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Class D Switching Power Amplifiers and Power D/A conversion

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Old 8th May 2006, 08:19 PM   #11
Pafi is offline Pafi  Hungary
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Eva! How do you paralell IGBT's? I mean how do you assure switching off in the same time, to keep equal Turn-off loss (considering typically positive turn off delay dependence with temperature)?
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Old 8th May 2006, 09:11 PM   #12
Tim__x is offline Tim__x  Canada
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Class-D with IGBTs? How do you get around the turn-off tail?
Could you show us your XOR dead time generator?
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Old 8th May 2006, 09:49 PM   #13
Eva is offline Eva  Spain
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Pafi:

It's all a matter of choosing the proper IGBT, see mine:
Click the image to open in full size.
They seem likely to get well together in groups


Tim:

Current tail length is shown as "tf" in the previous figure. I definitely can live with a 24ns tail of no more than 2A. Also, MOSFET devices show similar tails in practice because the internal resistance and inductance of the gate cell structure prevents the charge from being removed uniformly, and some parts of the die just continue conducting longer than others (usually the lower the Rds-on, the longer the current tail because lower Rds-on dies require more cells and are more complex).

Concerning the XOR splitter with dead time, you will have to figure out the schematic, but it's very straightforward. It's main feature is uniform propagation delays.
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Old 9th May 2006, 07:45 AM   #14
Pafi is offline Pafi  Hungary
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It seems to be optimized especially for paralelling! What type is it? Will you test if they can bear 2...3 times of a single device capability?
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Old 9th May 2006, 08:18 AM   #15
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IGBT´s suck for class d
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Old 9th May 2006, 10:11 AM   #16
Pafi is offline Pafi  Hungary
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Why do you think this? Don't you know up-to-date IGBTs? They have been designed especially for high freq class D! :-) (OK, not especially for audio.) They have advantages and disadvantages too. (Low conduction loss, low Qrr...) At high voltage IGBT is the most cost-efficient solution. And quality depends on controlling.
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Old 10th May 2006, 12:10 AM   #17
Eva is offline Eva  Spain
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Quote:
Originally posted by Pafi
It seems to be optimized especially for paralelling! What type is it? Will you test if they can bear 2...3 times of a single device capability?
Try to figure out first

I've already used a pair of them in a PFC front-end conducting up to 20A total during input rectified sine peaks, and there were no traces of hot spotting despite the two IGBTs being quite spaced in the heatsink.


Tekko:

The key is in actual device parameters. If you research further into the topic, you may discover that there are much more types of IGBTs other than the good old ones used to drive three-phase motors below 1Khz. You may also discover that MOSFETs actually suck for high voltage class D due to their clumsy and slow body diode, which becomes even worse for lower Rds-on devices. On the other hand, IGBTs don't have a body diode, so they require an external one that may be Hyperfast, SiC or whatever you like. Furthermore, there are also IGBTs with a built-in high performance diode, which come very handy to overcome space constraints.

On the other hand, most of these high voltage MOSFETs will just *explode* as soon as load current reaches 8A or 10A if you ever try to use them in a class D output stage with a 300V or 400V supply in the same way as people uses 100V devices in small amplifiers (which feature fast body diodes). Consider the body diode from the good old IRFP460 There are special high voltage MOSFETs with fast body diodes, but their Rds-on is two or three times higher than in standard devices (and the price increases by a even higher factor).

So long life to the high voltage switching bipolar transistor and its insulated-gate cousin.
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Old 10th May 2006, 04:45 AM   #18
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So long life to the high voltage switching bipolar transistor and its insulated-gate cousin.
Blaaaaaaasphemy!!!!!!!!!!

How 'bout those nice APT or whatever mosfets?

The body diode issue in mosfets does seem to force certain topologies /babysitting circuitry though.

I'm not sure it's restrictive otherwise?

I note your nice use of the buffers after the gate driver as well, you'll find most here dont' do that, when they probably should.

Regards,
Chris
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Old 10th May 2006, 05:42 AM   #19
Pafi is offline Pafi  Hungary
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How 'bout those nice APT or whatever mosfets?
Buy if you can! I can't.
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Old 10th May 2006, 05:47 AM   #20
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APT's IXYS are prohibitively expensive.....
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