JPS5812832B2 - transistor drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention speeds up the turning-off speed of switching power transistors used in DC-DC converters, etc.
This invention relates to a circuit that suppresses loss in switching power transistors.

Switching equipment used in DC-DC converters, etc.
In order to reduce loss in the base circuit, power transistors employ a so-called current drive method, in which a base current proportional to the collector current is fed back using a current transformer.

The conventional system is configured as shown in FIG.

In the figure, 1 is a DC power supply, 2 is a load, 3 is a current transformer, 4 is a switching power transistor, 5 is a control transistor, 6 is a diode, 7 is a control DC power supply, and 8 is a resistor.

n1, n2, and n3 are the respective numbers of turns of the current transformer 3.

The operation of this circuit will be explained.

It is assumed that the switching power transistor 4 is turned on because the base current is fed back by the current transformer 3.

If the collector current is i4, the base current is n1/n
It is 2.14.

At this time, transistor 5 is off.

Transistor 5 is turned on to turn off switching power transistor 4.

Since the n3 winding is short-circuited by the transistor 5 and diode 6, the base current jB becomes zero.

Therefore, i4 also becomes zero.

Thereafter, the DC power supply 7 is applied to the winding n3 via the resistor 8, so that an excitation current begins to flow.

This resets the core of the current transformer.

When the transistor 5 is then turned off, the energy stored in the current transformer 3 is released to the base of the switching power transistor 4, which turns on again and remains on due to current feedback.

In this circuit, when transistor 5 is turned on, the base and emitter of the switching power transistor are simply shorted, so the release of carriers accumulated in the base layer is delayed, and the fall time of the switching power transistor is lengthened. , has the disadvantage of increased loss.

The present invention proposes a switching system to eliminate these drawbacks.
The purpose of this invention is to forcibly apply a reverse bias voltage when the power transistor is off, thereby shortening the fall time and reducing the loss in the switching power transistor.The present invention will be explained in detail with reference to the drawings below. do.

FIG. 2 shows an embodiment of the transistor drive circuit of the present invention, in which 1 is a DC power supply, 2 is a load, 3 is a current transformer, 4 is a switching power transistor, 5 is a control transistor, and 6 is a transistor drive circuit according to the present invention. is a diode, and 7 is a control DC power supply, which are the same as in FIG.

9 is a reverse bias transistor. Also n1, n2
, n3 is the number of turns of each winding of the current transformer 3.

Thus, the load 2, the winding n1 of the current transformer 3, and the power transistor 4 are connected in series to the DC power supply 1, the winding n2 is connected between the base and emitter of the transistor 4, and the winding n3
A control transistor 5, a control DC power supply 7, and a reverse bias transistor 9 are connected in series to the transistor 5, and a diode 6 is connected between the emitters of the transistors 5 and 9.

FIG. 3 shows waveforms for explaining FIG. 2.

i4 is the collector current of the switching power transistor 4, iB is the base current of the switching power transistor 4, i9 is the collector current of the reverse bias transistor 9, i5 is the collector current of the control transistor 5,
i6 is the current of the diode 6, and E7 is the terminal voltage of the control DC power supply 7.

Next, the operation will be explained.

At time t0, the control transistor 5 is turned off, the switching power transistor 4 is turned on, and the collector current i4
flows, and the base current iB (iB=n1/
n2・i4) flows.

As a result, power is supplied to the load 2.

When the control transistor 5 and the reverse bias transistor 9 are turned on simultaneously at time t1, the voltage of the control power supply 7 is applied to n3.

At this time, since the beginning of each winding of the current transformer 3 is as shown in FIG.
is applied, the base current iB is the third
Reverse direction as shown.

When all the carriers accumulated at the base of switching power transistor 4 are removed at time t2, switching power transistor 4 is completely turned off.

Therefore, the switching power transistor 4 has a high impedance between its base and emitter.

However, since the control transistor 5 and the reverse bias transistor 9 are on, the winding n3 of the current transformer 3 has
An excitation current flows as shown in FIG.

This current increases linearly.

When the reverse bias transistor 9 turns off at time t3,
The current flowing through the winding n3 flows through a closed loop circuit including the control transistor 5 and the diode 6.

This current decreases linearly.

When the control transistor 5 turns off at time t4, the winding n
3 becomes zero, current flows through winding n2, and current transformer 3
The continuity of the magnetic flux in the core is maintained.

This operation causes switching power transistor 4
can be switched.

As described above, since the present invention can sufficiently apply a reverse bias voltage between the base and emitter when the switching power transistor 4 is turned off, the fall time of the switching power transistor 4 can be shortened. Therefore, it has the advantage of reducing loss and improving the efficiency of the switching circuit.

[Brief explanation of drawings]

FIG. 1 is a conventional transistor drive circuit diagram, FIG. 2 is a transistor drive circuit diagram according to an embodiment of the present invention, and FIGS. 3 i4 to i6 are waveform diagrams for explaining the operation of the circuit. 1...DC power supply, 2...Load, 3...
... Current transformer, 4 ... Switching power transistor, 5 ... Control transistor, 6
...Diode, 7...Control DC power supply, 8
...Resistor, 9...Reverse bias transistor.

Claims (1)

[Claims] 1 Configure a series circuit with a first DC power supply, a load, a primary winding of a current transformer having first, second, and third windings, and a switching transistor, and a secondary winding of the current transformer. In the circuit that supplies the base current of the switching transistor and turns the switching transistor on and off using a third winding, two switches and a second DC power supply are connected in series with the third winding. A transistor drive circuit comprising: a diode connected in parallel with a series circuit of the third winding and one of the switches.