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UCC28782: UCC28782- USING MP6809 for EPC GaN

robin gangopadhya18
Mastermind 6055 points
Part Number: UCC28782
Other Parts Discussed in Thread: TPS7A24

Hello

FYI: we will be sending this report to MPS Tech support as well.

MP6908A_GaN_UCC28782.pptx

We busted EPC2034, MP6908 along with TPS724xx device  in an implementation using UCC28782 controller.

It gives output as desired: but very fast , all secondary ICs popped!

Pl see the schematic screenshot attached.

The reason for the convoluted biasing of MP6908 is the datasheet does not specifically give any table for VG output values!

So we suspected that with HVC at any unknown voltage, VDD max could be 9V which is clear from datasheet. But nowhere any mention is made for VG value. Since they only show applications with MOSFET, most of which will take 12V, we assumed VG will be 9V- not acceptable for EPC GaN.

So we drop Vout from 20V to 5V & set up MP6908 with 5V.

Clearly, TPS724xx LDO cannot sustain charging current to VDD. 

We won't have to use this scheme if VG  pin value under all VDD conditions is specified to be 5V.

Is it?

Any help will be highly appreciated.

  • 0
    TI__Expert 7080 points

    Hi Robin,

    The schematic shows SR FET configured as low side , there is no description in the MP6908 datasheet to show the driver voltage when it configured at low side as you said . so please contact with MPS for some information.

    I recommend you change to MP6908A even through there is no difference description between MP6908 and MP6908A . But MP6908A is more popular in the market now .

    Thanks.

  • +1 in reply to Jaden Ning
    Mastermind 6055 points

    Really! But what do we do with lots we already bought- they are even installed under reflow...Still, thnx for pointing this out. Meanwhile, let me check.

    I am sure VG will track VDD: which maxes out at 9V.

    So our approach is correct for EPC GaN. The failure seems to originate from TPS7a24 LDO: its max input is 20V but the output showed to be 21v before all went boom....so, LDO went short, its output hits 21V  that goes to VDD, VG then hits 9V...EPC goes Boom....

    Will drop input by a zener( as it is done in the NV6117 circuit). That should fix it. It is a small change: 1 cut, & bridging by a SOD123 PKG. Passes our rework std.

    thnx a lot.

    r