LF: Killing FET finals...

["Pedro M.J. Wyns (netbook)" <pedro.wyns@telenet.be>]

Not relevant.  Most FET's are going QRT due to feedback issues in bad parallel circuits...

The golden rule is to put SUFFICIENT series resistance in every gate separately, do NEVER put gates in parallel.

Some nice literature on this is the fantastic book on HF amplifier design by Helge Granberg from Motorola.

Pedro ON7WP-AA9HX
Licensed transceiver doctor and radio guru :-)




mal hamilton schreef:

Chris

Use a separate pwr supply for the fan, not the same supply that runs the FETS or driver ccts. This is what I do and never had a problem.

mal/g3kev

 

----- Original Message -----

From: Chris Osborn

To: LF Group

Sent: Saturday, June 20, 2009 7:34 AM

Subject: LF: 500 kHz - Fans

LF,

I've nearly completed my new PA for 500 kHz and have now managed to

stop destroying FET's.

 

Unfortunately I have an unusual problem in that the heatsink cooling fans

are going U/S.

 

I have used 24 volts brushless PC-type fans and after a while they

either die completely or start to go backwards slowly. . .

 

The fans may be slightly over-run but only by a volt or so and on key-up

(unregulated supply) and are situated about 50 mm (2 inches) from the heatsink.

I have rf decoupled them from the supply but maybe not well enough !

 

Any suggestions in curing this problem would be most welcome.

 

73 Chris G3XIZ