JP4753381B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device for converting a DC voltage into a desired voltage and supplying it to an electronic device, and more particularly to a switching power supply device including a feedback control circuit.

  Conventionally, as disclosed in Patent Documents 1 to 3, a sub-winding is provided on the output choke coil of the forward converter so as to be coupled to the main winding, and the induced voltage generated in the output choke coil is supplied to the sub-winding. A switching power supply apparatus has been proposed in which a voltage signal is taken out from a diode as a voltage signal, and this voltage signal is input to a feedback control circuit to perform output voltage control. The power supply circuit disclosed in each patent document discloses an example in which a diode is used as a rectifying element of a rectifying / smoothing circuit including an output choke coil.

  On the other hand, in order to increase the efficiency of a switching power supply with a low voltage output, a synchronous rectifier circuit is generally used as a rectifier circuit in recent years. Therefore, in the power supply circuit of the conventional example disclosed in each patent document, it can be easily estimated that the synchronous rectifier circuit is applied to the rectifier circuit in order to increase the efficiency of the switching power supply.

  Therefore, a circuit of a power supply apparatus that performs feedback control by detecting an induced voltage generated in the conventional output choke coil in a switching power supply circuit having a synchronous rectifier circuit will be described with reference to FIG. In the switching power supply device shown in FIG. 7, the primary winding N1 of the transformer T1 and the main switching element TR1 are connected in series between the output terminals of the DC input power supply 10, and feedback control is performed on the control terminal of the main switching element TR1. The output of the main switching element driving circuit 14 that controls the output pulse width by the output of the circuit 12 is connected.

  The secondary winding N2 of the transformer T1 includes synchronous rectifying elements TR2 and TR3 such as MOS-FETs connected in series. The output of the synchronous rectifying element driving circuit 16 is connected to the gates of the synchronous rectifying elements TR2 and TR3. Has been. A synchronous rectifier circuit 18 is configured by the synchronous rectifier elements TR2 and TR3 and the synchronous rectifier element drive circuit 16. A smoothing circuit 20 including an output capacitor Co and an output choke coil Lo is connected between the drain and source of the synchronous rectifier element TR3, and a load 22 is connected to both ends of the output capacitor Co.

  The output choke coil Lo is provided with a main winding Nmain and a sub-winding Nsub, and an output choke coil induced voltage detection circuit 15 including a diode D1 and a capacitor C1 is connected to the output choke coil Lo. The capacitor C1 extracts and stores an induced voltage generated in the output choke coil Lo while the main switching element TR1 is off, and the voltage of the capacitor C1 is connected to the inverting input terminal of the error amplifier 13 of the feedback control circuit 12. ing. The reference voltage Vref is input to the non-inverting input terminal of the error amplifier 13.

In this switching power supply device, the main switching element drive circuit 14 controls the output pulse Vo of the switching power supply device by controlling the drive pulse width of the main switching element TR1. The main switching element drive circuit 14 determines the drive pulse width of the main switching element TR1 so that the feedback control circuit 12 makes the voltage VC1 of the capacitor C1 equal to the reference voltage Vref.
JP-A-6-284714 JP 2003-33024 A JP 2003-33025 A

  However, when a synchronous rectifier circuit is combined with the above conventional example, a problem occurs in output voltage control, and the characteristics as a switching power supply deteriorate. That is, in the switching power supply device of FIG. 7, since the influence of the temperature characteristic of the forward voltage VF of the diode D1 appears in the output voltage of the switching power supply apparatus, the switching of the characteristic in which the output voltage Vo changes due to the influence of the ambient temperature. There is a problem of becoming a power supply. The characteristic that the output voltage Vo changes due to the influence of the ambient temperature in the switching power supply device of FIG. 7 will be described below.

The induced voltage Vflsub generated in the sub winding Nsub of the output choke coil Lo is expressed by the following equation (1) from the turn ratio of the main winding Nmain and the sub winding Nsub of the output choke coil Lo and the output voltage Vo of the switching power supply. It is expressed in

The capacitor C1 of the output choke coil induced voltage detection circuit 15 stores the induced voltage Vflsub generated in the sub winding Nsub of the output choke coil Lo via the diode D1. Since the diode D1 generates a voltage drop of the forward voltage VF, the voltage VC1 of the capacitor C1 is expressed by Expression (2).

Since this switching power supply device performs control so that the voltage VC1 of the capacitor C1 of the output choke coil induced voltage detection circuit 15 is the same as the reference voltage Vref, Expression (3) is established.

