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UCC28950EVM-442: How to measure Lo-side MOSFET Ids and Vds waveform

Hiroshi Doi35
Expert 2845 points
Part Number: UCC28950EVM-442
Other Parts Discussed in Thread: UCC28951-Q1, UCC28950

Hi All,

My customer trying to measure of UCC28950EVM-442.

He want to measure the waveforms of Q3  both Vds and Ids.

He uses tex. P5205A differential prove  to mesure Vds.

And I think he lift up drain terminal and connect wire to board and use current probe.

But output load growing heavy and then FET broken.

I said to him,

1. Primary GND shoud be low imp. such as short length and Low AWG GND wire between power unit to EVM

2. Short and Low AWG GND wire should be used for current measurement.

Any other caution ?

He has UCC28951-Q1  sample IC also.

  • 0
    TI__Mastermind 25570 points
    Hello Doi-san

    The measurement can be done but with care. When the customer puts the current probe in the drain circuit of the MOSFET then the loop through the current probe must be kept as short as possible. This is very important because the loop will add some inductance to the MOSFET drain circuit and cause Vds spikes during switching which may become large enough to avalanche the MOSFET and destroy it.

    The MOSFET current could also be measured by looking at the transformer primary current - this current flows alternately in the Q1/Q3 and Q2/Q4 paths - the current in Q3 is simply half of the transformer primary current. Simply cut the connection from T2-pin3 and insert a current measurement loop there. This location is much less sensitive to the added inductance.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Expert 2845 points

    Hi Colin-san,

    The customer un do the current measurement.

    And confirm input short wire and primary GND.

    But MOSFET is still broken at heavy load.

    Something wrong with this board.

    But the customer is very clever than you have visited Japan customer.

    If he change the IC to UCC28951-Q1, he can expect something good result?

    Best Regards,

    H.DOI

  • 0 in reply to Hiroshi Doi35
    TI__Mastermind 25570 points
    Hello Doi-san

    The difference in the applications where the choice to be made is between the UCC28950 and UCC28951-Q1 are discussed in the app note at www.ti.com/.../slua853.pdf .
    The customer is of course welcome to try the UCC28951-Q1 device if they wish but to be honest I don't think that it would make much difference in this case. If however, they do decide to try the UCC28951-Q1 then they can do so by simply taking the controller card from the UCC28950EVM-442 , de-soldering the UCC28950 controller IC and replacing it with a UCC28951-Q1 controller. No other changes are needed. This process is described in www.ti.com/.../sluubj5.pdf . Note that sluubj5 refers to a slightly different EVM but the control card is identical to that in the -442 EVM.

    If you think that the customer's -442 EVM is damaged beyond repair, then perhaps the best solution would be to get a new one.

    Regards
    Colin