Switching power supply circuit and switching power supply system

Abstract

This invention provides a switching power supply circuit that can prevent excessive increases in output voltage regardless of whether or not there is a malfunction in the feedback circuit. [Solution] The switching power supply circuit 1 includes a switching circuit 10 to which an input node N1 is connected to an input power supply terminal 2a and an output node N2 is connected to an output power supply terminal 2a via an output switch 40; a feedback circuit 20 that supplies feedback information F to the switching circuit 10 based on the voltage appearing at a control node N4 provided in parallel with the output node N2; and a startup circuit 30 that turns on the output switch 40 after a predetermined time has elapsed since the voltage appearing at the control node N4 exceeded a predetermined value. Based on the feedback information F, the switching circuit 10 adjusts the voltages appearing at the output node N2 and the control node N4 to predetermined levels, respectively. The switching circuit 10 stops switching operation when it determines that the voltage appearing at the output node N2 is an overvoltage.

Description

[Technical Field]

[0001] The present invention relates to a switching power supply circuit and a switching power supply system equipped therewith, and more particularly to a switching power supply circuit suitable for configuring a switching power supply system by parallel connection. [Background technology]

[0002] A switching power supply system is known that consists of multiple switching power supply circuits connected in parallel. For example, the switching power supply system disclosed in Patent Document 1 synchronizes the restart timing of each switching power supply circuit by short-circuiting the on / off control terminals of the PWM control ICs included in each switching power supply circuit.

[0003] Patent Document 2 also describes a switching power supply circuit comprising a switching circuit, a feedback circuit that supplies feedback information to the switching circuit based on the voltage appearing at the output node of the switching circuit, and a startup circuit that turns on the output switch after a predetermined time has elapsed since the voltage appearing at the output node exceeded a predetermined value. The switching circuit adjusts the voltage appearing at the output node to a predetermined level based on the feedback information. The feedback circuit also includes an adjustment mechanism that can adjust the relationship between the voltage appearing at the output node and the feedback information. As a result, even when multiple switching power supply circuits are connected in parallel, no particular switching power supply circuit will enter an overcurrent or overload state. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-169471 [Patent Document 2] Japanese Patent Publication No. 2024-27839 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, in the conventional switching power supply circuit described in Patent Document 2, if an abnormality occurs in the feedback circuit, the switching circuit may malfunction and the output voltage may continue to rise, which is a problem as it is not possible to prevent an excessive rise in the output voltage. In addition, since the feedback circuit supplies feedback information to the switching circuit based on the voltage appearing at the output node, it is susceptible to the influence of the load connected to the output power supply terminal, and in some cases it may not be able to correctly read the voltage appearing at the output node and provide feedback.

[0006] Therefore, the present invention aims to provide a switching power supply circuit and a switching power supply system using the same that can prevent excessive increases in output voltage regardless of whether or not there is a malfunction in the feedback circuit. [Means for solving the problem]

[0007] The switching power supply circuit according to the present invention comprises a switching circuit in which an input node is connected to an input power supply terminal and an output node is connected to an output power supply terminal via an output switch; a feedback circuit that feeds back feedback information based on the voltage appearing at a control node provided in parallel with the output node to the switching circuit; and a startup circuit that turns on the output switch after a predetermined time has elapsed since the voltage appearing at the control node exceeded a predetermined value. The switching circuit adjusts the voltages appearing at the output node and the control node to predetermined levels based on the feedback information, and the switching circuit stops switching operation when it determines that the voltage appearing at the output node is an overvoltage.

[0008] According to the present invention, it is possible to prevent excessive increases in output voltage due to abnormalities in the feedback circuit, etc. Therefore, even when a switching power supply system is configured by connecting multiple switching power supply circuits in parallel and operating them in parallel, it is possible to prevent excessive increases in the output voltage of a specific switching power supply circuit and stabilize the system.

