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Old 18th December 2008, 11:29 PM   #1
b_force is offline b_force  Netherlands
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Default Simple class-d problems

I'm building a simple class-d design for a project for my study (Bachelor of Physics)

The goal is to understand the principles and make a working prototype.

The idea is very simple:

Click the image to open in full size.

The triangle generator is also very simple (standard), something like:
http://www.interq.or.jp/japan/se-inoue/e_ckt16.htm

The generator works fine, the comparator works fine (is there another way to get an output without the 1k resistor?, because of the DC), and the mosfet driver works almost fine.

This is the low side output on the scope:
Click the image to open in full size.

This is the high side output:
Click the image to open in full size.

Looks a little bit strange?
But I can't find out where the problem is.

the output of the comparator:
Click the image to open in full size.

There is also another problem.
When I try to use the output MOSfets, the powersupply gets shorted.

Do I miss a point ??




edit:
(Mistake in schematic changed)
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Old 19th December 2008, 01:41 AM   #2
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What are you doing wrong?

The input ground and output ground for the driver IC can only stand a very limited dc voltage difference between them (i.e., they often are connected directly together). You must use a level shifter to send the comparator output down to the negative output rail if you wish to use a center referenced input ground. - a.s.
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Old 19th December 2008, 01:45 AM   #3
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Caveat: I didn't check the driver IC spec sheet as all or most all IR parts do not include a center ground, but perhaps this is an exceptional newer one.
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Old 19th December 2008, 05:46 AM   #4
ssanmor is offline ssanmor  Spain
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Hello,
Your circuit can't work correctly:

The COM terminal of your 2108 should be tied to -20V, and you need a separate supply for the chip Vcc, that must be around 15V but referenced to -20V With such low voltages, it is quite easy (just a 7812 or 7815 whose input is connected to GND, ground is connected to -20V and output goes to Vcc of the IR2108.
Additionally, you need to level-shift the PWM input so it swings from -20V to -20V+15V. This can be done with a transistor and a couple of resistors.
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Old 19th December 2008, 06:37 AM   #5
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In other words, what spiceman said.

An alternative is of course to use unipolar supplies throughout and AC couple your I/O, as the other side of the same stick.
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Old 19th December 2008, 07:12 AM   #6
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if -20V is stable, you may connect COM to -20V and VCC to ground.
If comparator is rail-to-rail you may not need a level shifter.
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Old 19th December 2008, 12:38 PM   #7
b_force is offline b_force  Netherlands
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Thank you all for the reply's.

I have connected the COM to -15V (just because I hadn't -20V stabilized) and the result where two fried IRS2108.

The LM311 will not work without the 1k resistor to +15V (I really don't see why that is).
The comparator is connected like the schematic.
But on the other hand, if I just need a level shift, than that is done by the 1k pull-up resistor after the LM311

@ ssanmor
Yes I saw that type of regulator on another post at this forum.
I've simulated and tested is, but didn't understand the technique behind it.

The output was -7V or something.
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Old 19th December 2008, 12:55 PM   #8
ssanmor is offline ssanmor  Spain
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Exactly, you need 12V referenced to COM (-20V). The 7812 regulator provides a stable 12V output between its center and output pins, so if you connect the center to COM, you will get 12V from its output to COM, right?
And in order to work properly, the 7812 regulator needs at least around +15V at its input, again referenced to the center pin, that's why you connect the input to GND, because you have 20V from GND to -Vss.

You measure -7V from GND, and that's about right, because you have -20V + 12V= -8V. (perhaps you have -19V negative rail).

Please note that the max. input to output difference in a 7812 is around 35V. Besides, you have to multiply this difference by the current consumption to find how much power the 7812 is dissipating. It should be below 1.2W without heatsink in order to get a safe operating temperature. So the max. drive current you can draw is 1.2W / (20-12) = 150mA, more than enough for your purposes, as you should need something around 50mA).

Regarding the level shifter, your pull-up is not suitable, you need something that translated the 0 to 15V output of the comparator to -20 to -5V approx. to enter the IR2108. You will find a lot of schematics for doing so in these forums using a single transistor.

Besides, you are not providing dead-time (I don't know if the IR2108 already provides some), so you may have cross-conduction.

These are the basic points to put it to work, then there a lot of details that you should take care of after it is more or less working. But let's go step by step.

Hope these explanations are useful.
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Old 19th December 2008, 01:02 PM   #9
b_force is offline b_force  Netherlands
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Yes that's very clear to me, thank you!

Allright, so the COM must be connected to -15V and VCC to "+15V".

I mean not really +15V, but at a LM7815, where the reference =-20V and the input is gnd.

Hopefully that fixes the problem.
(The connection point will be between the diode and VCC and the 2,2uF I think? )

But why is the LO output correct and isn't the HI ?
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Old 19th December 2008, 01:17 PM   #10
ssanmor is offline ssanmor  Spain
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The COM should be connected to your -20V, together with the 7815 center pint. Vcc pin to the output of the 7815, and regulator input to GND, that's it.

You won't have it running unless you put a level shifter in the input(s).

Regarding dead-time, you don't have to worry about cross-conduction it as it has 500ns internally, but don't expect good quality audio, it is simply too much.
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