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Old 12th May 2007, 09:21 AM   #1
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Here I go again. In my first baby steps I learned that I was not really happy with the driver IC. But also not with selfoscillating in general.
The disadvantages of self oscilating are generally the possibility of lock ups and no chance to synchronize multiple chanels. This missing synchronization can lead to audiable beating frequencies if multiple chanels are used at the same time.

My new concept is avoiding this. Furtheron it is using standard components. And the synchronization is tranformer coupled in order tp get rid of HF errors in larger systems and may be when synchronizing with SMPS. In such systems GND at one end is never the same like GND at the other end...

The heart of the my new concept is a level shifting comparator which delivers the drive signal for a standard Halfbridge driver, i.e. RI2110.
Furtheron I am planning to split the feedback, which allows to have some db feedback even across the output filter without instability. If I look to some first simulations then I can achieve 5db...10db feedback from behind a 2nd order output filter.
Another clue is the logic supply for the IR2110. It is derived from the comparator output signals. This ensures that the IR signal treshholds are directly linked to the IR input signals. This is giving a very good stability of the dead time vs. temperature and supply voltage changes.

The following schematics are without output filters, without supply for the drivers and without rail caps.
This is keeping things simple and shows the principle.
Up to now still theory, but I am starting with the PCB these days.....
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Old 12th May 2007, 09:22 AM   #2
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...the first half bridge...
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Old 12th May 2007, 09:23 AM   #3
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...and the second halfbridge....

Feel free to reassemble all three circuit parts to full schematic.
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Old 12th May 2007, 09:25 AM   #4
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...
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Old 12th May 2007, 09:31 AM   #5
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The level shifting comparator has a propagation delay, below my measurement limit (10ns).
The falling/rising edges do slope within 50ns.
The attached picture shows the comparator outputs when operating the system at 1MHz.

Please note I used 1:10 probes. So the scale is 1V/grid.
Time base: 200ns/grid.
This speed slew rates are already at the limit of my 30MHz bandwith scope. So the real sloping might be faster....
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Old 12th May 2007, 09:33 AM   #6
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In fact it is nice to see that the comparator does provide this performance even in this set up
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Old 12th May 2007, 12:57 PM   #7
Eva is offline Eva  Spain
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Old 12th May 2007, 01:34 PM   #8
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Some more information:

The PWM signal is a traditional triangular signal. The design of that transformer should take two points into account. a) Providing a low HF impedance to drive the emitters. b) Low distorsions, because it is influencing the distorsions of the amp and there are only about 20db feedback to cure such things...
The magnitude at the secondary side of the transformer should be around 1Vp. This is resulting in a open loop gain of 55 . If we want 20db feedback the closed loop gain is 5.5 . To drive this amp I am planning to use some OP amp stage to deliver the required input signal levels.

Also I do not plan to run it with 1MHz. This was just for playing around with the comparator. 300kHz...400kHz should be OK in the end. This would also fit nicely with a synchronized SMPS, which could run with 1/8 of that.

The output filter is not fully finalized. I am also thinking about a 4th order filter. In that case I would run the 3rd&4th order completely outside the feedback loop.

...and of course I want to settle an overcurrent protection which is using the SD inputs - still thinking if I want it latched or with auto reset.
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Old 12th May 2007, 01:49 PM   #9
Pierre is offline Pierre  France
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Hello, ChocoHolic.
Thanks for sharing your design.
Have you tried with that mosfets in real-life? What switching times are you getting?

Also, how are you producing the dead-time?

Good luck with your project!!!
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Old 12th May 2007, 02:10 PM   #10
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Hi Pierre !

Up to it is not ready
But when design is ready and everthing is fine, then I offer to give also the gerbers for everybody who wants to have. Hm, needs patience... my projects are moving slowly. This class D thing will probably need one or two years..
Unfortunately I have to work to make my living...

Regarding switching and real life results with that MosFets you can have a look to my babystep thread.
In short form it turned out that the possible switching speed is by far higher than what I can handle with respect to self generated disturbances. di/dt around 1kA/us and higher are troublesome. Already the TO220 package inherent inductances do cause several volt inductive peaks.... So I am slowing down the switching speed on purpose and adjust the dead time with the gate drive circuit. Slow ON, fast OFF.
Typical half bridge sloping times will be around 50ns and current commutation time 20ns..30ns.
Dead time I will adjust at the limit to some decent cross conduction peaks. I found that even peaks up to 8A/60ns are OK from thermal stability (only minor thermal drift), but will probably adjust lower in order to need less cooling.

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