Actually its actually worse as we are dealing with dynamic waveforms.
BS170 s 24 (max 40) PF per device input capacitance.? times 2 devices.
Also the Crss or reverse transfer capacitance is 7pf (10 max).
IRF510? the Cin is 135PF (for some can be as high as 180pf).? X1 device
The reverse capacitance? is 20pf? (for that vendor, others are higher)
You have both input capacitance (gate to source) and what is known
as Miller capacitance which is essentially negative feedback
(the actual Crss capacitance) multiplied by the output voltage swing
(simple description).? So with IRF510 your really about? four times
greater load to drive with a tiny CMOS gate.
With all that, it makes the IRF510 much harder to drive than BS179s
in parallel.? How much difference?? BS170s are saturated RDS_on is
the limiting factor for power.? For Irf510 the only reason power out
happens is for drive above he gate threshold it has a gain of 1S
so for 5V drive you get about 2-2.5V gate drive not allowing for
capacitive load and you can maybe get to 2A drain current but
it is not saturated power its linear.? If it were saturated the .54ohm
RDS_on would make for a lot of power.?
FYI the current drive needed to hard switch a IRF510 at 13.55 mhz
is about 8A peak for?6 volt (measured) drive. I needed a 10V pulse
but no devices existed.? I was doing class D power conversion.?
Where does that power (current) go?? ?Charging that gate cap
then discharging it while the Miller coupling is working against you.
Allison
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