"Balance" two switch mode converters put in series on the PIRMARY side - Page 1
Forum section: electronics / projects

author: Lupin III.
author_rank: Contributor
timestamp: February 24, 2021, 01:21:20 pm
content: I'm wondering, if there is a way to put two switch mode converters in series on the primary side and somewhat keep the voltage to both the same.The situation is the following: I have ~30-50W LED panels (or will have, I have not selected/ordered anything yet, because it will depend, if this is possible), which have an integrated DC-DC converter to run the panels at any voltage from 12-30V. I'd now like to run the panels on 48V.One possibility would be to add another DC-DC converter before that. But I found that while there are plenty of converters available for up to ~30V input, above that they are pretty much non-existent (until you reach about 90V AC input). I guess that's because MOSFETs usually stop at 30V_ds.The other way I was thinking about, is connecting two of the LED panels in series on the primary side. The panels would of course be identical, even from the same batch, if I can guarantee that. But still, tolerances will mean that they will be slightly different and the same current will have to flow through both on the input side. The simple assumption is that the DC-DC converters just divide the voltage between them in a way that at a given current (should be around 2A) they get the power they need. They might need bigger input caps so they "see" as little of the other's switching noise. Other than that, could this work and if it doesn't, would their a simple way (as in less complex and cheaper than another DC-DC-converter) to keep them in balance?

author: richard.cs
author_rank: Super Contributor
timestamp: February 24, 2021, 02:35:20 pm
content: It won't work because the switch mode supplies have a negative input impedance, they draw more current as the voltage falls, so the moment the slightest imbalance occurs it will run away until one sees the whole voltage.

author: ajb
author_rank: Super Contributor
timestamp: February 24, 2021, 03:52:56 pm
content: Yeah, trying to put two switching converters together with their inputs in series is not a good idea.If you really wanted to for some reason or just wanted an weird power project, you could do it with an active rail splitter, such as are used for creating virtual grounds, but in a switch mode version for efficiency and power handling. You'd basically be building a a DC-DC converter except it has to be two quadrant, it would only have to handle up to half of the power versus a conventional step-down arrangement, and you'd likely have to build the loop from scratch. The half power bit comes from the fact that such a rail splitter would only need to handle the full power of one of the two LED modules in the worst case (the other one disconnected), but under normal operation the splitter only needs to handle the difference in power between the two modules, which should be fairly small if they're the same model and operating under the same conditions. If you could somehow control the input power of the two modules to match them maybe you could get away with a lower power splitter setup. If one of the converters fails with its input shorted, then the other one will see the full supply voltage so you'd probably want to incorporate some protection for this condition. But at this point, you're already building a ~50W 48Vin switching converter so you're not far off from building a more conventional step-down converter at higher power, so there's not much appeal to this approach unless you just really want to build a two-quadrant converter.For a more reasonable solution, there are plenty of DC-DC power supplies in the 100W range that will step 48V down to 24V. A quick look at digikey shows they start around $20 for PC Mount models and around $40 for enclosed models. There are also higher power step down modules designed for golf carts apparently that are pretty cheap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2021, 03:58:48 pm
content: Generally no.Such a configuration would probably work if they are dimmable type, i.e., output power depends on input voltage. Guessing this isn't the case though.If they have a control pin, it might have the correct sense to be wired as a voltage follower.Tim