By transforming equation (3), equation (4) is obtained.

Since the reference voltage Vref is normally a constant value, Equation (5) is obtained.

  From Expression (5), it can be seen that the output voltage Vo changes under the influence of the forward voltage VF of the diode D1.

  Here, the diode D1 of the output choke coil induced voltage detection circuit 15 is a PN junction diode or a Schottky barrier diode. These diodes have a low forward voltage VF when the ambient temperature is high. Is low, the forward voltage VF changes so as to increase. Therefore, the switching power supply having the configuration as shown in FIG. 7 has a characteristic that the output voltage Vo varies with the ambient temperature. In general, since the switching power supply device is required to have a constant output voltage Vo even when the ambient temperature changes, it is not preferable that the output voltage Vo change due to a change in the ambient temperature.

  This problem is particularly noticeable in a switching power supply with a low voltage and a large current output. For example, in a switching power supply with an output voltage of 1 V and a large current output, the main winding Nmain of the output choke coil Lo is often designed with one turn. This is a low-voltage output, so the inductance may be small, so the number of turns can be reduced and the number of turns can be reduced, so that the current flow path can be shortened and the resistance component can be reduced. This is because loss can be reduced.

  If the sub-winding Nsub of the output choke coil Lo of this switching power supply is one turn, a switching power supply in which the output voltage Vo changes by an amount corresponding to the change in the forward voltage VF of the diode D1. If a diode having a PN junction is used as the diode D1, the forward voltage VF of the diode D1 is about 0.8V at an ambient temperature of -40 ° C, about 0.6V at 20 ° C, and about 0.4V at 80 ° C. It has a temperature fluctuation of about 0.4 V from high temperature to low temperature. Therefore, at -40 ° C. to 80 ° C., 0.4V out of 1V output becomes a switching power supply whose output voltage varies.

Therefore, in order to reduce the output voltage fluctuation of the switching power supply, it can be improved by increasing the number of turns of the sub winding Nsub with respect to the main winding Nmain of the output choke coil Lo from the equation (5). If it is desired to make the fluctuation of the output voltage of the switching power supply device 0.1% or less, Equation (6) is obtained.

  If the output voltage fluctuation is to be reduced to 0.1% or less, the sub choke Nsub of the output choke coil Lo needs 400 turns, occupying a large winding space of the output choke coil, and the main winding Nmain by that much. This leads to an increase in the resistance component of the main winding Nmain. An increase in the resistance component leads to an increase in the loss of the switching power supply, and reduces the efficiency of the switching power supply.

  In addition, in recent switching power supplies, a structure that realizes miniaturization and high performance by forming a coil pattern of a transformer or an output choke coil on a printed circuit board and integrating it with a control circuit may be used. It is impossible to provide 400 turns of the sub-winding Nsub with the coil pattern on the printed circuit board. In the conventional circuit, this switching power supply cannot be made practically.

  The present invention has been made in view of the background art described above, and an object of the present invention is to provide a highly efficient and high-accuracy switching power supply device that is easy to downsize with a simple configuration.

  The present invention turns on / off a transformer having a primary winding and a secondary winding, an input power source connected to the primary winding of the transformer, and the input power source voltage applied to the primary winding. A main switching element to be turned off, a rectifier circuit connected to the secondary winding of the transformer and performing synchronous rectification of output power of the secondary winding, and a smoothing circuit comprising a choke coil and a capacitor for smoothing the output of the rectifier circuit A feedback control circuit for controlling the output voltage to a predetermined voltage by feedback of the output voltage of the smoothing circuit, and a main switching element driving circuit for turning on / off the main switching element by an output of the feedback control circuit A synchronous rectifying element driving circuit that operates in synchronization with an output signal of the main switching element driving circuit, and a synchronous rectifying element driving circuit. And a control pulse width of the main switching element is controlled by an output of the main switching element driving circuit to control the output voltage of the smoothing circuit to be a predetermined voltage. In the switching power supply device, the choke coil of the smoothing circuit is provided with a main winding and a sub winding, and a sub switching element and a capacitor are connected to the sub winding, and during the off period of the main switching element, By turning on the sub-switching element connected to the sub-winding, taking out the induced voltage of the choke coil generated in the sub-winding as a voltage signal, and inputting the voltage signal to the feedback control circuit, The switching power supply device controls the output voltage to a predetermined voltage.

