JP2002369495A - Drive circuit for voltage-driven element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively discharge the gate charges of a voltage-driven element (IGBT). SOLUTION: When the pulse outputted from a control circuit 102 shifts from high level to low level, a gate voltage Vg of the IGBT is lower than a reference voltage Vref. When the IGBT starts to turn off from an incompletely turned-on stage, transistors Q2 and Q3 are turned on to cause resistors R1 and R2, to make the gate charges of the IGBT discharged. When the IGBT starts to turn off from a completely turned-on state, the collector voltage Vc of the IGBT is monitored, and when the voltage Vc is lower than a prescribed value, the transistors Q2 and Q3 are turned on, to cause the resistors R1 and R2 to discharge the gate charges. When the collector voltage Vc becomes the prescribed value or higher as the IGBT turns off, the transistor Q2 is turned off, to only cause the resistor R2 to make the gate charged. Consequently, the gate charges can be discharged quickly, regardless of the width of the pulse and, in addition, the occurrence of a surge voltage can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for turning on and off a voltage-driven element, and more particularly to a circuit capable of effectively suppressing a surge voltage generated when the voltage-driven element is turned off.

[0002]

2. Description of the Related Art A field effect transistor having a MOS gate structure and an insulated gate bipolar transistor (IGBT)
Such a voltage-driven element can perform a high-speed switching operation, but since a high surge voltage is generated at the time of current interruption due to an inductance component present in a circuit, its suppression is necessary. Conventionally, in order to suppress the generation of a surge voltage, a method of gently discharging the gate charge when the voltage-driven element is turned off to reduce the switching speed has been used, but there is a problem that the switching loss is increased.

In order to solve the problem of switching loss, for example, in Japanese Patent Application Laid-Open No. Hei 6-291631, while the gate charge is rapidly discharged at the beginning of turn-off, the collector or drain voltage rising with the discharge is monitored. When the interruption of the current is detected and the interruption of the current is started, a method of slowing down the discharge speed of the gate charge is adopted to suppress the generation of the surge voltage without increasing the switching loss.

[0004]

However, when the pulse width of the supplied control signal is small, the voltage-driven element may shift from a state where it is not completely on to an off state. For example, in the case of driving with a pulse signal having a small width in order to obtain a small current by PWM control, the voltage-driven element shifts to a turn-off operation before being completely turned on. At this time, the collector or drain voltage is higher than a predetermined voltage. In the configuration in which the collector or drain voltage is monitored as described above and the discharge speed of the gate charge is determined, the gate charge is discharged at a slow speed from the beginning of turn-off. As a result, there is a problem that the turn-off time becomes longer than a desired time, and the switching loss becomes large. The present invention has been made in view of the above-mentioned conventional problems, and provides a drive circuit capable of rapidly discharging gate charges of a voltage-driven element even when a control signal having a small pulse width is supplied to the voltage-driven element. It is an object.

[0005]

For this purpose, the invention according to claim 1 has a first terminal, a second terminal and an insulating gate, the first terminal being connected to a power supply via a load, and the second terminal being connected to ground. A driving circuit for driving a voltage-driven element connected thereto, the control circuit outputting a control signal to an insulated gate of the voltage-driven element, and a first discharging speed or The voltage-driven element is completely turned on by comparing the discharge means for discharging at a second discharge rate lower than the first discharge rate with the voltage of the insulating gate or the first terminal of the voltage-driven element and the reference voltage. Comparing means for judging whether the state is turned on or not fully turned on, and holding for holding the judgment result of the comparing means when the voltage-driven element starts turning off based on a control signal from the control means. Means When the holding means holds a determination result indicating that the voltage-driven element is not completely turned on, the discharging means discharges at the first discharge rate, and the voltage-driven element is completely discharged to the holding means. When the determination result indicating the ON state is held, while the voltage of the first terminal of the voltage-driven element is smaller than a predetermined value,
A discharge control means for causing the discharge means to start discharging at a first discharge rate and discharging at a second discharge rate when the voltage of the first terminal becomes a predetermined value or more.

According to a second aspect of the present invention, the discharging means comprises the first
A discharge circuit having a first discharge circuit and a second discharge circuit when the discharge means discharges at a first discharge rate; When the discharge is performed, the discharge is performed in the second discharge circuit.

