Hi Stacking switchers is generally not a good idea. Even stacking them with non-switchers is not a recommended way to go. Can it be done? Sure, if you know a lot about exactly what’s inside the switcher ….
Best approach: Convert all the this and that to some voltage > 20 V and then do the “voting” from there. Diodes are one way, there are others. Let each supply do it’s thing with a nice normal ground on the low side of the supply. Also keep in mind that the 3801 supply is far from perfect. They have a tendency to blow input fuses for no apparent reason. Bob > On Jan 19, 2022, at 5:00 PM, ed breya <[email protected]> wrote: > > Since back in October, I've been working on getting the Z3801A in a 5065A > carcass project closer to completion. I mostly finished the clock control > board and other details, and moved on to the power system. I decided to > completely redo a lot of the mechanical arrangement, wiring, and the line > powered section with nicer layout and all the facilities for battery backup. > > A couple weeks ago, I had a major setback. I was really on a roll, having > wrapped up the external battery control section, and ready to start on the > internal battery interface and charging system. I had already built in the > facilities for external DC input years ago, so it was mostly ready to go, > once the controls were set up. I was doing some checkout on external > powering, when the DC-DC converter in the booster crapped out - specifically, > during hand-off tests between AC powered and external DC powered modes. I had > incorporated all sorts of protections against faults, but apparently not > enough, or I missed something that could happen in certain conditions. After > inspection and rethinking, I still can't determine for sure how the converter > could have blown out, but I have some theories. > > First, I should mention that this Z3801A is of the "BTS" type, which runs on > 20-30 VDC. This is handy for battery backing operation. The internal battery > system is two 12 V, 12 A-hr SLAs in series. When backing up, it is used > directly, without any extra conversion loss, just the forward drop of the > blocking diode. > > The AC power system uses the original 5065A transformer and choke, plus a > bunch of other stuff to make everything work. The main output is called "Vs," > and is the central point for all the power, running from about 21-30 VDC, > depending on line voltage and battery modes and conditions. Everything goes > into Vs. Its blocking diode is the rectifier bridge back at the transformer > output. > > The external source was intended to be 12 V nominal (11-15 range), like a car > battery. For this mode, I had stacked a DC-DC converter output on top of the > supply voltage, with all sorts of diodes to prevent (so I thought) damage > under all conditions. The regulated output of the converter was isolated 12 > V, and the input could run on anything from 9 to 36 V. It was really nice, > simply adding 12 V to the external supply voltage, giving around 23-27 V, > less the blocking diode drop. > > Anyway, it ruined my day when it crapped out. I had a pretty good deal going, > with exactly three of these identical DC-DC converters - two deployed, and > one spare. The other converter runs from Vs, to make the regulated 12 V wrt > chassis ground, that runs all the peripheral circuitry. The common line for > all the power side circuitry is separate from the chassis (but clamped by > diodes), to better control any ground loops. > > So, pondering the situation, I could have replaced the converter, and > scrutinized everything thoroughly and added better protection. Or, I could > have just dropped the feature for now, to be figured out some time in the > future (likely never). I chose a third option, to build a fresh boost > converter into it, where I know and control exactly what's going on inside, > rather than guessing the limits of the not all that well specified last DC-DC > converter module. > > I'm happy to report that I finally managed to squeeze everything into the > very limited space, and it's up and running fine so far. The new deal also > opens up other options for improving and simplifying operation. > > The converter is based on the LT1070, made civilized and protected by various > circuitry. I'll have more to say next time about what's in there, and details > of how all the stuff plays together. > > Ed > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
