AN601

Vishay Siliconix

Document Number: 70572

15-Feb-94

www.vishay.com

S FaxBack 408-970-5600

1

Unclamped Inductive Switching Rugged MOSFETs

For Rugged Environments

The evolution of the power MOSFET has resulted in a very

rugged transistor. The semiconductor industry defines this

ruggedness as the capability to withstand avalanche currents

when subjected to unclamped inductive switching. Historically,

MOSFET manufacturers chose to quantify ruggedness, not

based principally on individual performance, but rather on

comparative performance with other manufacturers. Siliconix

has optimized the cell structure of power MOSFETs, resulting

in a new class of extremely rugged devices. Today’s

avalanche-rated MOSPOWER FET exhibits a ruggedness

that far exceeds the performance of any power MOSFET of

earlier years.

This application note reviews the history of unclamped

inductive switching (UIS) and examines various theories

pertaining to failure. It further identifies what appears to be two

related mechanisms — thermal and bipolar — believed to be

responsible for failure during unclamped inductive switching

and concludes by recommending how a power MOSFET

should be qualified for ruggedness in the data sheet.

Two failure modes exist when MOSFETs are subjected to UIS.

In this article, these failure mechanisms are labelled as either

active or passive. The first, or active mode, results when the

avalanche current forces the parasitic bipolar transistor into

conduction. The second, or passive mode, results when the

this elevated temperature, a “mesoplasma”* forms within the

parasitic npn bipolar transistor and causes catastrophic

thermal runaway. In either case, the MOSFET is destroyed.

The passive mechanism is, therefore, identified as that failure

mode not directly attributed to avalanche currents.

Symbols and Definitions

Whenever possible, symbols and definitions established by

the JEDEC Committee, JC-25, are used in this article. To clear

up any discrepancies, however, the following list describes

symbols used frequently in this article.

the peak current reached during avalanche

L

the value of inductance

the breakdown voltage in avalanche

What is Unclamped Inductive Switching?

Whenever current through an inductance is quickly turned off,

the magnetic field induces a counter electromagnetic force

(EMF) that can build up surprisingly high potentials across the

switch. Mechanical switches often have spark-suppression

circuits to reduce these harmful effects that result when current

is suddenly interrupted. However, when transistors are used

as the switches, the full buildup of this induced potential may

thus resulting in catastrophic failure.

If we know the size of the inductor, the amount of current being

switched, and the speed of the switch, the expected potential

may be easily calculated as

V

=

(1)

where

L

=

the inductance (H)

di/dt

=

rate of change of current (A/s)

=

the supply voltage (V)

*A “mesoplasma,” according to Ghandhi, takes the form of a glowing red spot having an average temperature in excess of 650

_C and a peak

core temperature in excess of 1000

_C. This mesoplasma is a result of regenerative thermal runaway.