JPS63275222A - Pre-drive circuit - Google Patents

Abstract

PURPOSE:To prevent the malfunction of a power switching element by absorbing an unnecessary drive pulse caused by the residual magnetic energy of a pulse transformer, by turning a transistor (TR) connected to a primary side into ON, and attenuating a pulse caused in a secondary side. CONSTITUTION:When a pulse voltage is going to be produced by the residual magnetic energy of the pulse transformer T, the said pulse voltage is supplied to the TR Q4 through a diode D4 or the diode D5. Here, when the TR Q4 is turned into ON, a current i1 flows from a primary winding P to the diode D4 or the diode D5 the TR Q4 a resistor R4, and the pulse voltage of the primary side is absorbed, and an output from a secondary winding S is prevented from driving the power switching element Q1. Thus, the malfunction of the power switching element due to the unnecessary drive pulse can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a predrive circuit that operates a switching element using a pulse transformer in a DC/DC converter or the like.

(Prior Art) Predrive circuits that drive switching elements used in DC/DC converter circuits via push-pull type pulse transformers are widely used.

The circuit diagram shown in FIG. 2 is a circuit diagram showing an example of this type of predrive circuit, and includes a diode D2. The power switching element Q1 connected through D3 is controlled by the current of the primary winding P which turns on/on alternately the switching element connected in push-pull (P, P), and turns on the current from the input power Vin. / off control and output to Vout. In the figure, D1 is a diode, Ll and C1 are a smoothing choke coil and a capacitor.

(Problems to be Solved by the Invention) In the circuit of FIG. 2 described above, the switching element Q2
, Q3, as shown in the signal waveform shown in the figure, when each is on, current flows through the primary winding P, induces a voltage in the secondary winding S, and drives the power switching element Q+. During this time, the voltage was applied to the pulse transformer T at tl from when the switching element Q2 was turned off to when the switching element Q was turned on, and at t2 from when the switching element Q3 was turned off until the switching element Q2 was turned on. Since the current is cut off, a voltage in the opposite direction is induced in the secondary winding S due to residual magnetic energy. Since diodes D2 and D3 are connected to the secondary winding S, the rectifying action of the diodes D2 and D3 causes even the reverse voltage to become a pulse with the same polarity as the correct drive pulse, which is input to the power switching element Q+, and is input to the power switching element Q+. There is a possibility that the necessary drive pulse may cause the power switching element Q1 to malfunction.

The present invention was made in view of such problems,
The purpose is to provide a predrive circuit which attempts to eliminate the conventional problem by preventing unnecessary drive pulses caused by residual magnetic energy of a pulse transformer.

(Means for Solving Problems) According to the present invention, in a predrive circuit that drives a switching element of a switching converter, a constant voltage circuit of a base circuit using forward voltage drop of a diode, a resistor and A transistor circuit including diodes connected in parallel and a voltage control circuit of an emitter circuit using the forward voltage drop of the diode is connected in parallel to the primary side of a pulse transformer that drives a switching element, and a transistor circuit is connected in parallel to the primary side of a pulse transformer that drives a switching element. A predrive circuit is provided in which the flyback signal causes the transistor circuit to conduct and attenuate the flyback signal.

(Function) In the present invention, a voltage control circuit using a resistor and a diode is provided in the emitter circuit of the transistor connected to the primary side of the pulse transformer, and the current supplied to the switching element is passed. Since the voltage generated in the voltage control circuit increases and the emitter voltage increases,
The transistor does not turn on, so the drive signal from the pulse transformer is not absorbed, and in the case of unnecessary pulses caused by residual magnetic energy, the voltage generated in the voltage control circuit is low, so the emitter voltage is low compared to the base voltage. becomes low, the transistor becomes conductive, and unnecessary pulses flow, which has the effect of attenuating them.

(Example) Next, an example of the present invention will be described in detail using the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a predrive circuit according to the present invention, and the same parts as those shown in FIG.

