"Robert Morein" wrote in message
...
Permit me to clarify my comment. When I said MOSFETs are
ubiquitous in power
switching applications, I was not referring to audio. Power
converters,
inverters, motor drivers, and all other industrial applications
for power
control use MOSFETs, except for some rare IGBT apps.

Generally true but "rare apps" is a too wide sweeping of a
statement.

Power MOSFETs make up the bulk of the mainstream switchmode power
conversion. However, very large inverters, motor drives and
other apps routinely use other devices such as bipolars and
combination devices (MCTs and IGBTs to name a few). Many
actually use a BOTH device types such as MCTs or IGBTs for the
main power devices and MOSFETs either in parallel to reduce
switching loss or to force resonant switching. On the low power
side many switching regulator ICs (both offline and low voltage
DC-DC) use onboard bipolar transistors due to ease of integration
with control functions (although newer devices such as STs VIPer
uses onboard HV MOSFET) . I believe the vast majority of low end
TV sets still use bipolars for the high voltage flyback.
Electronic ballasts for florescent lighting are using more and
more MOSFETs but the majority still use bipolars in a self driven
architecture due to cost. Just about any application where
breakdown voltage exceeds 1200V is exclusively bipolar. Ditto
for high voltage and high current applications. You can get an
IGBT rated for 3,300V and 1,200A with 500ns switching in a small
module which just can't be done with current generation MOSFETs.

In many cases it boils down to cost. Bipolar structures use far
less silicon for the same current density. MOSFETs usually make
the most sense when either cost isn't the primary concern, fast
switching speed is required (without resonant techniques), or the
MOSFET die size can be large enough to have a lower conduction
losses than bipolar. In the commercial world MOSFETs usually
meet this critera when the power is more than a few watts but
less than a few kW.

Bipolar is the dominant technology for audio amplification.
However, thermal
runaway has never been solved. It cannot be protected against
by feedback or
any linear network.

Wrong. Many bulletproof protection methods are available and
have been for decades. Just because audio designers can be
ignorant and continually try to reinvent the wheel doesn't mean
the rest of the world hasn't figured out how to do it right.

I've designed kilowatt output switching power supplies with
bipolar devices which can withstand any overload you can throw at
it...even at a steady state operating temperature of 150C.

when pushed to the limit. By contrast, a MOSFET circuit is
simply immune to
thermal runaway, because the physical process does not exist in
the
semiconductor.

Simply immune is simply wrong.

Thermal runaway most certainly exists in a MOSFET. ON resistance
is a strong function of temperature. The hotter the MOSFET the
higher the ON resistance. Higher ON resistance causes more power
dissipation which causes temperature to rise which increases ON
resistance which causes temperature to rise which....BOOM!

In the case of a switching power supply you can easily get the
MOSFET in a state where it thermally runs away. I've had
prototypes where the MOSFET is running fine at a given ambient
temperature. Increase the ambient temperature by only 5C and the
MOSFET quickly runs away and exceeds the 175C rating and dies.

You may be confusing the situation where you have devices in
parallel. If an individual FET heats up the increased ON
resistance forces current to the other FETs which gives nice
current sharing. Bipolars in parallel don't share well by
themselves since as one heats up it's Vce decreases which allows
more current to flow in that device and can cause runaway.

It is for this reason that it has been universally adopted
for the above mentioned industrial apps.

Careful with the use of 'industrial'. Industrial usually means
high power and/or high voltage in which bipolar reins supreme
(steel mills, production facilities, etc). It is the commercial
world in which MOSFETs are most common (PC power
supplies/motherboards etc).