[Amps] LDMOS and VSWR

[Manfred Mornhinweg]

Jim,

> The videos on YouTube were all running CW at various frequencies. No
> pulse modes.

Unfortunately you are mistaken there. Jeff is right. They were pulsing 
the amplifier in that video.

If you look closely at the video, you will be able to confirm the 
following facts:

- When the 50Ω load is connected, they show the power supply only from a 
distance, so the exact current can't be read, but one can see that only 
one digit is on before the decimal point. So the current is less than 
10A, and thus the average input power cannot be more than 500W. Yet the 
RF power meter reads 1200W. Unless the amplifier is magical, it must be 
operating in pulsed mode.

- During the mismatch test, the power supply is shown close-up. 
Depending on the phase angle of the load, the current varies between 
about 1 and 10A, limiting the average input power to 550W at most.

- During the same test, we can read the model and ratings of that power 
supply. It's limited to 1000W. More evidence that the amplifier cannot 
possibly be putting out 1200W CW.

- The MOSFET is mounted on a water-cooled copper block. Most ham 
amplifiers get less good cooling. Although the MOSFET is clamped down 
dry to the copper surface. Soldering it down would be better.

Summing it up: The worst-case power dissipation during the tests shown 
was around 500W, which that LDMOSFET can take rather easily, on that 
watercooled block. But if a ham mounts that MOSFET on an air-cooled 
heatsink, and drives it to 1200W CW, using a power supply that can 
deliver 2500W, it wouldn't survive those high SWR tests.

Manfred


========================
Visit my hobby homepage!
http://ludens.cl
========================