In FIG. 1, a diode D4. D! 1 are connected to each other to conduct current to the collector of the transistor Q4,
Furthermore, resistors R2 and R8 are connected in series between the collector and base. A parallel circuit of two series-connected diodes Da and Do and a capacitor c2 is provided at the connection point between the resistors R2 and R3, and a primary winding P is connected to the other end of the parallel circuit. is connected to the midpoint of Therefore, the signal occurring at the secondary winding P is supplied via the diode D4 or D5 to the collector of the transistor Q4, and also via the resistor R2 and the diode Dis,
The voltage divided by the series circuit of D9 is applied to the base via the resistor R3. Also, transistor Q
A voltage Vcc, which serves as a power source for the switching elements Q2 and Qs, is applied to the emitter of No. 4, and the power is supplied to the midpoint of the primary winding P through a series circuit of a diode D8 connected to the emitter. In addition, take diode D6,
A resistor R is connected in parallel to the series circuit, forming an emitter bias voltage control circuit.

To explain the operation of the above circuit configured in this way,
When the residual magnetic energy of the pulse transformer T attempts to generate a pulse voltage at time tl or t2 when both switching elements Q2 and Q3 are turned off,
This pulse voltage is supplied to transistor Q4 via diode D4. Then, the current flowing through the diodes Da and De through the resistor R2 flows through the diode D.
A forward voltage of 8 + D is stored in the capacitor C2, and the transistor Q4 is turned on via the resistor R3. Here, when transistor Q4 is turned on, diode D4 or D5 → transistor Q4 →
A current 11 flows through the resistor R4, absorbs the pulse voltage on the negative side, and outputs the output from the secondary winding S to the power switching element Q.
+ is prevented from being driven.

On the other hand, when switching element Q2 or Q3 is turned on and a driving pulse is applied to pulse transformer T, current 12 from power supply Vcc is supplied to primary winding P through resistor R4. Since the current 12 during operation is larger than the current 11 due to the residual magnetic energy, the voltage drop occurring at the resistor R4 having a predetermined resistance value also increases, but since the diodes D6 and D7 are connected in parallel, , and this voltage becomes the bias voltage of transistor Q4. Therefore, the base voltage of this transistor Q4 is equal to the forward voltage of diodes D6 and D9, and the emitter voltage of transistor Q4 is equal to the forward voltage of diodes D6 and D9. Due to the forward voltage of D7, the transistor Q
4 is not turned on, and the drive pulse generated in the pulse transformer T is not absorbed by the transistor Q4.

Although the present invention has been described above using one embodiment, various modifications can be made within the scope of the gist of the present invention, and these are not excluded from the scope of the present invention.

(Effects of the Invention) According to the present invention, unnecessary drive pulses generated by the residual magnetic energy of the pulse transformer are absorbed by turning on the transistor connected to the primary side, thereby attenuating the pulses generated on the secondary side. This has the effect of preventing malfunctions due to unnecessary driving pulses of the element.

Note that a normal driving pulse generated by the switching element connected to the primary side of the pulse transformer has the effect that normal switching operation is performed without being absorbed since the transistor is turned off.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a predrive circuit according to the present invention, and FIG. 2 is a conventional predrive circuit diagram. Q,...power switching element, Q2, Q3...
- Switching element, Q4...transistor, Da. D2. Da, Do...diode, R4...resistance.

Claims (1)

[Claims] In a predrive circuit that drives the switching element of a switching converter, there is a constant voltage circuit in the base circuit that uses the forward voltage drop of a diode, and an emitter that connects a resistor and a diode in parallel and uses the forward voltage drop of the diode. A transistor circuit including a voltage control circuit of the circuit is connected in parallel to the primary side of a pulse transformer that drives a switching element, and a flyback signal generated in the pulse transformer causes the transistor circuit to conduct and attenuate the flyback signal. A predrive circuit characterized by