  The sub switching element is turned on after a predetermined first dead time after the main switching element is turned off, and the main switching element is turned on after a predetermined second dead time after the sub switching element is turned off. It is controlled to turn on.

  The first dead time from when the main switching element is turned off to when the sub switching element is turned on is that the synchronous rectifying element is turned on after the main switching element is turned off. The second dead time from the time when the sub-switching element is turned off to the time when the main switching element is turned on is the period until the sub-switching element is turned off. Is set to a period until the main switching element is turned on.

  The synchronous rectifying element and the sub-switching element are MOS-FETs, and a voltage ratio that can be generated in the forward direction of the parasitic diode of the synchronous rectifying element is set to a turn ratio of the main winding and the sub-winding of the choke coil. The turn ratio is selected so that the value multiplied by is equal to or less than the forward voltage of the parasitic diode of the sub-switching element.

  In addition, the induced voltage of the choke coil generated in the sub-winding may be extracted as a voltage signal, and the voltage signal may be input to the switching power supply protection circuit to be used for detecting an abnormal operation of the switching power supply. The choke coil is formed by forming a coil pattern on a printed board.

  The present invention performs output voltage feedback control using the sub-windings of the output choke coil, and uses a switching element such as a MOS-FET instead of the conventional diode in the output choke coil induced voltage detection circuit. By performing the control of the present invention, it is possible to easily reduce the size and increase the performance of the power supply device, and to provide a highly efficient and highly accurate switching power supply device. Furthermore, it is possible to improve the response of the feedback circuit of the switching power supply device to the output voltage drop when the load current of the switching power supply device changes suddenly.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a switching power supply apparatus according to a first embodiment of the present invention. The same members as those in the circuit shown in FIG. In this switching power supply device, a primary winding N1 of a transformer T1 and a main switching element TR1 such as a MOS-FET are connected in series between output terminals of a DC input power supply 10, and a control terminal of the main switching element TR1 includes The output of the main switching element drive circuit 24 that controls the output pulse width by the output of the feedback control circuit 12 is connected. The main switching element drive circuit 24 is also provided with a sub-switching element drive circuit 26 described later.

  The secondary winding N2 of the transformer T1 includes synchronous rectification elements TR2 and TR3 such as MOS-FETs connected in series. The synchronous rectification elements TR2 and TR3 are complementary to the gates of the synchronous rectification elements TR2 and TR3. The output of the synchronous rectifier driving circuit 16 that is turned on / off is connected to the output. The synchronous rectifier elements TR2 and TR3 and the synchronous rectifier element driving circuit 16 constitute a synchronous rectifier circuit 18 that synchronously rectifies the output from the secondary winding N2 of the transformer T1. A smoothing circuit 20 including an output capacitor Co and an output choke coil Lo is connected between the drain and source of the synchronous rectifier element TR3, and a load 22 is connected to both ends of the output capacitor Co.

  The output choke coil Lo is provided with a main winding Nmain and a sub winding Nsub, and the dot-side of the main winding Nmain is connected to the non-dot terminal of the secondary winding N2 via the synchronous rectifier element TR2. The side of the main winding Nmain without a dot is connected to the load 22. The sub winding Nsub is connected to the sub switching element Q1. The sub-switching element Q1 is installed in place of the conventional diode D1 shown in FIG. 7, and forms an output choke coil induced voltage detection circuit 25 together with the capacitor C1. The capacitor C1 is connected across the sub-winding Nsub via the sub-switching element Q1, one end of the capacitor C1 is connected to the inverting input terminal of the error amplifier 13 in the feedback control circuit 12, and the other end is grounded. Yes.

  The sub-switching element Q1 is connected to the output of the sub-switching element drive circuit 26 provided in the main switching element drive circuit 24. The output choke coil induced voltage detection circuit 25 stores the induced voltage generated in the output choke coil Lo in the capacitor C1 when the sub switching element Q1 is turned on while the main switching element TR1 is turned off. The voltage of the capacitor C1 is connected to the inverting input terminal of the error amplifier 13 in the feedback control circuit 12, and the reference voltage Vref is connected to the non-inverting input terminal of the error amplifier 13.

  In the operation of the switching power supply of this embodiment, the main switching element drive circuit 24 controls the drive pulse width of the main switching element TR1, thereby controlling the output voltage Vo of the switching power supply. The main switching element drive circuit 24 determines the drive pulse width of the main switching element TR1 so that the feedback control circuit 12 makes the voltage VC1 of the capacitor C1 of the output choke coil induced voltage detection circuit 25 equal to the reference voltage Vref.

