JP2002151989A - Clamp circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a clamp circuit that is provided with a protection function against an over-voltage. SOLUTION: A time setting section 2 is provided to a protection object element D2 in the clamp circuit 1. The time setting section 2 shuts off an input current to the circuit element D2 when an over-voltage is kept applied to the circuit element 2 being the protection object even a lapse of a sufficient time to clamp a voltage to be clamped after an over-voltage is applied to the element D2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clamp circuit for clamping an input voltage against a surge voltage, and more particularly, to a clamp circuit having a protection function against an overvoltage state.

[0002]

2. Description of the Related Art When a switching element is turned off, an instantaneous high voltage is generated due to parasitic inductance of wiring, which is called a surge voltage. The surge voltage may cause damage to the semiconductor element or shorten the life and cause noise. Therefore, a clamp circuit or a snubber circuit is provided as a protection circuit for the switching element to absorb the surge voltage.

FIG. 3 shows a configuration example of a general clamp circuit. The figure shows a transistor, MOS FET, IGBT
Of the power element Q1, the base current supply resistor R1, the bias supply resistor R2, the drive circuit A1 and the signal generator S1, the surge on the collector (drain) voltage (input voltage Vin). In an example in which the clamp circuit 100 is provided to protect the power element Q1 from a voltage, the input voltage Vin of the power element Q1 is clamped so as not to jump over a specific voltage value.

The power element Q1 has a collector (drain)
A signal flowing from the driver to the emitter (source) is supplied to the driving circuit A1.
ON / OFF by the drive pulse signal of the signal generator S1 input to the base (gate) via the resistor R1
A switching element that performs control, and is driven by a drive pulse signal to output a switching output Vout. The clamp circuit 100 is provided to clamp a surge voltage on a collector (drain) voltage (input voltage Vin) of the power element Q1, and the input voltage Vin is a zener voltage of the zener diode D2 and a base-emitter voltage of the power element Q1. Above the clamp voltage determined by
The diodes D1 and D2 are turned on, and a bias is applied to the base (gate) of the power element Q1, and the power element Q1 is turned on.
Is turned on to clamp the input voltage Vin.

The clamp circuit 100 is provided between the base (gate) and the collector (drain) of the power element Q1, and has a configuration in which a diode D2 for clamping and a diode D1 for preventing backflow are connected in series. The clamping diode D2 is a Zener diode, and is connected with the anode on the base (gate) side of the power element Q1 and the cathode on the collector (drain) side. The diode D1 is provided to prevent a current from flowing backward from the base (gate) side of the power element Q1 to the collector (drain) side. The diode D1 is connected to the anode of the power element Q1 in the opposite direction to the Zener diode D2. Collector (drain)
Side and the cathode are connected with the base (gate) side. Zener voltage of Zener diode D2 and power element Q
A value obtained by adding 1 to the base-emitter voltage becomes a clamp voltage of the clamp circuit 100. When the input voltage Vin becomes higher than the clamp voltage, the clamping diode D2 is turned on to clamp the input voltage Vin.

[0006]

As shown in FIG. 3, when the clamp circuit 100 is provided to clamp the input voltage Vin of the switching element Q1 below a specific voltage value to protect the switching element, the clamp circuit 100 When a voltage higher than the clamp voltage is applied to the input voltage Vin as well as the surge voltage, the circuit always operates and a current flows through the Zener diode D2.

In the case of a surge voltage, the voltage applied to the clamp circuit 100 is short because it is a high voltage but transient. However, when an abnormal voltage that is larger than the clamp voltage and equal to or less than the maximum rating of the element is applied as the input voltage Vin due to, for example, a use state of the power supply or a voltage increase due to an abnormality,
The electronic elements of the clamp circuit 100 are always in a state where an overvoltage is applied and a current flows. In such a case, the voltage is lower than the surge voltage, but is not instantaneous as in the case of the surge voltage, but is in a state of being applied to the clamp circuit 100 itself for a long time (always an overvoltage state). Could be.

[0008] When an electronic element is applied for a long time even at a voltage lower than the peak voltage specified in the maximum rating, heat is generated due to loss, resulting in damage and shortening the life of the electronic element.
Therefore, if the clamp circuit 100 is always in an overvoltage state,
A load greater than the permissible loss is applied to the electronic elements constituting the clamp circuit 100, which may damage the electronic elements or shorten the circuit life.

