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 )
