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Old 1st May 2008, 08:00 PM   #1
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Default Class D amplifier

Hello,

I would like to create a class D amplifier which have to work between 5 and 12V. The PWM signal will be generated by a ST7 micro-controller. The class D amplifier will be connected to a 4 ohms loudspeaker. I know how a class D works , but I do not know how to calculate the different values of the Low Pass Filter components, if I have to use a half bridge or full bridge ...and so on. My problem is that I don't know how to choose the components of the sytem(MOSFET ...)

Thank you very much for your help and advice.

Cisco
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Old 7th May 2008, 11:49 AM   #2
acid_k2 is offline acid_k2  Italy
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hi Cisco

there are some problems for that:

1 - single supply voltage between 5 and 12V is a bit low for MosFET, I suggest normal bipolar transistors (a simple H-bridge driven by the ST7 could do the work)

1a - for such low voltage, I suggest full bridge tipology (greater output power)

2 - we don't know the oscillator frequency (for output LPF calculation)

3 - we don't know the output bandwidth required (for output LPF calculation)

Ciao,
Fabio
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Old 7th May 2008, 02:11 PM   #3
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Hello Fabio,

Thanks for your reply.
The oscillator frequency is 1MHz.
At the end of the system, we want to rebuild the signal so it is a low frequency signal from 20HZ to 18 KHZ.

Would it be possible to have advice about the way of calculating the values of the components or schematics to help me ?

Thanks a lot in advance.

François
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Old 7th May 2008, 07:01 PM   #4
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are you running the output frequency at 1MHz or is that the operating frequency of the ST7?
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Old 7th May 2008, 09:05 PM   #5
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1MHZ is the operating frequency of the ST7 (frequency of the intern oscillator).

Here is what I have understood of the system I have to create:

A PWM signal is generated by the ST7 at a frequency I have chosen. This signal enters a class D amplifier which is supplied by a voltage which ranges from 0 to 12 V. I know that I have to use a Low Pass Filter in order to withdraw the high frequencies generated by the frequency which I have chosen to do the sample in the ST7.

So the output frequency has to be the same as the input signal as I would like to do an audio class D amplifier. I mean first, my input signal will go in an Automatic Gain Control, then the signal will go in the ST7 in order to have the PWM and at the end, the signal will go in the class D amplifier so as to rebuild the input signal.

Thank you very much for your help and advice.

Cisco
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Old 8th May 2008, 02:32 AM   #6
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Cisco,

The ST7 you are using might be unsuitable for a full audio spectrum class D amp depending on if you have one with a 32MHz clock for the timers. If the ST7 uses the main oscillator as it's clock source and the timers are 8 Bit, it will have 256 distinct points to represent the duty cycle, meaning to get your full 256 points you will get a max PWM frequency of 3.906KHz (1MHz/256). I don't know of too many class D amps that run at that low of a frequency, but you might be able to make a sub amp with very low frequency output, but your output filter will be very large with a 4 ohm load. In order to get a good full range reproduction of the audible band you should have a PWM frequency of >150KHz, at least in my experience. In order to get a full 8 bit modulation, with a 150KHz PWM frequency you will need a micro with a timer clocked by a 38.4MHz or faster clock. If you do have one of the ST7's that have the timers clocked with a 32MHz clock, this would allow you to have a PWM output frequency of 150KHz with just under 8bit accuracy(211 duty cycle points) or you could do full 8 bit accuracy (256 duty cycle points) at 125KHz.
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