JP2013115852A - Overvoltage protection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an overvoltage protection circuit that uses high withstand-voltage elements being easily manufactured and does not need to consider variation in impedance.SOLUTION: An overvoltage protection circuit includes: a drive section 10 that drives a switching element Q1 by a control signal; a differential circuit that is connected between a source and a drain of the switching element and in which a capacitor C2 and a resistor R1 are connected in series; an integration circuit that is connected to both ends of the resistor R1 and in which a resistor R2 and a capacitor C3 are connected in series; and an overvoltage protection section A1, R3, and C4 that determines that a voltage between the drain and source reaches more than or equal to a predetermined voltage when a voltage across the capacitor C3 reaches more than or equal to a reference voltage, and turns on the switching element to clamp the voltage between the drain and the source.

Description

  The present invention relates to an overvoltage protection circuit for protecting a transistor from a surge voltage, and more particularly to a gate drive circuit that drives a gate of a switching element.

  In recent years, the speed of switching operation by an output transistor used for a switching power supply or the like has been increased, and a back electromotive voltage generated by inductance of a load circuit using a solenoid or the like at the time of switching is increased. For this reason, a surge voltage may be applied between the drain and source of the transistor that is open at the time of turn-off, causing breakdown. For this reason, an overvoltage protection circuit for protecting the transistor from the surge voltage is necessary.

  Patent Document 1 describes a semiconductor device for a high-side switch intended to cut off an inductive load at high speed, and a method for suppressing a drain-source voltage of the high-side switch. In particular, a method is disclosed in which an active clamp circuit (active clamp circuit) in which a Zener diode and a diode are connected in series between the drain and gate terminals of the high side switch suppresses the drain-source voltage and protects the high side switch. ing.

  Patent Document 2 describes a power converter that aims to suppress an excessive surge voltage that occurs during high-speed switching of an IGBT or MOSFET to a low value. This power converter employs an active clamp circuit that can be applied to both a high-side switch and a low-side switch. The active clamp circuit in Patent Document 2 can quickly form a negative feedback path and clamp an overvoltage by connecting a capacitor in parallel to a diode in the configuration.

  As described above, the conventional overvoltage protection circuit detects the overvoltage using a Zener diode, a resistor, and a capacitor.

JP-A-8-288817 JP 2001-245466 A

  However, when detecting an overvoltage with a Zener diode, it is necessary to manufacture a Zener diode having a high breakdown voltage. In addition, when an overvoltage is detected by dividing two or more resistors, a loss always increases because a current always flows through the resistors. Furthermore, when two or more capacitors are divided to detect an overvoltage, the detection voltage accuracy is reduced due to impedance variations.

  An object of the present invention is to provide an overvoltage protection circuit that uses a high-breakdown-voltage element that is easy to manufacture and that does not need to take into account variations in impedance.

  In order to solve the above problems, an overvoltage protection circuit according to the present invention is a gate driving circuit that drives a gate of a switching element, a driving unit that drives the switching element by a control signal, and a first of the switching element. A differential circuit connected between one main terminal and a second main terminal, in which a first capacitor and a first resistor are connected in series, and connected to both ends of the first resistor; a second resistor and a second capacitor; And when the voltage of the second capacitor is equal to or higher than a reference voltage, it is determined that the voltage between the first main terminal and the second main terminal is equal to or higher than a predetermined voltage. An overvoltage protection unit configured to turn on the switching element and clamp a voltage between the first main terminal and the second main terminal.

  According to the overvoltage protection circuit of the present invention, the dV / dt of the voltage applied to the first main terminal of the switching element is detected by the differentiation circuit composed of the first capacitor and the first resistor, and is proportional to dV / dt. When the voltage across the first resistor is integrated by an integrating circuit comprising a second resistor and a second capacitor, a voltage proportional to the voltage between the first main terminal and the second main terminal of the switching element is generated in the second capacitor. To do.

  The overvoltage protection unit determines that the voltage between the first main terminal and the second main terminal is equal to or higher than a predetermined voltage when the voltage of the second capacitor is equal to or higher than the reference voltage, and turns on the switching element to turn on the first main voltage. Since the voltage between the terminal and the second main terminal is clamped, overvoltage can be prevented. Therefore, it can be configured with a high-voltage element that is easy to manufacture, and there is no need to consider variation in impedance.

It is a figure which shows the structure of the overvoltage protection circuit which concerns on Example 1 of this invention. It is a timing chart which shows operation | movement of the overvoltage protection circuit which concerns on Example 1 of this invention. It is a figure which shows the structure of the overvoltage protection circuit which concerns on Example 2 of this invention. It is a figure which shows the structure of the overvoltage protection circuit which concerns on Example 3 of this invention.

  Hereinafter, an overvoltage protection circuit according to an embodiment of the present invention will be described in detail with reference to the drawings.

