JPH11308862A - Switching power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce loss of choke coil during current transformation in relation to red-ease of induced energy, by turning ON the secondary side synchronous commutation element, and forming a parallel circuit of a couple of secondary side synchronous commutation elements when the primary side switch elements are in the OFF condition. SOLUTION: A drain terminal, of transistor TR3 for synchronous commutation is connected to one terminal, except for the center tap of the secondary winding of a main transformer T1, while connecting a drain terminal of transistor TR4 for synchronous commutation to the other terminal. The source terminals of transistors TR3, TR4 for synchronous commutation are connected with each other for output return. One side of the choke coil L1 is connected to the center tap of the secondary side of the main transformer T1, while the other side is used as the output terminal. One electrode of a capacitor C2 is connected to the output and the other electrode is connected to the output return. The output and output return are inputted respectively to a pulse width control circuit PWM. A smoothing circuit is composed of the choke coil L1 and capacitor C2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit, and more particularly to a PWM (Pulse Width Mo).
The present invention relates to a rectifier circuit of an output section of a switching regulator using a push-pull method for stabilizing an output by duration control.

[0002]

2. Description of the Related Art At present, switching power supply circuits are generally required to have high efficiency, small size and light weight in order to save energy and space. In order to meet this demand, as a conventional technique, as shown in FIG. 3, the switching transistors TR11 and TR12 connected to the primary winding of the main transformer T11 are connected to a Pulse Width Mod.
The pulse width control circuit PWM based on the pulsation control alternates the polarity of the input voltage and applies it as a primary voltage to the main transformer T11. The secondary winding of the main transformer T11 has a field effect transistor instead of a rectifying diode. A synchronous rectifier circuit is known which performs on / off control in accordance with the timing of the induced voltage of the secondary winding and performs full-wave rectification through a smoothing circuit including a choke coil L11 and a capacitor C11. In this circuit, the switching transistors TR11, TR1
2. Synchronous rectification transistor TR1 only with the same pulse width as 2.
3, TR14 is turned on, and when one of the synchronous rectification transistors TR13, TR14 is turned on, the other is turned off, so that both are not turned on at the same time. Further, in this circuit, the level shift circuits DRV1 and DRV1 that perform level shift to drive the synchronous rectification transistors TR13 and TR14.
2 required.

[0003]

In the prior art, when the synchronous rectification transistors TR13 and TR14 are both in the OFF state, the synchronous rectification transistors TR13 and TR14 release the induced energy of the choke coil L11.
The choke coil L via the parasitic diode of TR14
11 has a drawback that a large loss occurs in the parasitic diodes of the synchronous rectification transistors TR13 and TR14. Further, since the source voltages of the synchronous rectification transistors TR13 and TR14 are different, the level shift circuits DRV1 and DRV1
V2 is required, and as a result, the circuit of the driving system becomes complicated and large, and the stability of the circuit is lacking.

The present invention has been made in view of the above points, and reduces a loss at the time of current commutation due to emission of induced energy of the choke coil L11, simplifies a drive system circuit, and reduces the size of the drive system. Another object of the present invention is to provide a switching power supply circuit capable of reducing the weight and improving the stability of the circuit.

[0005]

SUMMARY OF THE INVENTION A switching power supply circuit according to the present invention comprises two primary-side switching elements which alternate the polarity of an input voltage and apply the same as a primary voltage to a main transformer; A synchronous rectifying element of a secondary-side output rectifier circuit that performs rectification in synchronization, comprising two field-effect transistors connected to a secondary side of a main transformer, a choke coil, and a capacitor. The driving of the two synchronous rectifying elements is performed by inverting the drive signal of the primary side switching element and connecting to the corresponding synchronous rectifying element, and one of them is turned on. When the primary switch element is both turned off by the drive signal of the first switch element and the drive signal of the second switch element, the other of which is off at the time of By the secondary-side synchronous rectifier element both in the on state, thereby constituting a parallel circuit composed of the two secondary-side synchronous rectifier element.

Further, the switching power supply circuit of the present invention comprises:
Source terminals of two field effect transistors used in the secondary side output rectifier circuit are connected together.

