JPH01194606A - Mosfet driving circuit - Google Patents

Abstract

PURPOSE:To expand a control range, to enhance a response speed, and to prevent a used transistor from being damaged by providing a transistor circuit on a secondary side of a transformer, discharging the charge of a gate electrode with a flyback voltage, further, providing two transistors on a primary side, connecting them in a push-pull type, and alternately conducting them. CONSTITUTION:When control pulses A and B are, for example, both in a low level, transistors 3 and 4 are turned off. The flyback pulse voltage is generated on a secondary coil side of a pulse transformer 16, from this, the base side of a transistor 2 becomes positive compared with an emitter side, and the transistor 2 is turned on. In addition, the charge in the gate of a MOSFET is discharged through the collector/emitter of the transistor 2, and the MOSFET is turned off. At such a time, since the maximum percentage for one cycle at the ON-time of the transistors 3 and 4 is 50%, the voltage impressed to the transistors 3 and 4 is at most the double value of a supply voltage, and the transistor with a low proof pressure can be sufficient.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to switching regulators, PWM (pulse code modulation), etc. as power sources for various devices.
Power M used for sine wave generation inverter etc.
O3... This relates to an FET drive circuit, and particularly relates to a MOSFET drive circuit with a good control range.

[Conventional technology]

As a drive circuit for MOS FET, (11) "Hitachi Power MOSFET Data Book"
There is one shown in FIG. 6-23, and (2) there is one disclosed in Japanese Patent Application Laid-Open No. 104830/1983.

The conventional circuits mentioned above are shown in FIGS. 3 to 5.

FIG. 3 is a circuit according to the published document (1) above. In the same figure, ■ is a MOSFET, 2 to 4 are transistors,
8 to 13 are resistors, 16 is a pulse transformer, 17.18 is a diode, and 19 is a capacitor.

In FIG. 3, when the control pulse input is set to low level, transistor 4 is turned off and transistor 3 is turned on.
MOSFETI gate C through pulse transformer 16
By supplying a positive voltage to M03FETI, M03FETI is turned on.

On the other hand, when the control pulse input is set to a high level, transistor 4 is turned on and transistor 3 is turned off, and the power supply to the transformer 16 is stopped, so a back electromotive force is output to the transformer winding (so-called flyback voltage is generated). Therefore, the base of transistor 2 becomes positive and transistor 2 is turned on. This allows the MOSFE
The charge stored in the gate of T (the gate of the MOSFET is capacitive) is discharged, and the MOSFET is turned off.

[Problem to be solved by the invention]

The advantage of this circuit is that even if the on/off ratio changes, the MO
The voltage applied to the gate of SFET does not change (MO
(The gate of the SFET needs to be driven at a constant voltage), so the control range is wide to some extent and the control response is fast. However, the disadvantage is that as the on period of transistor 3 becomes longer, the flyback voltage of the transformer described above increases. becomes higher, and the voltage VCE applied between the collector and emitter of the transistor 3 becomes higher. This situation is shown in Figure 6(b). Waveform voltage is shown. Therefore, a transistor with high withstand voltage is required, and if the on time is extremely long and the off time is extremely short, direct current will be applied to the transformer, causing an overcurrent to flow and damaging the circuits such as transistor 3. become.

FIG. 4 is also a conventional circuit, and in this circuit, parts similar to those in FIG. 3 are given the same reference numerals and their explanations are omitted. Also, 5,6
is a transistor, 7 is a photocoupler, and 14.15 is a power supply. The circuit shown in FIG. 4 uses a photocoupler 7 instead of a pulse transformer (for insulation). A drawback of this circuit is that the switching speed is determined by the limited response speed of the photocoupler.

-Statically, the current limit is about several KHz.

However, the switching power supply has a frequency of several tens of KHz to several hundred KHz, and this circuit is not currently employed. In addition, this circuit has a power supply 14 on the MOSFET side.
．． 15 is required, which has the disadvantage of complicating the circuit.

FIG. 5 shows the circuit disclosed in the above-mentioned publication (2). In this figure as well, parts similar to those in FIG. 1 are designated by the same reference numerals and their explanations will be omitted. Further, 20 is a capacitor. This circuit maintains that even if the on/off ratio changes, the diode 1
7, this circuit attempts to charge the capacitor 19 and keep the maximum and minimum values of the voltage applied to the gate G of the MOSFET constant.
As shown in the capacitor voltage waveform, the charging voltage to the capacitor 19 increases. At this time, if the ON time is suddenly shortened, the capacitor 19 will not be fully discharged (capacitor 19
The capacitance must be several times larger than the gate capacitance of the MOSFET. As shown in the gate voltage waveform in d), the gate voltage changes and there is a risk of damage to the MOSFET.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, have a wide dynamic and static control range, have a fast control response speed, prevent damage to MOSFETs and transistors used, and provide a power supply with a simple configuration. The present invention provides a drive circuit for a MOSFET.

[Means to solve the problem]

In order to achieve the above object, the MOSFET drive circuit of the present invention includes a transformer having a primary coil and a secondary coil, and first and second transistors that are connected to the primary coil side in a push-pull type and are alternately conductive. and a MOSFET whose gate electrode is connected to the secondary coil side and conducts when each of the first and second transistors is conductive.
When the first and second transistors connected to the secondary coil side are both non-conductive, the back electromotive force (flyback voltage) of the transformer conducts the MOS.
A transistor circuit is provided for discharging the charge stored in the gate electrode of the FET.

