JP2006060907A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve highly efficient operation in an entire range from a light load up to a rated load, in a switching power supply. <P>SOLUTION: A DC input switching element (SW3), which is connected in series to the primary winding of a first transformer (T1), is provided at the primary circuit side. At the secondary circuit side are provided a first gate drive circuit (11) inserted on load connection lines between the secondary winding of the first transformer (T1) and a load (16) to control a main switching element (SW1); a first gate drive circuit (12) that controls a sub-switching element (SW2) connected between the load connection lines; a second transformer (T2) between which and one of the load connection lines the primary winding is inserted; a control circuit (14) that generates and outputs control pulses on the basis of an output voltage supplied to the load (16); and a third transformer (T3) that controls the DC input switching element (SW3) on the basis of this control pulses. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching power supply device, and more particularly to a switching power supply device that enables high-efficiency operation over the entire range from a light load to a rated load.

  Switching power supply devices can be roughly classified into an RCC (ringing choke converter) method and a synchronous rectification method from the viewpoint of a control method. Among these control methods, in the RCC method, the excitation energy accumulated in the transformer during the ON period of the switching element is output to the secondary circuit during the OFF period of the switching element, and the control winding of the transformer is completed after the output is completed. The switching element is turned on again based on the feedback output of the ringing pulse generated on the line, so that a desired voltage is output. Therefore, the RCC method has a problem that power conversion efficiency is poor. On the other hand, the synchronous rectification method has a problem that standby power consumption cannot be ignored because switching is always performed.

  For example, in an RCC switching power supply device disclosed in Patent Document 1 below, a delay circuit provided between a control winding and a main switching element has a current of a rectifying element connected to a secondary winding of zero. The standby loss is reduced by prohibiting the turn-on of the main switching element for a predetermined period of time.

JP 2002-84748 A

  However, the RCC switching power supply device such as Patent Document 1 described above has a problem that it is difficult to maintain high-efficiency operation over an operation range from a light load to a rated load. Specifically, the RCC switching power supply device has a problem that the diode loss is large at the rated load and the transformer utilization factor is low, so that the operation efficiency at the rated load is low.

  In addition, as described above, the synchronous rectification switching power supply device has a problem in that standby power consumption cannot be ignored because switching is always performed.

  The present invention has been made in view of the above, and provides a switching power supply device that is not classified into a synchronous rectification method or an RCC method, which enables high-efficiency operation over the entire range from standby to rated load. For the purpose.

  In order to solve the above-described problems and achieve the object, the switching power supply device according to the present invention accumulates energy output from the DC power supply during the ON period of the main switching element in the choke coil while outputting to the load side, In a switching power supply device that outputs energy accumulated in the choke coil to a load side during an off period of the main switching element, one end is connected to either the positive electrode or the negative electrode of the DC power source, and the DC power from the DC power source is A DC input switching element for intermittent input, a primary winding and a secondary winding magnetically coupled to the primary winding, and one end of the primary winding connected to either the positive electrode or the negative electrode of the DC power supply A first transformer having the other end connected to the other end of the DC input switching element, a primary winding, and a secondary magnetically coupled to the primary winding. A second transformer in which the primary winding is inserted between one of the load connection lines connecting the secondary winding of the first transformer and the load; and the first transformer A first gate drive circuit for controlling conduction of the main switching element inserted in one or the other of the load connection lines based on a voltage generated in the secondary winding of the transformer, and one end of the first switching circuit A configuration in which one end of the secondary winding of the transformer is connected and the other end is connected to one end of the main switching element, or one end is connected to the other end of the main switching element and the other end is the first A sub-switching element connected in any one of the configurations connected to the other end of the secondary winding of the transformer, and a control circuit that generates and outputs a control pulse signal based on the output voltage supplied to the load When A second control circuit for controlling conduction of the sub-switching element based on an output voltage supplied to the load, a voltage generated in a secondary winding of the second transformer, and a control pulse output from the control circuit; A gate drive circuit; a primary winding to which a control pulse output from the control circuit is applied; and a secondary winding magnetically coupled to the primary winding, based on a voltage generated in the secondary winding. And a third transformer for controlling the DC input switching element.

  The switching power supply according to the next invention is characterized in that, in the above invention, the pulse width of the control pulse output from the control circuit is substantially constant.

