DCDC module has unusual input filter requirements? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 08, 2020, 08:57:01 pm
content: Hi,Please may I first state that Vicor DCDC modules are superb beyond belief…they are the best…we all know that …there are probably billions of them out there doing their wonderful job……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 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 guidehttp://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2020, 12:19:47 am
content: Used a whole handful of inductors that size, a few years ago. Multiple output converter, low voltages, high currents.They just don't want the filter Q being anything, so that noise currents might cause resonant peaking. Use the R+C.Tim

author: duak
author_rank: Super Contributor
timestamp: March 09, 2020, 08:55:34 pm
content: I've used Vicors but mostly the older DC-DC converters. I've never done anything with paralleling them. They have quite a high power density for a general purpose or modular packaging scheme. Based on anecdotal evidence from where I worked previously, make sure they are mounted and cooled correctly and leave an operating margin ie., don't run them 100% or in current limiting.The series inductance is small because Vicors have such a high switching frequency. If the inductance is greater or if the input LC network has significant Q, you can get significant ripple voltage and current on the module input at the network's resonant frequency.