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Old 14th November 2003, 05:47 PM   #1
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Default Complementary Power Mosfet Recommendations?

I have a prospective project where I'd like to make an optimal choice of complementary P & N channel power mosfets, probably @ 200V breakdown with 2 pair for each up to 200W/Channel 8 ohm in the 16 to 24 A range in a TO-247AC type package plus I'm hoping for the best short term SOA window without the Crss being too nasty? I'm planning to drive these two pair with what amounts to a class A push pull 12AU7 & considering bootstrapping. Would 150 volt MOSFETs be cutting it too fine for a +/- 65 volt supply?

So far, N channel parts by IRC & P channels by IXYS are the most suitable I've come across.

Also, I was thinking of making a temperature tracking bias network for the MOSFETs with a NPN transistor mounted to the output device heatsink as a 'zener' with an adjustable band gap shunt reference in series with the emitter to be able to trim the bias out over a temperature range. Comments on this?

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Old 14th November 2003, 09:16 PM   #2
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For power 200 W into 8 Ohms you will need minimal two ( rather three ) fets with dissipation 150 - 200 W each and Uds 200 V. Each of this fets have input capacity approximately minimaly 2500 pF, so summary input capacity will be very high. ECC 82 ( 12 AU 7 ) have Ri 7,7 kOhm, which is too high for this using - you get amp, which will be " very lazy " and which will have big distortion on higher frequecys. If you can use tube VAS, go on power ones, but don't wait any miracles .
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Old 14th November 2003, 11:33 PM   #3
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Hi, Upupa -

That's a good point, which is one reason that I'm giving primary consideration to only two pair instead of three, although if I find a complementary pair that has particularly low Crss at a lower maximum drain current, I can always change my mind.

Actually, my calculations indicate that I should be just about able to avoid slew limiting out to 20khz since I plan to operate the output devices as followers with an expected average Crss of about 400 - 500pf per each of four power mosfets. I also had the experience, a while back, of building a somewhat similar amp using a long tailed pair 12AT7 driver with only pullup resistors to drive the gates (5mA Ip/each side), and the thing slewed at 15V/uS at the zero crossing, which was significantly better than I expected. You might be wondering how drove power mosfets with a setup like that, but I did, and both halves of the totem pole were N-Channel, at that. (It's my very own driver/output topology and probably patentable, so I don't want to be too specific about how I made it work.)

But, why go with a 12AU7 totem pole driver, you might ask? Because if that works out I think I can do the input *and* driver of a 200 watt amp with a single compactron tube. Kewl.
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Old 15th November 2003, 07:26 AM   #4
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Default Slew limiting

It is not easy find fets with low input capacity and great Pd. Nearest are lateral types of BUZ 900 family or " historical " Hitachi's or their new " clones " in TO 247 case. If you can limit frequency characteristic for 20 kHz, 5 mA will be enough, but distortion will be very rising at this frequency ( look at calculation from Erno Borbelly in The Audio Amateur ). My recomedation is to use any " predriver " beside tube ( some lateral type such as 2 SK 213 ), which have input capacity much lower and becouse they are lateral, have better temperature coeficient of Ids.
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