum of the Zener voltages of the two Zener diodes, and the output voltage switches to the higher output voltage VCC.

[0029] As a result, the power supply voltage VCC of the control circuit 4 switches to a higher voltage, and when the power supply voltage VCC of the control circuit 4 exceeds the abnormal detection voltage value of the control circuit 4, the output of the switching circuit is stopped. In other words, when an abnormality occurs on the secondary side of the switching power supply 1, resulting in an overvoltage, the output of the switching power supply 1 can be stopped. Furthermore, when the induced voltage of the second winding Nsub2 falls below a predetermined value, the other transistor of digital transistor Q3 turns on, and the power supply voltage VCC returns to its normal voltage value.

[0030] Below, we will consider the number of turns of the auxiliary winding Nsub connected to the VCC switching circuit 5. The present invention relates to a switching power supply 1 with a switchable output voltage. The power supply voltage VCC for the control circuit 4 that operates the switching circuit 2 is obtained by rectifying and smoothing the induced voltage of the auxiliary winding Nsub and stepping it down to the voltage required for the operation of the control circuit 4 using a transistor Q2 (step-down regulator element). Here, the induced voltage of the auxiliary winding Nsub changes when the output voltage Vout switches. Even if it changes, the power supply voltage VCC of the control circuit 4 does not necessarily need to be kept constant, but it must be below the abnormal detection voltage value of the control circuit 4 and not below the operating limit voltage. Also, if an abnormality occurs on the secondary side and an overvoltage occurs, that voltage signal must be output to the control circuit 4. Therefore, setting the number of turns for the auxiliary winding Nsub is crucial.

[0031] The induced voltage of the auxiliary winding Nsub is calculated as follows: Nsub induced voltage = (Number of Nsub turns × Output voltage Vout / Number of secondary winding Ns turns). The switching power supply 1 in this embodiment is a power supply that switches between an output voltage Vout of 24V and 6V. In order for the voltage of the first winding Nsub1 to be supplied as the power supply voltage VCC of the control circuit, the recommended power supply voltage is 16V (up to a maximum of 24V), and if the power supply voltage VCC becomes 24.5V or higher (up to a maximum of 26.5V, which corresponds to the "threshold" in this invention), the control circuit 4 determines that it is an abnormal voltage and stops the switching operation. Note that the overvoltage detection function is only available when the output voltage is 24V.

[0032] Under the above conditions, we will consider the number of turns for the first winding Nsub1 and the second winding Nsub2. The number of turns is defined as the number of turns from the start of winding to the output terminal of each winding. The secondary winding Ns will have 6 turns. Note that the second winding Nsub2 is a terminal located in the middle of the auxiliary winding Nsub, and the number of turns for the second winding Nsub2 is included in the number of turns for the first winding Nsub1. When the output voltage Vout is 6V, in order for the induced voltage of the first winding Nsub1 to be at least 17V (considering the base-emitter voltage of transistor Q2 in addition to the recommended power supply voltage of 16V), if we use the formula 17V = (number of turns in Nsub1 × 6V / 6 turns), then the number of turns in Nsub1 must be 17 turns. If the second winding Nsub2 has 1 turn, the induced voltage of the second winding Nsub2 will be 1V.

[0033] Next, we consider the case where the output voltage is switched to 24V, assuming that Nsub1 has 17 turns and Nsub2 has 1 turn. Since the output voltage is 24V, the induced voltage of Nsub1 is (17 turns × 24V / 6 turns), which is 68V. Also, the induced voltage of Nsub2 is (1 turn × 24V / 6 turns), which is 4V. Even in this state, the power supply voltage VCC of the control circuit 4 is maintained at 16V by the step-down regulator. Although the input voltage of transistor Q2 increases, the output is maintained at the recommended power supply voltage of 16V, so heat dissipation measures should be taken to prevent the temperature of transistor Q2 from rising due to losses.

[0034] Here, if we assume that the voltage when the output voltage becomes abnormal is 30V, then the induced voltage of Nsub1 = (17 turns × 30V / 6 turns) becomes 85V, and the induced voltage of Nsub2 = (1 turn × 30V / 6 turns) becomes 5V. The circuit conditions are set so that the power supply voltage VCC switches when the induced voltage of the second winding Nsub2 reaches 5V. That is, the Zener voltage of Zener diode D4 is set to 5V.

[0035] With this setting, when the output voltage is 6V, the induced voltage of the second winding Nsub2 is 1V, and when the output voltage is 24V, the induced voltage of the second winding Nsub2 is 4V. At these voltages, the Zener diode D4 does not conduct, and the base voltage of transistor Q2 does not change, so the power supply voltage VCC does not switch.

[0036] When the output voltage Vout becomes abnormally high, exceeding 30V, the induced voltage of the second winding Nsub2 becomes 5V, causing Zener diode D4 to conduct, and Zener diode D3 to be released from the short-circuit state caused by the switch and return to normal function. As a result, the power supply voltage VCC rises from the normal 16V to 24.5V or higher, and the operating signal from control circuit 4 to switching circuit 2 stops. Note that the Zener voltage of a Zener diode is not fixed, but for example, the Zener voltage of Zener diode D2 can be set to 16V and the Zener voltage of Zener diode D3 can be set to 8V.

[0037] The operation of the VCC switching circuit 5 will be explained below with reference to Figure 4. Figure 4 is an explanatory diagram of the operation of the VCC switching circuit 5. Five waveforms are shown, from top to bottom: (A) output voltage Vout, (B) capacitor C4 voltage, (C) voltage Vce of the right-hand element of digital transistor Q3, (D) voltage Vce of the left-hand element of the same transistor, and (E) power supply voltage VCC. The horizontal axis represents time, and the period up to point (a) is at a normal level with no abnormalities occurring. At point (a), for example, an abnormality occurs in the shunt regulator IC11, and (A) the output voltage Vout begins to rise. At the same time, the induced voltage of the second winding Nsub2 rises, and (B) the capacitor C4 voltage also rises.

