Isolated, synchronous Cuk with dual primaries

If I have an isolated Cuk topology with dual primaries where both input voltages are floating, and I'd like to drive the single secondary from only one primary supply at a time, with the other shut down not sinking or sourcing any current.

I have a question about the series inductors on the primary and secondary side which in the single primary, single secondary case are often transformer-coupled to reduce output ripple. Is there a way to time the switches S1, S2, S3 (assume I can synchronize them in an isolated way via an optocoupler or something), and/or arrange the secondary-side inductors L3, L4 in series/parallel such that they can be coupled to their respective inductors on the two primaries, but only act at any given time as would be expected in a Cuk with a single primary winding, and don't back-feed the source on the other primary which is supposed to be shut down.

Or do something else clever that hopefully doesn't require custom magnetics. Or just don't couple them and have a single inductor on the secondary that's a compromise value and deal with it.

Reply to
bitrex
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The 1:n coupling transformer might be a low-profile planar type that's "wound" like the traces on a PCB with the core inserted through the stack, rather than on a traditional core.

Reply to
bitrex

bitrex wrote in news:FnR_F.13618$ snipped-for-privacy@fx29.iad:

You got no PC with a spreadsheet app like LibreOffice? You could hand trace(within the app) grid lines and lines in a spreadsheet and print it out and post a pic of that. Way better than that was. Hell, you could do a screenshot at that point.

I used to make signs for the lab in Excel all the time. Signs that I would print, and laminate and even cut out and place on the floor with the special floor tapes we were using for certain areas of the lab. Schematics would be easy. Albeit not captured.

Hell you could use a capture app, you dont have to have a full netlist.

What are you running, a 286? :-)

Reply to
DecadentLinuxUserNumeroUno

I like pen and graph paper, I feel my "drafting abilities" rank somewhere in-between Larkin and JanPan.

Reply to
bitrex

bitrex wrote in news:LKR_F.385162$ snipped-for-privacy@fx46.iad:

Yeah... where do I get off at, eh?

Reply to
DecadentLinuxUserNumeroUno

Can't do it. With all windings coupled together somehow or another, they're all (AC) powered. The switch will rectify passively -- assuming it's a single MOSFET.

What's wrong with two independent supplies, outputs in parallel? Possibly some control circuitry could be muxed, but even if not, you're just duplicating a controller, not a huge deal.

You're duplicating all the power components at least, and not batting an eye at that.

Or actually, if it's a secondary side control, with isolators to the primary side, you can still use just one control, and an opto to detect which primary is active to determine which secondary side to drive (if it should be synchronous), and the unpowered primary side drivers just don't do anything because, well, unpowered.

Chicken-and-egg of initial startup, but that's left as an exercise for the student. :^)

(Back in the day, there were TL494 based, PP forward converters -- your average ATX supply -- that did this, of a sort. Drive transformer for the bridge, secondary side controller. They solved startup either by making the bridge self-oscillating, or adding a small aux supply that also delivered 5V standby for the motherboard.)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

That actually might be OK, if the primaries are each connected to a battery in a series stack. Cuz the idea is a crude type of balancing-charger where the bulk charger connects to the top of the stack and charges until one battery in the stack is detected to hit a threshold SoC first, then the bulk charger cuts out and the "winning battery" is used as the energy source to push charge into the others. when it drops below a threshold and the others haven't hit theirs yet the bulk charger cuts back in. Wash, rinse, repeat.

So the "secondary" doesn't really need to have a switch I guess it can just have a free-wheeling diode and connect to the top of the stack when the bulk charger cuts out, then only the selected primary MOSFET runs and the other MOSFET body didoes and the diode on the secondary free-wheel.

The whitepaper I saw accomplished this with a multi-winding flyback but I'd prefer to use isolation scheme that doesn't have to store energy in the interest of compactness.

Reply to
bitrex

In that variation I think a high-side PMOS is used to drive the flyback on the primary sides which prevents the passive-rectification issue but isn't a particularly elegant solution

Reply to
bitrex

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