JPH08205533A - Rectifier circuit - Google Patents

Abstract

PURPOSE: To provide a rectifier circuit of a low loss by performing synchronous rectification by using a field-effect transistor for the free wheel element of a switching power supply. CONSTITUTION: The high level of the output voltage (j) of a second comparator 11 is applied to one input of an AND gate, and when the output voltage (i) of a first comparator 10 is changed from a low level to a high level, the output of an AND gate 13 becomes a high level. The output of the AND gate 13 is fed back to the input of an OR gate 12, and when the output voltage (j) of the second comparator 11 becomes a low level, the output level of the AND gate 13 becomes low. Therefore, the drive voltage of a free wheel element 6 can supply a constant gate-to-source voltage even when the reset period of a transformer 4 comes to an end and the voltage of a secondary winding becomes 0V, so that the effect of the low loss of a synchronous rectifier circuit may be obtained.

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, and more particularly to a rectifier circuit that uses a switching element as a rectifier element to perform synchronous rectification.

[0002]

2. Description of the Related Art FIG. 1 is a circuit configuration diagram of a switching power supply using a generally known synchronous rectification circuit, and its operation waveform is shown in FIG. In FIG. 1, 1 is a DC power supply, 2 is a main switching element drive circuit, 3 is a main switching element,
Reference numeral 4 is a transformer, 5 is a rectifying element such as a field effect transistor, 6 is a free wheel element such as a field effect transistor, 7 is a smoothing choke, 8 is a smoothing capacitor, and 9 is a load. In FIG. 2, a is a free wheel.
Gate source voltage of the device 6 for the routing, b is the drain source current of the device 6 for the freewheeling, and c is 2 of the transformer 4.
Next winding voltage, d (2) -e is the drain source current of the rectifying element 5 when the timing of the drive signal of the free wheel element 6 is delayed and the free wheel element 6 Drain source current.

The circuit operation of FIG. 1 will be briefly described. The switching power supply rectifies the input voltage from the DC power supply 1 through the main switching element 3 controlled by the main switching element drive circuit 2 through the transformer 4 and rectifies it by the rectifying element 5, smoothing choke 7 and smoothing. It smoothes by the capacitor 8 for supply, and supplies desired electric power to the load 9. When the main switching element 3 is not conducting, the energy stored in the smoothing choke 7 is rectified by the free wheel element 6 and smoothed by the smoothing capacitor 8 to obtain a desired power. Is supplied to the load 9.

[0004]

The use of a diode for the rectifying element 5 and the freewheeling element 6 in the circuit of FIG. 1 results in a high on-voltage, so that the loss is reduced as shown in FIG. It is common to use a switching element such as a field effect transistor. Regarding the driving method of the free wheel element 6, generally, the secondary winding voltage c of the transformer 4 is used. However, in the circuit of FIG. 1, the reset period of the transformer 4 ends and the secondary winding voltage c becomes 0V.
Then, the gate-source voltage of the free wheel element 6 becomes 0V as shown in FIG. That is, when the secondary winding voltage c becomes 0 V, the free wheel element 6 becomes non-conductive, and the parasitic diode of the free wheel element 6 which is a field effect transistor becomes conductive. That is, after the secondary winding voltage c becomes 0 V, the field effect transistor having a small forward voltage becomes non-conductive, so that the effect of lowering the loss by the synchronous rectification circuit is diminished.

The present invention was devised to solve the above-mentioned problems, and uses a switching element such as a field effect transistor in a switching power supply, and the secondary winding voltage c of the transformer 4 becomes 0V. Even if the free wheel element 6 becomes non-conductive (3), the parasitic diode does not become conductive, and it is an object of the present invention to provide a low loss rectifier circuit by performing synchronous rectification. Further, the drive signal of the free wheel element 6 is formed by the timing of the triangular wave of charging / discharging by the capacitor and the resistor which is the source of the drive signal of the first switching element 3 and the drive signal of the first switching element 3. By doing so, the timing of the drive signal of the free wheel element 6 can be arbitrarily selected, and due to the delay of the timing of the drive signal of the free wheel element 6, d.e in FIG. It is an object of the present invention to eliminate the short-circuit current shown in (1) and to provide the lowest loss rectifier circuit when performing synchronous rectification.

[0006]

FIG. 4 shows a synchronous rectification circuit according to the present invention for achieving the above object, and FIG. 3 shows a configuration diagram of a drive circuit for a free wheel element 6 of a switching power supply. 10
Is a first comparator, 11 is a second comparator, and 12 is an OR gate.
And 13 are AND gates. In FIG. 5, f is the first
, A triangular wave of charging and discharging by a capacitor and a resistance that is a source of a driving signal of the switching element 3, g is an output control amplifier signal, h is a variable voltage, i is an output voltage of the first comparator 10,
j is the output voltage of the second comparator 11, k is the OR gate 12
Output voltage, 1 is the output voltage of the AND gate 13,
The gate-source voltage of the free wheel element 6 is obtained.

