# Best topology for a 5kW DC-DC 48V input

1. ## Best topology for a 5kW DC-DC 48V input

Hi,

As the title says, I´m deciding on the power stage for a 5kW DC/DC application at 48Vin. I´ve always heard that a H full bridge should be used for applications above 1kW, however the push pull has the advantage of using only 2 switching elements which could potentially reduce the overall conduction & switching losses. I know it has some drawbacks like more voltage stress on the MOSFETs as well as more inefficient utilization of the transformer, since only one winding is conducting every time.

Does it make sense to consider push pull for this application or should I scrap the idea and go straight for an H bridge ?

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2. ## Re: Best topology for a 5kW DC-DC 48V input

Hii,

What is the output voltage of the converter??? We need some more information like output voltage,Efficiency & Variation in input & Output voltage etc

1) If the input & output voltage are almost constant or less varying. It is better to go with full bridge LLC with secondary side synchronous rectification or 2 half bridge LLC(Secondary side sync rectification) connected in parallel. You will get better efficiency & a good thermal profile.

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3. ## Re: Best topology for a 5kW DC-DC 48V input

The output voltage is variable from 100% to 10%, It can go from 400V at 12.5A to 40V at 125A and the load can also vary from 100% to 10%. I would like to prioritize efficiency. In the resonant section I was thinking LCC or LLC, need to investigate which one would more suitable. I guess from your response that paralleling multiple bridges is the best option.

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4. ## Re: Best topology for a 5kW DC-DC 48V input

In general the more switches you have the less power each switch needs to process and therefore dissipate. As you point out higher switch topologies can also make better use of the transformer.

In low power applications the 'cost' of a switch and associated gate drive is comparatively high compared to the small transformer. So 1 and 2 switch topologies make sense. But by 1kW you'd need a very large, heavy and costly push-pull transformer and if you study the problem you should find the full bridge is a big win both from the reduced switch dissipation and the transformer size reduction.

Honestly by 5kw even with full bridges you probably want to consider multiple in parallel.

If you are for some reason married to push pull then you'd certainly want to look at multiple in parallel (or series if you're really pushing 400V out).

5. ## Re: Best topology for a 5kW DC-DC 48V input

Dear Draytek, if you really require 400V 12.5A out, AND, at another time, 40V 125A, you will need either a serious tap on the transformer or some very thick windings on the 400V winding, - you are effectively seeking a 50kW secondary, giving both 400V, but able to carry 125A too.

If you could refine your real output requirements this would help greatly ...

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