  The switching power supply according to the next invention is characterized in that in the above invention, a delay circuit is further provided between the control circuit and the third transformer.

  The switching power supply according to the next invention is characterized in that, in the above invention, the control circuit varies the cycle of the control pulse based on an output voltage supplied to the load.

  The switching power supply according to the next invention is characterized in that, in the above invention, the control circuit comprises an astable multivibrator and a monostable multivibrator.

  According to the switching power supply device according to the present invention, since the switching frequency for intermittently connecting the DC input is a separately excited control method, it has a simple circuit configuration, but covers a wide range from a standby mode to a light load and further to a rated load. , There is an effect that the driving efficiency can be increased.

  Embodiments of a switching power supply device according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited by this embodiment.

  FIG. 1 is a simplified circuit diagram for explaining the operating principle of a switching power supply device according to the present invention. In the figure, a switching power supply 10 having an input end and an output end respectively controls three transformers T1 to T3, three switching elements SW1 to SW3, two capacitors C1 and C2, and switching elements SW1 and SW2. The gate drive circuits 11 and 12, the delay circuit 15, and the control circuit 14 for controlling the switching element SW3 through the delay circuit 15 and the transformer T3 are provided. Note that an input power supply (DC power supply) is connected to the input terminal of the switching power supply device 10 and a load is connected to the output terminal.

  Next, the circuit configuration of the switching power supply device shown in FIG. 1 will be described. Each of the transformers T1 to T3 includes a primary winding and a secondary winding, and each of the switching elements SW1 to SW3 is a predetermined part of the circuit as a three-terminal switch having one end, the other end, and a control end. Has been inserted.

  In FIG. 1, on the primary winding T1-1 side of the transformer T1, one end of the primary winding T1-1 of the transformer T1 is connected to the positive side of the input power supply (DC power supply) and the primary winding T1 of the transformer T1. -1 is connected to the other end of the switching element SW3, and one end of the switching element SW3 is connected to the negative side of the input power supply (DC power supply). That is, the primary winding T1-1 of the transformer T1 and the switching element SW3 form a series circuit and are connected to both ends of the input power supply. Further, the secondary winding T3-2 of the transformer T3 is connected to the control end and the other end of the switching element SW3, and the switching element SW3 is connected to the primary winding T3-1 via the delay circuit 15 from the control circuit 14. The switching element SW3 can be controlled based on the control signal. The capacitor C1 is a DC smoothing capacitor for smoothing a DC voltage applied to both ends of the series circuit of the primary winding T1-1 of the transformer T1 and the switching element SW3.

  On the other hand, on the secondary winding T1-2 side of the transformer T1, the transformer T2 and the switching element SW1 are respectively connected on the load connection line that electrically connects the secondary winding T1-2 of the transformer T1 and the load 16. Has been inserted. More specifically, the primary winding T2-1 of the transformer T2 is provided on a connection line between the one end of the secondary winding T1-2 and one end of the load 16 (hereinafter referred to as “the positive side of the load connection line”). The other end of the switching element SW1 is inserted into the secondary winding on the connection line between the other end of the secondary winding T1-2 and the other end of the load 16 (hereinafter referred to as “the negative side of the load connection line”). It is inserted so as to be connected to the other end of the line T1-2. A capacitor C2 connected in parallel to both ends of the load 16 is a DC smoothing capacitor for smoothing a DC voltage, like the capacitor C1.

  As described above, the switching element SW1 is inserted on the negative electrode side of the load connection line, whereas the switching element SW2 is inserted between the positive electrode side of the load connection line and the negative electrode side of the load connection line. More specifically, one end of the switching element SW2 is connected to one end of the primary winding T2-1 of the transformer T2, and the other end of the switching element SW2 is connected to the other end of the switching element SW1 (the other end of the load 16). Connected.

  Further, as described above, gate drive circuits 11 and 12 for controlling the switching elements SW1 and SW2 are provided. In these gate drive circuits, the gate drive circuit 11 controls the switching element SW1 based on the voltage generated in the secondary winding T1-2 of the transformer T1, and the gate drive circuit 12 applies the voltage applied to the load 16 (that is, the switching power supply). The switching element SW2 is controlled based on the output voltage of the device, the voltage across the secondary winding T2-2 of the transformer T2, and the control signal (control pulse) from the control circuit 14. That is, the gate drive circuit 11 is configured in a self-excited control system such that control over the switching element SW1 is autonomously performed. On the other hand, the gate drive circuit 12 is configured in a separate excitation control system in which the control over the switching element SW2 is performed in a different manner based on the control of the control circuit 14.