[0007]

The present invention provides a low-loss rectifier circuit by performing synchronous rectification by using a field effect transistor for the free wheel element 6 of the switching power supply.
By selecting the drive signal of the wheel element 6 arbitrarily, as shown in FIG.
This makes it possible to eliminate the short-circuit current indicated by d.e.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings. The output voltage of the output control amplifier signal g of FIG. 5 is input to the positive input of the first comparator (4) 10 of FIG. 3, and the triangular wave f is input to the negative input thereof, and the output voltage of the first comparator 10 becomes i. The place where the gate source voltage 1 of the free wheel element 6 becomes high level is determined at the place where it becomes high level.

The variable voltage h of FIG. 5 is input to the positive input of the second comparator 11, and the triangular wave f is input to the negative input thereof, and the output voltage of the second comparator 11 is j. Gate-source voltage 1 of element 6 for freewheel
Where the low level of is decided. The output i of the first comparator 10 is input to the input of the OR gate 12, and the output of the OR gate 12 becomes high level when i is high level.

This signal enters the input of the AND gate 13. At this time, one input of the AND gate contains the high level of the output voltage j of the second comparator 11, and when the output voltage i of the first comparator 10 changes from low to high, AN
The output of the D gate 13 becomes high level. The output of the AND gate is fed back to the input of the OR gate 12, and the output level of the AND gate 13 becomes low because the output voltage j of the second comparator 11 becomes low level. It will be time. As described above, a drive signal for the free wheel element 6 which is an AND gate output can be produced.

[0011]

As shown in 1 of FIG. 5, the drive voltage of the free wheel element 6 is constant even if the secondary winding voltage c becomes 0V after the reset period of the transformer 4 is completed. A source voltage can be supplied. Therefore, the effect of lowering the loss by the synchronous rectification circuit can be obtained.

Further, since the timing of the output voltage j of the second comparator 11 can be arbitrarily selected by the variable voltage h of the positive input of the second comparator 11, the timing of the drive signal of the free wheel element 6 can be set. It can be arbitrarily selected, the short-circuit current shown in d and e of FIG. 2 due to the delay of the timing of the drive signal of the free wheel element 6 can be eliminated, and the lowest loss occurs in the case of performing synchronous rectification. A rectifier circuit can be provided. (5)

[Brief description of drawings]

FIG. 1 is a conventional synchronous rectification circuit in a switching power supply.

FIG. 2 is an operation waveform of each part in a conventional synchronous rectification circuit

FIG. 3 is an element drive circuit for a free wheel according to the present invention.

FIG. 4 is a synchronous rectification circuit according to the present invention.

FIG. 5 is an operation waveform of each part in the present invention.

[Explanation of symbols]

1 DC power supply 2 Main switching element drive circuit 3 Main switching element 4 Transformer 5 Rectifier element 6 Freewheel element 7 Smoothing choke 8 Smoothing capacitor 9 Load 10 First comparator 11 Second comparison 12 OR gate 13 AND gate 14 Free wheel element driving circuit a Gate-source voltage of free wheel element 6 b Free drain element 6 drain Source current (6) c Secondary winding voltage of transformer d d Drain source current of rectifying element 5 when drive timing of free wheel element 6 is delayed e Free wheel Drain-source current of the freewheeling element 6 when the drive timing of the switching element 6 is delayed. F The charge and discharge by the capacitor and the resistor which are the source of the drive signal of the first switching element 3 are performed. Triangle wave g Output control Signal h variable voltage i output voltage of first comparator 10 j output voltage of second comparator 11 output voltage of OR gate 12 output voltage of AND gate 13 and free-wheel Gate-source voltage of the element 6 for gate m Voltage between the gate and source of the rectifying element 5

Claims (3)

[Claims] 1. A series circuit of a DC voltage source and a first switching element is connected to a primary winding of a transformer, and a rectifying element and a freewheel element are provided between a secondary winding of a transformer and a load. In a forward converter with a rectifying circuit consisting of
A rectifier circuit characterized in that the free wheel element is a field effect transistor which is a switching element. 2. The rectifying circuit according to claim 1, wherein the drive signal of the first switching element is formed by comparing a triangular wave of charging / discharging by a capacitor and a resistor with an output control amplifier signal. Rectifier circuit characterized by 3. A rectifier circuit characterized by having a freewheeling element as a rectifier circuit regardless of the rectifier circuit according to claim 1.