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flybuck design with eight isolated outputs

Dimit Ariti
Prodigy 110 points
Other Parts Discussed in Thread: LM25017

Hi

I need to design a circuit which can provide 8 isolated supply rails around 12 volt for isolated  gate drives. 0.25W is max power needed for each output and so total power is 2W. Obviously a simple design solution can be  two separate flybuck converter that each includes:  one flybuck  controller and one transformer  with 4 isolated outputs. This way I can have 8 isolated outputs. However, due to the cost and size constrains I’m looking for a design with only one controller. I have also small dual output coupled inductors available. Design solution  shown in below uses coupled inductors . In this picture as shown only one  controller is used and four output LCR are effectively in parallel. Since only one feedback is available the output  voltage of the first LCR filter is sensed and fed back to the controller. However, since the loads of all four LCR and secondary outputs   are identical we can assume the feedback system help also to regulate the other three primary outputs.

Now I'm wondering if this solution will work for me or not? Do I have to have a feedack at all, considering the fact that the  input is fixed regulated 15 volts?  Maybe this system can work open loop? If you there is any alternative solutions please let me know.

Thanks.

  • 0
    TI__Expert 7080 points
    Hello,

    Thanks for your interest in TI here.

    I think the solution you get from our design solution is work for you . but you still need a feedback.
    Because the control scheme of LM25017 is compare the FB with a internal 1.225V reference .
    If the FB voltage is below the reference, the internal buck switch is turned on for the one-shot timer period. Following the on-time the switch remains off until the FB voltage falls below the reference. so if open loop , after turn on time , there will a minimum off time .The switching frequency will run to a maximum set value which decided by the turn on time and minimum off time.
    You will lose the regulation of output . and OVP function.

    Here is one EVM reference design for your reference.www.ti.com/.../snva611b.pdf

    Thanks.