  FIG. 2 is a circuit diagram showing a specific configuration example of the switching power supply device shown in FIG. The switching power supply device shown in the figure shows a specific circuit configuration of each component of the gate drive circuits 11 and 12 and the control circuit 14 shown in FIG. 1, and in particular, the control circuit 14 is unstable. A multivibrator 21 and a monostable multivibrator 23 are included. In FIG. 2, the same parts as those in FIG. 1 are denoted by the same reference numerals.

  In the circuit configuration shown in FIG. 1, the delay circuit 15 is provided between the control circuit 14 and the transformer T3. However, in FIG. 2, this is omitted for convenience of explanation. The function of the delay circuit 15 will be described later.

  Next, the circuit operation of the switching power supply device shown in FIG. 2 will be described. First, the operation of the control circuit 14 shown in the lower part of FIG. 2 will be described. The output voltage (P5) of the switching power supply device 10 is fed back to the astable multivibrator 21 of the control circuit 14, and each base that is a common connection point of the pair of transistors TR11 and TR12 provided in the astable multivibrator 21 is provided. Input to the terminal. The astable multivibrator 21 changes its transmission frequency based on the fed back output voltage (P5). Specifically, when the output voltage (P5) decreases, the transmission frequency increases, and when the output voltage (P5) increases, the transmission frequency decreases. An example of the pulse signal output from the astable multivibrator 21 is shown in FIG. As shown by the pulse waveform in the figure, the pulse signal output from the astable multivibrator 21 is a signal having a constant duty ratio although the transmission frequency changes.

  The pulse signal output from the astable multivibrator 21 is input to a differentiating circuit 22 including a capacitor C3 and a resistor R15, and becomes a trigger signal responding to the rising portion of the input pulse signal as shown in FIG. This trigger signal is output to the monostable multivibrator 23 in the next stage. The monostable multivibrator 23 generates a pulse signal having a constant pulse width determined based on the time constants of the capacitor C4 and the resistor R16 based on the input trigger signal. Since the trigger period of the trigger signal output from the astable multivibrator 21 changes based on the change in the transmission frequency, the pulse signal output from the monostable multivibrator 23 is as shown in FIG. The pulse signals have a constant pulse width and different duty ratios.

  Explaining the above processing as a function of the control circuit 14, the control circuit 14 outputs a pulse signal with a constant pulse width and a large duty ratio when the output voltage (P5) of the switching power supply 10 decreases, and the output voltage (P5). When is increased, a pulse signal with a constant pulse width and a small duty ratio is output. When the duty ratio of the pulse signal output from the control circuit 14 is increased, the conduction time of the switching element is increased, so that the output voltage (P5) of the switching power supply device is increased and the duty ratio of the pulse signal is decreased. Output voltage (P5) decreases. These operations relating to the switching power supply 10 will be described later.

  Next, the operation of the circuit shown in the upper part of FIG. 2 will be described. When a pulse (positive pulse) signal from the control circuit 14 is applied to the primary winding T3-1 of the transformer T3, a voltage is generated in the secondary winding T3-2 for the duration of this pulse width, and TR10 is As a result, the current flows through the primary winding T1-1 of the transformer T1. At this time, a voltage having a positive polarity on the positive side of the load connection line is generated in the secondary winding T1-2 of the transformer T1 via the primary winding T1-1 of the transformer T1. The voltage P1 at the connection point of the pair of transistors TR4 and TR5 is set to a predetermined voltage (a forward voltage drop of a general semiconductor PN junction is “0.6V” by the resistor R5 and the transistor TR5 in advance (in a circuit). (Hereinafter also the same)), the collector voltage of the transistor TR4 (which is also the “drain voltage of the field effect transistor FET1”) P2 is slightly negative (for example, “−10 mV”). When the voltage changes to “−100 mV”, a voltage having a positive polarity on the positive side of the load connection line is generated at both ends of the transformer T1-2, so that the base current of the transistor TR4 flows. When the transistor TR4 is turned on, the base current of the transistor TR3 flows, the transistor TR3 is turned on, the transistor TR1 is also turned on, the gate potential of the field effect transistor FET1 is increased, and the field effect transistor FET1 is turned on. At this time, the transistor TR2 is turned off because its base potential is a positive potential.

