Improve output peak current capability of a DC/DC converter [16-140Vin -> 12/24 Vout]

Improve output peak current capability of a DC/DC converter [16-140Vin -> 12/24 Vout]

Hi all,
I need of a DC/DC converter with output voltage of 12 V or 24 V (selelctable by resistor network on FB pin) and current capability of 400 mA. The input voltage range is 16-140V.
It seems the only DC/DC regulator IC is the new LTC7138.
Maybe with the output voltage of the DC/DC converter I have to drive the coil of a contactor that sink 1.6 A of current for about 75 ms when the contact are closing, then it sinks about 100 mA to maintain close the contacts.
That DC/DC is capable to drive up to 400 mA of continuous current.
In the datasheet i see that it is possible to use multiple ic in parallel but it is an expensive solution.
Is it possbile to increase the value of the output capacitor (e.g by using 4*4700 uF capacitors)? Counterindications?
Thanks,

Luca

Re: Improve output peak current capability of a DC/DC converter [16-140Vin -> 12/24 V

Hi,

assuming the 1.6A are for 12V contactor. Then the coil resistance may be 7.5 Ohms.
Then the current from the capacitor is 1.6A -0.4A at the beginning.
The voltage drops to 3V (when the SMPS is still able to supply 0.4A)
Capacitor current drops from 1.2A to 0A when voltage drops from 12V to 3V = 9V.
The capacitor sees the resistance of 7.5 Ohms.
4 x 4700uF are 18.8mF, which gives a tau of 18.8mF x 7.5 Ohms = 141ms.
75ms are about 0.5 tau.
The voltage drops about 40% of 9V which is 3.6V

Beginning with 12V .. the voltage drops to about 8.4V wihtin the given 75ms.

But I think in reality it isn´t that bad, because the 1.6A will not be immediately after switch ON, because of the contactor´s inductance.
I expect a voltage of 9..10V after the 75ms.

Klaus

Re: Improve output peak current capability of a DC/DC converter [16-140Vin -> 12/24 V

First you want to know what the short circuit current
really is. It must be above the 400mA and may well
be above 1.6A depending on the design values. The
duty factor looks to be minuscule and if you are on
the right side of Abs Max ratings, should not impact
reliability.

May be no worse than charging up a bulk cap, as
that cap would accept charge at a pretty high
current also, once drawn down; the integral
"overcurrent times period" may be no different
reliability-wise.

If isolation is not an issue, I might suggest you
source the coil current via a large pass transistor
whose base "sees" the 12/24V and collector to
VIN, and energize relays from the low side (as
maybe you're already doing). The pass transistor
has only to supply coil current when "on", will do
it lossily of course but for sparse actuation this
may be cheaper and put less noise onto "other
stuff" than using a high quality limited current
DC-DC to power such a crude load.

Re: Improve output peak current capability of a DC/DC converter [16-140Vin -> 12/24 V

Klaus in post 2 had an interesting analysis. I think you could also add in the fact that during the 75 mS discharge the LTC7138 is trying to charge the capacitor back up.