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UCC27712-Q1: Unexpected high-side output with bootstrap power supply

Bryan Hall
Prodigy 10 points
Part Number: UCC27712-Q1


I constructed the following circuit using two UCC27712QDRQ1 driver chips and four IRF150P221 n-channel MOSFETs. I stuck as close as possible to the reference design guidelines, although Cboot may be a bit larger than needed as noted. Bias Vdd in is 12v, common ground. Vhv as tested on the bench was set at 60v, although changing this potential seems to have no significant effect on the high-side voltage.

What I am seeing is the expected output when LI is driven high (3.3v) where LO goes from 0v (all DC coupled to common ground) to close to the 12v supply over a single 50uS pulse (spaced at 1 second intervals for testing). What is unexpected (most likely due to my power design ignorance) is that HO is shown with about 10v with HI low until driven high (3.3v) where HO goes to 0v. I expected HO to be around Vhv with HI low, going to Vhv + ~12v when HI is driven high.

Am I misinterpreting the Bias supply input requirements using bootstrap capacitors causing the driver chip to enter some protection mode?

Puzzled, thanks in advance,

Bryan

  • 0
    Prodigy 10 points

    I should also note that these are high and lows from opposite sides. Also, all inputs on both sides are low until both of these are pulsed for 50uS, once every second for this test.

    In practice the single pulse width will be from about 25-50uS with 68uS - 125ms intervals depending on rotor speed.

  • 0
    Prodigy 10 points

    After I posted the Similar Topics suggested a thread that suggested the low side not being on long enough causing issues (missed that one in search somehow). As noted the low was normally off until required.

    I'm re-writting the MCU test code to leave both of the low side drivers on until the high side needs to be turned on, and then turn it back on after turning the high side off. Hopefully that will allow the boot caps to charge correctly and resolve the issue and allow this to function more as I expected. I'll update after more testing.

    Thanks,

    Bryan

  • 0 in reply to Bryan Hall
    TI__Genius 14145 points

    Hey Bryan,

    Thank you for your question regarding the UCC27712-Q1.

    It appears that you may have already found your problem and are on the right track. Once you have made this change please return here to confirm your findings or ask further questions.

    Thank you,

    William Moore

  • 0 in reply to William Moore
    Prodigy 10 points

    That would be it. The boot caps just weren't getting charged to follow Vhv. Looks like I'm getting about 11v above Vhv (which here is 60v), which work fine for this prototype. Life is good again...

    Thanks,

    Bryan