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Old 19th February 2012, 04:13 PM   #1
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Default clueless novice in need of advice

Hello I am attempting to design a discrete class D amp as part of a project and as the title of thread suggests I don't really know what I'm doing. Unlike most of the posters here I am not concerned with brilliant efficiency or high fidelity just a functioning amplifier. It would be greatly appreciated if anyone could give me a little guidance and hopefully some re-assurance I'm going in the right direction. Currently I'm trying to work out the output stage which is a full bridge design as shown below.
Click the image to open in full size.
*the load is an 8ohm speaker

If I have a PWM signal at 250 kHz alternating between +/- 5V what would be the best way to drive the MOSFET's? I know from what I've read that using N-channel MOSFET's for both the high and low side gives better results but the additional circuitry for level shifting seems a bit beyond me.
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Old 20th February 2012, 12:59 AM   #2
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MrDude,
Zetex used to have an eval board for their driver chip, using complementary pairs in a full bridge arrangement like you want.
Try locating zxcd50steval.pdf.
Blank out all the protection circuitry and notice how the pairs are driven via two caps from a 12V PWM signal.
E
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Old 20th February 2012, 01:40 AM   #3
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One way of doing it using a triangle wave generator and audio signal going into a comparator. The output is then level shifted into a phase splitter for the two pwm signals.
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Old 20th February 2012, 02:10 AM   #4
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Quote:
Originally Posted by mrdude View Post

I know from what I've read that using N-channel MOSFET's for both the high and low side gives better results but the additional circuitry for level shifting seems a bit beyond me.
Use IR2113 to drive N channel FETs
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Old 20th February 2012, 03:29 AM   #5
Eva is offline Eva  Spain
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Your concept schematic is only suitable for high supply voltages because it works as a power "voltage follower".

Swap N-ch and P-ch, and D and S connections, and analyze the resulting circuit.
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Old 23rd February 2012, 11:05 AM   #6
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Thanks for the replies, I had a look at the data sheet for the mosfet driver Nigel suggested and though I can't be sure I think maybe using just n-channel mosfets may be easier. I drew out a little diagram to try and get my head around what the driver actually did and how I could use it. Hopefully my understanding isn't too flawed.
Click the image to open in full size.

Thats for a half bridge, I don't know how a full bridge would be connected. I'm guessing you have a second driver but with the low and high outputs connections reversed or something. Also I couldn't see anything about it on the datasheet but does it have ample dead time? I noticed that this driver can handle very high voltages, I take it that it works fine with 5V or does it need a higher supply voltage? What sort of n-channel mosfets should be used with these drivers? Sorry I'm asking so many things but these things don't seem to mentioned on the datasheet.
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Old 23rd February 2012, 11:46 AM   #7
marce is offline marce  United Kingdom
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The easiest way to get into class D is find a reference design for your controller of choice and copy it. It will help you get to grips with the complexities and generally give you a layout to follow, as class D and SMPS's require the correct layout to functio and not pollute the world with EMI noise.
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Old 25th February 2012, 08:13 PM   #8
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mrdude,
you may want to check out this thread on a simple design using a complementary pair:
Just for fun: UcD on a shoestring
-E
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