| Electronics > Projects, Designs, and Technical Stuff |
| Choosing the best capacitor for switch mode power supply |
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| T3sl4co1l:
Heh, nice, LED strobe. That explains the bulk! Yeah, don't worry about the ESR, "good enough" is just whatever you need for the LEDs, not even the switcher. Again, if it's making a lot of noise, you'll most likely need filtering anyway. And with all those caps, you have, well let's see, about nine opportunities to make one heck of a strong CLCLC... filter. ;) (Mind, you'd use quite small L in this case, to keep the filter impedance sqrt(L/C) low.) Tim |
| Conrad Hoffman:
The numbers may be academic until you figure in trace length and the size of the loop. |
| arildj78:
If I'm adding an inductor in series with the output (as in an LC filter), wouldn't this contribute a lot to the emitted magnetic field? Also, would such an LC filter need me to adjust components in the SMPS such as the compensation network? |
| T3sl4co1l:
Inductors are small, and often shielded. No problem. If you wanted to use planar magnetics (PCB traces as inductors, with or without cores), that would tend to have more external field, and better shielding may be a concern. Indeed it may affect the compensation. You can do a few things: - Keep the cutoff relatively high (only somewhat below Fsw) so the filter still tends to act as one big capacitor as far as the control loop is concerned. - Draw AC feedback from the front of the filter, and DC feedback from the output (basically, regarding the voltage sense divider resistors: split the top one in half, and connect a relatively large capacitor (10nF+?) from the front of the filter, to the new resistor midpoint). Or really, since your load is pulsed, AC regulation doesn't really matter (there's no way the regulator can keep up with the load anyway, during a pulse), so you can just take the divider from the front in the first place. - Tweak the loop components for best results. These are typically a bit experimental, anyway -- it's hard to solve for these values in a first pass, but easy to dial in with an oscilloscope. And again, yeah, definitely make sure the filter is well damped. As noted, you don't have much room for inductance to remain reasonably well damped from capacitor ESR alone, so you'll probably need to add some ESR, either as a chip resistor on some of the capacitors, or using a selected ESR+C value of another chemistry. I did a project last year, with similar ratings (well, at lower output voltage), but continuous output, and with a low noise specification. I used a 5 pole filter, using a mix of ceramic and polymer capacitors, and achieved ~0.1mV output noise over a wide bandwidth (not just the 20MHz bandwidth that all the PS mfg's use). At this level, a carefully considered layout is as important as the filter order and values are. Tim |
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