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Class D Switching Power Amplifiers and Power D/A conversion

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Old 23rd March 2005, 07:55 AM   #31
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Quote:
Originally posted by lumanauw
Hi, Chris,

Thanks for the schematic. I have to study it further.

There are some components I don't understand.
-R78+D80, what are they for? Helping to fill C34 besides the work of D84?
-R71 and its lower 100k, what are they for?
-R75 = 40k, what is it for?

If it is to be used for higher rail (+/-50V), what values to be changed?

I tried IRF640N in my experiment cct, and they blow up far more easily than IRF640 (without N). For classD, IRF (without N) is more appropriate than the ones with N suffix?

A question. Is it a must to construct classD (any cct with fosc >250khz) with SMD component? Or classD can works with ordinary component too, like 1/4W resistors, TO-92/TO-220 transistors at the drivers?
Hi,

Yes, R78+D80 are part of the precharge circuit of the bootstrap capacitor, some delay is required before enabling output stage to make sure that cap is fully charged. During operation though, D84 does all the work.

The two 100k's only purpose are to increase PSRR.

R75 is another part of the bootstrap circuit, it ensures a path for the pre-charge circuit previously discussed, even under no load conditions.

For higher voltage.... hmmmmmm..... off the top of my head, R78 would obviously have to be increased accordingly.

Q142 and Q143 should be changed to ones with higher breakdown voltage.

I wouldn't recommend starting off trying to get one working with +-50V.

Start lower, work up.

Ideally you'd have a good PCB and SMD components all nicely layed out. At lower power levels like this circuit, you can easily get away with P2P, sound quality will likely still suffer somewhat, but it can sound very good.

I don't think anyone would want to do P2P with SMD but if they ever do I want to see it!

My circuit as it currently is using very ordinary parts, but my Mosfets are very high end.

I've had similar experience with this circuit in the past regarding mosfets. I really don't think newer generation types are worse, but they do burn out easier. I believe the older types take alot more abuse because of their greater parasitics, slowing everything down. They're harder to switch so it would take more noise to turn them on spurriously for example.

Hope that helps. Yeah, don't try doing 50 Volts with this, not at first.

Regards,
Chris
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Old 23rd March 2005, 09:35 AM   #32
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Hi, Chris,

Is it right that feedback before LC is more suitable for heavyworking classD (like for parrareled 1ohm subwoofer)?
Do you know where I can see a good design of selfoscilating with feedback before LC (like IRF amp) but with discretes like your UCD design?
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Old 23rd March 2005, 10:20 AM   #33
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Quote:
Originally posted by lumanauw
Hi, Chris,

Is it right that feedback before LC is more suitable for heavyworking classD (like for parrareled 1ohm subwoofer)?
Do you know where I can see a good design of selfoscilating with feedback before LC (like IRF amp) but with discretes like your UCD design?
Hi,

Yeah I forgot to mention R73 could also be scaled for higher voltage... that's not to say I didn't overlook something else as well.

I'm convinced pre filter feedback is inherently inferior so it's not something I've ever tried.

If you really want to drive 1ohm loads I'd say I'd say use a fully differential full bridge. It's easier to start small and work up though.

Sorry, you won't see many fully discrete class d's. I think you can learn alot from that example I gave you though. Try downloading LTspice for free and go to town with it. It will let you learn about how each part of the circuit interacts with the whole of it. Being fully discrete, every small change affects everything else to a certain extent, and a simulator is a great way to learn about it.

Cheers,
Chris
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Old 23rd March 2005, 07:00 PM   #34
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Quote:
Originally posted by analogspiceman


I have several concerns about your opamp circuit. It appears that nothing prevents in from over driving and severely Zenering the BE junctions of the following differential PNP pair. You really should consider grounding its positive input and running it in inverting mode with the minus input as a summing node at virtual ground (sum the input here as well). Then you could connect a couple of anti parallel diode strings (say two diodes in each string) from the output back to the minus input. This would ensure that the following PNP stage is never overdriven and would also keep the opamp from saturating and getting momentarily stuck to a rail. Once you have made these changes (and if you have hooked up your housekeeping supplies properly), then you should be able to (in fact should need to) add in an RC lead network in the "C2" position (something between 330pF and 1nF in series with 2.2k).
Hi analogspiceman,
the idea about the anti parallel diodes is cool.
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Old 24th March 2005, 02:39 AM   #35
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Quote:
You really should consider grounding its positive input and running it in inverting mode with the minus input as a summing node at virtual ground (sum the input here as well). Then you could connect a couple of anti parallel diode strings (say two diodes in each string) from the output back to the minus input.
I dont get it. From output to minus input is the feedback resistor place? What is the drawing looks like for the anti parrarel diode and its place?
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