Explanation Required for High Power Switching Power Supply (3-60v 40 Amps)

[treez]

The Bootstrap capacitor will charge through the primary inductor?

Speaking of using bootstrap half bridge drivers to drive two transistor forward converters, this is not without problems.
Basso, in his book "switch mode power supplies", on pages 760,761 states that some extra components are needed to be used aswell if you are going to use a bootstrap high side drive ic for a 2 transistor forward converter (as attached)

..as discussed, the alternative to the additional compoennts offered by Basso is to use a bootstrap half bridge drive but as in the linked post of post#31

 

[Warpspeed]

The Bootstrap capacitor will charge through the primary inductor?

Yes, but only at first initial power up where the bootstrap capacitor is fully discharged.

The upper diagonal mosfet initially gets zero gate drive voltage because there is no gate drive power available from the bootstrap.
The lower mosfet is directly powered off the main +12v power supply rail, so when it turns on, it pulls down the source of the upper mosfet through the primary of the transformer.

This only happens the very first switching cycle, charging the bootstrap. Second switching cycle and forever after, both lower and upper mosfets turn on and off together simultaneously.

The bootstrap then relies on the inductor flyback voltage to recharge each switching cycle. If it fails to do so, it will revert to the start up condition and restart anyway.

The upper gate drive circuit has an under voltage lockout so it cannot gradually wimp out from power starvation, it either works properly or it shuts down completely and reverts to start up mode.

Basso's solution is more elegant, but it will work fine without it.

 

[treez]

Warpspeed I believe you are right....and I think its very polite of you to say "elegant", because from your explanation, which I agree with, those components in pg760 and 761 look like in most cases being a total waste of time...not needed.

Anyway...
This hyper TFS2 2 transistor forward controller does a high side drive, but in a different way…….the diodes actually shovel the flyback energy into the RCD clamp of a flyback that’s also included……the 2 transistor forward controller then “drinks” its high side bias supply out of this flyback clamp “high side supply”.
This means that the usual problems for bootstrap drivers caused by potential ringing of the low side well below gnd due to pcb trace inductance, …well, this doesn’t affect this hyper tfs2 chip…so that nasty overvoltage on the bootstrap cap that you can get sometimes, well., as you can see, that couldn’t happen with the hypertfs2…because that low side diode cathode could ring well below ground and it wouldn’t mean any overvoltage on the bootstrap cap at all…

HyperTFS2 datasheet ( 2 tran forward controller)
https://www.mouser.com/ds/2/328/hipertfs-2_family_datasheet-271679.pdf

also here if the above link doesn't work
https://www.power.com/sites/default/files/product-docs/hipertfs-2_family_datasheet.pdf

 

[adnan012]

Thanks for reply.

Is there any design reference available for 2 switch forward converter?

 

[treez]

Do it like a (transformer adjusted) buck converter.

So first pick your max duty cycle, say D=0.4 (at min vin , max load)
Then “pretend” you are using a buck converter, and do “VIN” = VOUT/D

That “VIN” value is the VIN value that your “buck” converter needs….
-so find your min vin, and make the transformer turns ratio sufficient to give you that “VIN” value from your actual input voltage

remembering NP/NS=VP/VS