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Our SMPS is non isolated so we don't need Y capacitors in the common mode filter?


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We are doing a non isolated SMPS so please confirm we don’t need Y capacitors in our common mode filter?

Vin = 34V
Vout = 48V
Pout = 312W
Incidentally, 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:


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Most Helpful Member
Are you and your company why there are no power supply manufacturers in country, like you sometimes say?


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Yes i guess so.
And do you agree, that we can only use a common mode choke in our common mode filter?...because we quite literally have nothing to connect Y capacitors to...so can't use common mode filter capacitors (Y caps)?

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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.

....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?)

Also, ....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:
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Most Helpful Member
A question that is probably addressed in your country's electrical code. Getting permission from the net won't impress your inspectors.
what? I put " shortbus= " as the authoritative reference on many of my design drawings. Medical parts, high voltage parts, industrial control safety interlocks, all have your name on them. The authorities and, on one case, Lawyers, looked at it and all essential said, "he posts a lot, he must know his crap." See, all is good and everyone is safe.


Well-Known Member
A question that is probably addressed in your country's electrical code. Getting permission from the net won't impress your inspectors.
Thanks, rather than regulations being the guide here.... it all follows from the fact that the metal chassis..

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.
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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?

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