Using an external voltage feedback loop with a DCDC module....why not possible?

[treez]

We are investigating Vicor DCDC modules.
(Ie, the PRM* modules, and the DCM* modules.)

Page 30 of the PRM* module datasheet clearly states that the TRIM pin should not be used in an external analog feedback loop.
....However, App Note 35 from Vicor, clearly shows the TRIM pin of DCM* modules being used with an external analog feedback loop.
The TRIM pin has the same operation in DCM* and PRM* modules, so why can we not have the analog feedback loop to TRIM when using the PRM* modules, but we can evidently have this when using DCM* modules?

PRM* module datasheet
https://www.vicorpower.com/documents/datasheets/PRM48AF480T400A00_ds.pdf

DCM* module datasheet:
https://www.vicorpower.com/documents/datasheets/DCM3623x50M53C2yzz_ds.pdf

Vicor App Note 35:
https://www.vicorpower.com/document...ving_High_Accuracy_Voltage_Regulation_DCM.pdf

 

[Easy peasy]

It would need to be a very slow ( seconds ) and heavily damped feedback loop designed to current balance many modules

We have done this for battery chargers where the current o/p is very sensitive to even 10mV changes in Vout as, on a battery ( read zener diode with big Capacitance ) a tiny change in Vout can pull several amps from a charger with low Zout ... thus any active balancing network must be well designed.

 

[d123]

Hi,

DRM doesn't show Trim architecture, PRM sheet does, so guesswork a bit pointless. PRM says 'not recommended' rather than shouldn't.
Maybe causes instability of some kind with certain values of feedback loop components. Perhaps manufacturer would be willing to answer or let you know if PRM is also a resistive divider and a capacitor.

 

[treez]

Thanks, this is in the context of us wishing to use five Vicor PRM* modules in parallel for an 800W power supply.
Vout is 25V and Vin is 48V.
The application has fierce no_load to full_load transients.

Our contractor wishes to have one “master” PRM* module and 4 “slaves”. (all in parallel)

He is worried that the MASTER will conduct too much current, since the PRM* datasheet states that output current can be anything from 8A to 12.5A…….we wish to limit current to 8A.
Therefore, our contractor reasons that we can have an external current monitor feeding an error amplifier, which clamps the MASTERs current to 8A by pulling down the Master’s TRIM pin…….then , he reasons, the slaves will then also have their current output limited to 8A since they are connected to the masters SHARE pin.