Power module needed to slowly soft start

Hello,
We are doing a 48V to 1V5 SMPS using a vicor PRM48 power module feeding down to a VTM48 vicor “electronic transformer”. (the intermediate voltage is 36V).
On page 21 of the PRM48 datasheet, the output voltage of the PRM48 is shown to slowly ramp up at startup….also, during this startup, the PRM48 output current is being held very low. What is controlling this?.....i mean , we like it…because we want that soft start, but we want to assure we get it…but what controls it?……The PRM48 does not regulate below 20vout, …and we would really like it to rise slowly from 0vout to 20vout so that the output of the VTM48 can also slowly ramp up. …But how do we get the PRM48 to slowly ramp through 0vout to 20vout when that is outside its regulation range?
PRM48 datasheet
http://cdn.vicorpower.com/documents/datasheets/PRM48JH480T250A00-ds.pdf

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VTM48 datasheet
http://cdn.vicorpower.com/documents/datasheets/VTM48E_020_080A00.pdf

we want to assure we get it…but what controls it?…

Click to expand...

I do not know specifically how the Vicor works, but there is a trick to all this.

Most (?) supplies have a voltage error amplifier, and a current limit amplifier, usually arranged as some kind of proportional/integral type of control loop for stable operation.

The current limit loop overrides the voltage control loop, and overriding everything is often the soft start ramp up.

When you throw the big switch, the soft start ramp causes the output voltage to rise, but .... The voltage control loop is seeing severe undervoltage at initial power up, so slews to maximum full output.

As starting ramp up continues up to the crossover point where the voltage control loop takes over from the startup ramp, there is often a short overvoltage spike until the voltage control loop can slew back down to its correct set point.

The "Trick" is in how and where the start up ramp is applied.
Its always better to ramp up the master voltage reference, rather than try to control the output of the voltage error amplifier.

If you ramp the reference, the voltage control loop is always in full control, and as the reference voltage rises, the output rises with absolutely no overshoot.
If you do that to the current limit as well, it will also ramp up gracefully into a shorted output without any sudden spike.

It may all seem like a minor detail, but once understood, the over voltage spike at turn on will never be a problem again.

Thanks, indeed the soft start for the vicor does slowly bring up the error amp voltage reference...but we are wondering what limits the current so heavily at start up......the current as you can see on page 21 is very very low and nothing in the datasheet speaks of current being limited like that at start up

Current at start up has more to do with the nature of the load than the source.

Does the current at start up still remain amazingly low into a dead short ?

not sure yet, we havent got it made yet. I think the current would clamp at the clamp level though.