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Class D Switching Power Amplifiers and Power D/A conversion

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Old 22nd March 2005, 05:20 AM   #21
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The schematic is in post #9. http://www.diyaudio.com/forums/attac...amp=1111291589

It is an initial trial, after reading IRF classD amp, I wanted to try feedback before LC, seems more understandable to me (than self oscilating after LC-feedback like UCD).

I think for heavy loads (like 1ohm parrareled speaker played at full power), feedback before LC is more suitable.

I haven't make PCB yet, everything is P2P connection
This is the photo of the driver (IR2111). The emitors of MPSA06 of driver buffer is cut, and I insert 47ohm like Subwo1 suggest.

Analogspiceman, your Guestimate of UCD, is it working in real CCT or you just SIM it only?
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Old 22nd March 2005, 09:15 AM   #22
MikeB is offline MikeB  Germany
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Quote:
Originally posted by phase_accurate
Have you measured how fast your "hand-sewn" comparator and level-shifter works ? I don't know how the IR 2110's input logic reacts to slowly changing input signals. Maybe you should increase the current and decrease the resistors in said part of the circuit.

Regards

Charles
Hmm, according to the datasheet the ir2110 has schmitttriggers
at it's inputs, so unless the signals are not heavily shifted the
quality of these signals should not matter much ?

Mike
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Old 22nd March 2005, 12:13 PM   #23
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In this case the amp can be made hysteresis-controlled self-oscillating which wouldn't be a bad start. In this case also the 10 k can stay. Maybe the gain of the diff-stage could be reduced as well in order to make it behave less like a comparator (like Ivan's circuit).

Why not try it with open loop first, using an external "clock" in order to check the behaviour of the switching stage first. In a second step try it with pre-filter NFB. If everything is O.K. the loop can be rearranged for post-filter NFB.

If everything is working fine - but you are still not content because the circuit looks much too simple - you can add a double NFB-loop !

Regards

Charles
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Old 22nd March 2005, 09:05 PM   #24
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Hi lumanauw,

Here are a few observations about your circuit:

Small electrolytic capacitors are generally not suitable for use as part of a class d amp's output filter due to their being unable to handle the power lost in their series resistance. Their lifetime is loosely inversely proportional to internal temperature, so as long as the temperature is not too high, they will still work, but maybe for as short as hours or minutes. The series resistance does more than just cook the capacitor, however. It also serves to dampen the resonance of the output filter (which may be partially why you got immediate failures when using a 2.2uF film capacitor). You could check this by adding a small resistor (3 ohms or so) in series with the 2.2uF capacitor when using it with the 200uH inductor. Your circuit has other problems, however, that could lead to failures regardless of the damper resistor.

Your schematic does not show the details of your housekeeping power supplies. Exactly how are they implemented? With resistor-Zener strings? Or lab bench supplies? (Bad hookup here could also lead to failures.) By the way, I assume your NE5532 also has been provided with a -15 volt supply, correct?

Note that the IR2111 driver IC is contributing a lot of delay to the forward signal path. Its deadtime, its output transition times, plus the slewing time on the input required to overcome the hysteresis may add up to as much as 1us. Due to the equivalent phase shift this represents, your maximum possible operating frequency will be limited to well under 250kHz (your practical maximum operating frequency will probably be closer to 100kHz).

I have several concerns about your opamp circuit. It appears that nothing prevents in from over driving and severely Zenering the BE junctions of the following differential PNP pair. You really should consider grounding its positive input and running it in inverting mode with the minus input as a summing node at virtual ground (sum the input here as well). Then you could connect a couple of anti parallel diode strings (say two diodes in each string) from the output back to the minus input. This would ensure that the following PNP stage is never overdriven and would also keep the opamp from saturating and getting momentarily stuck to a rail. Once you have made these changes (and if you have hooked up your housekeeping supplies properly), then you should be able to (in fact should need to) add in an RC lead network in the "C2" position (something between 330pF and 1nF in series with 2.2k).

What is the configuration of the mosfet output power circuit traces and how do you connect in the drive signal?

There are more possible issues, but I don't want to overburden you and would rather wait for a little feedback before running off down a possible dead end, anyway.
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Old 23rd March 2005, 02:18 AM   #25
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Hi, Charles,

Quote:
(like Ivan's circuit).
In the Amploid schematic by IVX, I don't understand the level shifter section. Output of opamp is connected to emitor of BC856 while it's base is tied to -15V. How much VBE will this transistor experience? I think more of 0.6V will happen in that BC856.

Quote:
In a second step try it with pre-filter NFB. If everything is O.K. the loop can be rearranged for post-filter NFB
For NFB point taken before the LC filter, don't we need something like U1+C17+C18+R23+R26 in the iraudamp1.pdf? They call it Delta Sigma 2nd order integrator. What does this integrator do here?
I don't saw this on after LC feedback like UCD. Only need R+C parrarel with feedback resistor?
I feel that feedback before and after LC needs different CCT (this is from my current knowledge about classD). Or do you think they are the same, so with the same cct, we can take feedback before and after LC?

