Transistors｜Evaluation

About Destruction due to Avalanche Breakdown

2023.10.25

Points of this article

・When a voltage equal to or greater than the absolute maximum rating BVDSS is applied to a MOSFET, avalanche breakdown occurs.

・When avalanche breakdown occurs, a large current flows, and there is the risk of MOSFET destruction.

・MOSFET destruction due to avalanche breakdown may be short-circuit destruction or thermal destruction.

What is Avalanche Breakdown?

When a voltage equal to or greater than the absolute maximum rating BVDSS is applied to a MOSFET, breakdown occurs. When an electric field equal to or above BVDSS is applied, free electrons are accelerated and acquire large amounts of energy. As a result, impact ionization occurs, and electron-hole pairs are generated. The phenomenon of avalanche-like increased in these electron-hole pairs is called avalanche breakdown. At the time of an avalanche breakdown, the current flowing in the reverse direction of the MOSFET internal diode is called an avalanche current IAS (see (1) in the figure below).


Summary Diagram of current path (red) in a MOSFET destruction due to an avalanche breakdown

Destruction due to avalanche breakdown: Short-circuit Destruction

As indicated in the diagram, IAS flows to the parasitic base resistor RB of the MOSFET. At this time a potential difference VBE occurs across the base and emitter of the parasitic bipolar transistor, and if this potential difference is large, the parasitic bipolar transistor may switch to the on state. When the parasitic bipolar transistor turns on, a large current flows, and there is the possibility of a short-circuit destruction of the MOSFET.

Destruction due to avalanche breakdown: Thermal destruction

When avalanche breakdown occurs, in addition to short-circuit destruction due to erroneous turn-on of the parasitic bipolar transistor by the avalanche current, there is also the possibility of thermal destruction due to conduction losses. As explained above, when a MOSFET enters the breakdown state, an avalanche current flows. In this state, BVDSS is applied to the MOSFET and the avalanche current flows; the product of the two is the power loss. This power loss is called the avalanche energy EAS. An avalanche test circuit and test result waveforms are shown below. The avalanche energy can be expressed as in equation (1).


Avalanche tests: simplified circuit diagram


MOSFET voltage and current waveforms in avalanche tests

Equation representing avalanche energy

For MOSFETs guaranteed against avalanche breakdown, the specifications stipulate absolute maximum ratings for IAS and EAS, and these values should be referenced. In an operating environment in which avalanche currents occur, the actual values of IAS and EAS must be ascertained, and the device used such that the absolute maximum ratings are not exceeded.

As an example of occurrence of avalanche breakdown, consider the flyback voltage upon MOSFET turn-off in a flyback converter, the surge voltage due to a parasitic inductance, and the like. As means for dealing with avalanche breakdown caused by a flyback voltage, for example the circuit can be designed so as to lower the flyback voltage, or a higher-voltage MOSFET may be used. In the case of avalanche breakdown due to a parasitic inductance, it is possible to change to a MOSFET in a package with shorter leads, or improvements to the board layout can be made to reduce parasitic inductances, among other measures.

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