Switching Regulators in Series

looking for some help regarding switching regulators for a personal robot project.

The problem:

I have: two 18v 4.5Ah batteries (which when fully charged are actually more like 21v)

I need: 5v/1A for a USB hub, 12v/2A for a Udoo, and 30v-35v/2A for some stepper controllers.

Ideally, I'd just use three switching regulators to provide the required voltages at 90%ish efficiency, but I'm having trouble finding any that would cope with the 42v of the batteries in series.

What I'd like to do is add something like a 15v regulator to each battery and combine the outputs of the two regulators in series to make 30v. This would power the steppers and I could then add two further regulators to step down from 30v to 12v and 5v for the rest.

My question is: can this be done?!

• If you put them in series, put a reverse-biased diode across each output so the output voltage can't go too far negative. Commented Jul 17, 2014 at 12:38
• I don't know why you're having trouble finding regulators that can handle 42V in. There are literally tons of them out there that are designed to handle a nominal 48V bus, and they typically have inputs that can handle 36-72V. Commented Jul 17, 2014 at 12:51
• Perhaps I could clarify: I'm looking for pre-built circuit boards (ideally) that would operate through a wide range of the battery output - this will start at 42v and fall. I would like to maximise the amount of runtime (the steppers will actually run reasonably well at ~24v, so I guess that's my lower limit). However, I take your point that there probably are suitable devices out there if you know where to look! Commented Jul 17, 2014 at 17:50

If the switching regulators have common ground between the input and output, you cannot put them in series.

If the input and output grounds are isolated, you can put them series.

What is limiting your 42V input to a single regulator? If it is the input capacitor, replace it. If the switching device, MOSFET mostly, cannot handle 42V across its source and drain, replace it with a one with higher rating.

If there is no MOSFET but an IC doing all the stuff, connect a MOSFET (with proper rating) to the switching output of the IC and move the inductor and capacitor (filter) to the output of the MOSFET. Make sure that the pulses' peaks coming from the IC are within the specification of the MOSFET. Too low peaks will not turn on the MOSFET, too high pulses will damage the gate of the MOSFET.

• Thanks. I was hoping to use off-the-shelf pre-packaged regulators for this (e.g. cheap boards from ebay)... I think I'm right in saying these would not be isolated. Looks like I'll have to build something instead, which is a steep learning curve but might be fun! Commented Jul 17, 2014 at 13:24
• The stuff on Ebay is mostly rated because of capacitors. As I said before, look for the IC's input voltage specs. If those specs are OK, just replace the input capacitor by a one that is rated for higher voltage.
– nvd
Commented Jul 17, 2014 at 13:38
• That's interesting... Many seem to be LM2596 based, which says 40v in the datasheet. But I like your thinking, I'll keep looking! (and you're right... 35v caps on the boards) Commented Jul 17, 2014 at 13:44
• Sometimes, you can ask the seller to upgrade the capacitors. ;)
– nvd
Commented Jul 17, 2014 at 15:19
• @topher217 I think, I had assumed that he was charging the batteries in parallel. That would lead to shorts for both the switching regulators if non-isolated outputs were connected in series.
– nvd
Commented Dec 21, 2019 at 11:51

I think you'd be better off dropping your 36 volt battery output to 12 volts and running from there. 36 VDC is a fairly common golf cart battery voltage, and there are very cheap 12 volt buck regulators available for this application. Typically they have quite a wide input range. Try eBay.

• Thanks! I have been looking at eBay, can't find any that go above 40v - when fully charged my supposed 36v is actually 42v. Will keep looking though! Commented Jul 17, 2014 at 13:15
• Try "36v golf cart converter". Some have 36 volts as a lower limit, but a fair number have the range you're looking for. Commented Jul 17, 2014 at 16:47
• Confirmed... Literally the first one I saw would do the job for the 12v. I could step down for 5v, up for the 30v. Definitely a possibility, thank you. Commented Jul 17, 2014 at 18:03