  1 is a diagram illustrating a configuration of an overvoltage protection circuit according to a first embodiment of the present invention. In the overvoltage protection circuit according to the first embodiment shown in FIG. 1, a series circuit of a primary winding P of the transformer T1 and a switching element Q1 is connected to both ends of the DC power supply V1.

  The switching element Q1 is made of a GaN HEMT (High Electron Mobility Transistor). Alternatively, the switching element Q1 may be a bipolar transistor or an IGBT (insulated gate transistor). The switching element Q1 is turned on / off by a control signal from the control circuit 10.

  A series circuit of a diode D1 and a capacitor C1 is connected to both ends of the secondary winding S of the transformer T1. The diode D1 and the capacitor C1 constitute a rectifying and smoothing circuit, rectifying and smoothing the voltage generated at both ends of the secondary winding S of the transformer T1, and supplying a DC voltage to a load (not shown).

  The voltage detection circuit 12 detects the voltage across the capacitor C1, and outputs the detected voltage to the control circuit 10 as a feedback signal FB. The control circuit 10 controls on / off of the switching element Q1 according to the feedback signal FB from the voltage detection circuit 12.

  A differential circuit in which a resistor R1 (first resistor) and a capacitor C2 (first capacitor) are connected in series is connected between the drain (first main terminal) and the source (second main terminal) of the switching element Q1. Is done. The differentiating circuit detects dV / dt of the voltage applied to the drain of the switching element Q1.

  An integrating circuit in which a resistor R2 (second resistor) and a capacitor C3 (second capacitor) are connected in series is connected to both ends of the resistor R1. The integrating circuit integrates the voltage across the resistor R1 proportional to the detected dV / dt, thereby generating a voltage proportional to the drain-source voltage Vds across the capacitor C3 across the capacitor C3.

  The anode and cathode of the diode D3 are connected to both ends of the capacitor C3. The diode D3 is provided to protect the comparator A1.

  The non-inverting input terminal (+) of the comparator A1 is connected to the connection point between the capacitor C3 and the resistor R2 and the cathode of the diode D3, and the positive terminal of the reference power supply V2 is connected to the inverting input terminal (−) of the comparator A1. The The negative electrode of the reference power source V2 is connected to the negative electrode of the DC power source V1, the anode of the diode D3, one end of the capacitor C3, one end of the resistor R1, the one end of the capacitor C4, and the source of the switching element Q1.

  The comparator A1 compares the voltage of the capacitor C3 with the reference voltage of the reference power supply V2, and outputs the H level to one input terminal of the OR circuit OR1 when the voltage of the capacitor C3 becomes equal to or higher than the reference voltage of the reference power supply V2. To do.

  The other input terminal of the OR circuit OR1 is connected to the output terminal of the control circuit 10 and the anode of the diode D2, and the cathode of the diode D2 is connected to the gate of the switching element Q1. The diode D2 is provided to apply a control signal from the control circuit 10 directly to the gate of the switching element Q1.

  The OR circuit OR1 takes the logical sum of the output from the comparator A1 and the output from the control circuit 10. The cathode of the diode D4 and one end of the resistor R3 are connected to the output terminal of the OR circuit OR1, and the anode of the diode D4 and the other end of the resistor R3 are connected to the gate of the switching element Q1. A capacitor C4 is connected between the gate and source of the switching element Q1. The resistor R3 and the capacitor C4 constitute a delay circuit that delays the protection signal from the OR circuit OR1 by the time constant of the resistor R3 and the capacitor C4.

  The comparator A1, the OR circuit OR1, the resistor R3, and the capacitor C4 constitute an overvoltage protection unit.

  Next, the operation of the overvoltage protection circuit according to the first embodiment configured as described above will be described in detail with reference to the timing chart of each part shown in FIG. In FIG. 2, Vds (Q1) is the voltage between the drain and source of the switching element Q1, V (R1) is the voltage across the resistor R1, V (C3) is the voltage across the capacitor C3, A1out is the output of the comparator A1, and Vgs. (Q1) is a voltage between the gate and the source of the switching element Q1, and Id (Q1) is a drain current of the switching element Q1.

  First, when the switching element Q1 is turned off at time t0, the drain-source voltage Vds (Q1) of the switching element Q1 increases from time t0 to time t2. The time from time t0 to time t2 is determined by the time constant of the resistor R1 and the capacitor C2.

  Further, the differentiating circuit composed of the capacitor C2 and the resistor R1 detects dV / dt of the voltage Vds (Q1) applied to the drain of the switching element Q1. For this reason, the voltage V (R1) across the resistor R1 that is proportional to dV / dt of the voltage Vds (Q1) changes as shown in FIG.

  Next, when integration is performed by an integration circuit including the resistor R2 and the capacitor C3, a voltage V (C3) proportional to the voltage Vds (Q1) of the switching element Q1 is generated in the capacitor C3. The voltage V (C3) increases linearly as shown in FIG.

