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One Transistor Forward converter is a bogus SMPS toplogy?

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Flyback

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Hello,
The one transistor forward discharges the transformer magnetising flux by conduction in the auxiliary winding when the fet is off.
However, if N(aux) = N(pri), then the fet VDS goes to 2*vin during this flux discharge (transformer reset).
Not only that, but there will inevitably be leakage, and thus a RCD clamp will be needed across the primary, and this will mean even higher VDS voltage...as such, switch-on switching losses will be incredibly high.

Can we thus confirm that the 1 transistor forward is a seldom used academic postulation?
 
Hello,
The one transistor forward discharges the transformer magnetising flux by conduction in the auxiliary winding when the fet is off.
However, if N(aux) = N(pri), then the fet VDS goes to 2*vin during this flux discharge (transformer reset).
Not only that, but there will inevitably be leakage, and thus a RCD clamp will be needed across the primary, and this will mean even higher VDS voltage...as such, switch-on switching losses will be incredibly high.

Can we thus confirm that the 1 transistor forward is a seldom used academic postulation?
In its' simplest form, I'd say yes.

But, as you can see from this: **broken link removed**
there are practical applications when combined with more complex topologies.
 
Hi,

Yes, is he talking about a single transistor as in that is the only active device, or can there also be an IC used as the controller. Because if the latter, there are multiple types used around the world and you probably already have a few around your home in some low voltage regulated wall warts.
 
Yes they also seem to be popular in pc power supply 5v stand by circuits. I think for cheapness.
Also popular in scanner inverters to power the lamp strip, on older ones with fluorescents, they are slightly different design having an inductor in series with the supply so that the circuit operates in resonant mode, I hear very high effeciencys are possible, and being an approximate sine wave produce little emmisions.
 
thanks, I am speaking of a topology with a controller IC, not any smps that uses just one transistor.
I amsurprised to hear the 1TFC is used in low power apps that would normally be associated with a flyback, since the 1TFC needs the auxiliary coil as an extra.
 
A chip I played with recently that doesnt need an aux winding is the TNY 26x series, very good to use, theres a 700v fet built in the chip.
Topswitch do a range that seem very very similar I suspect the same.
 
on 230VAC, the vds(off) for a 1TFC is twice vin + the clamp voltage, which is probably above 700v
 
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