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.
Power
MOSFETs
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.
