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:
[email protected] [mailto:[email protected]] On Behalf Of PA0WFO
Sent: 04 February 2006 17:53
To: [email protected]
Subject: LF: Re: Re: More smoke...
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 )
