Si Power Device|Evaluation

About dV/dt Destruction


Points of this article

・dV/dt destruction is a phenomenon in which the charging current flowing through the parasitic capacitance Cds while a MOSFET is turned off flows through the base resistor RB, causing the parasitic bipolar transistor to switch to the on state and leading to short-circuit destruction.

・dV/dt is the voltage change per unit time; the steeper the rise of VDS, the more readily dV/dt destruction occurs.

・In general, the poorer the reverse recovery characteristic, the steeper dV/dt becomes, and the greater the tendency for destruction.

About dV/dt Destruction

As indicated in (2) in the diagram below, in dV/dt destruction, charging current that flows in a transient manner in the parasitic capacitance Cds while the MOSFET is turned off flows through the base resistor RB, thus causing a base-emitter potential difference VBE in the parasitic bipolar transistor, which switches to the on state and causes short-circuit destruction. In general, the higher the dV/dt (the more sudden the change in voltage), the larger is the potential difference VBE, so that the parasitic bipolar transistor is more easily turned on, and destruction occurs more readily.

Summary diagram of current path (blue) in dV/dt destruction

Further, in an upper-lower bridge configuration circuit such as an inverter circuit and a totem-pole PFC circuit, reverse recovery currents Irr flow in the MOSFETs. Due to the dV/dt caused by these reverse recovery currents, there is the danger of erroneous turn-on of the parasitic bipolar transistors, and this point also demands attention. The relationship between dV/dt destruction and reverse recovery characteristics can be checked through double-pulse tests. Shown below is a summary circuit diagram of double-pulse tests.

Summary circuit diagram of double-pulse tests

For detailed information on double-pulse tests, please refer to Evaluating MOSFET Recovery Characteristics Using Double-Pulse Tests in the Tech Web Basic Knowledge/Evaluation section.

Below, simulation results for dV/dt and the reverse recovery current are shown. MOSFETs ① to ③ were assumed, with the same gate resistor RG, power supply voltage VDD, and other circuit conditions, and only the reverse recovery characteristics different. The graph below indicates the drain-source voltage VDS and drain current (internal diode current) ID when Q1 transitions from free-wheeling operation to reverse recovery operation.

Simulation Results for Double-Pulse Tests

In general, MOSFET ③ can be said to be a product with “poor reverse recovery characteristics (large Irr, trr)” compared with MOSFET ①. From these simulation results, we see that the worse the reverse recovery characteristics, the steeper (larger) is dV/dt. This can also be understood from the fact that in general, transient currents flowing in a capacitor are represented by I=C×dV/dt. In the above simulations, the slopes of Irr (di/dt) are all shown for the same conditions. When di/dt is steep, dV/dt is similarly steep.

From the above, we may conclude that, when using MOSFETs in a bridge circuit or the like, the poorer the reverse recovery characteristics of the MOSFETs used, the greater the danger, in general, of dV/dt destruction.

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