  The switching power supply device of this embodiment operates as shown in the timing chart of FIG. 2 when the synchronous rectifying elements TR2 and TR3 operate ideally and the sub-switching element Q1 also operates ideally. When the main switching element TR1 is on, the synchronous rectification element TR2 is on and the synchronous rectification element TR3 is off. At the same time as the main switching element TR1 is turned off, the synchronous rectifying elements TR2 and TR3 are turned on / off.

  Further, the sub switching element drive circuit 26 performs control to turn on the sub switching element Q1 when the main switching element TR1 is off, and controls so that the sub switching element Q1 is turned off when the main switching element TR1 is on.

  According to the switching power supply device of this embodiment, the feedback control of the output voltage can be accurately performed using the sub winding Nsub of the output choke coil Lo, there is no voltage drop due to the conventional diode, and the fluctuation due to temperature. It does not occur. Furthermore, a high-efficiency power supply device can be provided even in a low-voltage and large-current switching power supply device.

  Next, a switching power supply device according to a second embodiment of the present invention will be described with reference to FIGS. Here, the same members as those in the above embodiment are denoted by the same reference numerals, and the description thereof is omitted. In this embodiment, the configuration and operation are performed in consideration of the fact that the synchronous rectifier driving circuit 16 in the actual synchronous rectifier circuit 18 has a time delay to turn on the synchronous rectifier TR3. In the output choke coil induced voltage detection circuit 25 of this embodiment, the drain of the sub-switching element Q1 of the N-channel MOS-FET is connected to the terminal on the non-dot side of the sub-winding Nsub, and the source is connected to the ground terminal of the capacitor C1. Connected.

  In the operation of the switching power supply element of this embodiment, as shown in FIG. 4, the driving of the synchronous rectifier element TR3 operates with a dead time relative to the operation of the main switching element TR1. Since this operation affects the output voltage accuracy of the switching power supply device, the operation is avoided. It is the operation of the synchronous rectifier element TR3 that affects the output voltage accuracy of the switching power supply. The synchronous rectifier element TR3 takes some time from when the main switching element TR1 is turned off to when the synchronous rectifier element TR3 is turned on. Has a delay (period A). This is because the actual synchronous rectifier driving circuit 16 has a time delay in turning on the synchronous rectifier TR3.

  The actual synchronous rectifier circuit 18 is controlled so as to have a slight time delay (period C) from when the synchronous rectifier element TR3 is turned off to when the main switching element TR1 is turned on. This is because both ends of the input power supply 10 are short-circuited via the main switching element TR1, the transformer T1, and the synchronous rectifying element TR3 when the synchronous rectifying element TR3 is in an on state when the main switching element TR1 is turned on. As a result, a large loss occurs. Therefore, in order to ensure that the main switching element TR1 and the synchronous rectification element TR3 are not simultaneously turned on, control is performed with a slight time delay.

  Period A and period C are periods in which the main switching element TR1 and the synchronous rectifying element TR3 are simultaneously turned off. At this time, an induced voltage is generated in the output choke coil Lo, and the parasitic diode Db3 of the synchronous rectifier element TR3 becomes conductive as shown in FIG. During this period, the induced voltage of the output choke coil Lo is a voltage obtained by adding the forward voltage VF3 of the parasitic diode Db3 of the synchronous rectifier element TR3 to the output voltage Vo of the switching power supply.

  In the period B in which the synchronous rectifying element TR3 is on, as shown in FIG. 5B, no current flows through the parasitic diode Db3 of the synchronous rectifying element TR3, so the forward voltage VF3 is not generated. Therefore, the induced voltage of the output choke coil Lo becomes the output voltage Vo of the switching power supply.

  Therefore, in the power supply circuit shown in FIG. 3, when the sub-switching element Q1 is controlled to be turned on at the same time as the main switching element TR1 is turned off and the sub-switching element Q1 is turned off at the same time as the main switching element TR1 is turned on, The fluctuation of the induced voltage of the output choke coil Lo in the period C appears as it is in the capacitor C1. On the other hand, the switching element drive circuit 24 controls the drive pulse width of the main switching element TR1 so that the voltage of the capacitor C1 becomes the same as the reference voltage Vref. If fluctuations in the induced voltage of the output choke coil Lo in the period A and the period C appear as they are in the capacitor C1, the average voltage of the capacitor C1 is increased as compared with the case where the influence of the period A and the period C is not affected. Become. Therefore, the switching element drive circuit 24 controls the drive pulse width of the main switching element TR1 so that the output voltage Vo of the switching power supply is lowered by the influence of the period A and the period C. Since the period A and the period C are short, control is not performed such that the output voltage Vo of the switching power supply becomes extremely low. However, the accuracy of the output voltage Vo of the switching power supply deteriorates.