[0009] In the present invention, the switching circuit includes a transformer including a primary winding connected to an input node, a secondary winding connected to an output node, and an auxiliary winding that generates an output voltage different from that of the secondary winding, a switching element connected to the primary winding, and a switching control circuit that controls the switching element. The power supply node of the switching control circuit is connected to the auxiliary winding. The switching control circuit may stop the switching operation of the switching element when it determines that the voltage supplied from the auxiliary winding to the power supply node is an overvoltage. Thereby, it is possible to monitor an excessive rise in the output voltage by using the switching control circuit.

[0010] In the present invention, the number of turns of the auxiliary winding may be less than the number of turns of the secondary winding, and the voltage level induced in the auxiliary winding when a current flows through the primary winding may be smaller than the voltage level induced in the secondary winding when a current flows through the primary winding. According to this, it is possible to supply an appropriate power supply voltage to the power supply note of the switching control circuit.

[0011] In the present invention, the switching control circuit may stop the switching operation of the switching element when it determines that the current flowing through the switching element is an overcurrent. According to this, the overcurrent state can be eliminated.

[0012] In the present invention, the switching control circuit may stop the switching operation of the switching element when it determines that the feedback information indicates an overload state. According to this, the overload state can be eliminated.

[0013] In the present invention, one end of the secondary winding is connected to the output node via the first diode and is also connected to the control node via the second diode. The other end of the secondary winding is grounded to the ground. The output node is grounded to the ground via the first capacitor. The control node is grounded to the ground via the second capacitor. One end of the auxiliary winding may be connected to the power supply node via the third diode, and the other end of the auxiliary winding may be grounded to the ground. The power supply node may be grounded to the ground via the third capacitor. In this case, the maximum rating of the second diode may be smaller than that of the first diode, and the capacitance of the second capacitor may be smaller than that of the first capacitor. According to this configuration, a predetermined voltage can be stably output from the output power supply terminal regardless of the magnitude of the load connected to the output power supply terminal.

[0014] In the present invention, the feedback circuit includes an adjustment mechanism that adjusts the relationship between the voltage appearing at the control node and the feedback information. The adjustment mechanism includes a voltage dividing circuit that divides the voltage appearing at the control node, and the voltage dividing circuit may include a variable resistor. By using an adjustment mechanism capable of adjusting the relationship between the voltage appearing at the control node provided in parallel with the output node and the feedback information, the level of the output voltage output from the output power supply terminal can be adjusted. Thereby, when a switching power supply system in which a plurality of switching power supply circuits are connected in parallel is configured, it is possible to prevent a phenomenon in which a specific switching power supply circuit preferentially enters an overcurrent state or an overload state.

[0015] The switching power supply system according to the present invention includes a plurality of the above-described switching power supply circuits. The output power supply terminals included in the plurality of switching power supply circuits are short-circuited to each other. The startup circuits included in the plurality of switching power supply circuits include startup control terminals that directly or indirectly control the output switch. The startup control terminals included in the plurality of switching power supply circuits are short-circuited to each other. According to this, it becomes possible to restart a plurality of switching power supply circuits simultaneously.

[0016] The switching power supply system according to the present invention may further include a plurality of power sources connected to the input power supply terminals of a plurality of switching power supply circuits. This ensures that even if there is a difference in the levels of the input voltages supplied from the plurality of power sources, no particular switching power supply circuit will preferentially enter an overcurrent or overload state. [Effects of the Invention]

[0017] Thus, according to the present invention, it is possible to provide a switching power supply circuit and a switching power supply system using the same that can prevent excessive increases in output voltage regardless of whether or not there is a malfunction in the feedback circuit. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a block diagram showing the configuration of a switching power supply system according to one embodiment of the present invention. [Figure 2] Figure 2 is a circuit diagram of a switching power supply circuit. [Figure 3] Figure 3 is a flowchart showing the monitoring and control of the power supply voltage by a switching control circuit. [Modes for carrying out the invention]

[0019] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings.