  When the field effect transistor FET1 is turned on, a closed circuit of T1-2 → T2-1 → C2 (or load (not shown)) → FET1 → T1-2 is formed, and a current I1 flows. At this time, since the terminal T2-2-2 side of the transformer T2-2 has a negative potential, the transistor TR6 is off. Therefore, the gate of the field effect transistor FET2 does not become a positive potential, and the field effect transistor FET2 continues to be off. However, the terminals A and B to which the field effect transistor FET2 is connected are not conducted.

  Next, when the application of the positive pulse signal from the control circuit 14 to the primary winding T3-1 of the transformer T3 is stopped (OFF state), the transistor TR10 is turned off, and a current flows in the primary winding T1-1 of the transformer T1. Since it does not flow, the voltage generated in the secondary winding T1-2 of the transformer T1 disappears, the transistor TR1 is turned off, and the field effect transistor FET1 is also turned off.

  In this state, a current I2 based on the electromagnetic energy accumulated in the transformer T2 flows instead of the current I1 flowing based on the energy supplied from the input power supply. More specifically, the terminal T2-1-2 side positive potential of the primary winding T2-1 of the transformer T2 becomes a positive potential, and the current flows along the path TR6 emitter → TR6 base → R10 → D2 → TR8 collector → TR8 emitter. The transistor TR6 is turned on, a current flows through the resistor R7, the gate potential of the field effect transistor FET2 rises, and the field effect transistor FET2 is turned on. At this time, a closed circuit of T2-1-2 → C2 (or load (not shown)) → FET2 → T2-1-1 is formed, and a current I2 flows.

  Next, when the positive pulse from the control circuit 14 is output again, the positive pulse is applied to the base of the transistor TR7 via the resistor R12, so that the transistor TR7 is turned on, and conversely, the field effect transistor FET2 is turned off. Become. Since the positive pulse is also applied to the transformer T3-1, the field effect transistor FET1 is turned on again as described above, and the above operation is repeated.

  The above operation is described assuming that the power is turned on, but the same operation is performed even when the power is not turned on. The reason is that after the power is turned on, the capacitor C2 is charged with a predetermined voltage, but the voltage generated in the primary winding T2-1 of the transformer T2 is higher than the voltage across the capacitor C2. This is because the current I2 flows from the primary winding T2-1 of the transformer T2 toward the capacitor C2 regardless of whether or not the power is turned on.

  When the electromagnetic energy accumulated in the transformer T2 is consumed in the repetition process of the operation described above, the voltage generated at both ends of the primary winding T2-1 of the transformer T2 is lower than the voltage at both ends of the capacitor C2. , The closed circuit of the positive electrode of C2 → T2-1 → FET2 → the negative electrode of C2 is formed, and the electric energy accumulated in the capacitor C2 may flow backward.

  The phenomenon of backflow of electrical energy as described above is not preferable for a switching power supply device. Therefore, in order to prevent this reverse flow phenomenon, a differential circuit centered on the transistors TR8, TR9 and the like is configured.

  As an example of a specific state, a case is assumed in which a voltage of 10V is charged in the capacitor C2, and the voltage across the primary winding T2-1 of the transformer T2 drops below 10V. At this time, the electric energy charged in the capacitor C2 tends to flow backward toward the transformer T2. On the other hand, as described above, when the potential of the terminal T2-2-2 of the primary winding T2-1 of the transformer T2 is positive, the transistor TR6 is turned on, and the gate potential of the field effect transistor FET2 is positive and the electric field is increased. The effect transistor FET2 is turned on, and a closed circuit is formed from the positive electrode of the capacitor C2 to the negative electrode of the capacitor C2 via the transformer T2. Therefore, in order to prevent the reverse flow phenomenon of electric energy from the capacitor C2 toward the transformer T2, it is necessary to prevent the transistor TR6 from being turned on.

