JPS58220473A - Drive circuit for field effect transistor - Google Patents

Abstract

PURPOSE:To break the high speed of an FET and to reduce the power consumption of a drive circuit by forming the third coil in a transformer, and abruptly discharging the charge of a gate capacity at the time of interrupting the FET. CONSTITUTION:A parallel circuit of the tertiary coil 33 of a transformer 3, a resistor 7 and a capacitor 8, and a switching element 6 are connected in series with a power source 1, the starting end of the coil 33 is connected to the collector of the element 6, and when the element 6 is conducted, a current which reversely excites a magnetic core is flowed. The element 3 is conducted simultaneously upon breaking of the switch element 2, a negative voltage is induced at the secondary coil 32, charge which is stored in the gate capacity of an FET 5 is abruptly discharged through the coil 33, and the FET5 is interrupted at a high speed. The resistor 4 can be sufficiently large or removed, the power consumption can be ingored, and power consumption of the resistor 7 may be sufficiently low, the power consumption of the drive circuit is reduced, the configuration is made simple and the operating characteristics can be preferably adapted for a practical use.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a drive circuit for driving a field effect transistor via a transformer.

Since the gate, which is a control electrode, of a field effect transistor is capacitive, a drive circuit that can charge and discharge a relatively large gate capacitance at high speed is required, especially in order to operate a power field effect transistor at high speed.

FIG. 1 shows an example of a conventional drive circuit for a field effect transistor, where I is a DC power supply, 2 is a switch element, 3 is a transformer, 4 is a resistor, and 5 is a field effect transistor.

Note that the black circle of the winding of the transformer 3 indicates the winding start end.

The DC power supply 1, the first winding 31 of the transformer 3, and the switch element 2 are connected in series, the second winding 32 of the transformer 3 is connected between the gate and source of the field effect transistor 5, and the resistor 4 is connected between the gate and source of the field effect transistor 5.

A signal induced in the second winding of the transformer 3 by the conduction of the switch element 2 is applied to the gate of the field effect transistor 5 to make the field effect transistor 5 conductive, and the switch element 2 is turned on.
The field effect transistor 5 is cut off because the gate signal of the field effect transistor 5 disappears due to the cutoff of the field effect transistor 5 .

FIG. 2 shows waveforms to explain the operation of FIG. 1. Here, an example is shown in which a non-polar transistor is used as the switching element 2, and an eno-insment type MO8 field effect transistor is used as the field effect transistor 5. I will explain about it.

A indicates the input voltage of the switch element 2, B indicates the collector-emitter voltage of the switch element 2, and C indicates the waveform of the gate-source voltage of the field effect transistor 5.

Time t. When switch element 2 becomes conductive, the collector and emitter of switch element 2 are short-circuited, and the voltage of waveform B becomes O.
Therefore, the voltage v1 of the DC power supply 1 is applied to the first winding 31 of the transformer 3, and the voltage v1 of the DC power supply 1 is applied to the first winding 31 of the transformer 3.
A signal voltage v2 higher than the threshold value vth necessary to make the field effect transistor 5 conductive is induced, and is applied between the gate and source of the field effect transistor 5 as shown in waveform C, thereby making the field effect transistor 5 conductive. .

When the switch element 2 is cut off at time t1, the second
A voltage of opposite polarity is induced in the winding 32, but if the gate capacitance of the field effect transistor 5 is large, it takes a long time to discharge the charge stored in the gate capacitance. Therefore, by reducing the value of the resistor 4, the discharge time can be shortened by the time constant determined by the product of the gate capacitance and the resistor 4, and the field effect transistor 5 can be turned off in a short period of time. However, if the resistor 4 is made smaller, the first thing that occurs is the time t when the switch element is conductive. The power consumed by the resistor 4 increases during the period from tl to tl, and secondly, from time t. The current accumulated in the first winding 31 of the transformer 3 during the period C,, tl is discharged for a longer period inversely proportional to the value of the resistor 4, so that the current accumulated in the first winding 31 of the transformer 3 is discharged from the switch element 2 again at time t2.
A problem arises in that the signal cannot be completely emitted by the time it becomes conductive, and the amplitude of the signal driving the field effect transistor 5 decreases as shown in the dotted waveforms B' and C'.

In order to solve these problems, the present invention provides a third winding 33 in the transformer 3, and rapidly discharges the gate capacitance charge of the field effect transistor 5 at the time when the field effect transistor 5 is cut off. The effect transistor 5 is cut off at high speed and the power consumption of the drive circuit for the field effect transistor 5 is reduced.This will be explained in detail below with reference to the drawings.

FIG. 3 is an embodiment diagram showing the configuration of a driving circuit for a field effect transistor according to the present invention, in which the transformer 3 has first, second and third windings 31, 32 and 33, and 6 is a switch. The elements, 7 is a resistor, 8 is a capacitor, and other symbols are the same as in FIG.