According to a third aspect of the present invention, the first discharge circuit includes a first resistor and a first transistor connected between an insulated gate of the voltage-driven element and a ground. A two-discharge circuit, a second resistor connected between an insulated gate of the voltage-driven element and ground, and a second
And the resistance value of the first resistor is smaller than the second resistor.

According to a fourth aspect of the present invention, the holding means includes:
The flip-flop circuit is configured such that an output of the voltage comparison means is input to a data terminal of the flip-flop, and a pulse signal is input to the clock terminal from the control means.

[0009]

According to the first aspect of the present invention, when the voltage-driven element starts to turn off, whether the voltage-driven element is completely turned on or not turned on by the holding means is determined. In the case where the turn-off is started based on the held state, for example, from a state where the voltage-driven element is not completely turned on, the gate charge is quickly discharged at the first discharge speed. So
Does not increase switching loss.

According to the second aspect of the present invention, the discharging means for discharging the gate charge has a first discharging circuit and a second discharging circuit, and the first discharging circuit and the second discharging circuit at the time of discharging at the first discharging speed.
Since the discharge is performed by the discharge circuit and the discharge at the second discharge speed is performed only by the second discharge circuit, the discharge speed can be changed only by switching the first discharge circuit, so that the circuit is simply configured. be able to.

According to the third aspect of the present invention, since the resistance value of the first resistor is smaller than that of the second resistor, the discharge speed of the first discharge circuit including the first resistor is faster than that of the second discharge circuit. Therefore, when the discharge speed is reduced and the discharge is performed only by the second discharge circuit, the discharge speed is greatly reduced, and the surge voltage can be effectively suppressed.

According to the fourth aspect of the invention, since the holding means is constituted by a flip-flop circuit, the output of the voltage comparing means is input to the data terminal of the flip-flop, and the pulse signal is input to the clock terminal from the control means. if,
The state of the voltage-driven element at the start of turn-off can be maintained by one component, and the circuit configuration is simplified.

[0013]

Next, embodiments of the present invention will be described with reference to examples. FIG. 1 is a diagram illustrating a drive circuit that drives a voltage-driven element. In FIG. 1, an IGBT is used as a voltage-driven element. IGBTQ1
Is a collector (first terminal) connected to the power supply Vb via the load 101.
And the emitter (second terminal) is connected to the ground.

As a drive circuit, resistors R5, R1, R2 are connected in parallel to the gate (insulated gate) of IGBT Q1. R5 is connected to the power supply Vcc via an NPN transistor Q5, the resistor R1 is connected to the ground via a PNP transistor Q2, and the resistor R2 is connected to the ground via a PNP transistor Q3. Here, the resistance R1 has a smaller resistance value than the resistance R2.

The bases of transistors Q5 and Q3 are connected to the output terminal of control circuit 102. Transistor Q5 is turned on when the output of control circuit 102 is at H level, and transistor Q3 is turned on when the output of control circuit 102 is at L level. It has become. The base of the transistor Q2 is connected to the output terminal of the control circuit 102 via the diode D3, and at the same time, to the power supply Vcc via the resistor R3, and to the collector of the IGBT Q1 via the resistor R4 and the diode D2. The base of the transistor Q2 is an NPN transistor Q
4 is grounded to the ground.

The gate of the IGBT Q1 is a comparator 10
3, where the gate voltage Vg is compared to a reference voltage Vref. The output terminal of the comparator 103 is connected to the data terminal D of the flip-flop circuit 104. Since the clock terminal CK of the flip-flop circuit 104 is connected to the output terminal of the control circuit 102, a comparison result between the gate voltage Vg and the reference voltage Vref is held when the output of the control circuit 102 falls. . An output terminal of the flip-flop circuit 104 and an output terminal of the control circuit 102 are connected to respective input terminals of the NOR gate 105, and an output terminal of the NOR gate 105 is connected to a base of the transistor Q4.

Next, the operation will be described. 2 and 3
FIG. 5 is a diagram showing changes in the voltage and current of the IGBT Q1 during the operation of the drive circuit. FIG. 2 shows the IGBT Q1
FIG. 3 shows a case where a signal having a pulse width shorter than the time required for completely turning on the IGBT is output.
2 and 3, Vin is the output of the control circuit 102, Vg is the gate voltage of the IGBT Q1, and Ic is the IGBT
The collector current Vc flowing through Q1 is a collector voltage generated in IGBT Q1.