Further, a switching power supply circuit of the present invention
A main transformer having a center tap is connected to each of the two primary terminals except the primary side center tap of the main transformer.
A field-effect transistor used as two primary-side switching elements to be applied to the main transformer as a secondary voltage, and an output and an output return of the secondary voltage as inputs,
Connected to each of the gate terminals of the secondary switch element,
A PW that drives the primary switch elements by a control signal that alternately turns on and off the two primary switch elements
A pulse width control circuit based on M (Pulse Width Modulation) control, and two electric fields each having a drain terminal connected to each of two secondary terminals except a secondary center tap of the main transformer as a secondary synchronous rectifying element. An effect transistor, which is connected to the gate terminals of the two secondary-side synchronous rectifiers, inverts the drive signals of the two primary-side switch elements, respectively, and outputs a corresponding drive signal of the secondary-side synchronous rectifier. A choke coil having one end connected to the secondary center tap of the main transformer and having the other end as an output,
One electrode is connected to the output, the other electrode is connected to the output return, and a capacitor is connected to the input DC power supply. A configuration is adopted in which the source terminals of the two primary-side switch elements are connected to the output return while the source terminals of the secondary-side synchronous rectification elements are both connected.

[0008]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing a switching power supply circuit according to the present invention as one embodiment. In the switching power supply circuit of the present invention, one electrode of the DC power supply E is connected to the center tap of the primary winding of the main transformer T1, and the other electrode of the DC power supply E is connected to the source terminals of the switching transistors TR1 and TR2. I do. Main transformer T1
The drain terminal of the switching transistor TR1 is connected to one of the terminals of the primary winding excluding the center tap,
The other end is connected to the drain terminal of the switching transistor TR2. One of the two outputs of the pulse width control circuit PWM is input to the gate terminal of the switching transistor TR1, and the other is the switching transistor TR1.
2 to the gate terminal.

One of the terminals of the secondary winding of the main transformer T1 except for the center tap is provided with a synchronous rectifying transistor T.
The drain terminal of R3 is connected, and the other terminal is connected to the drain terminal of transistor TR4 for synchronous rectification. The source terminals of the synchronous rectification transistors TR3 and TR4 are connected together to provide an output return. One side of the choke coil L1 is connected to the secondary side center tap of the main transformer T1,
The other is output. One electrode of the capacitor C2 connects to the output and the other connects to the output return. The output and the output return are input to the pulse width control circuit PWM. Note that a smoothing circuit is configured by the choke coil L1 and the capacitor C2.

The drive signal connected to the gate terminal of the switching transistor TR2 is input to the inverting circuit INV2, and the output signal of INV2 is input to the gate terminal of the synchronous rectification transistor TR3. Similarly, the drive signal connected to the gate terminal of the switching transistor TR1 is input to the inverting circuit INV1, and the output signal of INV1 is input to the gate terminal of the synchronous rectification transistor TR4.

The pulse width control circuit PWM is a circuit that monitors the output voltage and sets the ON time of the main switch so that the output voltage becomes the set voltage. The output pulse of the determined ON time is divided into two systems. This is a circuit that outputs alternately. The inverting circuits INV1 and INV2 are circuits that generate an off signal when an input signal is on and an on signal when the input signal is off, and can be configured by a transistor, a general-purpose buffer IC, or the like.

Next, the operation of the thus configured switching power supply circuit of the present embodiment will be described.

In the switching power supply circuit of the present invention, the switching transistors TR1 and TR2 are alternately turned on and off by an output signal of the pulse width control circuit PWM which is turned on and off alternately, so that the secondary winding of the main transformer T1 is connected. The induced voltage is full-wave rectified by the synchronous rectification transistors TR3 and TR4 and the smoothing circuit and output.

Next, the operation of each of the transistors TR1 to TR4 according to the control signal of the pulse width control circuit PWM will be described in detail with reference to the gate drive timing chart of FIG.

First, in the period of t1, the pulse width control circuit PW
In response to the M control signal, the switching transistor TR1
Is turned on, and power is transmitted to the secondary side via the main transformer T1. At this time, the pulse width control device PWM
Since the switching transistor TR2 is controlled to be turned off, the synchronous rectification transistor TR3 is turned on when the control signal is inverted by the inverting circuit INV2. As a result, part of the power transmitted to the secondary side via the transformer T1 is stored in the choke coil L1.

In the period of t2, both the switching transistors TR1 and TR2 are controlled to be turned off, and the supply of power from the primary side to the secondary side is stopped. At this time, the synchronous rectification transistors TR3 and TR4 are connected to the inverting circuit INV.
1, both are turned on by the inversion of the control signal by INV2, and the induced energy of the choke coil L1 flows through the parallel circuit composed of the synchronous rectification transistors TR3 and TR4.

In the period of t3, the pulse width control circuit PWM
, The switching transistor TR2 is turned on, and power is transmitted to the secondary side via the main transformer T1. Since the pulse width control device PWM controls the switching transistor TR1 to be turned off, the synchronous rectification transistor TR4 is connected to the inverting circuit INV1.
Is turned on by the inversion of the control signal by. As a result, part of the power transmitted to the secondary side via the transformer T1 is stored in the choke coil L1.