[0038] At point (b), the voltage of capacitor C4 reaches the voltage at which Zener diode D4 conducts. When this happens, the right-hand element of digital transistor Q3 turns on, and the left-hand element of digital transistor Q3 turns off. In other words, the switch connected in parallel to Zener diode D3 changes from on to off, and the base voltage of transistor Q2 increases from the Zener voltage of Zener diode D2 to the sum of the Zener voltages of Zener diode D2 and Zener diode D3.

[0039] The emitter voltage of transistor Q2, (E) the power supply voltage VCC, switches to a higher voltage, and the power supply voltage VCC of control circuit 4 becomes an abnormal detection voltage value. Because control circuit 4 has a delay time, the operation of switching circuit 2 stops at (c), and thereafter, (A) the output voltage Vout and (B) the capacitor C4 voltage decrease. At (d), the voltage of (B) the capacitor C4 decreases and falls below the Zener voltage of Zener diode D4 again, so each waveform from (C) to (E) returns to a normal level. Since the operation of switching circuit 2 remains stopped thereafter, at (e) the power supply voltage VCC can no longer maintain the recommended power supply voltage, and the voltage decreases further. At (f) the power supply voltage VCC drops to the lower limit of the recommended power supply voltage, and the operation becomes undefined. After the problem is resolved, the power will be turned back on and the system will return to normal levels.

[0040] Here, the induced voltage of the Nsub winding was explained using the formula related to the output voltage Vout mentioned above, but in reality, it also changes depending on the load on the secondary side. In this case, the induced voltages in the first winding Nsub1 and the second winding Nsub2 increase because the degree of coupling of the transformer T1 changes due to the increase in the output load current, causing the surge voltage between the windings to rise and raising the voltages across capacitors C5 and C4. As a result, the overvoltage detection level also becomes inaccurate due to the load current, but in this invention, the rise in the voltage across capacitor C4 is suppressed by implementing a resistor R7 in parallel with capacitor C4, thereby correcting the overvoltage detection level. Furthermore, although the Nsub1 voltage also rises with increasing load, the power supply voltage VCC is kept constant by the step-down regulator element of transistor Q2.

[0041] Figure 5 illustrates the temperature coefficient of Zener voltage. The horizontal axis represents Zener voltage (V), and the vertical axis represents the voltage-temperature coefficient. The unit of the voltage-temperature coefficient in graph A is (% / °C), and the unit of the voltage-temperature coefficient in graph B is (mV / °C). By setting the Zener voltage of the Zener diode D4 in the VCC switching circuit 5 to 5V, the Zener voltage of the Zener diode is less affected by temperature, as shown in Figure 5.

[0042] For example, if the ambient temperature around a Zener diode rises by 40°C, comparing the cases of a Zener voltage of 5V and 30V, optimizing the winding results in almost no change when using a Zener voltage of 5V, whereas a Zener voltage of 30V has an effect of about 1V (25mV / °C) × 40°C. The accuracy of voltage detection can be improved by using a circuit configuration that utilizes a 5V Zener voltage. [Explanation of Symbols]

[0043] 1. 10... Switching power supply, 2... Switching circuit, 3... Output voltage switching circuit, 4... Control circuit, 5... VCC switching circuit (voltage switching circuit for control circuit), 6... Feedback (FB) circuit, 7... DC conversion circuit (rectification and smoothing circuit).

Claims

1. A transformer having a primary winding, a secondary winding, and an auxiliary winding, A switching circuit connected to the primary winding of the transformer, A control circuit for controlling the switching circuit, A DC conversion circuit that converts the voltage induced in the secondary winding into DC, The system includes a feedback circuit that feeds back a signal corresponding to the output voltage to the control circuit, In a switching power supply in which a voltage induced in the auxiliary winding is applied to the power supply voltage terminal of the control circuit to drive the control circuit, and the control circuit controls the switching circuit based on a signal corresponding to the output voltage from the feedback circuit, The system further comprises an output voltage switching circuit that changes the signal from the feedback circuit and switches the output voltage by changing the resistance value of an output resistor to which the DC-converted voltage is applied based on the input of an external voltage switching signal, and a VCC switching circuit that switches the voltage applied to the power supply voltage terminal of the control circuit based on the voltage induced in the auxiliary winding. The auxiliary winding has a first winding with a high induced voltage and a second winding with a low induced voltage. The VCC switching circuit includes a regulator circuit, which steps down the voltage induced in the first winding and supplies it to the control circuit, while using the voltage induced in the second winding to switch the output voltage of the regulator circuit, thereby switching the voltage applied to the power supply voltage terminal of the control circuit. A switching power supply characterized in that, when the output voltage rises and an abnormality occurs, the VCC switching circuit switches the voltage applied to the power supply voltage terminal of the control circuit to a voltage equal to or greater than the threshold for the overvoltage protection operation of the control circuit.

2. In the switching power supply according to claim 1, The regulator circuit comprises a step-down regulator element and a plurality of Zener diodes connected in series with the control terminal of the step-down regulator element, and the switching power supply is characterized in that the output voltage of the regulator circuit is switched by short-circuiting at least one of the plurality of Zener diodes in accordance with the voltage induced in the second winding.

3. In the switching power supply according to claim 2, The VCC switching circuit is a switching power supply having a switch connected in parallel to at least one of the plurality of Zener diodes, wherein the switch is turned on when the voltage induced in the second winding is less than a predetermined value, and the switch is turned off when the voltage induced in the second winding is greater than or equal to a predetermined value.

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