Transformers for isolated DC-DC converters - Page 1
Forum section: electronics / beginners

author: bittumbler
author_rank: Contributor
timestamp: December 06, 2020, 12:12:02 pm
content: Hi,i want to design an isolated DC-DC converter for a sensor network.Input is 24V from an off the shelve mains power supply.Output is 24V (isolated), max 350mA.Problem is, i need 30 separate, isolated voltages, preferably on one PCB.The output voltage does not have to be regulated, however i need some short circuit protection / current limit.Isolation voltage should be min 1.5kV.I found some DC-DC converter modules, e.g. CUI PQQ6W-Q24-S24-S or Meanwell SKMW06F-24. They are a bit low on current, i can probably live with that. But i would prefer something lower cost. And also lower space than e.g. the SKMW06F-24.So i decided to design my own converter. And maybe learn something in the process.I found plenty of suitable switchmode controllers, e.g. NCP1030.My problem is with the isolation transformers. How do i find a suitable one? Is there some place i can order these at reasonable cost to custom specs, say in qty 100?Is there some better solution for 30 separate isolated voltages?

author: Ian.M
author_rank: Super Contributor
timestamp: December 06, 2020, 12:29:57 pm
content: You may be able to avoid the need for transformer isolation. It depends on the capacitance to ground you can tolerate at each output and the common mode dV/dt each output will be subjected to. You could use capacitor coupled (for DC isolation) Schottky bridge rectifiers, driven by a common pair of antiphase high frequency squarewaves from a H-bridge. You'll probably need a capacitor to ground from the negative side of each bridge, an order of magnitude larger than the coupling capacitors to swamp any common mode switching glitches. As its capacitor coupled, the energy that is transferred per switching cycle is inherently limited, so if correctly designed each individual output will be short-circuit proof. However it may be desirable to monitor the input current and reduce the switching frequency to handle many outputs being short-circuited without drawing excessive input current.

author: bittumbler
author_rank: Contributor
timestamp: December 06, 2020, 12:53:25 pm
content: Thanks for the idea.However i am not sure i can use capacitors for isolation.Some of the isolated voltages are for human input devices. Some are connected to/near mains voltage powered stuff.I do not have a strict dV/dt requirement. However in case of a direct mains connection of one of the voltages, i need to limit the current to 0,5mA to all the other voltages.

author: fcb
author_rank: Super Contributor
timestamp: December 06, 2020, 12:58:13 pm
content: I'd probably do it using 30 push-pull transformers driven sync'd together, very simple to build. If you've only got one to make, then probably handwind the transformers (ETD20/25 size) - easy to get the required isolation.You can buy transformers with 6 isolated windings, but they aren't typically rated at 1.5kV isolation.edit:https://www.we-online.com/catalog/en/WE-FLEX

author: msat
author_rank: Regular Contributor
timestamp: December 06, 2020, 07:23:16 pm
content: Might not be applicable as I don't know the details of your design, but there are isolated ADCs, some with integrated DC-DC converters.

author: tszaboo
author_rank: Super Contributor
timestamp: December 06, 2020, 11:09:18 pm
content: That is a lot of power. 300mA@24V is 7.2W, multiply that with 30, and suddenly you need a power supply with 250W peak power. Are you sure you need 350mA?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2020, 06:48:45 am
content: Pulse transformers of that size are plentiful, you'll even find some rated for mains isolation (2.5kV+ reinforced). They won't be terrifically cheap -- a few bucks each, so count on the whole thing costing upwards of $100.Planar magnetics are also kind of attractive, using the PCB for windings -- multilayer board counts as a "cemented" joint, so has high voltage rating and reinforced insulation type. Design is a rather advanced topic though -- just putting this out there, not so much as a practical recommendation at this time...Tim

author: bittumbler
author_rank: Contributor
timestamp: December 07, 2020, 09:25:17 am
content: Hi,regarding the total power: the 300mA@24V will not be used continuously and concurrently on all 30 "rails". This is just the upper limit per rail. So if i use separate DC-DC converters, these are my design parameters.regarding PCB coils: I have no idea how to design them, it seems a bit above my abilities. But thanks for the pointer.regarding the transformers with multiple windings: I did not know these exist. Thanks for the pointer. However for now i want to go with individual converters to have separate current limits on each rail.I did some more searching on transformer vendors websites and came across transformers intended for PoE. e.g. Pulse PA5099.003 or Coilcraft POE13P-24L. These are relative low cost, but 1:1.4-1.5 instead of 1:1.So the whole thing might be easier, if i start with 36V and do 36V to 24V isolated converters. Or use the PoE transformers in reverse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2020, 11:46:10 am
content: Can still implement per-rail limits using a clever rectifier -- perhaps something that disconnects above threshold (so, doesn't dissipate power continuously) and resets every cycle, giving a characteristic similar to a peak current mode converter perhaps.Might be easier to do on a forward converter also, in which case the primary side converter doesn't need control at all, that can all be done secondary side. Which also means no problems about cross-regulation, every output is individually regulated.In the old days, converters of this sort were made using saturable reactors -- the reactor switches on after some pulse width, controlled by a feedback loop. You'll probably have a hard time finding reactors of the right value (flux and current), let alone in an affordable price -- but MOSFETs are more than good enough to take over.Heh, I'm probably oversimplifying a bit, still. The control circuit would preferably be some IC that just drops in. I don't know any offhand, that are made specifically to implement this functionality. You might have to build your own from timers and comparators and such, which will end up taking up way more space (N controllers..).Anyway, it doesn't have to be a full common array either, if it's adequate to have a say 300mA limit, total between a pair of channels, you can just use dual secondary transformers (assuming the secondaries have adequate isolation between each other) and primary side limiting. Now you need N/2 controllers, etc. Every little bit helps.Quote from: bittumbler on December 07, 2020, 09:25:17 amIf those are flyback type transformers (relatively low magnetizing inductance), the exact voltages don't matter very much, you can use them higher or lower just fine. Or as forward converters, the extra ratio just as well helps provide adequate adjustable range (a half-wave forward converter loses half the secondary voltage due to PWM, so the required ratio tends to be higher than otherwise).Tim

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2020, 08:17:41 pm
content: I would look for pulse transformers with sufficient isolation and then use them in an inverter configuration rather than switching.