Thursday, 11 April 2013

Design for life - part 3

Last month we looked at film capacitors and saw how selecting the wrong material for the application can have very serious consequences. This week we will look at Power MOSFETS and uncover some surprising factors which have a big impact on long term reliability.


Generally speaking, power semiconductors are among the group of components least prone to ageing effects. Assuming they are used within their maximum ratings and are well thermally managed, they are very reliable. However they account for more than half of all service return failures.

Typically this is because their maximum ratings have been exceeded through knock-on effects of other component failures, poor circuit design, and environmental influences such as spike or surge, over-temperature or mechanical stress.

In terms of the circuit design however, there are subtleties that can contribute to a surprisingly large proportion of failures which tend to be far less well appreciated:

Problems can occur in MOSFETS where a high rate of rise of drain to source voltage (dVds/dt) causes capacitive charging of the FET gate. This can switch the FET back on while it is turning off—usually a destructive event!
This is especially problematic where the “off” drive connects the gate to a voltage slightly above zero, rather than to a negative potential. A negative drive holds the gate well below the threshold voltage as the drain-source cap charges and generally provides a much more robust solution. It should be noted that the gate threshold voltage typically reduces to less than 70% of its 25°C value at maximum junction temperature.

A high dVds/dt can also cause the parasitic transistor (present in the construction of all FET devices) to turn on, especially at high temperature where more thermally generated minority carriers exist within it. If the body diode of the FET is used to clamp the drain to source voltage (as in a zero voltage switching ‘ZVS’ resonant converter), its reverse recovery Time can be very long. This is due to the FET body diode only being moderately fast and the fact that the reverse voltage is only the “on” voltage the FET, typically around 1V.

As the body diode is in fact the collector-base junction of the parasitic transistor, the unrecovered charge carriers cause the parasitic transistor to turn on when Vds rises rapidly, allowing large currents to flow in the device. To make matters worse, the diode recovery time is even longer at higher temperatures.

There is a final scenario which sounds like it has come straight from science fiction! It is known as Single Event Burnout (SEB). SEB research carried out as long ago as 1996 showed that a high voltage MOSFET, biased off, supporting a voltage near to its maximum rating can suffer an avalanche failure caused by a single sub atomic particle colliding with a silicon nucleus.

Subsequent research has shown that even at ground level, neutrons from cosmic ray collisions in the upper atmosphere can cause random failures in high voltage. MOSFETs over and above the rate predicted by MTBF data from manufacturers life tests. Reducing the maximum Vds by even 6% has been shown to decrease SEB failures by an order of magnitude.

Advance Product Services Ltd

Paul Horner is Managing Director at Advance Product Services Ltd.