Favorite low isolation capacitance DC-DC modules/transformers - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: August 14, 2021, 06:57:12 pm
content: Does anyone have any favorite recommended low noise DC/DC modules that feature low isolation capacitance and have +/- 15V outputs > 150 mA? Also interested in off the shelf transformers that have electrostatic shields suitable for an isolated push-pull or full-bridge DC/DC converter.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2021, 10:05:41 pm
content: If you have to, you can also put two transformers in series and use the floating connection between them as the electrostatic shield.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2021, 10:57:38 pm
content: High capacitance..?Tim

author: ezalys
author_rank: Frequent Contributor
timestamp: August 15, 2021, 03:59:40 am
content: Sorry! Typo. Definitely low isolation capacitance! Recom seems to have the best I've seen -- anywhere between 5 and 15 pF. Aimtec seems to have a few as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2021, 05:52:55 am
content: That makes much more sense!IIRC the Murata line that's toroid cores embedded in PCB with super deep vias, is also quite low C. Recom are good, if you can swing for the price.If you don't mind homebrew, common mode chokes can give excellent isolation performance -- at the expense of terrible switching performance. Typical use case is a full throttle ("100%" PWM) H-bridge, with FWB or FWD on the secondary, and just vary the supply voltage/current into the bridge for regulation and limiting. The leakage is high enough that you don't really get more than a watt or two, at the maximum-output frequency.The MOF in turn is defined by saturation at the low end, and leakage at the high end. Both are close enough together, at typical V/I, that there's no real flat band "any frequency will do" range, you're really pretty stuck -- and this surprisingly doesn't scale much with core size, though when you think about it a bit, that kinda makes sense after all. Namely, a larger core needs fewer turns, so the saturation flux doesn't go up proportionally with size; meanwhile, leakage goes up with physical size, though down with fewer turns, so doesn't change very much. Overall, for a bigger core, you can't really drop frequency to get much more regulation / power capacity before saturation hits; meanwhile, leakage is fairly typical, implying you could transmit more power at higher voltage (and current) (i.e., at the same impedance), but that's not something you can vary in a given application, and anyway you're limited by the aforementioned saturation.The impedance limitation is essentially because the characteristic impedance of these transformers is very high, due to the intentional leakage -- typically 300-600 ohms. But any real power conversion application will require a tiny fraction of that (like your 15V 100mA supply, for 10% regulation, needs under 15 ohms of series impedance -- big difference!), so it kinda sucks.But since you're only doing a couple watts, it's still a possibility.Obvious counterindications include regulatory approval; they're only rated for the 1.5kV or so line-to-line rating, of course. In practice, they withstand much more (I once hi-potted a small split-bobbin type to breakdown at 12kV!), I wouldn't feel bad (ab)using one for a (functional isolation) gate driver say -- but, when you need the approval, yeah.Tim

author: voltsandjolts
author_rank: Unknown
timestamp: August 15, 2021, 08:33:06 am
content: I haven't used these yet (quite new chips, only 23 in stock at TI right now) but might be worth a look:https://www.ti.com/product/UCC25800-Q1The Wurth transformer for it has <1pF coupling capacitance:https://www.we-online.com/icref/en/texas-instruments/UCC25800-LLC-ResonantSplit bobbinTrifilar 1-3Bifilar 6-8

author: Echo88
author_rank: Frequent Contributor
timestamp: August 15, 2021, 02:25:51 pm
content: Nice find voltsandjolts! Interesting contender against the LT1533 and their brethren regarding low isolated EMI DCDCs.

author: ezalys
author_rank: Frequent Contributor
timestamp: August 15, 2021, 09:14:07 pm
content: Yeah this is an incredible find!

author: Terry Bites
author_rank: Super Contributor
timestamp: August 16, 2021, 03:08:35 pm
content: Look at medical grade units, leakage and i/o capacitance is well controlled. I'm a Traco fan- not cheap ($60) but excellent performance. Eg tracopower THB-3 or TIM series A bit cheaper RECOM REM series.Lots of chocieshttps://www.mouser.co.uk/Power/DC-DC-Converters/Isolated-DC-DC-Converters/_/N-brwkv?P=1y9f5fjZ1z0x0e1Z1z0x04hZ1yzveqbZ1z0wddoZ1z0wcjeZ1yxt7erZ1yxt7cyMooptastic