One Transistor Forward converter: Reset winding.

[cupoftea]

Ayk, a One transistor forward uses a "reset winding" to
reset the flux in the core after every primary switch-ON
cycle. However, the reset winding will not be perfectly coupled to the main primary winding.
As such, there will always be a small component of flux which cannot get reset by the
reset winding. So, in theory, staircase saturation will gradually occur.
However, it does not occur. Why not?

 

[betwixt]

I'm not sure I understand your question but if you are describing a blocking oscillator configuration, the gain if the switching transistor is what ensures complete cessation of the primary current. You seem to be implying it is the current in the 'reset winding' alone that does the work.

Brian.

 

[FvM]

The term "reset winding" is used for different features and to some extent ambiguous. If you are not considering a feature to actively reset transformer flux by applying a pulse, it's just a path for stored energy back to the dc bus. Flux "reset" is occuring passively driven by magnetizing inductance. The main point is to provide sufficient Vdt in the reset phase.

Leakage inductance has nothing to do with successful reset, it will however cause an overvoltage in other windings and is in so far unwanted.

 

[cupoftea]

Thanks, ayk, in the 1 tran forward, when the primary fet switches off, there is on path for the primary magnetizing current to discharge, (consider there is no clamp) so the reset winding is needed.
Though it cannot possibly be perfectly well coupled with the primary. As such, it cannot reset all that flux supplied into the core from the primary coil.
Good point about leakage, this is another problem, and does need some clamping....typically if N(reset) = N(pri) then the leakage reset will be usually made to occur above 2 x vin.....so a TVS style clamp would be useful to burn up the leakage energy.

 

[Easy peasy]

You only need the flux ( and hence magnetising current ) to go to a low value ( the same low value ) during reset for the thing to work.

The fact that reset ckts do work for the forward - shows your thinking is flawed in some respect

When the mosfet turns off - its Vds goes to something like 2 x Vin, during reset - current flowing in the reset wdg - when the Vds starts to fall - pretty much all the energy in the core has been transferred to the input vin, Cin

Due to Lleak - there will always be a small spike on the mosfet turning off - this may need to be snubbed

If the ON time is too large ( typ over 45% ) then staircasing can occur on a falling Vin ( such as power down with full load connected ) and blow the mosfet - if there is no UVLO to stop the thing - newbie designers often overlook such fine detail ( the gotcha's ) required for a successful forward converter ! ! !

 

[cupoftea]

the fact that reset ckts do work for the forward - shows your thinking is flawed in some respect

Thanks, i definitely appreciate that these circuits work...and i believe you kindly confirm how.....they work by simply resetting most of the flux......how much they reset, depends on the degree of coupling between reset winding and main winding.
So now we would look into whether its worth interleave winding the reset winding and the main winding.
If we do not reset all the flux, then we have that little less margin before saturation.

And/or, do we add a super-thin shim to give a wee gap....so that the onset of saturation is that bit less sharp.

If the ON time is too large ( typ over 45% ) then staircasing can occur on a falling Vin ( such as power down with full load connected ) and blow the mosfet - if there is no UVLO to stop the thing

Thanks, this is excellent....and we look for a control chip with max duty cycle which is 45% and they are nowhere...i think demo'ing how far behind the (western) world is when in the field of SMPS.....the demand isnt there...for SMPS...just import it from China.

I think a ucc28c43 can be made to have a max duty below 50% , but not far below...and ucc28c43 has lousy 1V CS reference.

 

[cupoftea]

You bring up great points, because ayk, with say a 50% max controller (most are all the way to 50% unfortunately), the DutyCycle can go to 50% in overload (before overload shutdown occurs), and during non soft starts.......so yes, staircasing saturation is lurking in there. There have been long discussions on edaboard on micro thin shim gapping of forward converters because of this.
..Though i have somewhat hijacked the thread there, since its wondering about eg bifilar winding of the reset coil, or interleave winding of it with the pri...or just stuffing it in the sandwich between the two primary halves.

 

[Easy peasy]

how much they reset, depends on the degree of coupling between reset winding and main winding

this is not strictly true - it is a time thing, also reset windings are meant to be intertwined with the main pri wdg to get max coupling - to reduce overshoot spike on the mosfet.