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Old 26th October 2009, 01:06 PM   #1
ndsonu is offline ndsonu  India
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Default Class D amplifier Power Output

Hi all,

I want to design a class D amplifier using the audio controller from IRF. I want a 25W X 2 amplifier.I want to find out how we can design an amplifier for a given output power requirement.My question is like is there any mathematical expressions that give the output power from amplifier based on parameters like supply voltage,modulation index(i guess this is correct..:-)) etc....Could anyone please help me..
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Old 26th October 2009, 01:34 PM   #2
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Quote:
Originally Posted by ndsonu View Post
... mathematical expressions that give the output power from amplifier...
Yup. Ohm's Law.
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Old 27th October 2009, 02:05 AM   #3
ndsonu is offline ndsonu  India
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Default Class D amplifier Power Output

i agree that (Vrms)^2/RL gives power.But how to get the Vrms of a PWM signal?
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Old 27th October 2009, 02:41 PM   #4
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A PWM signal is just a higher frequency AC signal than a normal audio signal, nothing special in that.

What is special and much harder to calculate is the efficiency of the amplifier.

The power consumption of any amplifier will be the same all else being equal but the output will vary with efficiency.

P(max drain) = V^2 / 2Z

P(max out) = P(drain) x effeciency

Efficiency of all amplifiers vary with output power and load (which in turn varies with both frequency and output power).
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Old 27th October 2009, 08:35 PM   #5
Pafi is offline Pafi  Hungary
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Quote:
P(max drain) = V^2 / 2Z
Sorry, but this is not true. There are 3 errors in it at least.

Quote:
But how to get the Vrms of a PWM signal?
The RMS value of a bi-polar PWM equals to its amplitude, but this is irrelevant. What is relevant, is the RMS of the filtered output signal, wich is practically the same as the output of a linear amp. If modulation index is not limited (I mean 0...1), then the peak value (almost) reaches the supply voltage. If limited... guess!
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Old 28th October 2009, 07:06 AM   #6
ndsonu is offline ndsonu  India
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Thanks for your replies....
Have a look at this link Understanding Class-D amplifier power supply requirements | Audio DesignLine
I also thought the way Pafi thought about it.
About filtered signal in the case of a single ended amplifier (assuming I fed a sinewave as input..) the output across the Loudspeaker will be = (Vsupply*Modindex*.707) ..Am I correct?
If I am correct then why many parameters contribute RT = RLOAD + ROUT + RIND + RPCB + RPS in the link ....i am a bit confused..Am I slightly wrong somewhere...???
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Old 28th October 2009, 04:08 PM   #7
Pafi is offline Pafi  Hungary
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ndsonu!

Vsupply is not constant in reality, it drops if you load it, this is manifested in RPS. And peak output voltage is only near the supply voltage, but doesn't reach it (normally). There are drops on MOSFET, inductor, etc... which are proportional to their resistance, however normally they are not very significant (0.5...5 %). These are what I referred to when I wrote "almost".
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