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Class D Switching Power Amplifiers and Power D/A conversion

Power Transistor Choices (New to Class D Design)
Power Transistor Choices (New to Class D Design)
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Old 7th February 2012, 06:03 AM   #1
PabloPhoenix is offline PabloPhoenix  United States
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Default Power Transistor Choices (New to Class D Design)

Hi all. I'm not at all new to electronics, but I'm new to the design of audio amplifiers. I've gotten it into my head that I'd like to attempt to design a class D amp. And by "design" I mean without the use of someone else's class D chip. The more I read on this topic, the more I think that it may be wrong-headed to think that this is a good starting point in trying amplifier design. But I haven't changed my mind quite yet.

I've come to the conclusion that a good starting point in choosing components might be to narrow down my choice of power transistors, since this will ultimately determine the timing and voltages required from the previous stage. I'm currently considering a full bridge design that would use some specific Infineon transistors I've selected, primarily based on their rise and fall times, input capacitances, low cost, and some other specs. The two options I'm currently considering are as follows:

1. 4 of IPP034N03L (N channel)
2. 2 of IPP034N03L (N channel) and 2 of IPD50P03P4L-11 (P channel)

If I were considering a half-bridge design, I'd probably be unsure if these transistors were matched closely enough. Given that it would be a full-bridge design, I believe they probably are, but my worst concern about pairing them up is that the PMOSFETs might sort of drag down the quality of the whole system.

So my questions are:
1. Do any of you have any experience using Infineon's offerings in the OptiMOS line? If so, are they very good in the power stage of a class D amp?

2. Given the rise and fall times of these transistors, am I correct in assuming that shoot-through would be less of a risk in a design based on them, as long as timing is done in a reasonably good way?

3. Is it reasonable to think that these two specific transistors would work pretty well together in a full bridge, so long as I'm not trying to push them harder than what the PMOSFETs can do?

4. Supposing I'm completely wrong about this, what are some better options and (if it won't be obvious) why?
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Old 7th February 2012, 05:46 PM   #2
darkfenriz is offline darkfenriz  Poland
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1. These are reasonable transistors, however logic level is a bit of a problem. Low 'logic' Vgs threshold favours fast switch on and slow switch off, opposite of what you'd like to have in a class D amplifier. Miller capacitance can also pose some issues although this is not a big issue at 30V. P channel is much worse in every aspect, but the worst is very limited Vgs swing, +5V/-16V of Vgs can get induced over internal inductances when switching fast and kaboom for no apparent reason.

2. You can get no shoot through with either slow or fast ones, it' more a matter of tweaking rise/fall times vs. dead time. BTW charges are more informative compared to switching times.

3. Yes, aside from the possible exceeding of Vgs of a Pmos.

4. You're not, generally. I suggest you went for all N channel though.

Regards,
Adam
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Old 7th February 2012, 08:44 PM   #3
PabloPhoenix is offline PabloPhoenix  United States
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Adam,

Thanks for your input.

At the moment, I think I'm leaning toward using four NMOSFETs. The ease of generating the signals for well-matched NMOS and PMOS is still very appealing and it seems like generating the signals for 4 NMOSFETs would be kind of a pain, but finding a really good NMOS/PMOS pair seems difficult.

Do you have any specific recommendations for transistors that work well on the output stage of a class D?
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Old 8th February 2012, 12:46 AM   #4
MOER is offline MOER  United States
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Default Infineon

Dear Pablo,

We use the Infineon IPP200N15NG3 with rail voltages up to +/-65v in a totem output configuration.

These are the best 150v N channel MOSFETs available for class D. Low Qg, high current, low RDs.

We have used tens of thousands and they are super reliable. We run our class D amplifiers at between 400-500KHz.

No matter what MOSFET you use, dead time is a must.

Other parts we use are IRFB4212 (100v), IRFB4227 (200v), FDP2572 (150v), FDP3682 (100v) FDP42AN15Ao (150v)

Steve

Zed Audio
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