Push Pull MOSFET with OT

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JazzGuitarGimp
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Push Pull MOSFET with OT

Post by JazzGuitarGimp »

I had some free time on my hands today, so I decided to play around with Mouser's Multisim Blue simulator. What we have in the screen shot is a push pull MOSFET power stage using IRF820 MOSFETs and an OT with an 8k Zpri to 8-ohm secondary. B+ is 460V and the gate bias voltage is +40mV.

Multimeter XXM1 is displaying DC current at the OT CT (1mV = 1mA). So, 147mA required from the supply at full output.
Multimeter XXM3 is displaying RMS AC voltage across the 8R dummy load. 20.867V across 8-ohms is 54.43 watts output.

In the oscilloscope display, the larger waveform is the plate-to-plate voltage. Scope input sensitivity for this channel is 500V/div. The smaller waveform is the output voltage across the 8R dummy load. Scope input sensitivity for this channel is 50V/div.

The stage requires a 0.5V RMS input signal to achieve full output.
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xtian
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Re: Push Pull MOSFET with OT

Post by xtian »

Very cool. But are you reinventing the wheel?

Actually, I do like the idea of a compact 50 watt amp, tube preamp, solid-state power amp, fits in rack unit...oops--there I go, reinventing the wheel.
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Re: Push Pull MOSFET with OT

Post by JazzGuitarGimp »

Methinks just being silly more than anything... :-) Though I've always wondered about subbing MOSFETS for output tubes. It looks workable, but the bias voltage as well as the input sensitivity are much different for MOSFETS as compared to output tubes.

Another interesting tidbit: At the present 40mV bias voltage, output transistors are dissipating 4.3 watts each at idle, and at full, undistorted output, they are dissipating 24.3 watts each.
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pompeiisneaks
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Re: Push Pull MOSFET with OT

Post by pompeiisneaks »

I've been thinking of something class D like this: http://www.infineon.com/cms/en/product/ ... 4e281c7e50

Then into mosfets for the output, but this is super efficient because the mosfets are getting the equivalent of a P-P wave at them, on or off fully for the output, therefore much more efficient, and the word I hear is that Class D is pretty damn nice so long as you do the high/low pass filters right to remove the hash/noise correctly.
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MakerDP
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Re: Push Pull MOSFET with OT

Post by MakerDP »

What kind of heat sinking would that require? What would I look at on the datasheet to figure that out?
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Re: Push Pull MOSFET with OT

Post by pompeiisneaks »

Which do you mean? The mosfets or the one I linked? Generally there should be something related to max power dissipation at ambient. From there you have to calculate how much higher you want to go, and what kind of heat sink and/or fan combination you'd need to keep it in that same range. I have seen it explained once myself, but I can't recall now how you do the maths :P I definitely need to get that figured out too.

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MakerDP
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Re: Push Pull MOSFET with OT

Post by MakerDP »

The OP schem with the IRF820's.
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Re: Push Pull MOSFET with OT

Post by tubeswell »

That T220 package for those IRF820s can handle about 1W without a heatsink. You wouldn't get much output power out of that. If you can keep the junction temp under 25 deg celsius (with all sorts of caveats), the Vishay datasheet claims a 50W dissipation limit. There's the rub.
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Re: Push Pull MOSFET with OT

Post by JMFahey »

I find that simulation weird ... and that´s an understatement.

By the way, I *do* commercially build and sell Transformer output MosFet powered amps and my experience runs diametrally opposite to that simulation:

1) Vertical Mosfets , the kind those are, need about +3 to +3.5V DC bias just to start passing any current.
Anything less and they are OFF, period.
Maybe you used a Lateral Mosfet model?

2) you should have a ton of crossover distortion.

3) at lower currents they are *extremely* non linear (they are meant for switching after all), no way they will give you those nice sinewaves.
You ned to pass at least 300 to 400 mA idle current just to get distortion/non linearity down enough to be corrected (somewhat) by NFB.
Open loop? No way!!!

Sorry but I do actually scope them, not simulate.
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Re: Push Pull MOSFET with OT

Post by txbluesboy »

Does anyone have a schematic or plan for a mosfet phase inverter?
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