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Class D Switching Power Amplifiers and Power D/A conversion

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Old 11th May 2003, 12:24 PM   #21
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Quote:
You need a duty cycle limiter to prevent shoot through on clipping.
The cct isn't subject to "normal" shoot-through, in fact at zero output both fets are on 50% duty cycle, but are on *at the same time*, not alternately. That would be curtains for a normal totem pole pwm output stage. If I have missed your point please tell me.
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Motorola has an app note with a discrete design, Linear Technology has a driver IC with it built in.
This I would like to see. Could you supply a link to save me searching?

Meanwhile I'll go out to the shed (it's about 9:25pm Sunday night here and heading toward Winter) and take a few photos like I promised. Should have them posted in 20 minutes or so.
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Old 11th May 2003, 02:03 PM   #22
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Old 11th May 2003, 02:04 PM   #23
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Old 11th May 2003, 02:07 PM   #24
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Both TO-247 100v schottky's shown. Note no heatsink. Output filter cap is 100nF MKP.
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Old 11th May 2003, 02:11 PM   #25
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Default ILLUDIUM Q-36 explosive space-modulator.

PWM controller chip. Vertical pcb has other stuff but I am only using two voltage regs.

/Circlotron - never uses simulators or builds vapourware.
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Old 12th May 2003, 07:56 AM   #26
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Quote:
You need a duty cycle limiter to prevent shoot through on clipping.
NO, you definitely don't !!!

Regards

Charles

P.S. The duty cicle limit in the Motorola AN1042 is due to the fact that the output devices are not DC coupled which is a bad idea generally, because you need to limit duty cycle .......
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Old 12th May 2003, 01:18 PM   #27
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"The cct isn't subject to "normal" shoot-through, in fact at zero output both fets are on 50% duty cycle, but are on *at the same time*, not alternately. That would be curtains for a normal totem pole pwm output stage. If I have missed your point please tell me."

You're right, its been a while since I read the Crown white paper.

For information purposes:

https://www.onsemi.com/pub/Collateral/AN1042-D.PDF

Fig. 6 pp. 13:

http://www.linear-tech.com./pdf/11602fa.pdf

For most switching amplifier topologies shoot through is a problem.

Tripath, for instance, really lowers the switching frequency as the duty cycle approaches 50%.

The LT part actually senses when the off state drain to gate charge has dropped below the gate to source threshold voltage and inhibits the turn on of the other side.

The Crown topology doesn't need this, brain-fade on my part (I already knew the answer but couldn't come up with it in time to keep from sticking foot-in-mouth).
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Old 12th May 2003, 01:37 PM   #28
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Aww, that's cool djk. Anyway, fired it up again tonight with a more robust 40-0-40 volt supply, changed the current sense resistors to 40 mR and organised a better quality audio feed. Power output is now 133 watts into 6 ohms (my test resistor) woohoo! That was 40 volts peak. from about a 41 volt rail. There is still something tricky though. I could only get that power output on a tone burst. If I ran it continuous tone I could get perhaps half that power and then it would go sort of unstable. One clue though is if I touch the metal tab of the schottky that is under the pcb furthest from the edge, the one who's tab is flapping up and down 80 volts, the fuzzy distortion I spoke of the other day gets worse, and if I touch it harder the amp cuts out till I let go. The input wires to the pwm chip are unshielded and only about 2 inches away. Maybe some HF is getting back into there. Have to have a look-see another day.

80v p/p on the load resistor was a sight for sore eyes. Oh yeah.
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Old 12th May 2003, 01:42 PM   #29
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Quote:
For most switching amplifier topologies shoot through is a problem.

Tripath, for instance, really lowers the switching frequency as the duty cycle approaches 50%.
Shoot through is a problem for most switching amplifiers, that is right - but it is almost indepandant of duty cycle.

Regarding the Tripath amps:
It is vice-versa, i.e. for no input signal the switching frequency is high and it is getting lower with increasing signal level (as usual for sigma-delta modulators).

Regards

Charles
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Old 13th May 2003, 02:37 AM   #30
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Good man.......!

The Tripath parts seem to have the reputation of

1.) Not sounding good, and then

2.) Being fixed to solve that problem, but at the expense of shoot-through. I can not confirm that though.

Jocko
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