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  1. #1
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    Non isolated SMPS does not need Y capacitors?

    Hi,
    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.

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    Re: Non isolated SMPS does not need Y capacitors?

    We quite literally have nothing to connect Y capacitors to...so cannot use them. Would you agree?

    - - - Updated - - -

    ..*....*..*....*..*....*..*....*..*....*..*....*.. *....*..*....*..*....*..*....*..*....*..*....*..*. ...*..*....*..*....*
    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.

    - - - Updated - - -

    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?



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  3. #3
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    Re: Non isolated SMPS does not need Y capacitors?

    Everything is relative.....

    If your chassis is ground and your reference is ground, there is no antenna effect.

    Brian.
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    Re: Non isolated SMPS does not need Y capacitors?

    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.
    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/...uide-VICOR.pdf



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    Re: Non isolated SMPS does not need Y capacitors?

    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.



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    Re: Non isolated SMPS does not need Y capacitors?

    If I understand the problem correctly:

    1. you have a 'chassis' (presumably the outer casing of the base unit) that should have no signals on it referenced to ground.
    2. You have two unshielded wires from the box to carry power to the drone, one positive, one negative.
    3. Your concern is that both these wires behave as antennas and will radiate EMC from the PSU in the base unit.

    When I say 'base unit' I am referring to the fixed part of the system rather than the flying part.

    Questions:
    CAN the chassis be grounded - is there a reason it must be isolated?
    What powers the base unit, is it external power (AC line) or internal (ie isolated) power from batteries.?
    Putting aside other considerations, CAN one side of the tether be connected to chassis?

    My questions are to see if the problem really exists and if so how to resolve it in the simplest way. If EMC really is an issue, the situation seems to me to be rather like a signal source sitting between ground and a vertical antenna and should be treated as such. The solution would be to filter the two (or one if it can be directly connected) power wires so they are at the same signal potential as the chassis. Whether 'Y' or any other capacitors are needed is a different issue and related to the initial power source feeding the entire thing.

    Brian.
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    Re: Non isolated SMPS does not need Y capacitors?

    CAN the chassis be grounded - is there a reason it must be isolated?
    the chassis cannot be grounded to the circuit negative, otherwise it will become a circuit conductor
    What powers the base unit, is it external power (AC line) or internal (ie isolated) power from batteries.?
    Base unit on ground is ultimately powered by an AC mains generator
    Putting aside other considerations, CAN one side of the tether be connected to chassis?
    No we cannot connect tether to chassis, as the chassis would then be a circuit conductor

    The main reason for putting in common mode noise filtration, is to stop noise which could interfere with the electronics on the drone.



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    Re: Non isolated SMPS does not need Y capacitors?

    What I'm not grasping is WHY the chassis can't be a circuit conductor. Making it so would greatly simplify the EMC problem as there would be no voltage difference across the critical points.

    Nevertheless, if the chassis has to be isolated, your problems is the EMC voltage is generated between the tether cable and the chassis itself. The solution is to use common mode filters in the cable, as close to the chassis as possible. You want the DC to the cables to be intact but effectively short out any AC between either wire and the chassis. You could use 'Y' capacitors but standard types may not be adequate when the tether acts like a vertical antenna. I don't know how long the tether is likely to be but I would think of it as a half wave at RF and design filters to suppress frequencies in that region rather than treat it as a normal SMPS AC line filter.

    I presume the chassis itself will have an adequate 'real' Earth on it, by a stake in the ground or similar. If it doesn't, you will have all kinds of problems if you rely on EMC being conducted away through the generator wiring.

    Brian.
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    Re: Non isolated SMPS does not need Y capacitors?

    Thanks, the drone won't have a real earth on it as such...the drone will be at the end of a 100 metre tether...this just comprises the go and return from the 700vdc power supply thats on the ground.
    The chassis of the drone will accomodate the PSU, the computer box, the BLDCs and their inverters and all the heatsinks. Each will need to be coupled to chassis, but if chassis was connected to circuit ground at each point, then for example, common mode chokes and sense resistors, etc etc in the circuit ground wire may get bypassed by the chassis connection.
    Also, since there is a 250 Amps of current flowing, there may be voltage drops in the chassis if it was a conductor, and the chassis would not be at an equal potential at each point.

    The vicor documentation has a chassis ground not connected to circuit ground, (eg as in #4 post above) and i believe they have done this because of the problems i speak of here.



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    Re: Non isolated SMPS does not need Y capacitors?

    Please consider this riddle of common mode filtering 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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    Re: Non isolated SMPS does not need Y capacitors?

    I presume the chassis itself will have an adequate 'real' Earth on it, by a stake in the ground or similar. If it doesn't, you will have all kinds of problems if you rely on EMC being conducted away through the generator wiring.
    Thanks, ..it would be too heavy to have an extra earth wire in the tether, along with the go and return wires.



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  12. #12
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    Re: Non isolated SMPS does not need Y capacitors?

    Hi,
    Regarding an SMPS with just a two wire feed (go and return), it is a fact that generally a non-isolated design will give a lesser common mode noise problem than an isolated design.
    Why is this fact not stated anywhere, in any book or website?
    ..............................*................... ...........*.............................*........ ......................


    Bottom of page 2 (2nd paragraph up) of this document
    http://www.ti.com/lit/an/snva489c/snva489c.pdf
    Says that ….

    QUOTE
    In a typical IC based non-isolated DC-DC SMPS, only two lines connect to the input port. So any current going in through one terminal has to go out through the other. In this configuration, the common mode noise VCOMM will always be zero.
    UNQUOTE

    This is simply not true. Even a two wire non isolated SMPS can radiate energy into its surroundings and thus the “go” current will not equal the “return” current.



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