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Old 8th January 2008, 11:28 AM   #11
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Hi, BWRX, Kanwar,

I've simmed both your suggestions. Unfortunately, none can help.
R//with back diode can only delay about 10'ns, that's all.

Is there any other "passive trick" that can delay up to 100ns?
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Old 12th January 2008, 10:52 AM   #12
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You cannot push your circuit harder...om... your circuit seems as a lazy circuit Don't expect too much.
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Old 12th January 2008, 06:35 PM   #13
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Hi, Kartino,

You're right, it is a simple and cheap classD output . But the more I study the cause, I found out that this cross-conduction will happen to all classD output topology, since it is caused by body dioda of a mosfet, not due to a certain topology. If you use adjustable "on" dead time for upper and lower mosfet, this Trr can be pursued (not to happen) in expense of wider dead time (=more distortion).
In 100's of khz switching, this body dioda reverse recovery is really a problem. It makes cross conduction of 10's of ampere (although the time is very narrow, but enough to heat up the mosfet). This will not show up at idle, you have to burden the output stage to make it show up. I think the heat caused by this body dioda cross conduction is higher than Rds heat.
The only remedy so far is to choose a better mosfet, with low Trr and low Qrr (like IRF540Z), or choosing the difficult way, bypassing internal body dioda and use external 30ns dioda.
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Old 13th January 2008, 12:50 AM   #14
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Hi, Kanwar, whats your email?
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Old 4th February 2008, 02:52 AM   #15
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Default bypassing body dioda

Reading SMALA amps (Steven's URL), I saw this interesting picture.

Imagine this is an output stage of classD amp. There will be C // with Rload (to complete LC filter).

To bypass body diode, it is possible with this configuration. It saves 2 schottky series diode.

But it needs 2 inductors.

IC7+D1 works independently towards IC8+D2. We can pick 30nS diodes for D1 and D2, so the internal body diode of mosfets are not used.

My question is about the inductors.

Is it possible to use only 1 core, bifiliar winding for making both L1 and L2, or these needs independent cores?
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File Type: jpg bodydiode2.jpg (37.7 KB, 176 views)
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Old 4th February 2008, 04:04 AM   #16
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Thats BCA!!!!!!!!
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Old 4th February 2008, 05:39 AM   #17
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Hi, Kanwar,

Yes, it is BCA output stage. I guess the answer to my question is "No", BCA needs independent cores (cannot be on one core, bifiliar, made a transformer that way).

So, the body dioda is naturally unused in BCA output stage.

Anybody has tried this output stage with ordinary loop, the one used for half bridge (not BCA loop)?
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