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  1. #41
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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Draining the static sounds like a good idea… at 300m we would see a potential of 30kV as you kindly say.
    It is a worry that we could stress the insulation of the 700V isolated power supply on the ground.

    Even on a nice sunny day, we could go up into a cloud which happened to be highly charged…and create a nice lightning bolt down the tether.

    This tethered drone looks like it has an armoured cable…and I suspect it is to act as a lightning conductor…just in case one strays into highly charged clouds (even on a sunny day).
    https://www.youtube.com/watch?v=GzTMaXfLFb4
    Last edited by treez; 8th March 2020 at 11:44.



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    a tethered drone has an armored cable so the armor takes the forces/stress/strain of tethering and
    the power/control/data wires do not

    the armored cable can also relieve the static build up to earth - it should not take the static
    all the way to the power and control, but should have a path to earth a few meters before
    the power and control to protect said power and control


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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Thanks, the tethered drone cables have a fibre jacket, (not metal) as in the attached
    https://www.gore.com/system/files/20...eet_013020.pdf
    I can see a problem here because the fibrous jacket cannot be used to take an earth connection to the tethered drone…and so we therefore cannot do common mode filtering.
    the armored cable can also relieve the static build up to earth - it should not take the static
    all the way to the power and control, but should have a path to earth a few meters before
    the power and control to protect said power and control
    We also cannot relieve the static build up to earth due to the fibrous nature of the outer jacket…so I believe we have problems.



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    The discharge current due to altitude is tiny and doesn't suddenly appear out of nowhere. As the drone climbs it stays discharged along it way so there is no build-up it suddenly has to survive. As long as there is a conductive route, even a high resistance one, from the outside to the power wires you should not have a problem. Even a resistor of say 1 to 10M will be enough to stop it charging up and a similar resistance at the ground end, it it isn't already Earthed will also suffice.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Thanks, i take it you mean say a 1meg resistor from drone chassis to drone negative_return, and on the ground, a 1 meg (say) resistor from drone negative_return to earth?

    As a summary........

    We are making an SMPS which will be at the end of a 300m cable.
    The Vin is 700VDC. This is from an isolated 700VDC source.
    Pout = 12Kw. (48vout)
    It will comprise “units” comprising of a vicor BCM module feeding four vicor DCM modules…..and these “units” paralleled in a whopping group of 10 to give the 48V, 12Kw output.
    We need to connect the chassis of the BCM modules to chassis ground or earth………also, the App notes state that under each DCM module there must be a copper plane, which is connected to either chassis ground or earth.
    The thing is, we don’t have an separate earth wire going to our product, and cant have this.
    We will simply be using 700VDC negative as our chassis ground.
    Also, we plan to connect the 700VDC negative to Earth…(so as to avoid this isolated output getting induced up to some high voltage).
    Do you think these measures are appropriate?

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

    Hi,
    This is about the Vicor DCM module. (a DCDC module).
    Page 87 of the DCM design guide (below) shows a chassis ground layer in the PCB, below the DCM module.
    We don’t have a chassis ground, (we also don’t have an earth connection) we just have a “power” and “return” from the 700VDC isolated power supply. Do you agree we should therefore put the power return connected copper under the DCM module?
    DCM design guide:
    http://www.vicorpower.com/documents/...uide-VICOR.pdf



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Hi,
    We are doing a 12Kw power supply. Vout = 48V, iout = 250A.
    We are planning to use ten paralleled combo’s of vicor module “Units”.
    The vicor module “unit” we have made is is one vicor BCM module feeding 4 paralleled vicor DCM modules.

    We will need to filter the inputs to the DCMs very well in order to stop beat frequencies between the 40 (!) vicor DCM modules from stopping the DCMs from operating.

    ..So we see on page 23 of the vicor “DCM design guide” (below) that there are options for damped input filters….all well and good so far……..super!................

