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Class D Switching Power Amplifiers and Power D/A conversion

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Old 13th February 2005, 08:41 AM   #1
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Default What is UCD?

Many threads here talking about UCD amp. But never saw any schematic about this. Is this DIYable, or must buy from Hypex, Netherlands?

I also don't understand looking at Philips patent. It seems that ordinary power amp (class AB/B) with LC filter at the ouput, and feedback taken after the LC filter, will automaticly becomes self-oscilating classD?

Oh, yes. One more thing. In many patents it is said Class AD and Class BD, for class D power amps. What is the difference between AD and BD?
First I tought it was about the pulse when 0 output (more narrow or more wide with 0 output). Then I think it has to do with toggling the supply rail. But what is AD/BD really mean?
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Old 13th February 2005, 09:24 AM   #2
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Hi,

I is DIYable but buying from Hypex gives you the real McCoy for a very reasonable price. Besides, it is patented technology, although not everywhere in the world. Here is the link to Hypex document describing the technology.

Basically you are correct. Very (ultra) fast classB amp could be transformered into ClassD amplifier. But look at it another way. Difference between Class A/AB and class D is like difference between opamp and comparator.

Output stage of class AD can have only two states Vsupply+ and Vsupply-, while Class BD has three possible states: Vsupply+,GND and Vsupply-. With zero input, Class AD has circulating current in output filter, while class BD does not.

Best regards,

Jaka Racman
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Old 13th February 2005, 11:23 AM   #3
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Quote:
Difference between Class A/AB and class D is like difference between opamp and comparator.
Thanks for the analogy. This explains it.
But from this analogy, I think what is needed not an amp with high speed, but more to high OL gain. Is this right?

Based on this, can any power amp with mosfet finals be treated with LC filter and feedback taken after LC filter to make it classD auto oscilating?

There is also difference between classD when 0 signal occur. Having wide pwm width or near zero pwm width. What is this difference called (Different class maybe?)
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Old 13th February 2005, 12:10 PM   #4
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Hi,

based on linear amp analogy, the most important parameters you are looking for in classD amp are:
low saturation voltage when output is driven into rails (clipping)
very fast rise and fall times (less than 50ns typically)
no storage time (instantaneous recovery from clipping)

Class AD has 50% duty cycle with zero input. With classBD you can have less than 50% but this requires 3state modulation.

Best regards,

Jaka Racman
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Old 13th February 2005, 12:15 PM   #5
ghemink is offline ghemink  Netherlands
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Quote:
Originally posted by lumanauw


Thanks for the analogy. This explains it.
But from this analogy, I think what is needed not an amp with high speed, but more to high OL gain. Is this right?

Based on this, can any power amp with mosfet finals be treated with LC filter and feedback taken after LC filter to make it classD auto oscilating?

There is also difference between classD when 0 signal occur. Having wide pwm width or near zero pwm width. What is this difference called (Different class maybe?)

You need very high speed since the amp needs to oscillate at 400kHz or so and need to switch the output transistors in 10-20ns or so. Say you have +50V and -50V power rail voltages and the output switches from one rail to the other in 20ns, then you talk about a slew-rate of 5000V/us, there are for sure not many amps that can do that. Besides, that you also need an amp with quite a bit of gain since you need to have "gain" for the feedback loop to work with.

So better not try this with any MOSFET amp.

Gertjan
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