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UCC29002 load sharing issue

Kum Hoo Lee
Expert 1855 points
Other Parts Discussed in Thread: UCC29002, UCC39002

Hi,

Currently I am doing three power supplies paralleling test with this IC, But circuit seems not working fine, current is not balanced when loading is high(>50A).

Our power unit is universal input / 12VDC 30A output / 350W.

Issue now is : can not share current during power on with load(>20A). If loading >40A, each power unit will OCP one by one. can not power on eventually. What will be causing this issue? 

Thanks,

KH Lee

  • 0
    TI__Mastermind 25570 points
    Hello KH
    I'm afraid that this behaviour is inherent in a sharing system where one module cannot supply the entire load. As you correctly point out, each unit goes into OCP as the system starts. If they go into some kind of hiccup or stop/retry mode you will have to make sure they are enabled at the same time so the first one on doesn't come up into an overload condition, starting a hiccup cycle for all the other modules to come up into to.

    The UCC29002/1 device has a different start-up behaviour and you may find that it works better in your application - it is a drop-in replacement for the UCC39002.

    If you have a possibility to reduce the load during the start-up phase then this should remove the OCP conditions.

    The converter module must be able to start into a pre-biased output. Due to manufacturing and design tolerances, paralleled modules will invariably have slightly different start up profiles and the module that comes up first will bias the output that the other modules are starting up into. - effectively this means that a converter that uses synchronous rectification must temporarily disable the freewheeling MOSFET so it does not sink current from the power plane. It isn't clear from the datasheet whether the XP power GCS350PS12 does this or not. If not then you will need to add some external isolating diodes to prevent current flow into the module output.
    The load share loop must be slow - a decade below the cross over frequency of the modules being controlled.

    Regards
    Colin