On 26/05/2011 13:39, Olin Lathrop wrote:
>> What effect will the capacitance have?
> It takes large current to switch quickly.  For example, let's see what it
> takes to change a 1nF capacitor by 15V in 50ns.
>
>    15V * 1nF / 50ns =3D 300mA
>
> Take a look at the effective gate capacitance of some large power FETs an=
d
> you will see why high end FET drivers are rated for 1A or more switching
> current.
>
A 900V 15A FET may a have 0.8nF to 2nF gate capacitance. The lower=20
capacitance ones cost a lot more!

Higher current rating is often achieved by paralleling FET structures or=20
even entire FETs on the chip. So gate capacitance is roughly related to=20
IdMax
I've tried design/building some RF amplifiers using cheap $1 SMPSU=20
instead of $50 RF FETs. At 400W you are limited to about 4MHz. At 100W=20
to about  50Mhz and at 20W or more  over 70MHz is possible from parallel=20
IRF510 in push pull.
The trick is to drive at 12.5 Ohm or even 3 Ohm

50 Ohm 2:1 transformer gives 12.5 Ohm. You start needing significant=20
power to drive it. Still for a 137KHz 1W ERP you may need a 2KW PA and=20
cheap $1 SMPSU fets running direct off full wave bridge/Capacitor on the=20
mains will do that with suitable isolated input and output transformers.=20
Here with 230V mains.
--=20
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