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UCC256402: IC doesn't work below 26V

Rajendra Goswami
Prodigy 225 points
Part Number: UCC256402

We are using UCC256402 for half bridge LLC converter, where Vhv = 150V.

According to datasheet, it is mentioned that Vcc supply voltage range is Min: 13V, Nom: 15V, Max: 26V in recommended operating condition table. But IC didn't work when provided 24V at Vcc pin, then there is another table which mentions - VCCStartSelf: min :25V, TYP: 26V,  Max: 28V , Which is quite contradictory to the earlier statement.

So the question is if IC doesn't work below 26V then what is the meaning of recommended operating condition ?

When we reduce (after start up) Vcc voltage from 26V to even 24V, IC stops generating switching waveforms. IC always needs 26V supply to Vcc pin in order to make it work.

  • 0
    TI__Guru* 85616 points

    Hello,

    The UCC256402 should start at VCC = 26 V typically.  The data sheet does show there is a spread in VCCStartSelf voltage and it is below the absolute maximum.

    The UCC256402 will stop switching for also

    -Bulk voltage to low, BLK pin, BLKSartTH = 3 V ,BLKStopTH = 2.2 V, check data sheet for variation

    -Over Current Protection, ISNS pin, OCP1 = 4 V, OCP2 = 0.6V Avg, OCP3 =0.43V Avg, check data sheet for variation

    • Only trigger when ISNS is positive.  Averaging is only done when ISNS is positive

    -Output Over Voltage Protection BW pin

    • This is measure with a voltage divider off the aux winding. 
    • OVP = 4V, check data sheet for variation.

    -VCC not being biased correctly through a bias supply or aux winding.

    You can determine which fault by probing the VCC, ISNS,  BW and BLK pin with an oscilloscope.

    Thinks that could help you with your design and evaluating performance.

    Evaluation Module:

    https://www.ti.com/lit/ug/sluubx3b/sluubx3b.pdf

    Excel Design Tool:

    The following application notes:

    There is also a Webench design tool that will generate a schematic and calculations for designing a transformer for this application.  You can find the Webench tool with following link. Just click on it and search for WEBENCH.

    https://www.ti.com/product/UCC256402?keyMatch=UCC256402&tisearch=Search-EN-everything&usecase=GPN#design-development

    Regards,