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OK  -  there does seem to be some slight good news on the transformer front   -   since the turns ratio appears to be  0.75 pri : 1 sec  ( from data sheet )

this can only be achieved with 3 : 4 turns  <-   this is the min turns that can give this turns ratio,  the next is  6 : 8  and so on  . . .

[ and I can almost count  4+4 turns on the sec from the pictures ]

So for 3 turns on the pri,  and 2 x E55/28/21 cores  of active area 722 mm^2  and  say   55V volts driving,  the  delta B in the cores is:   +/- 63mT  @ 100kHz

Depending on how crappy the core material is  ( and it ain't  3C98,  that's for sure )  if you drop the sw freq to 50kHz the flux swing will double to  +/- 0.126Bpk

which  may well be just manageable for the core  -  likely  120mW / cm^3   when it gets up to temp  ( ~ 80 deg c ?  )  or possibly better  -  see below.

This halves the sw losses straight away  -  so down to 200W  per diode snubber in the snubbers  -  at full power.

[   N.B.  output current will be DCM for longer in L1, and twice as ripply  in  CCM  -  so check the temp rise of the dual L,  L1  -  p.s.  inductors in series like this is a bad idea  -  as the mid point can ring like crazy and easily over volt the wdgs  -  so check this and perhaps add snubbers across each  L  -  or a bigger single part  -  indeed a current doubler secondary with two chokes would have been a sitter here as only  45A in each choke  and  only  45A in Tx sec  ].

but I digress slightly,    you have D2,  D3  as diodes to a sort of clamp ckt,   now what you do is add a controlled buck as the load on C5  ( make C5 bigger, and the diodes while you're at it  )  -  a 400W buck converter if you haven't been following the discussion  -  and control its pwm in step with the pwm on the pri side  -  or using a Vmax control on C5  -  any way it runs all the time but harder for higher load  -  this then gets your snubber energy to the output  -  and increases the efficiency of the whole ox cart quite a bit as well.

A good idea to leave  the 9 x zeners on each of the two sec side mosfets as prev described  -  better safe than sorry from over volt spikes.

Since C5 is soaking up the energy  -  the buck can be further away from the action  -  which should help with overall layout.

A dummy load on the output,  say   100mA at 48VDC,   say   470 ohms,  rated to easily handle 5W  ( i.e. properly heatsunk )  -  will help keep the buck output to 48VDC max at lighter loads.

Again  -  you're welcome.

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TP4A   -  not  TP44 as typed in Tx data sheet

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It is largely a lost cause  -  unless you can get the tandem buck going  -  as suggested above.   Just looking at it I suspect the figure of 250nH for leakage is a lil optimistic ( in fact if you measure Lpri with all sec's shorted,  I would be surprised if the L leak measured < 1uH )  -  why did they not gap/glue the cores to get 35uH pri L  ?  -   that would have been easy enough.

You are certainly on a learning curve here.