Quote:
looks much too simple
Charles, because I wanted to learn about classD, I feel that this cct is too complex. Too many non-basic parameter that makes the amp fails. I will post my idea about very basic classD.

Hi, Analogspiceman,

Quote:
Small electrolytic capacitors are generally not suitable for use as part of a class d amp's output filter
Ahh, that explains why they heat up so quickly (in seconds)

Quote:
Your circuit has other problems, however, that could lead to failures regardless of the damper resistor.
Yes, I know something(s) are not right, I just dont know what they are.
Quote:
Your schematic does not show the details of your housekeeping power supplies. Exactly how are they implemented? With resistor-Zener strings? Or lab bench supplies? (Bad hookup here could also lead to failures.) By the way, I assume your NE5532 also has been provided with a -15 volt supply, correct?
NE5532 have -15V, I forgot to draw it. The 12V (relative to -50V) is generated by LM317 (from 0 to -50V) for powering the IR2111.
Quote:
(your practical maximum operating frequency will probably be closer to 100kHz).
It's OK with me for learning. IR2111 has automatic dead time ( no need to adjust) about 650nS. For F=100khz, what value of LC filter do you suggest?
If I have understand about how classD works, I dare to make 400khz cct.
Quote:
I have several concerns about your opamp circuit
You're right. I feel that opamp add much complexity for learing about classD. I the next post, I will post a very simple classD, just to learn how classD works.
Quote:
then you should be able to (in fact should need to) add in an RC lead network in the "C2" position (something between 330pF and 1nF in series with 2.2k).
This is my big question. I cannot put anything in C2 position that not-making the mosfets destroyed. From 100pf, 1k+220pf, they all makes my experimental amp failure. I can only put R1 or R3+C3 to adjust the switching frequency without destroying the mosfets. This is very contradictive to UCD papers.
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Old 23rd March 2005, 02:27 AM   #26
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Hi lumanauw,

I now understand the 12 volt supply on the negative rail, but you neglected to mention where the +/-15 volt opamp supplies come from or how they are hooked up. - a.s.
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Old 23rd March 2005, 02:41 AM   #27
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NE5532 (opamp) in my first experiment classD seems to add complex bananas. I should be able to do self-oscilating without using any opamps. This is the very basic classD I draw to learn how a classD works. Will figure A works as self-oscilating classD?
NFB is taken after LC, so it might have to use R3-C3 in the feedback resistor.

I couldn't resist (even I have not knowing wheter A will work or not), but I'm intrigued by the possibility of this design. It can make NelsonPass's concept of Susy, like I draw in B. This option works only if A works.

SuSy can eliminate absolute distortion that the speaker sense, because the speaker outputs are taken floating from both outputs, if both outputs have the same distortion, the speaker wouldn't sense it.
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Old 23rd March 2005, 02:45 AM   #28
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Hi, AS,

Sorry I forgot. +/-15V comes from emitor followers with NPN/PNP transistor which their bases is hold at 15V with zeners, relative to 0/ground. (the exact voltage is not +/-15V, but 14V4 more precisely). The transistors are TIP31/32C, to hold 14V4 from +/-50V only for powering the opamps.
After reading about BUS-Pumping in iraudamp1.pdf, I'm afraid this method of generating +/-15V from main rail is not good. Or is it OK to use this way to power the opamps?
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Old 23rd March 2005, 03:31 AM   #29
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Hi Lumanaux,

Try something like this, it's about as simple as UCD gets, not best, but simple. It works. Sorry it's a bit of a mess, I think R40=1K. I'm just using an air coil, I don't really care about EMI right now, it's low power, doesn't heat up, doesn't saturate.. does the job.

I have the output stage working in P2P right now, not with the exact values shown but it's also not the exact mosfets shown.

The comparator portion is still on a breadboard, will be doing that in P2P tonight as well.

I find it's nice to start with something that you know works, and then go from there.

Regards
Chris
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Old 23rd March 2005, 06:00 AM   #30
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Hi, Chris,

Thanks for the schematic. I have to study it further.

There are some components I don't understand.
-R78+D80, what are they for? Helping to fill C34 besides the work of D84?
-R71 and its lower 100k, what are they for?
-R75 = 40k, what is it for?

If it is to be used for higher rail (+/-50V), what values to be changed?

I tried IRF640N in my experiment cct, and they blow up far more easily than IRF640 (without N). For classD, IRF (without N) is more appropriate than the ones with N suffix?

A question. Is it a must to construct classD (any cct with fosc >250khz) with SMD component? Or classD can works with ordinary component too, like 1/4W resistors, TO-92/TO-220 transistors at the drivers?
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