  Further, the transistor TR9 is always kept on by the voltage of the capacitor C2. Therefore, the potential (P4) at the upper end of the resistor R11 is about 9.4V. On the other hand, when the backflow phenomenon as described above becomes a problem, the potential of the terminal T2-1-2 of the primary winding T2-1 of the transformer T2 is less than 10V, and the turn ratio of the transformer T2 is set to 1: 1. For example, the potential of the terminal T2-2-2 of the secondary winding T2-2 is also less than 10V. Therefore, the transistor TR8 is not turned on, and at the same time, the transistor TR6 is not turned on. Therefore, the FET2 is turned off, and the backflow phenomenon of the electric energy from the capacitor C2 toward the transformer T2 can be prevented.

  Further, the electromagnetic energy accumulated in the transformer T2 disappears, and the charge voltage 10V of the capacitor C2 is applied to the positive side of the load connection line to which the secondary winding T1-2 of the transformer T1 is connected. However, as described above, when no current flows through the primary winding T1-1 of the transformer T1 and no voltage is generated across the secondary winding T1-2 of the transformer T1, the secondary winding T1-2. Since the collector potential (P2) of the transistor TR4 connected to is also maintained at a potential of about 10 V, the field effect transistor FET1 is not turned on.

  In addition, the base potential of the transistor TR8 is adjusted by the variable resistor VR. With this configuration, the timing for turning on the field effect transistor FET2 is accurately controlled. That is, in FIG. 2, when the potential at the point P3 becomes equal to or higher than the potential at the point P5, the transistor TR8 is turned on, the transistor TR6 is turned on, and the FET 2 can be turned on. Specifically, assuming that the charge voltage of the capacitor C2 is 10V, for example, the base potential (P3) of the transistor TR8 can be set to about 10.01V. This is because, as described above, when an electric current is caused to flow through the capacitor C2 and the load by the electromagnetic energy accumulated in the transformer T2, the potential of the terminal T2-1-2 of the primary winding T2-1 of the transformer T2 is changed to the capacitor. It is sufficient if the charge voltage of C2 is exceeded. However, if the FET 2 is not turned on at this time, a closed circuit of the current path is not established. Therefore, as a specific countermeasure, the winding ratio of the primary winding and the secondary winding of the transformer T2 is set to 1: 2, for example, so that a sufficient voltage is generated in the secondary winding of the transformer T2, and the resistance By adjusting R9 and the variable resistance VR, it is possible to generate an appropriate voltage for immediately turning on the transistor TR8 at the base potential (P3) of the transistor TR8. If such a voltage is set, the transistors TR8 and TR6 are immediately turned on when the potential of the terminal T2-1-2 of the primary winding T2-1 of the transformer T2 slightly exceeds the charge voltage of the capacitor C2. Further, the FET 2 can be turned on, and the electromagnetic energy generated in the transformer T2 can be efficiently supplied to the capacitor C2 (or load) without waste.

  Finally, the function of the delay circuit 15 shown in FIG. 1 will be described. Up to now, the operation of the switching power supply device according to the present invention has been described in detail. However, these descriptions are based on the premise that the switching power supply device maintains an ideal operating state. On the other hand, before the transistor TR7 is turned on and the field effect transistor FET2 is turned off by the positive pulse output from the control circuit 14 due to the type of load, the wiring length of the circuit, or the operation delay of the transistor element. It is conceivable that there is a slight time zone in which the voltage generated in the secondary winding T1-2 of the transformer T1 flows to the field effect transistor FET2 when the transistor TR10 is turned on. In such a case, the voltage generated in the secondary winding T1-2 of the transformer T1 returns to itself without being accumulated in the transformer T2, which causes a reduction in operating efficiency. In such a case, as shown in FIG. 1, by inserting the delay circuit 15 between the control circuit 14 and the transformer T3, it is possible to obtain an effect that a decrease in operating efficiency can be suppressed.

  Note that the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, various modifications as exemplified below are possible. Is possible.

(Modification 1)
FIG. 3 is a circuit diagram showing a modification of the present invention shown in FIG. The switching power supply device 10a shown in the figure is obtained by changing the connection of the switching element SW3 connected to the DC power supply and the transformer T1 in the primary circuit section of the switching power supply device shown in FIG. That is, one end of the primary winding T1-1 of the transformer T1 and the other end of the switching element SW3 are connected, and one end of the switching element SW3 is connected to the positive electrode side of the input power supply (DC power supply). The other end of the winding T1-1 is configured to be connected to the negative electrode side of the input power supply (DC power supply). Even in such a configuration, the same operation as described above is performed. Note that the conductivity type of the switching element shown in FIG. 3 is different from that of the switching element of FIG.