[Effect]

The operation based on the above configuration will be explained.

According to this circuit configuration, like the conventional technology shown in Figure 3 above, a capacitor is not used on the secondary side and the charge on the gate electrode is discharged by a flyback voltage, so the on/off ratio changes. However, the voltage applied to the gate of the MOSFET does not change, and the MOSFET is driven at a constant voltage, resulting in a wide control range and fast control response. Furthermore, two primary-side driver transistors are provided, and these are connected in a push-pull type to alternately turn on (
conduction), and during either of these alternating on periods the MOSF
By configuring the ET to be conductive, the ratio of the on period to the total period of each transistor can be suppressed to at most 50%.

As a result, compared to the prior art shown in FIG.
Even if the total on-time (of the ET) is made sufficiently long, high voltage is not applied to each transistor, so high voltage transistors are not required. In addition, MOS FET is
The charge on the gate electrode is discharged by the flyback voltage, thereby changing from conduction to non-conduction.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

In FIG. 1, l is a MOSFET, 2 to 4 are transistors, 8 to 10 are resistors, 16 is a pulse transformer, and 17a
~17d is a diode. The first and second transistors 3 and 4 are for driving and are connected in a push-pull manner to the primary coil of the pulse transformer 16, and control pulses A and B are alternately applied to these transistors 3 and 4, respectively. They are starting to join. The secondary coil of the transformer 16 has one end coupled to the diode 17c and the resistor 8, and the other end coupled to the diode 17d and the resistor 10, and the coupling point is connected to the gate electrode G of the MOSFET I. The midpoint of the secondary coil of transformer 16 is diode 1
It is connected to the source electrode of MOSFET I via 7e. Both ends of the secondary coil of the transformer 16 are connected to each other by two diodes 17a and 17b connected in series in opposite directions, and a resistor 9 is connected between the connection point of the diodes 172 and 17b and the source electrode of the MOS FET. is connected. Further, the transistor 2 has a base connected to the middle point of the secondary coil, an emitter connected to the source electrode, and a collector connected to the gate electrode.

Next, the operation of this embodiment will be explained.

Now, in FIG. 1, if the control pulse A input is set to high level, transistor 3 is turned on, and pulse transformer 1
Upper end and middle point of secondary winding 6, diodes 17c, 17e
, a positive voltage is supplied to the gate of MOSFET I through resistor 8, and MOSFET I is turned on. On the other hand, when the control pulse A is set to a low level and the control pulse B is set to a high level, the lower end and middle point of the secondary winding and the diodes 17d and 17e are similar to the case described above.

Through the resistor 10, MOSFET I is turned on.

Further, when control pulse A and control pulse B both become low level, transistor 3. Transistor 4 is turned off, and a flyback voltage is generated on the secondary coil side of pulse transformer 16, which causes the base side of transistor 2 to be positive compared to the emitter side, and transistor 2
is turned on, the charge charged in the gate (capacitive) of the MOSFET is discharged through the collector-emitter of the transistor 2, and the MOSFET is turned off. Figure 2 Ha, system? fll pulse A, control pulse B, MOSF
ETI gate voltage waveform (vGs) and collector-emitter voltage waveform applied to transistor 3 or 4 (
'Vex□).

In the circuit of this embodiment, the pulse transformer 16 is driven by two transistors in a push-pull type, and the ratio of the on-time of each transistor to the period of one cycle is at most 50%. The voltage applied to the VCE voltage waveform in Figure 2 is as shown in Figure 2, and the limit is at most twice the power supply voltage (+24V).
Compared to the circuit shown in FIG. 1, a transistor with a lower breakdown voltage may be used.

Also, by alternately inputting two control pulses with different phases,
By changing the on-ratio of each pulse from 0% to 50%, the on-ratio of MOS FET can be changed from 0% to 100%.
It can be changed.

〔Effect of the invention〕

As explained in detail above, according to the MOSFET drive circuit of the present invention, the conventional technology (
In addition to maintaining the advantages of (Fig. 3), the two low-voltage transistors in push-pull configuration can be reduced by up to 50% each.
Just by turning on, the on ratio of M05FET can be changed from 0% to 10
Since it is possible to change it over a range of 0%,
It has excellent effects such as a wide control range and good drive that can be realized with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the drive circuit of the MOS FET of the present invention, FIG. 2 is a voltage waveform diagram of each part of FIG. 1,
3, 4, and 5 are conventional MOSFET drive circuits, and FIG. 6 is a voltage waveform diagram of each part of the conventional circuit. 1...---MOS FET, 2~6---
------ Transistor, 7---Photocoupler, 8-13--One resistor, 14゜15--Power supply, 16--Pulse transformer, 17゜18 −・
---1 diode, 19, 2 (L------
---Capacitor. Figure 1 Figure 2 Figure 4 Figure 5 Figure 6 □ Japanese question

Claims (1)

[Claims] 1. A transformer having a primary coil and a secondary coil, first and second transistors connected in a push-pull type to the primary coil side and conducting alternately, and the transistor having a gate electrode connected to the secondary coil side. A MOSFET that is conductive when each of the first and second transistors is conductive, and a MOSFET that is conductive due to a back electromotive force of the transformer when the first and second transistors connected to the secondary coil side are both non-conductive. 1. A drive circuit for a MOSFET, comprising: a transistor circuit for discharging charges stored in a gate electrode of a MOSFET.