Re: flybacks & transformer leakage
- From: cbarn24050@xxxxxxx
- Date: 17 May 2005 02:01:01 -0700
Adam. Seychell wrote:
> I'm building a 25W off line flyback SMPS, and wondering what are
typical
> leakage inductances for the gapped ferrite E core transformer. I'm
> winding the secondary sandwiched between a split primary and getting
> about 2.0% leakage. Without interleaved windings then leakage
inductance
> gets too embarrassing to mention.
>
> Another question: Is flyback my best choice.
>
> Can I ask why flyback is common for < 50W power levels ? Sure,
flyback
> converters eliminate the largish output inductors and use a single
> switch. However its simplicity seems superficial because the designer
is
> challenged with high peak currents, EMI/ripple filtering, and
magnetics
> design.
>
> The forward converter reduces these high peak currents but requires
> output inductors. Energy stored in the transformer leakage inductance
> still gets wasted.
>
> Ok, so leakage inductance energy can be utilized with the two switch
> forward converter, and additionally halving the peak voltages across
the
> switches. But this topology adds one gate drive transformer to the
system.
>
> With the need for a gate drive transformer, then seems almost no more
> effort to go for half bridge topology and gain the benefits of even
> better transformer core utilization. Comparing the half bridge to the
> flyback converter in the 25W .. 100W range;
>
> Flyback disadvantages:
> * high peak currents and ripple
> * large ferrite core with air gap
> * high switch peak voltages
>
> Full bridge disadvantages:
> * needs a gate drive transformer
> * needs output inductor
> * an extra FET switch
> * slightly more complex control circuitry
>
>
> When does one topology over the other get more economical/easier to
build ?
>
>
>
>
>
>
>
> The half bridge seems simpler.
> Adam
Hi Adam, leakage is relatively high in flybacks because the windings
conduct at different periods, this means that the fields of the 2
windings cannot cancel each other therefore split sandwiching doesn,t
help very much unlike in a forward transformer.
Why are they common? because they use fewer parts. If you a making
1000's of supplies the part count is more importamnt that the extra
problems in the design phase. For many applications where the load is
constant(ish) its not too hard to get a design that is reasonably
efficient.
Half bridge makes the transformer design much simpler, it's now allmost
impossible to wind 1 that wont work, the downside is more power
switches but thats not a problem as they are so cheap now. The problem
now is the driver transformer/drive configeration. As there are now
good halfbridge drivers, made for the motor drive industry, around use
1 of those instead.
I gave up making supplies years ago as they are now so cheap, sometimes
cheaper than the cost of the parts, why are you bothering?
.
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