Development of a "reference" class D starting point

[classd4sure]

Originally posted by subwo1


The comparison sounds interesting.

See what I can do It might be asking a bit much for the first day with a new program..If I can manage it though it will say alot for it.

Regarding being impatient...I already started skipping sections of the help files ...

I'd hate to have it cost me two coffees

Chris

[phase_accurate]

How did you go about playing with the gain of the loop without effecting the overall gain of the amplifier? Just a change in the RC portion of the loop?

By adding an LTF of the following form into the forward path of the circuit :


H = A0 * (jwT1 + 1)/(jwT2 + 1)

The netlist I sent to Bricolo already contains it
, hope he didn't get too confused. The additional circuit has a gain of 5 up to 20 kHz approx and then falls to 1 at about 100 kHz. Even though it is basically flat above that it has enough phase-shift at the switchibg frequency that the feedback cap has to be redimensioned if one doesn't want the switching frequency to drop too much.

I wasn't able to find Johan's double loop topology but I assume he does something similar.

For better understanding I post the schematic here (PSPICE users can mail me for the .SCH file).

Regards

Charles

[classd4sure]

Hi,

For your reference:

http://www.diyaudio.com/forums/attac...&postid=427287

He's not sure if that can work, neither am I. That actually is somewhat similar, isn't it?

Regards,
Chris

[phase_accurate]

Aaaaaah this one !!!!

Yes, definitely !!!!!!
You just have to use a resistor in series with C2 and another resistor parallel to said series RC circuit.
This would give an elegant all-IC solution (apart from the output stage of course) when a gate-driver IC is used. I would use a dual Op-AMP so there is one left to be used as a differential input buffer.

BTW the very same circuit can be used for carrier-based class-d amps with feedback takeoff from the filter, if dimensioned differently (hint: PID controller) !


Regards

Charles

[phase_accurate]

Playing around a little with the simulator and using ideal switching stages (i.e. voltage controlled voltage source) I had difficulties to get an UcD running.
With nonideal components however this is quite easy.
It is never a bad idea to develop a circuit that makes proactive use of the inevitable non-idealities of real-world components.
Since the intrinsic circuit delays help you building up the oscillation in an UcD you don't have to fight for extremely small delays like in a carrier-based class-d amp.

Regards

Charles

[classd4sure]

Hey!

I was just fighting with that myself, if I understand your french (It's 5am), we came to the same conclusion, I wasted all night.

Also not exactly profficient on Ltspice yet "singular matrix" errors....that's new.


Here's a few thoughts I had maybe you can confirm them unless you already have.

-If we use a comparator IC instead of the discrete version, we should use one with some delay to use to our advantage, (50ns) or have to add some hysteresis and that's ugly.

These little 10nS comparators dont' seem to do the job well.

Note the one Bruno said he used, 45ns...it's that ideal component argument.

-The other is perhaps an argument for going with a non inverting type (mixing the feedback with the input signal before the comparator input). These comparators dont' seem to have a very high common mode voltage range? I can't even find it in the datasheet for the Lt1116 I tried......and it is 5am so I could be all wrong

@Subwo1
I'm not in love with it yet, but I dont' hate it either
It's got some real nice features that's for sure.
Time for a break, went braindead hours ago.

Regards,
Chris

[phase_accurate]

These little 10nS comparators dont' seem to do the job well.

That's my thought as well ! But don't hesitate to use them for carrier-based amps.

Regards

Charles

[subwo1]

Originally posted by classd4sure


[snip]These little 10nS comparators dont' seem to do the job well.

Note the one Bruno said he used, 45ns...it's that ideal component argument.

-The other is perhaps an argument for going with a non inverting type (mixing the feedback with the input signal before the comparator input). These comparators dont' seem to have a very high common mode voltage range? I can't even find it in the datasheet for the Lt1116 I tried......and it is 5am so I could be all wrong

@Subwo1
I'm not in love with it yet, but I dont' hate it either
It's got some real nice features that's for sure.
Time for a break, went braindead hours ago.

Regards,
Chris

Originally posted by phase_accurate


That's my thought as well ! But don't hesitate to use them for carrier-based amps.

Regards

Charles

Hmm, I have to give the idea of some comparators seeming to be too fast additional thought. Definitely, some can be too slow. Bruno said something like there being a possibility of including an additional RC time constant somewhere before the mosfet drive.

Chris,
If it did not have at least somewhat of an intuitive user interface, it would not have a chance with me.

[johanps]

Let me sum up what I've gathered so far from the occasional requirements debate.

* All-in-one Class-D chips are not interesting.
* Single-ended output with dual supplies should be OK. Although H-bridges has distinct advantages, they come at a cost people seem unwilling to take for the "starting point" circuit.
* As for power levels, limiting it is a good idea for beginners, although some expressed that there's no point with class D if power ability is below some level. Break-point seems to lie around 100W (?).
* Double-sided PCB is not a requirement per se, but a must for performance! Try to keep upper-side for ground plane mostly.
* Try to use only through-hole components (?). SMD has size/space advantages, and most likely performance ones too, but save that for the "ultimate" circuit ?
* Separate input+control loop from output stage, for easy replacement/refinement of each.

Did I forget something important? Or misrepresent something/someone?

To comment on the power requirement debate, I believe that we can come up with something that does handle up to 100-200W, although it *will* require much more care to wire things up and get it going. (EMC 'niceness' may be lost somewhere on the way up there :-)
That doesn't preclude a beginner to start with +/- 15, or +/-25 volts and get the circuit going without a hitch. So it's more of a documentation/positioning thing...

Regards / Johan

[johanps]

Originally posted by phase_accurate
According to Bruno it's not a good idea at all to use low ESR caps since they tend to increase ringing instead of reducing it.

I guess there's a time and place for everything - could someone provide some insight about when to use low-ESR caps, and when to avoid them?
(Or on second thought - don't; there's a another forum for that .
Perhaps someone has seen a good thread about it, or wants to start one?)