In order to cope with such an overvoltage state at all times, a method of using an electronic element having a large withstand voltage can be considered. However, a semiconductor element having a high rated voltage generally has a large chip size and is expensive, so that the circuit is expensive. Larger and more expensive. In view of the above problems, an object of the present invention is to provide a clamp circuit having a protection function against an overvoltage state.

[0010]

The present invention presupposes a clamp circuit for clamping an input voltage of a switching element from a surge voltage, and includes a clamp means and a time setting means for solving the above problems.

Clamping means When the input voltage becomes equal to or higher than a specific value, the input voltage is clamped. The time setting means sets the clamp means to a non-operating state when a specific time has elapsed since the clamp means started clamping. The time setting unit includes, for example, a switching unit that is provided in series with the clamping unit and switches ON / OFF of a current input to the clamping unit, and a timer unit that turns off the switching unit when the specific time has elapsed. Have.

According to the present invention, when an overvoltage state occurs for a specific time or more after the clamping means starts clamping, the time measuring means puts the clamping means into a non-operation state. As a result, it is possible to prevent the circuit elements constituting the clamp circuit from being broken or deteriorated due to the application of overvoltage for a long time.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of a clamp circuit according to the present embodiment. 1 and 2 show FIG.
The same reference numerals are given to the same components. In FIG. 1, a clamp circuit 1 is shown in FIG.
Input voltage Vin of power element Q1 for the same circuit as
It is provided in order to absorb the surge voltage.

The clamp circuit 1 according to the present embodiment has the same diodes D1 and D2 as the clamp circuit 100 shown in FIG.
A time setting unit 2 is provided to always protect the Zener diode D2 against an overvoltage state. The time setting unit 2 includes a circuit element to be protected (the zener diode D in FIG. 1).
2) against an overvoltage (in the case of FIG.
If the overvoltage remains applied for more than a specific time after the voltage reaches the zener voltage or more, the input current to the circuit element to be protected is cut off.

The surge voltage is equal to the turn O of the power element Q1.
Since the high voltage is instantaneously generated at the time of FF, the clamp circuit 1 only needs to operate during that time to clamp the input voltage Vin, and does not need to be operated thereafter. Since the overvoltage applied to the clamp circuit 1 after the surge voltage is clamped damages the electronic element, the clamp circuit 1 according to the present embodiment is damaged.
In the case where an overvoltage is applied to the clamp circuit 1 for a specific time or more after the surge voltage is generated and the clamp circuit 1 enters the operating state, the time setting unit 2 inputs the electronic element to be protected. The electronic element is protected by interrupting the current to make it inactive.

The time setting section 2 can be realized by various configurations, and one form thereof is shown in FIG. In the time setting unit 2 shown in FIG.
The switching of the clamp operation enabling / disabling element Q2 is performed by changing the gate voltage of the clamp operation enabling / disabling element Q2 that switches ON / OFF of the input to the input terminal 2 by a transient phenomenon of a CR differentiating circuit that receives the input voltage Vin. .

The time setting unit 2 shown in FIG.
An ON / OF of an input to a Zener diode D2, which is a device to be protected, for a clamp operation enabling / disabling device Q2 such as an SFET.
F as a switch for switching, and a capacitor C for timer
1, having a resistor R3 and a Zener diode D3 as constituent elements. The clamp operation enabling / disabling element Q2 is provided between the diodes D1 and D2, and is an N-channel MOS FET.
In this case, the drain is connected to the cathode of the diode D1, and the source is connected to the cathode of the Zener diode D2.

The capacitor C1 and the resistor R3 are provided in series with each other, and one end of the capacitor C1 is connected to the power element Q1.
And the anode of D1 and the other end of a resistor R
3 and the gate of the clamp operation enable / disable element Q2. The resistor R3 has an end opposite to the side connected to the capacitor C1 connected to the cathode of the Zener diode D2. The Zener diode D3 is a diode for protecting the clamp operation enabling / disabling element Q2, is provided in parallel with the resistor R3, and has an anode connected to the cathode of the Zener diode D2 and a cathode connected to the gate of the clamp operation enable / disable element Q2. .

The operation of the circuit shown in FIG. 1 will be described below. The surge voltage generated when the power element Q1 is turned off is superimposed on the input voltage Vin and the clamp voltage (the Zener diode D
2 or more and the voltage determined by the base-emitter (gate-source) voltage of the power element Q1)
A transient current flows through the path of the capacitor C1, the resistor R3, and the zener diode D2. As a result, a voltage is applied to the gate of the clamp operation enabling / disabling element Q2 via the capacitor C1, and the clamp operation enabling / disabling element Q2 is turned on.

