| Electronics > Beginners |
| SMPS Ringing Help |
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| exe:
I'd suggest start with the layout proposed in the datasheet. Basically, the idea is to minimize "loop area" where high current/voltage is switched. So, on your layout the diode should be much closer to the coil and ground polygon (see suggested layout in datasheet). There is plenty of documents in Google about this ("smps noise"). Perhaps, not a bad start would be this article: https://www.eetimes.com/document.asp?doc_id=1279232 . There is also "PCB LAYOUT GUIDE" in the datasheet. BTW, I don't think compensation will help here, it does a different thing. I think there are only two ways to reduce the noise of a buck smps: reduce parasitics, and/or reduce switching speed. Hard switching edges will produce more noise. Just my two cents, never built an SMPS myself. |
| T3sl4co1l:
The layout is actually really good, assuming you don't have anything else tied into it (like an enable wire). I would like the diode closer to the regulator, like wedged inbetween the reg and boot cap, but it's really not terrible where it is. (It could actually affect compensation, in that the compensation network is laying over the regulator-diode switching loop, which makes it susceptible to its own noise. Hopefully, it's made to deal with this, and will be okay.) What's good about it? You have the input and output at the same place, with caps stacked up around ground. This shunts as much high frequency content as possible, to ground, at a single point, so that there is little difference in the RF voltages at the three pins. There will still be some RF leftover, which I guess is what you're seeing. Maybe it's quite a lot after all, since it's getting through the 20MHz BW limit. The simplest solution is to follow up the input and output with an LC filter. Not much will be needed, maybe 1uH and 10uF (ceramic)? Tim |
| exe:
BTW, what is the load? I'd suggest try with a purely resistive one for tests. With minimal wire length. |
| mgbolts:
Thanks T3sl4co1l and exe. I appreciate the input. The reg will have a typical load of around 0.7amps at 5v although its designed to take up to 1.5amps with active cooling. My other switching reg designs have always put the reg to the left, inductor upper right and diode lower right..... not that different to the datasheet incidentally! I did not have the same ringing issues with this approach. Given I got an improvement by 'upgrading' the diode, it does seem to be indicator of something going on with the diode. I will move it and report back in a couple of weeks. BTW, part of the application is for RF TX/RX so its good hear your comments on RF characteristics of the caps..... Cheers |
| mgbolts:
Seems I misread your question. It was tested under a load of 0.5amps with an electronic load, the ZKE Tech EBD-A20H. Have to say I love this thing, works a treat, great build quality compared to other cheap loads out there. I am surprised that it has not been given more attention. I think I paid around US$65 on taoboa http://www.zketech.com/pd.jsp?id=18 |
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