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coupled buck topology

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Newbie level 4
Apr 5, 2017
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Hi all

I found out, in my compagny archive, such a topoly

does someone know the common name for it?
after that, I'll google the name and probably be able to found out some equations.

The main advantage I could see , is a lower input current ripple compared to a buck 's one.
inductor are coupled

thanks for your interest


  • buck coupled.jpg
    buck coupled.jpg
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Its an interesting one....and by the looks though...this appears (please do lambast me if im wrong) to offer nothing to improve on the related ones, the coupled Cuk and the coupled SEPIC....both of these have an advantageous low side fet drive, and low side switch current sense. Coupled SEPIC can also offer very low input ripple with suitable lower coupling.
Should you wish attached is coupled cuk and sepic sims in ltspice.

It would also be interesting to see if the coupled Buck needs a leakage snubber....AYK, the coupled SEPIC doesnt need this.


  • CUK and
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in CCM , basic transfert fonction seems same as "standard" buck one's.
my hypothesis is : when in steady state, voltage across capacitance is equal to Vin as it could be if switch is off for a long time.

well... maybe, by simulation I cannot point out flaws. I'm not very confident with coupled inductor and there "magic"
what I see , is a much lower current ripple seen by the input.
I found this schematic in a project where input filter cannot be build with chemical caps (especially the big one used with a resistor to damp the ringing).
the high side mosfet isn't a pb because duty is around 50% and gate coupling can be done with a cap

attached the little simulation. I hadded a small RC snubber Feel free to make comments, I'm a newbee



  • buck coupled inductor.rar
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As I said, I'm not very confidend with coupled unductor, that's why I finally change simulation model from coupled inductor (with K spice factor fixed to 1) to a inline Xformer .
And I don't know why, simulation isn't the same now....
I push two package , first one with coupled inductor, the next one with a xformer

if one of you could be kind enough to spot me the fault...


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  • buck with xformer.rar
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okay I finally agreed with simulation
I'm confident with some basic math , so I found output current (seen by cap and resistor) is exactly as inductor current.
what I cannot find is the "flying cap" current, if fany of you got an idea ... helpfull
--- Updated ---

funny.... when flying cap ESR value gets very high (1R, I tryied to damp spikes) the topolgy behaves like a standard buck.

so to make fun I change ESR to almost zero and ecreased cap value to 480µ, now input current gots a very low ripple...



  • buck coupled inductor.rar
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