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| resistor wattage for hotplugging snubber? |
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| Siwastaja:
--- Quote from: k8943 on April 07, 2019, 10:32:05 am ---Totally on board now. May I just double check something? "For a snubbing capacitance in parallel to your 2.2uF MLCC, 100uF is of course already an order of magnitude more than required." I thought we were taking 30uF as the capacitance required for snubbing on the grounds that experimentally that seemed to be what was necessary - using ceramics - to balance the impedance of the system. (And that value was characterised by the power supply, the cables and so forth - rather than the convertor.) Which would mean that 100uF was about 3 times more than "required"? --- End quote --- Well, you can see that if you snub 2.2uF low-ESR cap with another 2.2uF + R (1x), you'll reduce the overshoot from 100% to somewhere around 20-30%. At 14x (30uF) and properly tuned value of R, you are already at practically completely snubbed system with no overshoot (less than a few percent) - it's already overkill, from pure capacitance viewpoint. I have seen "rules of thumb" vary between 1x and 10x! I tend to go with 3xC, but you may need more for other reasons (when choosing an elcap for different parameters than C - namely ESR). If you are interested, research the math, or just go with something. In any case, as you have the 2.2uF on the BOM already, it would make sense to construct the snubber by placing at least two more (4.4uF or more). They could share a common series resistor, or you could use separate resistors (regarding the wattage; this would spread the heat as well). This kind of solution is often flexible in miniatyrized layouts, if you happen to have some "dead real estate" somewhere - compared to using one big component. |
| Siwastaja:
--- Quote from: k8943 on April 07, 2019, 12:03:42 pm ---Since the convertor is not yet switching <1ms there's no inrush current in this period. --- End quote --- Note that the converter itself doesn't take a lot of inrush, because it's a current mode controller (well, at least with a current limit!), so that it never goes over the specified limit by active limiting (mere 1.2A max by the datasheet). The inrush is from the input caps - the 2.2uF plus the snubber caps you add. For example, the 2.2uF cap would have an inrush current of about 2400A (assuming 24V input and 10mOhm ESR), but only for a very short time (time constant of 22 nanoseconds), and from a zero inductance supply - which doesn't exist, hence lower actual inrush current, for a longer actual time - and the overshoot and ringing you see! Your 30uF + 2R would have inrush current of 12A at 24V. This is for a very short time as well. Approximate with RC time constant of 60 microseconds. Now this is in a meaningful time scale, which is a real possibility with your cable inductance. Hence, it works for snubbing. For this inrush current reason, don't go overboard with your snubber caps. If you put, for example, a honking 10000uF electrolytic there, it would indeed give you a stable voltage rise with no overshoot, but chances are that you'll blow some small fuse later, or when you try to control your circuit through a MOSFET "rated for 10A", it will die on your 100A inrush because that capacitor would have ESR way below 1ohm, and capacitance large enough so that the spike last for a long time. Just some examples. Inrush limiting is more often needed in AC power line switchers, because they need to have large input capacitors to store energy because your AC supply is "dead" for such long time during every 50 or 60 Hz cycle. They have no choice but to store the energy if they want to output DC, necessitating the big cap. |
| Ice-Tea:
--- Quote from: Siwastaja on April 07, 2019, 11:52:05 am --- --- Quote from: Ice-Tea on April 07, 2019, 11:22:29 am ---Have you considered an NTC? They are made for this kind of application, so the surge rating shouldn't be an issue. And you could dimension it so that losses in steady state are a lot lower than a fixed 2-3 \$\Omega\$ resistor. --- End quote --- Sorry but you have completely misread the subject. *snip*. --- End quote --- No need to be pedantic about it. You have read the OP and have concluded it's standard snubber. I did not. That's all there is to it. |
| Siwastaja:
--- Quote from: Ice-Tea on April 07, 2019, 12:14:48 pm --- --- Quote from: Siwastaja on April 07, 2019, 11:52:05 am --- Sorry but you have completely misread the subject. *snip*. --- End quote --- No need to be pedantic about it. You have read the OP and have concluded it's standard snubber. I did not. That's all there is to it. --- End quote --- Sorry; in any case I hope being overly pedantic did give real information to the OP and the others. |
| Ice-Tea:
--- Quote from: Siwastaja on April 07, 2019, 12:21:13 pm --- --- Quote from: Ice-Tea on April 07, 2019, 12:14:48 pm --- --- Quote from: Siwastaja on April 07, 2019, 11:52:05 am --- Sorry but you have completely misread the subject. *snip*. --- End quote --- No need to be pedantic about it. You have read the OP and have concluded it's standard snubber. I did not. That's all there is to it. --- End quote --- Sorry; in any case I hope being overly pedantic did give real information to the OP and the others. --- End quote --- Heheh, that it did :-+ |
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