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| Normal electrolytic capacitor in place of low and super low ESR electrolytic cap |
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| samimi_electronics:
Hi, I'm working on designing a SMPS. I used the software PI Expert to get a base design based on what I need. I'm currently building the schematic presented by the software based on a TNY278PN TinySwitch-III. Anyhow, there are three filtering caps along one inductor for filtering the output after the rectifier (schematic attached), two of which (C6/C7) are specified super low ESR and one (C8) specified low ESR. The problem is here in Tehran, low ESR and super low ESR caps are not available. My question is that: Can these capacitors be replaced by general purpose electrolytic capacitors? Would it cause instabilities? Why are they specified so? I'm not proficient at SMPS design and this is my first attempt at building one. |
| Miyuki:
You will get huge ripple voltage (ESR x Transformer output peak current) And also this ESR will cause heating of caps Can be replaced with more general purpose ones or help with something like film cap |
| samimi_electronics:
Oh, that makes sense. Thanks. In other words, the low ESR caps are specified to reduce heat losses at the high frequencies and to increase working age of the capacitor. But I don't understand why would it cause a huge (bigger) ripple voltage compared to low ESR? (ESR x Transformer PK-Current) Why is it proportional to the transfo's peak output current? |
| T3sl4co1l:
--- Quote from: samimi_electronics on December 08, 2018, 08:27:41 pm ---(ESR x Transformer PK-Current) Why is it proportional to the transfo's peak output current? --- End quote --- Yes, that is the definition of Ohm's law, V = I*R. It is also exacerbated by the flyback topology, which has the transformer current going between zero and full, every cycle. (The improvement there would be a push-pull, usually forward converter, design, or a multiphase design.) Do you not have access to Chinese sources? Western suppliers are pretty obviously off limits, but you're still not exactly a deserted island, as far as I know! Film caps aren't very useful here on account of the large values required (>10uF?). General purpose electrolytics can still be used, but you need many more of them in parallel, maybe >5,000uF worth, and then the compensation has to change, and everything becomes a bulky mess. Tim |
| DaJMasta:
Haven't tried myself and don't know if it applies to this use case, but what about using standard electros for the replacement and then bypassing them with a lower ESR cap or two in parallel (maybe a small film and/or a ceramic)? It wouldn't really help with the low frequency ripple, but if this is an SMPS secondary side, the frequency would be pretty high for a 1500uF electro to have that low impedance, so the smaller value low impedance caps would help for filtering out the higher frequencies. |
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