[0020] Figure 1 is a block diagram showing the configuration of switching power supply system 1 according to one embodiment of the present invention.

[0021] As shown in Figure 1, the switching power supply system 1 according to this embodiment has a configuration in which a plurality of switching power supply circuits 2 are connected in parallel. Each switching power supply circuit 2 has an input voltage V in The input power supply terminal 2a is supplied with power, and the output voltage V outIt has an output power terminal 2b that outputs power and a startup control terminal 2c. The output power terminals 2b included in each switching power supply circuit 2 are short-circuited with each other and connected to a common load 4. The input power terminals 2a included in each switching power supply circuit 2 are connected to different power sources 3. The power source 3 may be a power pickup transformer that extracts power using the magnetic field generated around high-voltage transmission lines. The startup control terminals 2c included in each switching power supply circuit 2 are also short-circuited with each other.

[0022] Figure 2 is a circuit diagram of the switching power supply circuit 2.

[0023] As shown in Figure 2, the switching power supply circuit 2 includes a switching circuit 10, a feedback circuit 20, a startup circuit 30, and an output switch 40. The switching circuit 10 includes a dual-output type transformer T with a primary winding 11, a secondary winding 12A, and an auxiliary winding 12B, a switching element 13 connected to the primary winding 11 of the transformer T, and a switching control circuit 14 that controls the switching element 13.

[0024] One end of the primary winding 11 constitutes the input node N1 of the switching circuit 10 and is connected to the input power supply terminal 2a. The other end of the primary winding 11 is connected to the switching element 13.

[0025] One end of the secondary winding 12A is connected to the output node N2 via diode 16A (first diode) and to the control node N4 via diode 16B (second diode). The other end of the secondary winding 12A is grounded via ground node N6.

[0026] One end of the secondary winding 12A is connected to the anode of diode 16A, and the cathode of diode 16A is connected to the output power terminal 2b via the output switch 40. The cathode of diode 16A is also connected to the ground node N6 via capacitor 17A (first capacitor). Diode 16A and capacitor 17A constitute a smoothing circuit.

[0027] One end of the secondary winding 12A is also connected to the anode of diode 16B, and the cathode of diode 16B is connected to the start circuit 30. The cathode of diode 16B is also connected to the ground node N6 via capacitor 17B (second capacitor). Diode 16B and capacitor 17B constitute a smoothing circuit.

[0028] The characteristics of diode 16B may be the same as or different from those of diode 16A. For example, the maximum rating of diode 16B may be smaller than that of diode 16A. The characteristics of capacitor 17B may be the same as or different from those of capacitor 17A. For example, the capacitance of capacitor 17B may be smaller than that of capacitor 17A, and an electrolytic capacitor may be used for capacitor 17A while a multilayer ceramic capacitor is used for capacitor 17B.

[0029] The transformer T further includes an auxiliary winding 12B for generating the power supply voltage for the switching control circuit 14. The auxiliary winding 12B, like the secondary winding 12A, is magnetically coupled to the primary winding 11 and generates an electromotive force corresponding to the current flowing through the primary winding 11. The number of turns of the auxiliary winding 12B may be less than the number of turns of the secondary winding 12A. That is, the voltage level induced in the auxiliary winding 12B when current flows through the primary winding 11 may be lower than the voltage level induced in the secondary winding 12A.

[0030] One end of the auxiliary winding 12B is connected to the power supply node N of the switching control circuit 14 via diode 16C (third diode). 11is connected. The other end of the auxiliary winding 12B is grounded to the ground. One end of the auxiliary winding 12B is connected to the anode of the diode 16C, and the cathode of the diode 16C is connected to the power supply node N 11 is connected. The cathode of the diode 16A is also connected to the ground via a capacitor 17C (third capacitor). The diode 16C and the capacitor 17C constitute a smoothing circuit.

