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Old 2nd September 2009, 02:42 PM   #1
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Default class d with multiple rails

Hi,

does anybody know of any Class D amp with multiple rails switched like in Class H?

just wondering...

thanks,

Tamas
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Old 2nd September 2009, 03:09 PM   #2
nitrate is offline nitrate  United Kingdom
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Switched power rails?? Doesnt seem logical in classD, there is next to no advantage in it due to the rail to rail switching nature of the classD final output stage. In theory the quiescent disipation is very low anyway, unlike other classes where the savings in quiescent disipation justifies having switched power rails.

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Old 2nd September 2009, 03:11 PM   #3
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Check "Class-L".
There was a discussion about it over at the 41Hz forums but I doubt there is a readily available product atm..
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Old 2nd September 2009, 05:40 PM   #4
Pafi is offline Pafi  Hungary
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Tamás!

There are multi-level PWM power stages with multiple rails in industry for high power motor drive applications. For audio purposes it's not really good.
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Old 2nd September 2009, 10:07 PM   #5
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Quote:
Originally Posted by Pafi View Post
Tamás!

There are multi-level PWM power stages with multiple rails in industry for high power motor drive applications. For audio purposes it's not really good.
Hi PAFI,

do you think, it s not good enough for audio applications becouse of the relative slow correction of the global feedback?
(becouse the same in a class H where the basic topology is class AB, is just quicker, but elswhere very similar)

thanks,

Tamas
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Old 2nd September 2009, 10:21 PM   #6
wg_ski is offline wg_ski  United States
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Those motor drives are already going to be operating at a low frequency compared to a class D audio amp. Any global feedback employed would be painfully slow, even for a subwoofer amp.

If you wanted to eke out more efficiency in a sub amp (or get away with slow MOSFETs) lower the PWM frequency first before doing anything more elaborate. A sub amp may be quite happy with 10-20 kHz and switching losses should fall dramatically and there would be no need for any rail stepping. Unless of course, you were using those big industrial FET packs, and putting out 50kW (100 hp). You're not thinking about running 20 lab horns off a single amplifier, are you?
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Old 2nd September 2009, 10:49 PM   #7
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Hi,

In Fact, I dont want to have 50kW :-)

But maybe something around 4kW @4 ohms
The feedback is not really a problem, becouse you can use local (quick) feedback for the compensating of the rail switching.
but some benefits: lower voltage mosfets could be used (lower Rds on, lower gate charge, etc) and this could make efficiency better and overall part costs lower...
of course there could be used many other techniques used at the same time...

any toughts?

Tamas
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Old 2nd September 2009, 11:08 PM   #8
Pafi is offline Pafi  Hungary
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What I referred to is not really similar to Class H.

http://www.ece.utk.edu/~tolbert/publications/apec99.pdf fig.3

The problems are: it is difficult to drive the gates, it needs many transistor, and the diodes makes loss and distortion higher. The controll method is more problematic too.

If you really want to switch the supply rails of a conventional ClassD amp, then there are other problems. The switchings will go out. Class AB is not only much faster, but its supply rejection is basically much better, even without feedback. ClassD is basically a modulator, while an emitter-follower is a voltage generator, it's operation is essentially independent from supply voltage (as long as supply is significantly higher then output).

And finally: it doesn't worth the effort.
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Old 2nd September 2009, 11:24 PM   #9
Pafi is offline Pafi  Hungary
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Quote:
The feedback is not really a problem, becouse you can use local (quick) feedback
The cutoff freq of feedback cannot exceed half of the switching freq.

The best you can do is error feedforward.

Quote:
some benefits: lower voltage mosfets could be used (lower Rds on, lower gate charge, etc)
Yes, but you will need many other mosfets on supply rails, many capacitors, rectifiers, and complicated control circuit.
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Old 2nd September 2009, 11:50 PM   #10
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by ttako View Post
Hi,

In Fact, I dont want to have 50kW :-)

But maybe something around 4kW @4 ohms
Tamas
4kW at 4 ohms shouldn't be any real problem using conventional class D techniques. You just won't be able to buy a kit or a one-chip-wonder solution. Full bridge, 220-250V rail, 8 state-of-the-art TO-220 PDP-panel HEXFETs ($3 each) and appropriate driver circuts, 100 kHz. No need to resort to anything crazy - just a few months of homework and research.
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