  Therefore, in the power supply device of this embodiment, as shown in FIG. 4, the sub-switching element Q1 connected to the sub-winding Nsub of the output choke coil Lo is slightly shorter than the period A after the main switching element TR1 is turned off. Main switching element having a dead time T2 which is turned on with a long dead time T1 and controlled so that the sub switching element Q1 is turned off at a timing slightly earlier than the period C before the main switching element TR1 is turned on. TR1 turns on.

  By performing this control, the influence of the period A and the period C can be eliminated, and the control accuracy of the output voltage Vo can be increased. Note that, when the sub-switching element Q1 is in the off state, only the parasitic diode Dbq1 is in a conductive state, and therefore, the fluctuation in the induced voltage of the output choke coil Lo in the period A and the period C is the sub-switching element Q1. The capacitor C1 is affected only when the forward voltage of the parasitic diode Dbq1 is exceeded. Regarding this, the fluctuation of the induced voltage of the output choke coil Lo is a value obtained by multiplying the forward voltage VF3 of the parasitic diode Db3 of the synchronous rectifier element TR3 by the turn ratio of the main winding Nmain and the sub winding Nsub of the output choke coil Lo. Therefore, a value obtained by multiplying the voltage that can be generated in the forward direction of the parasitic diode Db3 of the synchronous rectifier element TR3 by the turn ratio of the main winding Nmain and the sub-winding Nsub of the choke coil Lo is a parasitic value of the sub-switching element Q1. If the turn ratio is selected so as to be equal to or lower than the forward voltage of the diode Dbq1, fluctuations in the induced voltage of the output choke coil Lo during the period A and the period C do not affect the capacitor C1 at all.

  Further, as in the conventional example, when the induced voltage of the output choke coil Lo is detected using the diode D1, even if the output voltage Vo of the switching power supply device decreases when the load current of the switching power supply device suddenly changes, the voltage of the capacitor C1 is reduced. It does not drop immediately. This is because the diode D1 cannot flow current from the cathode to the anode. Therefore, in the conventional circuit, when the output voltage Vo of the switching power supply device decreases, the induced voltage of the output choke coil Lo decreases. However, the induced voltage of the output choke coil Lo before the output voltage Vo of the switching power supply device decreases decreases. Since it is stored in the capacitor C1, the reduction in the induced voltage of the output choke coil Lo cannot be reflected instantaneously. That is, in the conventional circuit, the feedback control circuit in the switching element driving circuit cannot instantaneously detect the decrease in the output voltage Vo of the switching power supply device.

  On the other hand, in the power supply device of this embodiment, a power supply device that can instantaneously follow a sudden change in load current can be formed. That is, in the power supply device of this embodiment, the diode D1 is replaced with the sub-switching element Q1 to be a MOS-FET, so that the current is bidirectional from the drain to the source and from the source to the drain while the MOS-FET is on. Can flow. For this reason, when the induced voltage of the output choke coil Lo decreases, the voltage of the capacitor C1 decreases instantaneously, and the feedback control circuit in the switching element drive circuit instantly detects the decrease in the output voltage Vo of the switching power supply device. It is possible to improve the response of the feedback control circuit to the output voltage drop at the time of sudden change in load current.

  In addition, the circuit of the power supply device of the present invention is a switching power supply having a configuration that realizes miniaturization and high performance by forming a coil pattern of a transformer and an output choke coil on a printed circuit board and integrating it with a control circuit. This is particularly effective, and the output voltage of the switching power supply can be controlled with high accuracy with a small number of sub-windings of the output choke coil.

  Note that the sub-switching element Q1 of this embodiment may be configured by a circuit using a P-channel MOS-FET as shown in FIG. In this embodiment, the same effect as that of the above embodiment can be obtained.

