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Class D Switching Power Amplifiers and Power D/A conversion

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Old 30th May 2005, 03:19 PM   #121
MikeB is offline MikeB  Germany
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I'm sorry that i am a bit out of time...
Maybe you should discard c7, now your deadtime is ~150ns,
this might be a bit much ? I added it to increase deadtime...

Mike
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Old 31st May 2005, 01:04 AM   #122
sovadk is offline sovadk  Denmark
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By using some more components, I've been able to get some better results.

Output slew rate is between 2V/ns to 4V/ns with +/-36V supply. The transistors are rated to 5V/ns.
Worst rise time 38ns, best rise time 30ns
Worst fall time 64ns, best fall time 46ns
I believe that the difference between rise times or fall times are due to the output inductor helping or not with the switching. If this it right, the deadtime is somewhere arround 64ns-46ns=18ns. Looking at the waveforms I've spotted what looks like 30ns of deadtime.
Better results can probably be achived by fitting with the zenor voltages.

I've changed the comperator stage a bit, to improve the slew rate.
Schottky diodes has been added to the output stage, to improve current spikes while switching. These can probably be left out, since it dosn't matter that much.


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Old 31st May 2005, 03:05 AM   #123
sovadk is offline sovadk  Denmark
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I've also made a N-ch version of the output stage. It performs as well as the N-ch + P-ch stage with two IRF540 transistors. Switching times and dead times are about the same.
As simulation with two modern IRF6665 N-Ch transistors gave me switching times down to 10ns and deadtimes in the range of 20ns, without any optimizingn.
It seams like these fully descrete amps can be made to perform very well, at least in simulations.

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Old 31st May 2005, 02:31 PM   #124
sovadk is offline sovadk  Denmark
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Heres a version using two n-ch IRF6665 mosfets in the output stage. I've replaced the mosfet in the driver logic with a bjt. It actually performs better.
By looking at the waveforms, Deadtime + switching time seems to be arround 20ns for both rising and falling edges.
I've very surprised that this is possible with a fully descrete design. Now does anybody know where mortals can get these IRF6665 transistors? Fairchildsemi makes the FDD3682 which performs very well (low gate charges and Ron), but IRF6665 has half the gate charge.
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Old 31st May 2005, 05:49 PM   #125
bcarso is offline bcarso  United States
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The 6665's should be available. They were pitching them to me last year. What was still not released was the reference design---there was still some work being done on the protection circuitry associated with the gate driver IC, in which the lower FET's voltage drop is being sensed on a cycle-by-cycle basis.

They do have astonishingly low parasitic inductances as well as the optimized gate charge vs. on R. A bit tricky to solder for DIY use however. They showed some data that dramatized the improvement in reduction of EMI between what had been considered low-L packages and the new package.

The claim was also made that a lot of the improvement in a prototype amplifier was due to the pin-strappable deadtime gate driver ICs they have come out with, and the role that feature plays in reducing jitter/noise in the gate drive.

I saw bench performance that was exemplary out of a relatively simple hysteretic converter using an LM318 in the front end. I speculated that a quieter front end, perhaps a hybrid discrete/IC design, could bring their noise floor down even further, and they agreed.
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Old 31st May 2005, 07:19 PM   #126
sovadk is offline sovadk  Denmark
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I really would like to try those fets. Do you know where they can be bought?
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Old 31st May 2005, 07:29 PM   #127
bcarso is offline bcarso  United States
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I have been meaning to give the rep a call anyway, so I will ask him. They ought to be in distribution by now, although I don't recall offhand who is on top of the new parts. Of course Digikey will have them eventually, but I don't think they are high on the priority list for relatively new parts.

Maybe I could cadge a few samples out of them.
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Old 31st May 2005, 07:56 PM   #128
sovadk is offline sovadk  Denmark
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Please do all you can.
I'm just a student, working with this stuff in my spare time. Not a very good status if I'm going to convince sales rep.
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Old 31st May 2005, 07:57 PM   #129
MikeB is offline MikeB  Germany
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Hi sovadk !

That's strange, in my sims i had better switchingtimes with the
schmitt-trigger in the comparator ?
You will find out that realworldcircuits behave much more complicated
than in sims... Do you have a scope ?

Mike
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Old 31st May 2005, 09:06 PM   #130
sovadk is offline sovadk  Denmark
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After I added C25 and Q5 the difference wasn't that big. I've also tryed to add a two feedback 33pF capasitors to the comperator to give it a hysteresis behavior, but it didnt make any difference.
I could imagine though, that hysteresis would be a good thing IRL.
Anyway I do have a scope (200Mhz), but I don't have the time to build this the next few days.
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