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Unregulated (No Feedback) Isolated DC/DC Controllers

Bryan McLaughlin
Intellectual 985 points
Other Parts Discussed in Thread: UCC28700, LM5160, UCC28089, LM5109

This EDN article from TI recommends half-bridge, push-pull, and full-bridge as candidates for an unregulated design. Could you please recommend PWM controllers suitable for an unregulated DC/DC, as well as any app notes on unregulated DC/DC?

We require 12Vin, 12Vout @ 12W. Output voltage can be +/- 2V over load (10mA-1A). 

A center tap on either side is highly inconvenient, so Push-Pull isn't a great solution.

We'd like to avoid an Aux winding, so the PSR Flyback (UCC28700) isn't very attractive.

We are looking at Fly-Buck as well, but it's getting close to the LM5160 power limit.

  • 0 in reply to Bryan McLaughlin
    Prodigy 5 points

    Hello Bryan, 

    This type of design can be done with a Push Pull controller.
    In our design archives we have a PMP2279, 48Vin 48Vout @ 2Amps.
    The design is not public.


    The UCC28089 Primary-Side Push-Pull Oscillator with Dead-Time Control is used.
    Bang Bang 50% duty cycle, with a little dead time to avoid cross conduction, creates the DC transformer effect.
     

    The output regulation would be a function of; Vin and resistive losses in the transformer and inductor.

    As is the circuit could work for you, but you could modify it to closer match your voltages.
    Transformer is a PULSE PA0905 off the shelf.

    Let me know if you need anything else.

  • 0 in reply to Ed Walker
    Intellectual 985 points

    Hi Ed! Thanks for the reply. Push-Pull is basically not going to work; due to physical constraints we can't fit many winding connections.

    Haifeng Fan's article mentions that half- and full-bridge converters are sometimes used for this "DC transformer" application. A circuit like this would be ideal since it doesn't require any center taps or aux windings:

    Any suggestions down this path would be appreciated. Thanks!

  • 0 in reply to Bryan McLaughlin
    Prodigy 5 points

    Bryan, good to hear from you.

    A push pull controller can drive;
    push pull
    Half Bridge
    Full Bridge

    So the UCC28089 could work for your full bridge design.
    I made a sketch as an example.

    UCC28089 has 2 outputs, A B
    A drives top left, bottom right
    B drives top right, bottom left

    Since the UCC28089 has only ground referenced outputs, you need a driver to be able to get the top mosfet gates above 12Vin.
    LM5109 was the lowest cost driver that could work.
    LM5109 boot strap will work great since the duty cycle will be ~50%

    I couldn't find a full reference design for 12Vin 12Vout with a full bridge.
    Usually at that low voltage, and low power, a push pull is used.

    But this drawing below shows how it could be done.
    Of course there are details to you to work out.