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| resistor wattage for hotplugging snubber? |
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| k8943:
Have various boards with a small buck convertor at the input. Over a few weeks blew 3 of them whilst plugging into a 24v line, despite the fact that the max input voltage of the buck is 28v continuous, 30v peak. Checked on the scope and indeed was getting some serious ringing at the moment of hotplugging, the first spike going well above 30v. After some experimentation discovered that 3 * 10uF 50v 1206 ceramic capacitors in series with a 2-3 ohm resistor does a pretty good job of snubbing this in a variety of circumstances. (Those ceramics seem like a space and cost effective solution.) Thing is, during testing was using chunky through-hole resistors because it was easy to chain them together. The whole charging thing is all over in 50-100uS so am guessing that not enough energy flows to do much damage to any kind of resistor but nonetheless did the following approximate calculation: watts = total energy stored in capacitor / time = 0.5 C * V*V / time = 0.5 * 3 * 10uF * 24 * 24 / 100uS = 0.5 * 30 * 10e-6 * 24 * 24 / 100 * 10e-6 = 86 watts That is quite a large number and so feel a bit silly looking at 0.125W 0603 resistors on Digikey. But then why take a 0.5W resistor? How should I validate a resistor wattage decision? (Only calculations found on the net pertain to steady switching in convertors rather than a one off hotplugging event.) Taking 86W and setting it equal to I * I * R where R = 3 ohms, get a current of 5A (for a very short time). Hmmm. Or do we take the total energy and use the mass and specific heat of the resistor....? Or just try and if a little 0603 resistor doesn't blow it's OK? |
| Siwastaja:
If you want to do it properly, look at the single pulse (non-repetitive) handling characteristic curves on the actual resistor datasheet. Of course, not all manufacturers provide such full curves, and sometimes they are hidden behind another application note sheet, but by digging around a bit, you'll find curves for a very similar (same package, thin film vs. thick film, same nominal power rating...) product, which you can use as a reference by applying a bit of derating margin, even if it for a different manufacturer, or a different product series. If you are space restricted, special resistors that can handle much more pulse current than the typical same package counterparts exist. They are more expensive, though. Note that often the option of using a bog standard electrolytic capacitor does exactly the same, but in a cheaper, smaller and simpler package (one integrated component). The ESR is used as the R. This is important because typically for such snubber, large capacitances (at least 3x the total ceramic no-ESR capacitance) are needed. |
| k8943:
Excellent. Exactly as you said found curves (attached - for anyone reading along!) after a few tries. Will now re-look at electrolytics. Had already noticed that 100uF crushes it, so now it's a search cost, can size and possibly a smaller value. Most of the time they don't bother to give an ESR so guess it's down to trial and error. Thanks a lot! |
| Siwastaja:
Electrolytics tend to be "inherently stable", i.e., their capacitance - resistance ratio never makes them oscillate/overshoot on their own; this is even true for most "low ESR" electrolytics, which still have considerable ESR. Over full temperature range and product lifetime (i.e., increased ESR at cold temperature, and/or very old capacitor), you may want to use a bigger (than the minimum suggested 3x ceramic C) elcap so that the ESR won't be too high. But elcaps tend to have a lot of excess capacitance available very cheaply, so often just designing in 10x the ceramic C, while also derating voltage, is not a problem. Do note that if the device you are snubbing is a switch mode converter, e.g. a buck converter which produces quite heavy input capacitor ripple, and if the ceramic capacitance you are snubbing is not quite large enough, the added elcap could be significantly contributing as the switcher bypass cap, heating up internally due to its ESR. (The same issue is possible for your discrete resistor case!) Make sure this is not an issue, since this power dissipation would be repetitive and continuous. Testing is often the only way here, because it's very hard to simulate all the parasitics correctly to see how the ceramics and electrolytics share the AC ripple current. |
| k8943:
Drawing 150mA current at 5v from this 0.5A max buck, driven at 24v, measured circa 240mV p-p ripple on its input cap and about 1/3 of that on a 100uF electrolytic. The input cap is a tiny 2.2uF ceramic as recommended by the TI web designer for this configuration. Not sure how to quantify 80mV p-p on the electrolytic as too much or too little but had already internalised that: a) it's better to put 2 or more electrolytics to achieve target capacitance "just in case"; b) case size and price does increase moving from 100 -> 300 uF; c) perhaps larger electrolytics are better suited to through-hole mounting since it's rather against their nature to build them for a stint in a (hot) reflow oven? In this application, hotsplugging of boards is an infrequent event (essentially linked to maintenance or configuration), it would seem desirable to minimise investment in input protection. If the strategy is to use the minimum cost/component volume possible, is there a case for the ceramic setup based on: 1) if there is heating it's in an external resistor and not drying out electrolytic => if the energy loss can be tolerated then has no importance and so the capacitance can be designed to handle the hotplug-spike but under-designed from the perspective of the convertor input interaction; 2) no worries about finite lifespan linked to inherent component properties; 3) natural component redundancy (since 3 times 10uF ceramic far cheaper than a larger ceramic or tantalum); ? |
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