  When the voltage V (C3) of the capacitor C3 reaches the reference voltage V2 at time t1, the output A1out of the comparator A1 becomes H level and is supplied to the resistor R3 via the OR circuit OR1. The resistor R2 and the capacitor C4 apply the H level from the OR circuit OR1 to the gate of the switching element Q1 after being delayed by the time constant of the resistor R2 and the capacitor C4, that is, the time t1 to t2.

  Then, the voltage Vgs (Q1) between the gate and the source of the switching element Q1 increases linearly from time t1 to t2, and when the ON start voltage Vth is reached at time t2, the switching element Q1 is turned on.

  Then, a current Id (Q1) flows between the drain and source of the switching element Q1. For this reason, since the rise of the voltage Vds (Q1) of the switching element Q1 is suppressed and falls, the voltage V (R1) becomes a negative voltage. Along with this, the voltage V (C3) of the capacitor C3 also decreases, and when the voltage V (C3) becomes less than the reference voltage V2 at time t3, A1out of the comparator A1 becomes L level, so that the current Id ( Q1) becomes zero.

  Then, when the voltage Vds (Q1) of the switching element Q1 rises and reaches the reference voltage V2, the output A1out of the comparator A1 becomes the H level. As described above, the intermittent ON operation of the switching element Q1 is performed at time t4. Repeat until. At time t5 to t7, the voltage Vds (Q1) is lowered to a voltage sufficiently lower than the voltage VP, and therefore the switching element Q1 is not intermittently turned on.

  As described above, in the overvoltage protection unit including the comparator A1, the OR circuit OR1, the resistor R3, and the capacitor C4, when the voltage of the capacitor C3 becomes equal to or higher than the reference voltage V2, the voltage Vds (Q1) becomes equal to or higher than the predetermined voltage VP. Since the switching element Q1 is turned on and the voltage Vds (Q1) is clamped to the predetermined voltage VP, overvoltage can be prevented.

  Further, since the current Id (Q1) does not flow during the time t0 to t2 and the time t4 to t8, the loss is reduced. Therefore, a high voltage element that is easy to manufacture can be used, and variation in impedance need not be taken into consideration.

  FIG. 3 is a diagram illustrating the configuration of the overvoltage protection circuit according to the second embodiment of the present invention. The overvoltage protection circuit according to the second embodiment shown in FIG. 3 is characterized in that a Zener diode ZD is provided instead of the capacitor C2 of the overvoltage protection circuit according to the first embodiment shown in FIG.

  The cathode of the Zener diode ZD is connected to the drain of the switching element Q1, and the anode of the Zener diode ZD is connected to one end of the resistor R1. In other words, since the junction capacitance of the Zener diode ZD plays the role of the capacitor C2 by setting the Zener diode ZD in the reverse bias state, the overvoltage protection circuit according to the second embodiment also has the effect of the overvoltage protection circuit according to the first embodiment. Similar effects can be obtained.

  FIG. 4 is a diagram illustrating the configuration of the overvoltage protection circuit according to the third embodiment of the present invention. The overvoltage protection circuit according to the third embodiment shown in FIG. 4 is characterized in that a capacitor C5 is connected in parallel to the Zener diode ZD of the overvoltage protection circuit according to the second embodiment shown in FIG.

  When a desired capacitor value is not sufficient for the junction capacitance of the Zener diode ZD, a differentiation circuit can be configured by connecting the capacitor C5 in parallel to the Zener diode ZD. Accordingly, the same effect as that of the overvoltage protection circuit according to the first embodiment can be obtained in the overvoltage protection circuit according to the third embodiment.

  The present invention can be used for a gate drive circuit.

V1 DC power supply V2 Reference power supply T1 Transformers D1 to D4 Diodes C1 to C5 Capacitors R1 to R3 Resistor ZD Zener diode A1 Comparator Q1 Switching element OR1 OR circuit 10 Control circuit 12 Voltage detection circuit

Claims (4)

A gate drive circuit for driving the gate of the switching element,
A driving unit for driving the switching element by a control signal;
A differential circuit connected between a first main terminal and a second main terminal of the switching element, wherein a first capacitor and a first resistor are connected in series;
An integrating circuit connected to both ends of the first resistor, wherein a second resistor and a second capacitor are connected in series;
When the voltage of the second capacitor is equal to or higher than a reference voltage, it is determined that the voltage between the first main terminal and the second main terminal is equal to or higher than a predetermined voltage, the switching element is turned on, and An overvoltage protection unit for clamping a voltage between the terminal and the second main terminal;
An overvoltage protection circuit comprising: The overvoltage protection unit compares the voltage of the second capacitor with a reference voltage, and outputs a protection signal when the voltage of the second capacitor is equal to or higher than a reference voltage;
A delay circuit for delaying the protection signal from the comparison unit by a predetermined time and applying the delayed signal to the control terminal of the switching element;
The overvoltage protection circuit according to claim 1, further comprising:   The overvoltage protection circuit according to claim 1, wherein the switching element is a GaN HEMT.   4. The overvoltage protection circuit according to claim 1, wherein the first capacitor includes a junction capacitance of a Zener diode. 5.

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