Dear William, LF Group,

Power MOSFETs work like big zener diodes anyway – if you exceed the drain-source breakdown voltage, avalanche occurs and the voltage is clamped, usually about 20 – 40% above the BVdss value (60V in this case). This does not harm the MOSFET directly, but does result in high power dissipation – if breakdown occurs at 80V, and the clamping current is 10A, 800W will be dissipated. This is OK if it only occurs during short, infrequent voltage “spikes”, but if the high voltage transients occur with a large duty cycle, the MOSFET will quickly overheat. It might be possible to connect high-power zeners across the MOSFET with a slightly lower breakdown voltage than the MOSFETs, but then the problem would be how to protect the zeners!

The current rating of these devices is so high it is unlikely the power supply will be able to deliver enough sustained current to cause them any problem. Power dissipation under mis-matched conditions could be a problem – although each device is rated at 150W, this refers to a case temperature of 25C. In reality, the case temperature will get much higher, limiting continuous power dissipation to maybe 50W or less per device without exceeding the junction temperature rating. If it is somehow possible for large transient voltages (greater than about 20V) to occur between the gate and source terminals, this will blow a hole in the gate oxide and destroy the device immediately.

Cheers, Jim Moritz

73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of PA0WFO
Sent: 04 February 2006 17:53
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Re: More smoke...

Dear All ,

In my Ropex transmitter I have a class D PA with 2 power fets Fairchild RFP 70N06 with rather generous ratings 70 Amps each and a max voltage of 60 Volts the supply voltage is 13.8 Volts and a measured output of 154 Watts . When the antenna is mismatched for whatever reason will the Fets break down on voltage or current peaks ? ( the DC input of 13.8 Volts is properly fused ) and is it possible to prevent voltage breakdown by fitting power zener diodes over the drain and source of both Fets without spoiling their efficiency . A fast overcurrentprotection is included in the circuit . The reason for this question is that we had a breakdown in the Ropex Fet final of PA0VHF while tuning the antenna matching unit out of resonance ( while doing this the input was rather low ) and are wondering what caused it .( The heatsink was not hot at all while tuning up to the rated output )

73,William PA0WFO