The third winding 33 of the transformer 3, the resistor 7 and the capacitor 8
and the switch element 6 is connected in series with the power supply 1, the third winding m33 of which is connected to the transformer 3.
A current flows through the magnetic core to excite it with opposite polarity.

4 is an operating waveform explaining the operation of FIG. 3, where A is the input voltage of the switch element 20 as in FIG. 2, D is the input voltage of the switch element 6, and B' is the voltage between the collector and emitter of the switch element 2. Voltage C' is the gate-source voltage of the field effect transistor 5. Switching element 2 and switching element 6 perform an operation of conducting (but still cutting off) alternately, and waveforms A and D are in a complementary pair relationship. Time t. When the switch element 2 becomes conductive, a voltage V2 is induced at the gate of the field effect transistor 5 as shown by waveform C', causing the field effect transistor 5 to conduct. At this time, since the switch element 6 is cut off, its operation is exactly the same as that shown in FIG. When the switch element 2 is cut off at time t1, the switch element 6 becomes conductive at the same time, so that a negative voltage is induced in the second winding 32 of the transformer 3 and stored in the gate capacitance of the field effect transistor 5. The charges are rapidly released, and the field effect transistor 5 has fast cut-off characteristics. Since the discharge current due to the release of the charge stored in the gate capacitor flows through the series circuit of the capacitor 8, the third winding 33 of the transformer 3, and the switch element 6, the capacitor 8 is rapidly charged at time t1, and then At time t2, the switch element 2 becomes conductive again, and the resistor 7 is turned on until the switch element 6 is cut off.
and capacitor 8 through a parallel connection circuit of transformer 3.
Since the excitation current flows through the third winding 33, the voltage between the third winding gradually decreases, and as a result, the gate-source voltage of the field effect transistor 5 approaches time t2, as shown in waveform C'. The characteristic is that the value approaches zero. When the switch element 6 is cut off at time t2, the charge stored in the capacitor 8 can be released through the resistor 7.

As is clear from the above description, in this embodiment, the electric charge accumulated in the gate capacitance of the field effect transistor 5 cannot be discharged through the third winding 33 of the transformer 3. Unlike the operation shown in FIG. A sufficiently large value is sufficient, or resistor 4
may be removed, the power consumption of the resistor 4 can be ignored. Furthermore, most of the charge accumulated in the capacitor 8 is at time t.
1 is only used to change the gate-source voltage of the field effect transistor 5 from the positive potential (v2 of waveform C') to the negative potential, so the power consumption of the resistor 7 that discharges the charge of the capacitor 8 is also sufficient. A small value is fine. Also, the period during which the switch element 2 is cut off (for example, from time t1 to t
During period 2), the magnetic core of the transformer 3 is excited with the opposite polarity to the excitation current flowing through the third winding 33, so when the switch element 2 is conductive, the gate-source voltage of the field effect transistor 5 becomes more It is clear that there is no decline.

Further, in the above embodiment, the capacitor 8 was connected in parallel with the resistor 7, but one end of the capacitor 8 is connected to the negative polarity side of the DC power supply 1 instead of the positive polarity side, that is, the third terminal of the transformer 3. It is clear that the effects of the present invention do not change even if a capacitor is connected in parallel with the series circuit of the winding 33 and the switch element 6.

As explained above, the present invention uses a three-winding transformer,
Switch elements 2 and 6 are operated alternately to alternately excite the magnetic core of the transformer to opposite polarities, and field effect transistor 5 is
In addition to rapidly discharging the gate capacitance load of the gate capacitor to enable high-speed shutoff, it also aims to reduce the power consumption of the drive circuit.The structure is simple and the operating characteristics are good, making it highly effective for practical use. There is something.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an example of a conventional field effect transistor drive circuit, Fig. 2 is a waveform diagram for explaining the operation of Fig. 1, and Fig. 3 is a diagram showing an example of a field effect transistor drive circuit of the present invention. FIG. 4 is a waveform diagram for explaining the operation of FIG. 3, which is a diagram showing one embodiment of the configuration. 1 ・・・・・・DC power supply, 2, 6 ・・・・・・
...Switch element, 3...Transformer, 4,7 ・Curved resistor, 5...Field effect transistor, 8 ・Per°° capacitor. Figure 1 Figure 2 1oH12

Claims (1)

[Claims] In a field effect transistor drive circuit in which a DC power supply, a first winding of a transformer, and a switch element are connected in series, and a second winding of the transformer is connected between the gate and source of the field effect transistor, The DC power source, the resistor, the third winding provided in the transformer, and another switch element are connected in series, and the third winding and other switch element are connected in parallel with the resistor. A drive circuit for a field effect transistor, characterized in that a capacitor is connected in parallel with the series circuit of the above, and the above switch element and the above other switch element are operated alternately.