(Modification 2)
FIG. 4 is a circuit diagram showing another modification different from the modification shown in FIG. The switching power supply device 10b shown in the figure has the switching element SW1 connected to the negative electrode side of the load connection line connected to the positive electrode side of the load connection line in the secondary circuit section of the switching power supply device shown in FIG. Is. Even in such a configuration, the same operation as described above is performed. Although illustration is omitted, the transformer T2 connected to the positive electrode side of the load connection line in FIG. 1 can be connected to the negative electrode side of the load connection line for the same purpose as the switching element SW1.

  As described above, according to the switching power supply device of the present invention, the first gate drive circuit controls conduction of the main switching element inserted on the load connection line, and the second gate drive circuit controls the load connection line. The sub-switching elements connected in between are controlled in conduction, and the control circuit determines the control frequency of the DC input switching element based on the output voltage of the switching power supply device. Operation efficiency can be increased over a wide range from time to light load and even to rated load.

  As described above, the switching power supply device according to the present invention is useful as a highly efficient and inexpensive power supply device.

It is the circuit diagram simplified for demonstrating the principle of operation of the switching power supply device concerning this invention. FIG. 2 is a circuit diagram illustrating a specific configuration example of the switching power supply device illustrated in FIG. 1. It is a circuit diagram which shows the modification of this invention shown in FIG. It is a circuit diagram which shows the other modification different from the modification shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Switching power supply device 11,12 Gate drive circuit 14 Control circuit 15 Delay circuit 16 Load 21 Astable multivibrator 22 Differentiating circuit 23 Monostable multivibrator C1, C2, C3, C4 Capacitor FET1, FET2 Field effect transistor SW1, SW2, SW3 Switching element T1, T2, T3 Transformer TR1-TR14 Transistor R1-R4, R6-R12, R15, R16 Resistor R5, VR Variable resistance

Claims (5)

The energy output from the DC power supply during the ON period of the main switching element is stored in the choke coil while being output to the load side, and the energy stored in the choke coil is output to the load side during the OFF period of the main switching element In the switching power supply device
A DC input switching element having one end connected to either the positive electrode or the negative electrode of the DC power supply, and interrupting DC input from the DC power supply;
A primary winding and a secondary winding magnetically coupled to the primary winding, wherein one end of the primary winding is connected to either the positive electrode or the negative electrode of the DC power supply and the other end is the DC input A first transformer connected to the other end of the switching element;
A primary winding and a secondary winding magnetically coupled to the primary winding, the primary winding between one of the load connection lines connecting the secondary winding of the first transformer and the load A second transformer into which the winding is inserted;
A first gate drive circuit for controlling conduction of the main switching element inserted in one or the other of the load connection lines based on a voltage generated in the secondary winding of the first transformer;
One end is connected to one end of the secondary winding of the first transformer and the other end is connected to one end of the main switching element, or one end is connected to the other end of the main switching element and the other A sub-switching element connected in any one of the configurations in which the end is connected to the other end of the secondary winding of the first transformer;
A control circuit that generates and outputs a control pulse signal based on an output voltage supplied to the load;
A second control circuit for controlling conduction of the sub-switching element based on an output voltage supplied to the load, a voltage generated in a secondary winding of the second transformer, and a control pulse output from the control circuit; A gate drive circuit;
The DC input switching element comprises a primary winding to which a control pulse output from the control circuit is applied and a secondary winding magnetically coupled to the primary winding, and the DC input switching element based on a voltage generated in the secondary winding A third transformer for controlling
A switching power supply device comprising:   The switching power supply device according to claim 1, wherein a pulse width of a control pulse output from the control circuit is substantially constant.   The switching power supply device according to claim 1, further comprising a delay circuit between the control circuit and the third transformer.   4. The switching power supply device according to claim 2, wherein the control circuit varies a cycle of the control pulse based on an output voltage supplied to the load. 5. 5. The switching power supply device according to claim 1, wherein the control circuit includes an astable multivibrator and a monostable multivibrator. 6.

Images