  In the above embodiment, only an example in which the induced voltage generated in the output choke coil is input to the feedback control circuit via the sub-switching element Q1 and the capacitor C1 and used for stabilization control of the output voltage of the switching power supply device is shown. However, it is also possible to detect the abnormal operation of the switching power supply device by inputting the voltage of the capacitor C1 to the switching power supply protection circuit and use it to control the protection circuit of the switching power supply device such as an overvoltage protection circuit or an undervoltage protection circuit. is there.

  The circuit in the above embodiment is based on a single forward converter, but the same applies to any converter circuit including an output choke coil and a synchronous rectifier circuit such as a push-pull converter, a full bridge converter, and a half bridge converter. An effect is obtained. The synchronous rectifier circuit can achieve the same effect even when a current doubler synchronous rectifier circuit is used.

1 is a schematic block diagram of a switching power supply device according to a first embodiment of the present invention. It is a timing chart which shows operation | movement of the switching power supply device of 1st embodiment of this invention. It is a schematic circuit diagram of the switching power supply device of 2nd embodiment of this invention. It is a timing chart which shows operation | movement of the switching power supply device of 2nd embodiment of this invention. It is a schematic circuit diagram which shows the generated voltage of the output choke coil of the switching power supply device of 2nd embodiment of this invention. It is a schematic circuit diagram of the modification of the switching power supply device of 2nd embodiment of this invention. It is a schematic block diagram of the conventional switching power supply device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 DC input power supply 12 Feedback control circuit 16 Synchronous rectification element drive circuit 18 Synchronous rectification circuit 20 Smoothing circuit 22 Load 24 Feedback control circuit 25 Output choke coil induced voltage detection circuit 26 Sub switching element drive circuit TR1 Main switching element TR2, TR3 Synchronous rectification Element Q1 Sub-switching element

Claims (6)

A transformer having a primary winding and a secondary winding, an input power supply connected to the primary winding of the transformer, and main switching for turning on / off the input power supply voltage applied to the primary winding A rectifier circuit connected to the secondary winding of the transformer and performing synchronous rectification of output power of the secondary winding; a smoothing circuit including a choke coil and a capacitor for smoothing the output of the rectifier circuit; A feedback control circuit for controlling the output voltage to a predetermined voltage by feedback of an output voltage of the smoothing circuit, a main switching element driving circuit for turning on / off the main switching element by an output of the feedback control circuit, and the main The synchronous rectifier driving circuit that operates in synchronization with the output signal of the switching element driving circuit, and the synchronous rectifier driving circuit. A switching power supply device comprising a synchronous rectifying element that is turned off, and controlling the drive pulse width of the main switching element according to the output of the main switching element drive circuit so that the output voltage of the smoothing circuit becomes a predetermined voltage In
The choke coil of the smoothing circuit is provided with a main winding and a sub winding, a sub switching element and a capacitor are connected to the sub winding, and connected to the sub winding during an off period of the main switching element. The sub-switching element is turned on, the induced voltage of the choke coil generated in the sub-winding is taken out as a voltage signal, and the output voltage is set to a predetermined voltage by inputting the voltage signal to the feedback control circuit. The switching power supply device characterized by controlling to.   The sub switching element is turned on after a predetermined first dead time after the main switching element is turned off, and the main switching element is turned on after a predetermined second dead time after the sub switching element is turned off. The switching power supply according to claim 1, wherein the switching power supply is controlled to turn on.   The first dead time from when the main switching element is turned off to when the sub switching element is turned on is that the synchronous rectifying element is turned on after the main switching element is turned off. The second dead time from the time when the sub-switching element is turned off to the time when the main switching element is turned on is set to a period until the sub-switching element is turned off. The switching power supply device according to claim 2, wherein the switching power supply device is set to a period until the main switching element is turned on thereafter.   The synchronous rectifier element and the sub-switching element are MOS-FETs, and a voltage that can be generated in the forward direction of the parasitic diode of the synchronous rectifier element is multiplied by the turn ratio of the main winding and the sub-winding of the choke coil. 4. The switching power supply device according to claim 1, wherein the turn ratio is selected so that the value is equal to or less than a forward voltage of a parasitic diode of the sub-switching element.   The induced voltage of the choke coil generated in the sub-winding is taken out as a voltage signal, and the voltage signal is input to a switching power supply protection circuit to be used for detecting an abnormal operation of the switching power supply. The switching power supply device according to 2, 3, or 4. 6. The switching power supply device according to claim 1, wherein the choke coil has a coil pattern formed on a printed circuit board.

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