    However, we then look at page 35 of the vicor “DCM design guide”…and we see an input filter inductor called “LDM_1” being used….this is just 220nH in value…the text states that its used because otherwise there would be high frequency noise currents and voltages in the DCM module input which would over-stress the input capacitors of the DCM, and cause it to fail prematurely…..

    ……Uuuuuuugh?

    …....how many SMPS’s have you seen where such a low value inductor like that is needed…and for that reason?.....this is creating serious concerns over the vicor DCM modules for us, and we are thinking of using Paralleled synchronous Boost converters instead..(eg using LTC3784).

    Another point about vicor DCM modules is that we are stuck with their 20khz crossover frequency…….and this places unfortunate constraints on the input filter, in that the output impedance of the input filter must be 10 times less than the Zin of the DCM right up to the crossover frequency……if we used Sync boosts with lower crossover, then we would be able to ditch this constraint to an extent, and not need the unusual “LDM_1” inductor aforementioned.

    The use of the “LDM_1” does tend to suggest to us some kind of "hypersensitivity" of DCM modules….no doubt they are great in single useage….but for our requirement of 40 of them operating together…I think we are ill-advised using DCMs…would you agree?

    Vicor DCM design guide
    http://www.vicorpower.com/documents/...uide-VICOR.pdf
    Last edited by treez; 8th March 2020 at 21:35.



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Hi,
    The output overcurrent limit for the Vicor BCM4414xG0F4440yzz DCDC module is stated as 42A to 70A. Do you know what happens when this level is breached? The datasheet doesn’t say..[it only says what happens if short circuit level (55A) is breached]
    We wonder if it latches off?
    ..Or maybe it turns off for a certain amount of time, then re-trys?
    ..or maybe it clamps its output current to 42A?

    Our contractor wants to go ahead building a PSU costing £20000 worth of vicor modules, and wants to use the BCM modules at 39A nominal load......when overcurrent threshold is 42A....and we may just get transients which take us above 42A....datasheet doesnt say anything...please advise us?

    Vicor i doubt will help us as they will know from our application that we dont have big sales volumes.

    BCM4414… module datasheet:
    http://www.vicorpower.com/documents/...G0F4440yzz.pdf



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Quote Originally Posted by treez View Post
    We are making an SMPS which will be at the end of a 300m cable.
    The Vin is 700VDC. This is from an isolated 700VDC source.
    Pout = 12Kw. (48vout)
    It will comprise “units” comprising of a vicor BCM module feeding four vicor DCM modules…..and these “units” paralleled in a whopping group of 10 to give the 48V, 12Kw output.
    We need to connect the chassis of the BCM modules to chassis ground or earth………also, the App notes state that under each DCM module there must be a copper plane, which is connected to either chassis ground or earth.
    instead of building the whole thing, build a scaled down version
    assemble a 1.2 KW version - about 1/10th the power, so 1/10th the modules

    one vicor BCM feeding 2 DCM
    parallel to a not so whopping group of 2 or 3

    no, it won't answer the ultimate questions, but it will yield experience at
    paralleling and balancing

    add a few more modules and test again
    step by step to build confidence and experience


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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Hi,
    The signal pins of the vicor DCM3623T50M53C2T00 DCDC module are referenced to the –IN pin.
    Presumably there is a physical connection inside the module to connect the –IN pin to the internal signal ground.
    So why don’t they bring this internal signal ground out to a pin on the module?…rather than requesting the user connects the –IN pin to the signal ground sections outside the module.
    After all, there could be ground bounce problems if the user does not connect the external signal ground to the –IN pin as close as possible to the –IN pin.

    DCM3623T50M53C2T00 datasheet
    http://www.vicorpower.com/documents/...53C2yzz_ds.pdf



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Why does the Vicor DCM3623T50M53C2T00 DCDC module say in its datasheet that the output capacitance shoudl be limited to 2mF....

    I just started one up with 6mF and on 150W load......it stared up fine and was just as stable as when with the recomended amount of output capacitance.
    What is the limit of output capacitance?
    There was also a 150nF inductor upstream of 5mF worth of the output capacitance....surely an LC resonance frequency at less than crossover frequency of the SMPS...but no stability problem.

