small high frequency mosfets

[mook johnson]

gents,

I'm looking for a high sped, low capacitance mosfet in a package the
size of a to-92 or smaller than a SOIC-8 with the following characteristics.

application:

Attached to a 1:1 push pull transformer with 5V on the CT to generate ~
10V p-p on the output. This means the switching transistors will see at
least 10V when the other one is one + leakage inductance spike.

Each transistor will turn on between 25 and 50nS each. The peek current
during each pulse will be ~ 300mA

When both transistors are off, it is desired that the reflected
impedance from the secondary is high hence the reason for low
capacitance mosfets.


The MOSFETS I've seen that fit this description usually have very
voltage. (Vds ~7V)

Wondering if you guys know of a higher voltage "RF" mosfet for an
application like this one.

thanks

 

[mook johnson]

2N7000.

Thanks,

If you know of a few more. I'm all ears.

 

[Tim Williams]

What kind of transformer? Most circuits like that, I'm running, say,
200kHz (2500ns turn-on is a lot more than 25-50, of course), and the
transformer probably has 20-30 turns of fine wire on it, having about as
much capacitance as the transistors themselves.

As far as creative approaches, I suppose J105 is still an option, but
using JFETs for switching is probably suboptimal, and not common at least.

2N4401 can be made to switch that fast. Use a small base-emitter
resistor, and a fair amount of drive (or better still, active base drive
in both directions). That'll have lower saturation and capacitance, but
you make up for it in switching time and losses.

Otherwise, like John said, 2N7000 (or 2N7002, or BS170, or a zillion
others) is hard to beat for generic purposes. There are a few similar
parts in the same range, 0.5-2A, 30-60V, "logic" level, Digikey or Mouser
can help find them. "Complement" is BSS84 or so.

Tim