Minto,
14 nF gate capacity can easily be driven by a IXDD414, at 500 kHz and
even at 1800 kHz. I've built a 600W class E for 160m running at 36 VDC
(for /p, 3 car batteries in series ;-) ), using 4xIRFP260N. Efficiency
is above 90% but mostly due to losses in the coils ( i like to built it
compact :-) ).
73, Stefan/DK7FC
Am 13.01.2011 19:08, schrieb Minto Witteveen:
Mal,
Yes, putting several in parallel means that the total capacitance that
the driver stage sees (sum of gate capacitances and the reverse
transfer capacitances) increases so more power is needed to
hard-switch the fets. I use a TIP31/32 totem-pole to accomplish this.
And the capacitance becomes quit big, Ciss = 4200 pF, Crss= 135 pF
(translates roughly to approx 135 * 20 = 2700 pF) so total capacitance
the driver sees is somewhere near 14 nF!!
Cx @ 500 KHz = 22.7 Ohms.
The IRFP360 is even worse.
Now image 4 of these in parallel.
Regards,
Minto pa3bca.
--------------------------------------------------
From: "mal hamilton" <[email protected]>
Sent: Thursday, January 13, 2011 18:50
To: <[email protected]>
Subject: Re: LF: FET RDS
Minto
One approach is to use a few in parallel like they do in plasma tv's but
there must then be other considerations to hinder the application.
mal/g3kev
----- Original Message -----
From: "Minto Witteveen" <[email protected]>
To: <[email protected]>
Sent: Thursday, January 13, 2011 5:42 PM
Subject: Re: LF: FET RDS
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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