Yup that is correct. High(er) voltage fets usually have the higher RDS-on
values... Tradeoff based on physics...
I started my 500-600 Watts 500 KHz transverter with two IRFP360's in
parallel.
Later I switched to IXFH26N50 (cheaper at EUR 1 a piece and slightly better
than the IRFP360).
The IXFH26N50 has a VDSS of 500 Volts, and a RDS-on of 0.23 Ohms and an Id
of 25A.
With two of these in parallel the efficiency is > 90%. DC supply is (max) 54
Volts.
Peak voltage on the drains is somewhere around max 250 Volts. So I might
search for Fets with a somewhat lower RDSon and a lower max voltage, but
these fets are indestructible in my setup, they survive open and
short-circuited antennas without a problem for several minutes until heat
becomes a problem.
For more info wrt my setup see www.pa3bca.nl
Regards,
Minto pa3bca (500 KHz in PA idle at the moment, alas...)
From: mal hamilton
Sent: Thursday, January 13, 2011 17:25
To: rsgb
Subject: LF: FET RDS
LF/MF
It seems to me if you are working with low V high current FETS the RDS seems
reasonable 0.02 for example but when a High V low current device is need the
RDS of these devices seem to be around 0.4 considerably higher.
therefore the efficiency of the amplifier will never reach the 90% plus that
some claim.
I stripped a plasma tv recently and found banks of FETS (6 per bank) and
wondered why the application neederd so many and have come to the conclusion
that because of the high RDS lots were required in parallel to reduce the
losses.
Maybe there are low RDS fets about that will handle several hundred volts at
modest currents ie 10A at 1000 volts
Room for thought
de mal/g3kev
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