Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2001-04-25
2003-10-14
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S058000
Reexamination Certificate
active
06633473
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a technique of stabilizing a gate-voltage control type power semiconductor element, such as an insulated gate bipolar transistor (hereinafter, referred to as IGBT) and a vertical power MOSFET, in an overcurrent limiting.
BACKGROUND ART
Hereinafter, an overcurrent limiting of an IGBT will be discussed, as an example.
FIG. 16
shows an equivalent circuit of a semiconductor device
100
P consisting of an IGBT
1
P and its overcurrent limiting circuit
10
P, as an example of the background art. In an emitter region of the IGBT
1
P of this figure, a plurality of emitter cells are connected in parallel in order to pass a predetermined emitter current (main current) i. Further, the IGBT
1
P incorporates a current detecting element (current sense portion) for detecting the emitter current i as well as a main portion, and outputs a current sense current is from a current sense terminal S connected to the current detecting element. Reference characters G, C and E represent a gate terminal, a collector terminal and an emitter terminal of the IGBT
1
P, respectively.
Though a planar gate type IGBT which is obtained by microfabrication and a trench gate type IGBT have been developed as a high-performance IGBT in recent years, these IGBTs each have a number of channel regions per unit area and supposing a load short circuit state occurs, very large main current flows to increase energy loss, causing significant characteristic deterioration of the element. For this reason, in the IGBT, it is necessary to limit the main current by monitoring the current sense current is and lowering a gate voltage when an overcurrent flows. A monitoring circuit used for this purpose is the overcurrent limiting circuit.
In
FIG. 16
, the overcurrent limiting circuit
10
P having a general configuration is shown. When a voltage between the current sense terminal S and the emitter terminal E, which is the product of the current sense current is similar to the emitter current i and the resistance value R
S
of a sense resistor
3
P becomes equal to or higher than the threshold voltage of a current limiting n-type MOSFET
2
P, the MOSFET
2
P is turned on and electric charges accumulated in a gate region of the IGBT
1
P is bypassed, to lower the gate voltage of the IGBT
1
P and bring the IGBT
1
P into an OFF state, thereby suppressing an increase in main current i. Further, this circuit
10
P has an advantage of changing an overcurrent detection level by changing the resistance value R
S
of the sense resistor
3
P and the threshold voltage of the MOSFET
2
P.
Furthermore, when a reverse bias is applied across the gate and emitter of the IGBT
1
P, as shown in
FIG. 17
, the reverse bias is sustained by providing a diode
8
P between the gate terminal G of the IGBT
1
P and a drain of the MOSFET
2
P.
Further, though the n-type MOSFET
2
P is used as a current limiting transistor in
FIGS. 16 and 17
, a bipolar transistor may be used instead to produce the same effect.
The overcurrent limiting circuit
10
P can achieve a stable overcurrent limiting when the emitter current i and the current sense current is show the same behavior even in a transient state.
On the transition of turnon and turnoff in a switching operation of the IGBT, however, there is a case where these currents i and is do not show the same behavior due to various factors. For example, there are cases (1) where the threshold voltage (Vthm) of the main portion and the threshold voltage (Vths) of the current sense portion are different from each other for internally structural reason of the IGBT and the relation Vthm>Vths is hold and (2) where the time constant which is defined by an internal gate resistance (Rgm) and a gate capacitance (Cm) of the main portion and the time constant which is defined by an internal gate resistance (Rgs) and a gate capacitance (Cs) of the current sense portion hold the relation (Rgm×Cm)<(Rgs×Cs) for designing reason. Then, in such cases (1) and (2), it is reported that there is a case where the attenuation of the current sense current is becomes slower than that of the main current i at the turnoff and the current sense current is momentarily increases (see “Analysis and Suppressing Method of Transient Peak Current In Current Detecting Unit Cell of IGBT With Current Sense”, The Transactions of the Institute of Electrical Engineers of Japan. C, Vol. 115, No. 1).
In such a case, the overcurrent limiting circuit
10
P illustrated in
FIGS. 16 and 17
early becomes not able to perform the stable overcurrent suppressing function. Specifically, when the current sense current is momentarily increases due to the factors (1),(2) and the like, since the voltage which is the product of the current is flowing in the current sense portion and the resistance value R
S
of the sense resistor
3
P rises to be over the voltage at the time when these currents i and is show the same behavior even in the load short circuit state, a voltage applied to the gate electrode of the current limiting MOSFET
2
P becomes higher and the energizing capability of the MOSFET
2
P becomes much higher than necessary. For this reason, the speed of lowering the gate voltage of the IGBT
1
P becomes faster. When the speed of lowering the gate voltage becomes faster thus, the turnoff speed of the IGBT
1
P becomes faster, and as a result, a surge voltage which is defined by a circuit inductance and the rate of change in current at the turnoff becomes higher and therefore there may be a case where the surge voltage exceeds the element breakdown voltage, depending on conditions.
These problems rise both in cases where the bipolar transistor is used as a current limiting transistor instead of the MOSFET
2
P and where a vertical power MOSFET is used as a power semiconductor element.
DISCLOSURE OF INVENTION
The present invention is intended to solve the above problems, and an object of the present invention is to always achieve a stable overcurrent limiting operation without raising a turnoff speed of a power semiconductor element in an overcurrent limiting under any condition and in any state.
The present invention is directed to an overcurrent limiting circuit. According to a first aspect of the present invention, the overcurrent limiting circuit of a power semiconductor element which has first and second regions for passing a main current, a third region for controlling the main current which flows from the first region towards the second region, a current detecting region for passing a current sense current from the second region, and a first electrode terminal, a second electrode terminal, a third electrode terminal and a current sense terminal connected to the first region, the second region, the third region and the current detecting region, respectively, comprises: a resistor connected between the second electrode terminal and the current sense terminal; a transistor comprising a first main electrode, a second main electrode and a main control electrode connected to the third electrode terminal, the second electrode terminal and the current sense terminal, respectively, which comes into an ON state to pass a current from the first main electrode towards the second main electrode when a voltage not lower than a first control voltage is applied to the main control electrode; and a voltage clamping circuit connected between the main control electrode and the second main electrode of the transistor, for clamping a voltage applied to the main control electrode to a second control voltage not lower than the first control voltage when a voltage which is the product of the current sense current and the value of the resistor becomes a predetermined value not lower than the first control voltage.
According to a second aspect of the present invention, in the overcurrent limiting circuit of the first aspect, the voltage clamping circuit comprises a diode having a first electrode and a second electrode connected to the main control electrode and the second main electrode of the transistor, res
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