When the clamp operation enabling / disabling element Q2 is turned on, the diodes D1 and D2 are simultaneously turned on, and a voltage is applied to the base (or gate) of the power element Q1 so that the collector voltage Vin of the power element Q1 becomes constant. And the collector voltage Vin is clamped. (Equivalent state to the circuit of FIG. 3) However, the gate voltage of the clamp operation enabling / disabling element Q2 gradually decreases as the timer capacitor C1 charges. As a result, the bias voltage is reduced, so that the clamp operation enabling / disabling element Q2 is provided with an overvoltage greater than the clamp voltage applied to the collector voltage even after the time required for absorbing the surge voltage at turn-off has elapsed. Is OFF. Also, when the clamp operation enabling / disabling element Q2 is turned off, the power element Q1 is also turned off because the bias by the clamp operation enabling / disabling element Q2 is eliminated.
The time during which the clamp operation enabling / disabling element Q2 is turned off is determined by the time constant τ of the capacitor C1 and the resistor R3.
Since this time is determined by C1, this time can be freely set by selecting an appropriate value of the capacitor C1 and the resistor R3.

When the power element Q1 is turned on by the drive pulse signal, the drain voltage of the element Q2 for enabling / disabling the clamp operation is reduced, thereby the Zener diode D2
Anode voltage rises. As a result, the capacitor C1 is discharged through the discharge path of the zener diode D2 → the zener diode D3 → the capacitor C1, and returns to the operable state at the next turn-off.

As described above, the clamp circuit 1 according to the present embodiment is designed so that the clamp circuit 1 according to the present embodiment can withstand a constantly overvoltage state such as an abnormal rise of the power supply voltage.
Since unnecessary clamping is not performed in the inactive state, the electronic circuit of the component can be always prevented from being damaged by overvoltage. Although the clamp circuit described so far is of a type in which a voltage is clamped by a zener diode, the present invention is not limited to this type and is similarly applied to other clamp circuits. I can do it.

For example, in FIG. 1, even in a clamp circuit in which a large-capacity capacitor is connected instead of the Zener diode D2 and a surge voltage is clamped by this capacitor, an overvoltage is applied to this capacitor for a specific time or more. In such a case, the circuit element can be protected from destruction or deterioration due to overvoltage by shutting off the circuit and making the clamp circuit inoperative.

[0024]

As described in detail above, the present invention provides
Clamp below a specific value against surge voltage,
The clamp circuit can always be reliably turned off in response to overvoltage. This can prevent the circuit elements constituting the clamp circuit from being destroyed or deteriorated due to the overvoltage.

Further, the present invention can be realized with a simple circuit configuration without requiring a condition determining IC such as a timer and a comparator. Therefore, it can be realized as an inexpensive and compact configuration.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of a clamp circuit according to an embodiment.

FIG. 2 is a diagram illustrating a configuration example of a clamp circuit of the present embodiment illustrating a configuration example of a time setting unit.

FIG. 3 is a diagram showing a configuration of a conventional clamp circuit.

[Explanation of symbols]

 1, 100 clamp circuit 2 time setting section A1 drive circuit D1 diode D2, D3 Zener diode R1, R2, R3 resistor Q1, Q2 switching element S1 signal generator Vcc drive circuit power supply

Claims (5)

[Claims] In a clamp circuit for clamping an input voltage of a switching element from a surge voltage, a clamp means for clamping the input voltage when the input voltage exceeds a specific value, and the clamp means starts clamping. And a time setting means for setting the clamp means to a non-operating state when a specific time has elapsed since the start of the operation. 2. The time setting means according to claim 1, wherein said time setting means is provided in series with said clamping means and is configured to supply a current to said clamping means.
2. The clamp circuit according to claim 1, further comprising a switching unit that switches N / OFF, and a timer unit that turns off the switching unit when the specific time has elapsed. 3. 3. The clamp circuit according to claim 2, wherein the switching unit is a CR differentiating circuit configured by a capacitor and a resistor and having the input voltage as an input. 4. The clamp circuit according to claim 3, wherein the switching unit further includes a zener diode connected in parallel with the resistor. 5. A clamp circuit comprising one or more electronic elements, wherein a voltage of a specific value or more is applied to an electronic element to be protected, which is one of the one or more electronic elements, for a specific time or more. A circuit protection means for interrupting a current flowing through the electronic element to be protected when the electronic circuit is to be protected.