In a message dated 17/07/2004 20:32:33 GMT Standard Time, [email protected] writes:

Any thoughts on if a matching transformer should 'click' when the key is released... Obviously it is likely an inductive reaction to the rapid change in current supplied... When I connect the 48V supply to the system the finals don't last long... OK at 12 and 24V so I feel it is a inductive kickback that's blasting the 200V rated FETs...  The system works great into a 6 ohm load driver pushpull without the match xfmr...  It must be the inductive kickback...

 

Perhaps the inductance of the windings it too great?

 

Damn physics... To quote a power engineer friend of mine "it's a damn shame that FETs behave the way they do, otherwise they would be the perfect device..."

200V FETs probably are being hit by some fairly high voltages. I have seen 200V spikes from a 35V supply line, particually when the load impedance is incorrect.

On one TX here that operates from a 48 - 56V SMPSU, I had to fit 400V FETs. That solved the problem and gave 600W output. Other choice is to to fit 200V 'transorbs' (?) across the devices. If the spikes are narrow this may be OK. If not, it's only one more thing to catch fire (hi)

 

 

73

 

David  G0MRF