Gate resistor for lateral power MOSFET?

I don't remember the s.e.d. subject, and just now I couldn't find the folder I made on DropBox, for you s.e.d. blokes and a DIY-audio discussion, so I created a new one, see AMP-70A under s.e.d.

--
 Thanks, 
    - Win

dl=0

That was so nice for you to post this, thanks! I remember seeing it now, though I'd forgotten it before I began the project with the power amp requirement.

One prominent note in your schematic warns about using the 1-ohm output apparently due to lack of protection. You may not have needed the low- resistance output (though that seems strange given the 5A - losing over

1kW is a lot of heat!) - but did you find that active shutdown schemes were too slow to protect your amplifier? (I haven't yet done this with my amplifier)

** And you won't need to.

From the capacitance figures you quoted, I see you have gone for the high power "double die" devices rated at 200V,16A and 250W.

Full protection only requires gate zeners of about 3.3V to limit current to about 5amps.

In the event of high dissipation, when the chip temp reaches about 165C, max current falls preventing further heating and the device saves itself - amazing but true.

.... Phil

Another CAD schematic version includes a fast shutoff scheme, but I didn't implement that on this prototype. The amplifier is (almost) fast enough to be a 50-ohm RF source, but the real usefulness comes from its high-current output, despite what the drawing says. We've been using two of them that way for almost a year, and nothing failed. Actually, we fried the electrode system, but the amplifier survived. Knock on wood.

Using BJTs at high currents and high Vce invites disaster; the 180V 2SC4883A 2SA1859A have a much smaller SOA than I'd like. That's because they're small die parts with fT = 60 and 120MHz. We've been lucky so far. I also designed a fast SOA-sensing protection circuit, but haven't implemented that either. As it stands, the AMP-70A design is just a high-speed test bed. I have modifications extending its slew rate to 2 or 3kV/us. Keep up this kind of business long enough and eventually something will blow out!

--
 Thanks, 
    - Win

Wow. Too good to believe? Gonna have to check it. 3.3V zeners are so soft, but the NFB may hide some of that. Great observation, Phil, will have to check further.

That kind of speed from a power amplifier? Wow! The first vertical deflection amplifier I designed for a Tek 'scope only had ~10kV/us, obviously only for driving a well-defined load (deflection plates).

I don't need this kind of bandwidth or slew rate for my current application. So far the lateral MOSFET design is simpler and considerably more compact. So long as I can make it reliable!