small high frequency mosfets

Oct 22, 2012 4 Replies
M

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


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J

2N7000.
John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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M

Thanks,

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

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J

We have pushed a 2N7000 (2N7002, the SOT23 version actually) to 50 volts and 1 amp, driving a transmission-line transformer. Rise and fall times are around 1 ns if you drive the gate hard.

We've also used a pair of them to drive a cheap ISDN transformer to make a DC/DC converter, 16 of such on one PC board.

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So if you can stand the drain capacitance, it should work.

If cost doesn't matter, there are some SOT89 gaasfets that would work, much faster and with less drain capacitance. CLY2, or the Avago ATF-series enhancement parts. The ATFs actually zener around 25 volts.

John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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T

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

-- Deep Friar: a very philosophical monk. Website:

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