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Old 28th October 2006, 11:11 AM   #1
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Lightbulb A new (Stupid?) class d idea

I got this idea last night of using a GDT instead of lets say a IR2010 which would spare the gate driver ICīs if the fets blow.

Now i only have IRFP460 but heres the schematic of my idea:
Suggestions are welcome.

Click the image to open in full size.
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Old 28th October 2006, 11:16 AM   #2
djQUAN is offline djQUAN  Philippines
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but how would you introduce dead time?

and also, remember the switching frequencies are also modulated by low freq audio so the driver transfo might saturate......
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Old 28th October 2006, 11:24 AM   #3
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I have ran this output stage setup with other stuff and havent experienced any problems with cross conduction, but that was a SSTC and induction heater so i dunno how it will behave here.

Only one way to find out.

But i am not sure on which input of the comparator the feedback should go, actually im not really a fan of self oscillating class d circuits as they tent to lock onto the rails when you push them hard.

Another idea i have is no GDT but putting an optocoupler on the highside gate driver. Also that would require a deadtime circuit, but i think i have one of those laying around on the drive, that one with XOR gates and RCD circuits.
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Old 28th October 2006, 11:26 AM   #4
djQUAN is offline djQUAN  Philippines
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I also thought about optocouplers before but it is very hard to find ones that can switch at high enough frequencies.
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Old 28th October 2006, 11:31 AM   #5
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Yah and they are also expensive.
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Old 28th October 2006, 11:41 AM   #6
djQUAN is offline djQUAN  Philippines
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so the best options are mosfet drivers specifically designed for the purpose or the UCD concept but we already know that.
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Old 28th October 2006, 12:20 PM   #7
Pafi is offline Pafi  Hungary
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Tekko!

If you limit output voltage to 40...60% of supply voltage, and if you modify the feedback, then theoretically it can work. Somehow...

"i am not sure on which input of the comparator the feedback should go"

It depends on the polarity of the transformer, but this fedback method is basically wrong. 2 correct way:
- 2nd order LPF in feedback loop with 200-300 kHz cutoff freq, or
- feedback from after LC, and phase compensation in feedback network. Check UcD!
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Old 28th October 2006, 12:29 PM   #8
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I was also thinking about a transformer drive for the power stage. But djQUAN's note about the low frequency modulation was exactly the reason why I skipped this idea. Already from theory the resulting number turns were unpractical.

Please note the transformer has to transfer the low frequency signal of music. Especially bass is causing issues, when driving at strong modulation, means high music signal levels.
Consider up to 95% modulation, close to clipping.
Consider about +/-22V peak output of your drivers... means your transformer has to handle 0.95x22V/1.41=14.8Vrms at may be 20Hz. Unsaint conditions for small signal transformers. This massive low frequency signal is existing in the gate drive signal in the same way as in the half bridge output....
If you ignore this, then at high levels you might run into trouble with poor gate signals and in worst case with killed MosFets. At low levels probably nothing bad would happen.
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Old 28th October 2006, 12:42 PM   #9
Pafi is offline Pafi  Hungary
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Maybe this way, or something like this.

Though I'm not a big friend of gate transformers. I tried everything, but I couldn't reach low enough leakage inductance. I ordered a sample from a factory, but it was much worse then mine.
Attached Images
File Type: gif trafo gate drive.gif (3.9 KB, 808 views)
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Old 28th October 2006, 01:13 PM   #10
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Iīll just sample a few HIP4080īs

There problem solved.
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