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

To: cq contest <CQ-Contest@CONTESTING.COM>
Subject: [CQ-Contest] LDMOS claims re: KPA1500 vs OM 2000A
From: Dick Frey <k4xu@bendcable.com>
Reply-to: k4xu@arrl.net
Date: Wed, 11 Mar 2020 23:25:34 -0700
List-post: <mailto:cq-contest@contesting.com>
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

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