About dV/dt 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 C_ds while the MOSFET is turned off flows through the base resistor R_B, thus causing a base-emitter potential difference V_BE 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 V_BE, 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 R_G, power supply voltage V_DD, and other circuit conditions, and only the reverse recovery characteristics different. The graph below indicates the drain-source voltage V_DS and drain current (internal diode current) I_D 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.