First, a description will be given of a case where the device is turned off from a completely on state. When the IGBT Q1 is completely turned on, as shown in FIG.
Since g is higher than the reference voltage Vref, an H-level signal is applied to the D terminal of the flip-flop circuit 104. When the output of the control circuit 102 changes from the H level to the L level at the time t1, the transistor Q5 is turned off and the transistor Q3 is turned on. At this time, since the gate voltage Vg of the IGBT is equal to or higher than the reference voltage Vref, the output of the flip-flop circuit 104 is held at the H level. Thus, the output of NOR gate 105 is at L level, and transistor Q4 is off.

At the start of turn-off, IGBT Q1
Since the collector voltage Vc of the transistor Q2 is low, the transistor Q2 initially has a resistor R4, a diode D2,
By the base current flowing through IGBT Q1,
It turns on. Therefore, at the beginning of the turn-off, both the transistors Q2 and Q3 are on, and the resistors R1 and R3 are turned on.
The gate charge of the IGBT Q1 is discharged with a small resistance value in parallel with the two. Therefore, the gate charge is rapidly discharged.

As the gate charge of IGBT Q1 is discharged, at time t2, when IGBT Q1 starts to cut off collector current Ic, collector voltage Vc increases. Time t
3, resistors R3 and R3 between the collector voltage Vc and the power supply Vcc.
When the relationship between the base potential of the transistor Q2 determined by the divided voltage of the transistor 4 and the gate voltage of the IGBT Q1 is reversed,
The transistor Q2 is turned off, and thereafter the gate charge of the IGBT Q1 is discharged only by the transistor Q3 via the resistor R2. Since the resistance value of the resistor R2 is larger than that of the resistor R1, the discharge speed is suddenly reduced.

Therefore, the rate of change (di / dt) of the current when the current is completely cut off is reduced, and the surge voltage generated at the collector of IGBT Q1 is suppressed. In this way, the gate charge is rapidly discharged at the beginning of turn-off, and when the current starts to be cut off, the discharge speed is slowed down.
The effect of suppressing the turn-off delay while suppressing the surge voltage can be obtained.

Next, an operation of turning off the IGBT from a state where it is not completely turned on will be described with reference to FIG. FIG. 3A is a diagram showing changes in the current and voltage of the IGBT during the operation of the drive circuit.
(B) is a comparative example similar to the conventional example, in which the comparator 103, the flip-flop circuit 104, the NOR gate 105, and the current with the transistor Q4 removed from FIG.
The change in voltage is shown.

First, a description will be given of the comparative example shown in FIG. 2B. At time t2 when the turn-off starts, the collector voltage Vc 'of the IGBT Q1 does not decrease and the operation shifts to the turn-off operation, so that the transistor Q2 remains off. , The gate charge is discharged only by the resistor R2 from the beginning of the turn-off operation. For this reason, the time required to cut off the collector current Ic 'is greatly delayed, and as a result, the time for energizing the load greatly exceeds the desired pulse width. This means that when controlling the load by PWM control or the like, accurate control is difficult in a small current region.

On the other hand, in this embodiment, as shown in (a), first, when the IGBT Q1 is off, Vg is zero and the collector voltage is at the maximum value. At time t1, when the output of the control circuit 102 transitions from the L level to the H level,
When the transistor Q5 is turned on, charging of the gate of the IGBT Q1 is started via the resistor R5, and the gate voltage Vg increases. Then, the collector current Ic is supplied to the IGBT.
Flows, and the collector voltage Vc starts to decrease.

Since the pulse width T is shorter than the time required for turning on the IGBT Q1, before the gate voltage Vg reaches the reference voltage Vref, the control circuit 1 is turned on at time t2.
02 changes to the L level. At this time, an L-level signal is applied to the D terminal of the flip-flop circuit 104, so that the output of the flip-flop circuit 104 is low.
Retained on level. Since the output of the control circuit 102 and the output of the flip-flop circuit 104 are both at L level, the output of the NOR gate 105 is at H level. Therefore, transistor Q4 is turned on and transistor Q4 is turned on.
2 is also turned on.

Since the transistor Q3 is turned on when the output of the control circuit 102 goes to the L level, both the transistors Q2 and Q3 are turned on during the turn-off operation regardless of the collector voltage Vc of the IGBT Q1. The gate charge of the IGBT Q1 is discharged with a small resistance value in parallel with the resistances R1 and R2. At this time, the cutoff of the collector current Ic does not become gentle, but the cutoff current itself is small, so that the current change rate (di / dt) is small, and the generated surge voltage does not increase.