[0031] The switching control circuit 14 consists of, for example, a controller IC, and has a power supply node N 11 , a ground node N 12 , a control node N 13 and a feedback node N 14 , N 15 . The switching control circuit 14 operates by the voltage supplied between the power supply node N 11 and the ground node N 12 , and controls the frequency and duty of the switching signal S output from the control node N 14 based on the feedback information F supplied to the feedback node N 13 to adjust the voltage appearing at the output node N2 to a predetermined level. The switching element 13 to which the switching signal S is supplied consists of, for example, an N-channel type MOS transistor.

[0032] Also, the current flowing through the switching element 13 is supplied to the feedback node N 15 of the switching control circuit 14 via the resistor 15. The switching control circuit 14 is based on the feedback node N 15The current supplied to the switching element 13 is monitored, and if it is determined that the current flowing through the switching element 13 is an overcurrent, the switching signal S is fixed to an inactive level (e.g., ground level) to stop the switching operation of the switching element 13. This prevents damage to the switching element 13 due to overcurrent. Furthermore, if the switching control circuit 14 determines that the feedback information F indicates an overload condition, it also fixes the switching signal S to an inactive level to stop the switching operation of the switching element 13. Once the switching operation stops, the switching control circuit 14 is restarted and resumes the switching operation after a predetermined time.

[0033] The voltage supplied to the power node N11 by the switching control circuit 14 is approximately equal to the voltage generated across the auxiliary winding 12B of the transformer T. As shown in Figure 3, the switching control circuit 14 monitors the power supply voltage supplied to the power node N11 (step S101), and if it determines that the power supply voltage is overvoltage (step S102Y), it stops the switching operation of the switching element 13 by fixing the switching signal S to an inactive level (e.g., ground level) (step S103). This not only prevents damage to the switching control circuit 14 but also prevents an excessive rise in the secondary voltage of the transformer T.

[0034] The level of overvoltage at which the output of the switching signal S should be stopped is determined based on the withstand voltage characteristics of the switching control circuit 14 and the allowable level of the secondary voltage. If the number of turns of the auxiliary winding 12B is less than the number of turns of the secondary winding 12A, the voltage generated from the secondary winding 12A will be greater than the voltage generated from the auxiliary winding 12B, and the problem of excessive rise in output voltage will be significant, making it highly significant to stop the switching operation. If the switching operation is stopped for a certain period of time, the switching control circuit 14 is restarted and the switching operation resumes (steps S104Y, S105).

[0035] The feedback circuit 20 generates feedback information F based on the voltage appearing at the control node N4, which is provided in parallel with the output node N2, and feeds this back to the switching circuit 10. The voltages appearing at the output node N2 and the control node N4 are, when the output switch 40 is ON, the output voltage V out This is almost identical. The feedback circuit 20 includes fixed resistors 21, 24, a variable resistor 22, a shunt regulator 23, a photodiode 25, and a phototransistor 26. The fixed resistor 21 and the variable resistor 22 are connected in series between the control node N4 and the anode of the shunt regulator 23 to form a voltage divider circuit, and the voltage at the connection point is supplied to the reference node of the shunt regulator 23. The fixed resistor 24 and the photodiode 25 are connected in series between the control node N4 and the cathode of the shunt regulator 23. The photodiode 25 and the phototransistor 26 form a photocoupler, which transmits feedback information F from the secondary side to the primary side while ensuring isolation between the primary and secondary sides.

[0036] The level of feedback information F changes not only with the voltage appearing at the control node N4, which is equivalent to the voltage appearing at the output node N2, but also with the voltage division ratio of the voltage division circuit consisting of the fixed resistor 21 and the variable resistor 22. In the design phase, the voltage division ratio is determined so that the voltage appearing at the output node N2 is at a predetermined level (e.g., 24V). However, due to individual differences caused by manufacturing variations, the voltage actually appearing at the output node N2 may deviate from the predetermined level. The variable resistor 22 is an adjustment mechanism to eliminate such deviations. By adjusting the resistance value of the variable resistor 22, the relationship between the voltage appearing at the control node N4 and the feedback information F is fine-tuned after manufacturing, thereby adjusting the voltage appearing at the output node N2 to a predetermined level (e.g., 24V).

[0037] The startup circuit 30 includes fixed resistors 31 and 32, a timer IC 33, a capacitor 34, and an N-channel MOS transistor 35. The fixed resistors 31 and 32 are connected in series between the control node N4 and ground to form a voltage divider circuit, and the voltage at the connection point is set to the input node N of the timer IC 33. 21 It is supplied to the input node N. 21 If the voltage supplied to the output node N exceeds a predetermined value, after a predetermined time has elapsed, 22 A high-level activation signal A is output from the control node N2. Activation signal A is supplied to the gate electrode of transistor 35. Transistor 35 is connected between the gate electrode of output switch 40, which is made up of P-channel MOS transistors, and ground. Therefore, when activation signal A is activated to a high level, the gate electrode of the P-channel MOS transistor constituting the output switch 40 is grounded, and the output switch 40 turns on. Consequently, the output switch 40 turns on after a predetermined time has elapsed since the voltage appearing at output node N2 exceeded a predetermined value. Here, the predetermined time measured by timer IC 33 is measured at adjustment node N2. 23 This can be adjusted by the capacitance of the capacitor 34 connected to it.

[0038] Furthermore, the startup signal A is also supplied to the startup control terminal 2c. As described above, the startup control terminal 2c is commonly connected among multiple switching power supply circuits 2. Therefore, after the switching circuits 10 included in each of the multiple switching power supply circuits 2 are started up, the startup signal A, which was activated first, is supplied to each switching power supply circuit 2 in common via the startup control terminal 2c. As a result, the output switches 40 included in each switching power supply circuit 2 are turned on almost simultaneously. In the example shown in Figure 2, the startup signal A indirectly controls the output switch 40 via the transistor 35, but it may also directly control the output switch 40.

[0039] As shown in Figure 1, a switching power supply system 1 can be constructed by connecting multiple switching power supply circuits 2 with this configuration in parallel. In this case, by adjusting the variable resistor 22, the output voltage levels of each switching power supply circuit 2 can be made nearly identical, so that some of the switching power supply circuits 2 do not become overloaded.

[0040] Furthermore, if an overcurrent or overload condition occurs in a switching power supply circuit 2, the switching operation in that switching power supply circuit 2 will stop, and the output voltage V out The voltage becomes zero. As a result, an overcurrent or overload condition occurs in the remaining switching power supply circuits 2, and ultimately the switching operation stops in all switching power supply circuits 2. Subsequently, the switching control circuit 14 is restarted, causing the voltages of the output node N2 and control node N4 in each switching power supply circuit 2 to rise. Then, the start signal A is activated in the switching power supply circuit 2 in which the voltage of the control node N4 reaches the predetermined value first, and this is supplied to the other switching power supply circuits 2, causing all switching power supply circuits 2 to start outputting power almost simultaneously. For this reason, no overcurrent or overload condition occurs in any of the switching power supply circuits 2 during startup.

[0041] Furthermore, in the example shown in Figure 1, each switching power supply circuit 2 is connected to a different power source 3, so the input voltage V input to each switching power supply circuit 2 in The levels do not necessarily coincide with each other. Even under such conditions, according to the switching power supply system 1 of this embodiment, the output voltage V of each switching power supply circuit 2 out Since the levels are almost identical, some of the switching power supply circuits 2 will not be overloaded.

[0042] Output node N3 is one end of the secondary winding 12A of transformer T, that is, the anode of diode 16A. The voltage level of output node N3 is higher than output node N2 by the forward voltage of diode 16A, and higher than the voltage level of control node N4 by the forward voltage of diode 16B. However, since the voltage levels of output node N2 and control node N4 are almost synchronized with output node N3, control node N4 and output node N2 can be considered almost identical in terms of the operation of the feedback circuit 20 and the startup circuit 30. For this reason, the startup circuit 30 may be connected to control node N4.

[0043] In this embodiment, the input node N5 of the feedback circuit 20 is connected to the control node N4 connected to the start circuit 30, rather than to the output node N2 connected to the output power terminal 2b via the output switch 40. By connecting the input node N5 to the control node N4, the output voltage V is not affected by the load 4 connected to the output power terminal 2b. out It can correctly read the signal and output the specified voltage even under no load.

[0044] Ideally, the feedback information F should be read from output node N2. If the feedback information F is read from control node N4, the loss of diode 16A is not considered, and variations in individual diode 16A can cause fluctuations in the output voltage level of each switching power supply circuit 2, potentially leading to unstable operation during parallel operation. However, the output voltage level of the switching power supply circuit 2 can be adjusted by adjusting the output voltage V that appears at output power terminal 2b. out This is done by adjusting the variable resistor 22 while observing the output, and the adjustment result is also reflected in the output node N2. Therefore, there is no problem in referring to the voltage of the control node N4 as feedback information F.

[0045] Malfunctions of the feedback circuit 20 caused by reading the voltage reversed from another switching power supply circuit 2 during parallel operation can be prevented by inserting a diode between the output switch 40 and the output power supply terminal 2b. However, in this embodiment, it is not necessary to implement such a diode, which is advantageous in terms of component cost, output efficiency, and board size.

[0046] As described above, if the secondary voltage of transformer T rises or falls for any reason, the feedback circuit 20 detects this and notifies the primary side, and the switching control circuit 14 controls the switching element 13, thereby lowering or raising the secondary voltage.

[0047] Here, if the feedback circuit 20 fails to operate correctly due to a failure of the diode 16B or capacitor 17B connected to the control node N4, or a break in the wiring, the switching control circuit 14 will not be able to receive the feedback information F, and the secondary voltage may continue to rise due to incorrect control by the switching control circuit 14. In particular, if the voltage to the feedback circuit 20 is supplied through the control node N4, which is a separate system from the output node N2, the excessively high secondary voltage will appear at the output power terminal 2b through the output node N2, which may damage the circuit board in other switching power supply circuits 2 connected in parallel. Even if the feedback circuit 20 is equipped with an overvoltage protection circuit, the same thing can happen if the overvoltage protection circuit fails.

[0048] However, in this embodiment, the power supply node N of the switching control circuit 14 11 The voltage from the auxiliary winding 12B is supplied to the secondary winding 12A, and the overvoltage protection function of the switching control circuit 14 is used to monitor the secondary voltage, so an excessive rise in the secondary voltage can be detected regardless of whether there is a malfunction in the feedback circuit 20. Normally, the terminal voltage of the auxiliary winding 12B is proportional to the terminal voltage of the secondary winding 12A, and when the terminal voltage of the secondary winding 12A rises, the terminal voltage of the auxiliary winding 12B also rises. In other words, the switching control circuit 14 is connected to the power supply node N 11The system monitors the voltage appearing at output node N2 and stops its operation if it determines that the voltage is an overvoltage, thereby reliably preventing excessive increases in the secondary voltage.

[0049] Although preferred embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the invention, and these modifications are also included within the scope of the present invention.

[0050] For example, in the above embodiment, the case where the input node N5 of the feedback circuit 20 is connected to the control node N4 was given as an example, but the input node N5 may also be connected to the output node N2. [Explanation of Symbols]

[0051] 1. Switching power supply system 2. Switching power supply circuit 2a Input power terminal 2b Output power terminal 2c Startup control terminal 3 Power source 4 load 10 Switching Circuits 11 Primary winding 12A secondary winding 12B Auxiliary winding 13 Switching elements 14 Switching control circuit 15 Resistors 16A diode (first diode) 16B Diode (Second Diode) 16C diode (third diode) 17A Capacitor (First Capacitor) 17B Capacitor (Second Capacitor) 17C Capacitor (Third Capacitor) 20 Feedback Circuit 21 Fixed resistance 22 Variable resistors 23 Shunt Regulator 24 Fixed resistance 25 Photodiode 26 Phototransistors 30 Startup Circuit 31 Fixed resistance 32 fixed resistance 33 Timer IC 34 Capacitors 35 transistors 40 Output Switches A Activation signal F Feedback Information N1 input node N 11 Power node N 12 Grounding node N 13 control node N 14 Feedback node N 15 Feedback node N2 Output Node N 21 input node N 22 Output node N 23 Adjustment node N3 Output Node N4 control node N5 input node N6 Grounding S switching signal T Transformer Vin Input Voltage Vout output voltage

Claims

1. A switching circuit in which the input node is connected to the input power terminal and the output node is connected to the output power terminal via the output switch, A feedback circuit that feeds back feedback information based on the voltage appearing at a control node provided in parallel with the output node to the switching circuit, The system includes a startup circuit that turns on the output switch after a predetermined time has elapsed since the voltage appearing at the control node exceeded a predetermined value, The switching circuit adjusts the voltages appearing at the output node and the control node to predetermined levels based on the feedback information. The switching circuit is a switching power supply circuit that stops switching operation when it is determined that the voltage appearing at the output node is an overvoltage.

2. The aforementioned switching circuit is A transformer including a primary winding connected to the input node, a secondary winding connected to the output node, and an auxiliary winding that generates an output voltage separate from the secondary winding, A switching element connected to the primary winding, The switching control circuit includes the switching element, The power supply node of the switching control circuit is connected to the auxiliary winding, The switching power supply circuit according to claim 1, wherein the switching control circuit stops the switching operation of the switching element when it determines that the voltage supplied from the auxiliary winding to the power supply node is an overvoltage.

3. The number of turns of the auxiliary winding is less than the number of turns of the secondary winding. The switching power supply circuit according to claim 2, wherein the voltage level induced in the auxiliary winding when current flows through the primary winding is smaller than the voltage level induced in the secondary winding when the current flows through the primary winding.

4. The switching power supply circuit according to claim 2, wherein the switching control circuit stops the switching operation of the switching element when it determines that the current flowing through the switching element is an overcurrent.

5. The switching power supply circuit according to claim 4, wherein the switching control circuit stops the switching operation of the switching element when it determines that the feedback information indicates an overload condition.

6. One end of the secondary winding is connected to the output node via a first diode and to the control node via a second diode. The other end of the secondary winding is grounded. The output node is grounded via the first capacitor. The control node is grounded via a second capacitor. One end of the auxiliary winding is connected to the power node via a third diode. The other end of the auxiliary winding is grounded. The switching power supply circuit according to claim 2, wherein the power supply node is grounded to ground via a third capacitor.

7. The aforementioned feedback circuit is The control node includes an adjustment mechanism that adjusts the relationship between the voltage appearing at the control node and the feedback information, The adjustment mechanism includes a voltage divider circuit that divides the voltage appearing at the control node. The switching power supply circuit according to claim 1, wherein the voltage divider circuit includes a variable resistor.

8. A plurality of switching power supply circuits according to any one of claims 1 to 7 are provided, The output power terminals included in each of the plurality of switching power supply circuits are short-circuited to each other. Each of the plurality of switching power supply circuits includes a startup control terminal that directly or indirectly controls the output switch. A switching power supply system in which the startup control terminals included in each of the plurality of switching power supply circuits are short-circuited to each other.

9. The switching power supply system according to claim 8, further comprising a plurality of power sources connected to each of the input power supply terminals included in the plurality of switching power supply circuits.

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