[CQ-Contest] LDMOS claims re: KPA1500 vs OM 2000A

[Dick Frey]

There have been some claims made in this thread about the VSWR 
capabilities of LDMOS vs older MOSFETs (VDMOS) vs vacuum tubes.

It's basic physics. The silicon in an RF power transistor is less than a 
quarter of a postage stamp and about as thick. When a 'FET is abused 
under conditions of mismatch, it will break down, like a Zener. The 
current through it times its breakdown voltage (BVdss) is the power 
dissipated in the device. Under these conditions, the device will melt 
in a few milliseconds. Your semiconductor will turn into a 
conductor,theinternal bond wires will evaporate, and you are off the air 
...unless there are protective measures in place.

There are marketing videos on the web showing MOSFET devices being 
stressed with nasty VSWR -- shorts and longs, all phase angles, etc. And 
voila! The device takes a licking and keeps on ticking. It is a feat of 
RF legerdemain designed to demonstrate ruggedness to the unwary. I know 
this because I made one of those videos 
<https://www.youtube.com/watch?v=bMEsEATudgM>. Here's another 
<https://www.youtube.com/watch?v=8ziYqjMQGEQ>by NXP.

Here's how it works. An amplifier is built that will put out a kW or 
more. It is mounted on a water-cooled heat sink. The drive signal is 
whatever is required to produce the intended output BUT the duty cycle 
of this drive is typically 5%, and the pulse repetition frequency is 
longer than the device's thermal time constant. If you look at one of 
these demos carefully you will see that the power meter is reading peak 
power but  the current on the power supply reads average. Those numbers 
don't work unless you factor in the duty cycle. But it makes great sparks.

The peak breakdown power deposited in the device, while very high,  
represents a manageable amount of energy per pulse with an average power 
which is still within the thermal dissipation capabilities of the device 
as mounted.

So here's what is important. Your xDMOS amplifier must have devices 
which are well cooled, and protected by fast-acting over-current and 
VSWR monitors. They will be happy no matter how dumb you are.

A tube has a very large thermal mass and voltage headroom. It can 
withstand most amateur operators, especially so if protected from high 
VSWR. But, try as you might, it's not "no tune".

-- 
Dick Frey, K4XU