    DCM3623T50M53C2T00 datasheet:
    DCM3623T50M53C2T00 datasheet



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Page 23 of the “DCM design guide” states that the SRF of the input filter capacitor must be higher than the switching frequency of the DCM module.
    However, page 35 of the “DCM design guide” states that a small inductor is needed just downstream of the input filter capacitor. This would seem to work toward cancelling out the effect of having a high SRF input filter capacitor bank. After all, adding the aforementioned inductor, effectively reduces the SRF of the input capacitor bank as “seen” looking back from the DCM module’s input.

    “DCM design guide"
    http://www.vicorpower.com/documents/...uide-VICOR.pdf



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Hi,

    Quote Originally Posted by treez View Post
    Why does the Vicor DCM3623T50M53C2T00 DCDC module say in its datasheet that the output capacitance shoudl be limited to 2mF....

    I just started one up with 6mF and on 150W load......it stared up fine and was just as stable as when with the recomended amount of output capacitance.
    What is the limit of output capacitance?
    There was also a 150nF inductor upstream of 5mF worth of the output capacitance....surely an LC resonance frequency at less than crossover frequency of the SMPS...but no stability problem.

    DCM3623T50M53C2T00 datasheet:
    DCM3623T50M53C2T00 datasheet

    It says 2,000uF max. in both sections of the electrical specifications, Cext andCext trim, the wording lower down in the datasheet is vague but presumably implies you follow max. values in specs. As the un-generous datasheet shows no internal workings of what the pertinent two pins are outputs/inputs of, can't guess at why that value is stated. Something about a max discharge time or perhaps not exceeding an inrush current at whatever is behind +Out. Who knows... If it operates correctly with 6,000uF, great.


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    Surely a signal should be filtered with a capacitor at the pin?

    Hi,
    We are using the DCM3623T50M53C2T00 as a DCDC module. This module has some signal pins. Its usual to provide noise-proofing for signal pins with a capacitor connected right at the pin.
    However, page 39 and 40 of the “DCM design guide” show the “TR” and “EN” pins being filtered with a resistor at the pin…..the resistor then leading into a capacitor…why is the capacitor not at the pin itself?

    …the “DCM design guide” says that the resistor is there to damp oscillations between the lead inductance and the external filter capacitor…but all ICs have lead inductance…it doesn’t mean you put a resistor in there…the filter cap is always connected right at the pin…surely?

    Interestingly, on page 39, the filter capacitor at the “TR” pin is recommended to be 100nF. However, on page 77 of the “DCM design guide”, this same filter capacitor is recommended to be 1nF. Do you know which is the best value?

    I hope it isn’t 100nF….because we are connecting our external feedback loop to the TR pin and 100nF would slow things down too much.

    DCM design guide:
    http://www.vicorpower.com/documents/...uide-VICOR.pdf



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Hi,
    We find that when we have more than 220uF of C(out) on our DCDC module the output current gets unstable as attached.
    It’s a DCM3623T50M53C2T00.
    The Eval board for this module only has 220uF.

    Our spec is vin = 34v, vout = 48v, pout = 312w
    Transients are 0-100% load

    However, when we add say 470uF or 2mF or more, the current is unstable, but the instability is always bounded and never gets worse than the attached, no matter how much extra capacitance is added.
    The vout ripple remains fine.
    Its still able to supply max load no problem.
    Do you think it’ll be OK to use it with >220uF?
    We cannot adjust the feedback loop components, its internal.

    And what about having 40 of these in parallel, each with the same instability?

