Hello,
That IRF540 will be very difficult to drive with increasing frequency.
The input will be very reactive and will require a more complex
network to get the needed RF power (and voltage) impressed across
that gate capacitance. That takes greater power and the driver may
not be up to the task. A first order calculation says driving
impedence will be 1/10th (.1) times that of the IRF510. None of the
input or output networks used will perfom as well with the irf540
due to it's differences. At 80m this will not be as bad as 20 or 10m.
I'd try it but do not be surprized if the results require redesign
to achive adaquate gain. If you are successful you may be rewarded
with greater power than the IRF510. A scratch evaluation suggests
if accomodated the IRF540 could be capable of 60-80w output with
about 4-5W drive at the lower HF frequencies aassuming it's not
bandwidth limited.
There are several parameters to look at besides those quoted.
Gate turn on voltage (lower the better). Output capacitance
as well as Drain to Gate capacitance are also functional
parameters of importance. The drain to gate capacitance is
can be thought of as a negative feedback capacitor from Drain
to gate and limits gain with increasing frquency. The result
is that you need to feed both the gate and extract power from the
drain at low impedences or the implied feedback will reduce gain to
below zero (less out that in). At 20m these will look like around 7
and 2 ohms for the IRF540 where the IRF510 they are around 65
and 10-12 ohms. Accomodating these new conditions were not
anticipated in the original design.
I hope this gives a starting point to a successful design.
Allison
--- In BITX20@..., Heinz Schnait <oe5eep@q...> wrote:
Hi Kumar,
The datasheets give the following parameters:
IRF510: RDS = 0.54 Ohms; ID(continous) = 5.6 A; maxPower = 43W; Input
capacitance = 180 pF
IRF540: RDS = 0.077 Ohms; ID(continous) = 28A; max Power = 150W; Input
capacitance = 1700 pF
The IRF540 is a more robust version than the IRF510. It looks to me
as if
the 540 has more junctions wired internally in parallel. This is also
reflected in the input capacitance, which is much higher for the
540. I am
afraid this might cause problems for the gate driving circuit,
especially
at higher frequencies.
Is anybody out there with a working IRF510 amp who would like to do
some
experiments with a IRF540 replacement? Please share your results!
73 Heinz, OE5EEP
Am Freitag, 22. Juli 2005 07:33 schrieb kumar gautam:
HI,
HERE IN MARKET IRF510 IS NOT AVAILABLE, ONLY IRF540 IS AVAILABLE. ANY
ONE HELP ME THAT IRF540 IS EQUIVALENT OF IRF510 OR I CAN
SUBSTITUTE THIS
FET. WITH 73'S
DE KUMAR GAUTAM
