smps voltage ripple for feeding UcD amplifiers - diyAudio
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Old 6th October 2008, 11:42 AM   #1
zilog is offline zilog  Sweden
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Default smps voltage ripple for feeding UcD amplifiers

I am trying to evaluate before drawing the prototype PCB whether I have designed for too much output voltage ripple or not.

The smps is supposed to feed a pair of 200W UcD modules (not hypex, but very similar in design in all important areas). Both modules are fed from the same capacitor bank at the output from the SMPS, and are thus exposed to cross-talk (I dont know if this is a problem since I have not tested for it yet). The SMPS is supposed to work in an automotive environment, and I have calculated worst case temperature swing for the ESR of the output filter capacitors, which will give a worst case ESR-based ripple of 200mV, best case ESR ripple of 70mV, and a constant C-based ripple of 24mV. This totals to a worst case ripple of 224mV, and best-case of 94mV (less ripple the warmer it gets). The frequency of this ripple is 106kHz.

Is this too great an amount of ripple? I can meet all rms-ripple demands for the output filter with this little capacitance, and adding more capacitors to keep ripple down makes routing the pcb difficult. Handling rail pumping is not an issue since I have provided measures for dealing with that by way of an additional balancing smps.

The output filter is 50uH/680uF, Ton*Von/L is held constant at 3.46A by the control loop.

In addition to my calculations, I have 680uF capacitance per rail per amplifier module as well, but I have chosen not to include that into the calculation as the amplifier modules also might change in the future, and I want to have the option of applying filtering that might decouple this storage from the control loop.

If forced to, I think I can fit 2x680uF per rail before going out of PCB space, but that will also be at the expense of a few more cm^2 of current loop area :/

Any input on this?
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Old 7th October 2008, 02:37 PM   #2
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Bruno Putzeys posted something about current in local capacitor http://www.diyaudio.com/forums/showt...313#post662313
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Old 7th October 2008, 02:51 PM   #3
BWRX is offline BWRX  United States
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What is the PSRR of the modules at 106kHz?
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Old 7th October 2008, 03:29 PM   #4
zilog is offline zilog  Sweden
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Quote:
Originally posted by BWRX
What is the PSRR of the modules at 106kHz?
I have no idea, I guess what I am curious about is how the feedback loop of the UcDs will handle this, and what kind of phase locking that might result. Also interested in whether beating might take place or not.

I actually managed to route in 2x680uF per side, so the ripple is expected to be only half of that during high output power, and lower at low output load (the ripple is calculated for the case when the output inductors are on the verge of going dry)
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Old 7th October 2008, 07:40 PM   #5
BWRX is offline BWRX  United States
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Beating probably would not cause any noticeable effect unless the switching frequency of the UcD-like module and the SMPS get close to one another (about 20kHz or less).

When Hypex releases the specs for their SMPS check them out and see how your SMPS compares to them.
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Old 10th October 2008, 06:42 AM   #6
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Anyone ever tried synchronizing SMPS to sefl-oscillating class D amp?
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Old 10th October 2008, 02:18 PM   #7
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I think one could do it by asynchronous divide-by-2 circuit followed with a wide band PLL.
A divider eats most of a modulation, a PLL provides "clock" at startup, overdrive and other strange occasions, eats rest of modulation/jitter too.
What do you think?
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Old 10th October 2008, 04:01 PM   #8
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Hi, Darkfenriz,

Your post is too complicated for me. Do you have a schematic for that? Who is the master, the smps or the classD amp?
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Old 10th October 2008, 04:35 PM   #9
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Sorry for the quality of MS Paint schem.

P.S. You are a moderator now?
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File Type: png 1.png (15.2 KB, 399 views)
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Old 11th October 2008, 02:30 AM   #10
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Hi, Darkfenriz,

I got your idea now. The amp will be still selfoscillating and smps will be the slave. Will the classD amp able to maintain constant switching frequency?

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