    (the current in the attached is (v-0.33)/0.264 Amps
    (its the output of a ACS722 current monitor)



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    ..I must admit this noise in the output current is the most unthreatening noise i have ever seen...it doesn't get any worse no matter how much extra c(out) is added.....it is always bounded to +8% to -8% of average...and the vout ripple is fine...and the load service is fine even when 0-100% transients.....the noise involves smooth changes in current. It really is the mildest noise ever.
    Do you think it could get worse under some condition?...we cannot change the feedback compensation as its inside the module internally. Looks like we're stuck with this noise if we want c(out) > 220uF.....but then again...it hardly seems a problem. The DCM3623T50M53C2T00 datasheet says if we want to be able to serve transients down to <10% load then we need >220uF for c(out).

    - - - Updated - - -

    The interesting point is that we also have an external error amplifier to control the TRIM pin of the dcdc module, and interestingly, when that is functioning, it gives a perfectly stable TRIM pin voltage, and successfully controls the output current to the correct average value…… but yet still the noise occurs on the output current………so in that case, due to the stable TRIM pin voltage, it cannot be said to be a feedback loop problem. I think this noise is some kind of thing going on in the internals of the module. Would you agree?



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    This “noisy output current feature”….is possibly not a bad thing….the module still supplies the load perfectly well with max/off transients served very well.
    Also, I have just put 12mF of C(out) on the output, and the “noise” is no worse than with 470uF on the output…so I am changing my view, and saying now that this “noise” is actually possibly a special feature of the vicor DCM3623T50M53C2T00 module, and is part of the scheme of it being able to handle very wide ranging amounts of C(out)…would you agree?

    - - - Updated - - -

    The following scope shots show the output current “noise” on various timescales…as you can see, it does not look like noise, and looks like some kind of active regulation scheme going on inside the DCDC module……these shots were with the external error amp doing the regulation , and that feedback loop was perfectly stable with a smooth , stable TRIM pin voltage…so there is no unstable feedback loop here.
    (The noise is the same when I have the module on static TRIM).
    These shots were taken with vin=31v, vout = 40v, iout = 3.5A (external error amp)

    Also, its notable that the noise looks very similar to the Burst mode output current pulsing that goes on when the module is on no load…..
    The "noise" does not happen when c(out) <220uF.



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Please find block diagrams of two 3.7kW power supplies. One is isolated. One is not. They are otherwise the same.
    Which one is more likely to suffer from noise issues which may stop the power supply from operating properly.? (supplying the load)
    (I am not so interested in which one would be more likely to fail EMC regulations.)

    DCM3623T50M53C2T00 DCDC module datasheet:
    http://www.vicorpower.com/documents/...53C2yzz_ds.pdf

    BCM4414BG0F4440 DCDC module datasheet:
    http://www.vicorpower.com/documents/...53C2yzz_ds.pdf



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    I'm assuming this is your tethered drone application and I don't understand how isolation would be of any benefit. Everything is at the end of the same long cable and unless there is a large source of static electrcity the 1meg resistors should keep the two sides of the isolation at the same potential anyway.

    If there is a 'large' (in relation to 1meg) source of static I don't see much benefit in letting the secondary float. In fact that seems risky. But it's a pretty unique application..



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    I'm assuming this is your tethered drone application and I don't understand how isolation would be of any benefit.
    Thanks, yes, the sole reason for isolation is that it “breaks” the current loops in which high frequency noise currents can be induced in to….these currents can possibly afflict the power modules and stop them working………ie, the point discussed in this post…..
    https://www.edaboard.com/showthread....alleled-SMPS-s
    Would you say this is a good reason to isolate?



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    Re: Large numbers of paralleled DCDC modules?...more than datasheet recomends

    Hi,
    The attached is a non isolated 12kW 48Vout power supply.
    It is made up of Vicor BCM and DCM modules.
    Why won’t it work?
    Why will it be incapable of providing 48V +/-1V at 12Kw?

    (Please note, I am not asking why it would fail EMC)

    By the way, it would cost £12170 for all the DCDC modules.


    BCM4414BG0F4440
    http://www.vicorpower.com/documents/...G0F4440yzz.pdf

    DCM3623T50M53C2T00
    http://www.vicorpower.com/documents/...53C2yzz_ds.pdf



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