In the period of t4, the switching transistors TR1 and TR2 are both controlled to be turned off, and the supply of power from the primary side to the secondary side is stopped. At this time, the synchronous rectification transistors TR3 and TR4 are connected to the inverting circuit INV.
1, both are turned on by the inversion of the control signal by INV2, and the induced energy of the choke coil L1 flows through the parallel circuit composed of the synchronous rectification transistors TR3 and TR4.

From t1 to t4, a one-cycle operation is performed.
The same operation as t1 is repeated again from t5.

As described above, by turning on the synchronous rectification transistors TR3 and TR4 during the periods t2 and t4, the choke coil L1 is induced by the parallel circuit of the field effect transistors used as the synchronous rectification transistors TR3 and TR4. Energy can be transferred,
Power loss during commutation can be reduced.

Further, since the driving of the field effect transistor has a property that it can be turned on and off by a gate voltage based on the source voltage of the field effect transistor, the source terminals of the synchronous rectification transistors TR3 and TR4 are connected together. As a result, the voltage references for driving them are the same, and the level shift required when the source voltages are different is not required.

In another embodiment of the present invention, the basic configuration is as described above. However, even in a bridge type switching regulator, the same result as that of the present invention can be obtained by making the secondary side configuration the same. can get.

[0024]

The commutation of the induced energy of the choke coil L1, which has been conventionally performed by the parasitic diode of the field effect transistor, is performed through the parallel circuit of the field effect transistor, as is generally known. Since the loss is smaller when a field-effect transistor is used than when a diode is used, power loss during commutation can be reduced.

Further, since the source terminals of the synchronous rectification transistor are connected together, the level shift required when the source voltage of the synchronous rectification transistor is different is not required, so that the circuit can be simplified and the source can be further reduced. When the voltages are the same, the influence of noise or the like is reduced, and the drive circuit can be stabilized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a switching power supply circuit according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating gate drive timing of a switching power supply circuit according to an embodiment of the present invention.

FIG. 3 is a diagram showing a conventional switching power supply circuit.

[Explanation of symbols]

E DC power supply T1, T11 Main transformer TR1, TR2, TR11, TR12 Switching transistor TR3, TR4, TR13, TR14 Synchronous rectification transistor L1, L11 Choke coil C1, C11 Capacitor INV1, INV2 Inverting circuit PWM Pulse width control circuit DRV1, DRV2 Level shift circuit

Claims (3)

[Claims] 1. In a push-pull switching power supply circuit, two primary-side switch elements for alternating the polarity of an input voltage and applied as a primary voltage to a main transformer, and in synchronization with the primary-side switch element An element for synchronous rectification of a secondary-side output rectifier circuit for performing rectification, wherein the two field-effect transistors are connected to the secondary side of a main transformer;
And a smoothing circuit composed of a choke coil and a capacitor and connected to the synchronous rectifying element. In driving the two synchronous rectifying elements, a driving signal of a primary-side switch element is inverted, and the corresponding synchronous circuit is inverted. The driving signal is connected to a rectifying element, and when one is on, the other is off.
When both of the primary-side switch elements are turned off by the drive signal of the switch element, the two secondary-side synchronous rectifier elements are both turned on, so that the two secondary-side synchronous rectifier elements are turned off. A switching power supply circuit comprising a parallel circuit. 2. The switching power supply circuit according to claim 1, wherein the source terminals of two field effect transistors used for the secondary side output rectifier circuit are connected together. 3. A main transformer having a center tap and connected to each of two primary-side terminals of the main transformer except for a primary-side center tap. 2 applied to
A field-effect transistor used as two primary-side switching elements; an input and output of a secondary voltage and an output return connected to respective gate terminals of the two primary-side switching elements; PWM (Pulse Width Modulatio) that drives the primary-side switch element by a control signal that alternately turns on and off
n) a pulse width control circuit by control; and two field effect transistors each having a drain terminal connected to each of two secondary terminals except a secondary side center tap of the main transformer as a secondary synchronous rectifying element; Two drive terminals connected to the gate terminals of the two secondary-side synchronous rectifiers and inverting the drive signals of the two primary-side switch elements, respectively, to serve as drive signals of the corresponding secondary-side synchronous rectifiers. An inverting circuit, a choke coil having one end connected to the secondary side center tap of the main transformer and having the other end as an output, and a capacitor having one electrode connected to the output and the other electrode connected to the output return. , And one of the poles of the input DC power supply is connected to the center tap on the primary side of the main transformer, and the other pole is the source terminal of the two primary side switch elements. Is connected, the switching power supply circuit to adopt a configuration that connects to both output return the source terminal of the secondary-side synchronous rectifier element.