As described above, in this embodiment, the gate charge of the voltage-driven element is rapidly discharged even with a small pulse width.
Also, since the circuit has a very simple configuration,
It can be realized at low cost. In this embodiment, the transistor Q2 and the resistor R1 are connected to the first transistor of the invention.
A discharge circuit is formed, and the transistor Q3 and the resistor R2 are connected to the second
It constitutes a discharge circuit. Further, the comparator 103 constitutes a voltage comparison unit, and the flip-flop circuit 104 constitutes a holding unit. Then, a signal line from the control circuit 102 to the bases of the transistors Q2 and Q3, the resistors R3 and R4, the diode D2, and the NOR gate 10
5. The transistor Q4 constitutes a discharge control means.
Although the IGBT is used as the voltage-driven element in the present embodiment, a field effect transistor having a MOS gate structure can be used.

In the above embodiment, the gate voltage V of the IGBT
Using g, the comparator 103 determines whether the IGBT is completely turned on by comparing it with the reference voltage Vref, and determines the timing for turning on the transistor Q2. As a modification, as shown in FIG. In comparison with the comparison between the collector voltage Vc and the reference voltage Vref *, the IGBT
By judging whether or not Q1 is completely turned on, the timing for turning on the transistor Q2 can be determined. In this case, the collector voltage Vc is connected to the minus terminal of the comparator 403, and the reference voltage Vref * is connected to the plus terminal. Other configurations are the same as those in FIG. The same effect can be obtained by this.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment.

FIG. 2 is a diagram illustrating changes in current and voltage of an IGBT when the IGBT is turned off from a completely on state.

FIG. 3 is a diagram showing changes in current and voltage of the IGBT when the IGBT is turned off from a state where it is not completely turned on.

FIG. 4 is a diagram showing a modification.

[Explanation of symbols]

 Reference Signs List 101 load 102 control circuit (control means) 103 comparator (comparison means) 104, 404 flip-flop circuit (holding means) 105 NOR gate R1 resistance (first resistance) R2 resistance (second resistance) Q1 IGBT (voltage driven type) Element) Q2 PNP transistor (first transistor) Q3 PNP transistor (second transistor)

Claims (4)

[Claims] 1. A drive circuit having a first terminal, a second terminal, and an insulated gate, the first terminal being connected to a power supply via a load, and the second terminal being connected to ground, for driving a voltage-driven element. Controlling means for outputting a control signal to the insulated gate of the voltage-driven element; and controlling the charge of the insulated gate of the voltage-driven element to a first discharge rate or a second discharge rate lower than the first discharge rate. Discharging means for discharging at
The voltage of the insulated gate or the first terminal of the voltage-driven element is compared with a reference voltage to determine whether the voltage-driven element is completely turned on or not completely turned on. Comparison means, based on a control signal by the control means, holding means for holding the determination result of the comparison means when the voltage-driven element starts to turn off,
When the holding means holds the determination result indicating that the voltage-driven element is not completely turned on,
When the discharging means discharges at the first discharging rate and the holding means holds a determination result indicating that the voltage-driven element is completely turned on, the voltage-driven element is turned off. While the voltage at one terminal is smaller than a predetermined value, the discharging means starts discharging at the first discharging speed. When the voltage at the first terminal becomes higher than the predetermined value, the discharging means starts discharging at the second discharging speed. And a discharge control means for causing the voltage-driven element to drive. 2. The discharge means includes a first discharge circuit and a second discharge circuit, and the discharge control means includes a first discharge circuit and a second discharge circuit for discharging the discharge means at the first discharge rate. 2. The driving circuit according to claim 1, wherein the discharging is performed by the second discharging circuit, and when the discharging is performed at the second discharging speed, the discharging is performed by the second discharging circuit. 3. The first discharge circuit includes a first resistor and a first transistor connected between an insulated gate and a ground of the voltage-driven element, and the second discharge circuit includes: A second resistor connected between the insulated gate of the voltage-driven element and the ground, and a second transistor, wherein the resistance value of the first resistor is smaller than the second resistor; 3. The driving circuit for a voltage-driven element according to claim 2, wherein: 4. The holding means is constituted by a flip-flop circuit such that an output of the voltage comparing means is input to a data terminal of the flip-flop and a pulse signal is input to the clock terminal from the control means. 4. The driving circuit for a voltage-driven element according to claim 1, wherein the driving circuit comprises: