No Y capacitors in our common mode filter because  the SMPS is non isolated? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 12:06:46 pm
content: Hi,We are doing a non isolated SMPS so please confirm we don’t need Y capacitors in our common mode filter?Vin = 34VVout = 48VPout = 312WIncidentally, we are using the Vicor DCM3623T50M53C2T00 DCDC module, though this post is not about that module.The SMPS will power a drone on a tether. The tether merely comprises “go” and “return” wires…no earth wire. The “return” wire will actually be earthed at the power supply that’s on the ground.Since our SMPS is non-isolated (ie we have directly connected the primary and secondary grounds of the DCM3623T50M50C2T00 module), then our Y capacitors are effectively infinite in value…..since a short is basically an infinite capacitance. Therefore, we obviously don’t need Y capacitors. Would you agree with this? We will just have a common mode choke at the module input.Incidentally, the Circuit ground will be connected directly to chassis of the drone.DCM3623T50M53C2T00 datasheet:http://www.vicorpower.com/documents/datasheets/DCM3623x50M53C2yzz_ds.pdf

author: wraper
author_rank: Unknown
timestamp: March 24, 2020, 12:31:48 pm
content: Non isolated PSU output does not mean you can get away without them. Actually when compared with PSU which has it's output grounded, it does not make any difference. Only when output is floating you might need additional Y capacitor.QuoteWhat the point if you don't need isolated output?EDIT: somehow missed it's low voltage input.

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 03:23:52 pm
content: QuoteThanks, ..in our case, we literally have nothing to connect the Y capacitors to....so we cannot use them. We can only use a common mode choke.QuoteIts the only DCDC module on the market which serves our required Vin, Vout and power.

author: wraper
author_rank: Unknown
timestamp: March 24, 2020, 03:28:23 pm
content: You don't use earth wire?

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 04:08:46 pm
content: Thanks, ..no we dont...well, our return wire is earthed on the ground.But we dont have a "3rd" earth wire, so to speak.

author: madires
author_rank: Super Contributor
timestamp: March 24, 2020, 04:24:35 pm
content: Quote from: treez on March 24, 2020, 12:06:46 pmSince the input is low voltage DC I don't see any reason for a class Y filter cap.

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 05:08:48 pm
content: QuoteThanks, but for some reason the "DCM design guide" , on page 37, shows Y capacitors being used with a DCM module.All the various Vicor DCM modules have low voltage DC outputs of 48V or less.DCM design guide:http://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*..*....*Woops, I’ve just realised that if I make our negative return conductor our chassis ground, then this will mean that the chassis and frame of the drone will end up conducting current along with the negative return conductor.Also, the downstream Brushless DC motors and inverters are likely to be using a chassis ground, which will likely be coupled to their power and return conductors via Y caps, as well as 1 Meg (say) resistors to stop chassis ground from getting induced up to a high voltage. Indeed the Inverters and BLDCs will need a chassis ground as they won't be able to connect their power return directly to chassis for the same reason that the chassis would end up conducting return current.As such, we will need to have, would you agree, a separate chassis ground on the PCB, directly under the DCM module, and hence we will need to have Y capacitors connected to this. -And use Common mode chokes aswell.

author: madires
author_rank: Super Contributor
timestamp: March 24, 2020, 06:23:59 pm
content: And suddenly there are much more things to consider. You should start by defining the design, e.g. if the drone's chassis should be the return path, different from the circuit ground or completely isolated. And when you figured out all the details you'll be able to answer your question yourself.

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 06:29:14 pm
content: Thanks,Please confirm that if (say) we connected circuit negative return directly to the metal outer chassis of the power supply in one place only....that would still be a nightmare as the chassis would then basically act like an antenna, spitting EM noise out all over the place?Do you agree that with our metal outer framed power supply, chassis ground and circuit return must really be separate? (ie not directly connected, but only connected with Y caps and high value resistors to stop chassis from getting induced up to high voltage?)

author: madires
author_rank: Super Contributor
timestamp: March 24, 2020, 07:20:54 pm
content: I'm not an EMI expert but I'd guess it's easier to reduce EMI when the drone's metal chassis isn't part of the circuit ground. In your first post you've mentioned that the PSU is located on the ground and that the negative terminal is earthed. If the PSU's metal case is also earthed (directly or via the negative terminal) there shouldn't be much trouble with the PSU itself. But the tether is a nice antenna. So I would add filters at both ends.

author: wraper
author_rank: Unknown
timestamp: March 24, 2020, 08:40:21 pm
content: Your application asks for boost converter. Not isolated module.

author: ocset
author_rank: Super Contributor
timestamp: March 24, 2020, 09:07:53 pm
content: Thanks, we are going to be using 40 of these in parallel, and can't afford all the components etc for 40 boosters....so just use 40 modules instead.....on page 87 of the DCM design guide, it shows chassis ground in yellow. Chassis ground is deliberately not connected to any circuit ground (neither primary or secondary ground). This I believe is deliberate, because nobody wants their chassis becoming a circuit power rail conductor.The yellow chassis ground pour is also shown directly under the noisy DCDC module, this is deliberate as it must “catch” noise escaping from the DCDC module and guide it back into the circuit conductors via the Y capacitors…would you agree?DCM design guide:http://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf

author: wraper
author_rank: Unknown
timestamp: March 24, 2020, 09:40:57 pm
content: Are you sure you want to connect 40 of them in parallel? If something wrong happens with load, consequences might be quite catastrophic. Also 12kW from single 36V power source sounds questionable.

author: ocset
author_rank: Super Contributor
timestamp: March 25, 2020, 05:49:24 am
content: thanks, the source is ultimately 700vdc......then theres a 6r cable ....then 10 "Blocks" in parallel.Each block is one BCM module feeding one DCM module.As attached......*......................*..............................................*Thanks, and rather than regulations being the guide here.... it all follows from the fact that the metal chassis of the power supply, ..1...can't be allowed to float (otherwise if could get induced up to high voltage)2...Can't be directly connected to circuit ground...otherwise chassis would be a circuit conductor...these two things tell that chassis must be connected to circuit ground via a 1meg resistor, and Y capacitors for EMC.

author: dmills
author_rank: Super Contributor
timestamp: March 25, 2020, 03:35:43 pm
content: Say rather that the chassis should be connected to the circuit negative at ONE point.This is very standard in audio doings (Possibly not with 300A in play, but still), took a while longer then it should have done for the right way to do this to be figured out, but resistors and caps are NOT the sane way to connect these things.Also a direct connection gives you some chance when the skystatic decides to smite you.

author: ocset
author_rank: Super Contributor
timestamp: March 25, 2020, 03:59:19 pm
content: Thanks, on page 87 of the DCM design guide, it shows chassis ground in yellow. Chassis ground is deliberately not connected to any circuit ground (neither primary or secondary ground). This I believe is deliberate, because nobody wants their chassis becoming a circuit power rail conductor.........A connection at one point seems good, but there are so many things on the drone that would need a local connection, eg 6 * BLDCs, 6 * BLDC inverters, multiple heatsinks, PSU, computer boxThe yellow chassis ground pour is also shown directly under the noisy DCDC module on page 87, this is deliberate as it must “catch” noise escaping from the DCDC module and guide it back into the circuit conductors via the Y capacitors…would you agree?DCM design guidehttp://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdfSo the vicor documentation has a chassis ground not connected to circuit ground, and i believe they have done this because of the problems i speak of here.

author: dmills
author_rank: Super Contributor
timestamp: March 25, 2020, 05:35:09 pm
content: Chassis ground and exactly what to do with it is a SYSTEM level design issue, it is way outside the scope of a supply vendor.You design your power distribution network connecting everything that deliberately sources or sinks current together and then connect that negative rail to chassis at one point. That eval board will likely be assuming some kind of isolated use case rather then non iso buck/boost.I am guessing at some sort of AUV rather then aerial vehicle?If so there is a lovely trap in that the sonar systems often operate in the same kinds of frequency ranges as the power converters, and class II dielectrics are piezo electric... Oh how we laughed when we hit the light switch and the sonar image disappeared to starboard (Lighting supply converter was mounted on the starboard side beside the sonar head).

author: ocset
author_rank: Super Contributor
timestamp: March 26, 2020, 08:53:00 am
content: Please consider this riddle of common mode noise in a non isolated SMPS...We have a non-isolated SMPS in a metal box, where chassis (metal box) ground and all the heatsinks are directly connected (at only one place) to circuit ground. (The SMPS only has a “go” and “return” connection (no separate earth wire).In this circumstance, our contractor says we cannot use common mode filtering. We say that we can, and that we can do common mode filtering with a common mode choke. We admit that Y caps are completely pointless, because there is nothing to connect them to. In fact, the circuit intrinsically has Y capacitors….the Differential mode caps now effectively act as Y caps, and the shorting together of primary and secondary grounds of the SMPS is like an infinite Y capacitor.Would you agree with us?

author: ocset
author_rank: Super Contributor
timestamp: March 26, 2020, 12:03:53 pm
content: QuoteThanks, this sounds like the best idea.…and since our system is quite physically large…its 50 DCDC modules in total for 12Kw and has multiple heatsinks, which all will be connected to circuit ground (at the one place)…..we realise that some of the modules may not be physically near to the single point where circuit ground and chassis are shorted together….as such, Really high frequency noise will not “see” this as a short, and will be connected to chassis via too high an impedance……..for these cases, I believe that we will still need local 1nF caps to couple “locally” to chassis from circuit ground…would you agree?

author: madires
author_rank: Super Contributor
timestamp: March 26, 2020, 01:35:37 pm
content: Have you thought about what will happen when all 50 DC/DC converters start at the same time?

author: ocset
author_rank: Super Contributor
timestamp: March 26, 2020, 03:26:18 pm
content: Thanks, yes, they have a very soft start....many 10's of milliseconds to get up to voltage.We need them to all start together otherwise if one starts first, it may see too much output capacitance.