12v to 24v converter

[fsmyth]

Anyone have a good print for a 24-28v boost
converter? Application will be for a vehicular
auxiliary 24v system - probably 200 AH or so.
Need ~10-15A max. current for charging.
I'm thinking something similar to:

http://hmin.tripod.com/als/andysm/pages/sw-regs7.html#UPC71

with a bit more smoke.
Not adverse to building one with F/F drivers, if
I could find one robust enough for the environment,
but the two I have built went T/U after a couple of
weeks.
TIA
<als>

 

[Warpspeed]

Have you considered pulling some ac volts direct out of the alternator, and doing something with that ?

 

[fsmyth]

Yep. Problem here is that Dodge is controlling
the alternator field in the ECM, not with a
standard regulator. And there is not much room
left for an extra alternator in a pickup.
If it was not a diesel (with 2 paralleled batteries)
I would replace the alternator and run a tapped
24v system. Lots of reasons not to do it (there
are 2 50-60A intake heaters, electric seats/windows,
fuel heaters and pumps, etc...)
An isolated 12v-12v converter will do it, but
I would like to keep the charge the same on
both series-connected 12v batteries - monitor
circuits here would be added complications.

In the past, I have converted older service
trucks using a 200A, 12v Leece-Neville alternator
which was controlled by an external converter that
ran the alternator at 24v, and had an internal
transformer for the 24v-12v down conversion.
(I believe the original application for these
boxes was ambulances/fire trucks).
But... this stuff is getting harder and harder
to find, and to fit into today's crowded engine
compartments.

 

[Warpspeed]

Why is it a problem to steal some three phase ac voltage from the existing alternator and regulator ?

It is already ac, maybe 14 volts peak to peak (plus a couple of diode drops). A voltage doubler would come pretty close to producing 28 volts dc output.

 

[OperaHouse]

This is looking more homebrew than product. I would do it by buying a 2000/4000W Harbor Freight inverter off ebay for about $25. Sure its broken. I've bought almost a dozen of these and it is almost always a shorted FET in the high voltage parallel H bridge that I just wiggle off. In your case, you don't care about HV. This unit has seven HV inverters with four diodes on each transformer making a bridge. Cut the traces and connect these in parallel to the both legs of primary coil. Change the feedback resistor to set the voltage. This should easily put out 20A with existing diodes. All the parts and a case for cheap. Good hour project. I would add a little current sense to limit current. You got those resistors too.