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UCC27211: HO stays low after a N channel mosfet is connected

Remco Hardeveld
Prodigy 30 points
Part Number: UCC27211
Other Parts Discussed in Thread: TINA-TI,

Hi for everyone who is reading this,

For a project I am experimenting with the UCC27211 high and low side mosfet driver. I want to make a "power" amplifier using the stated driver in combination with a n channel trench mosfet (SQP90142E). But it seems that I can't get the driver working properly. As a reference I used the "UCC27211 transient analysis" that is available for Tina-TI (see picture below). I adjusted the capacitor load (CloadHO & CloadLO) with a rough estimation of the N channel mosfet I intend to going to use. After building the circuit I measured with a scope the HO and LO outputs. The graph is in my opinion correct (minus the huge transients on the flanks, but that is probably due to that I use a bread board (A)).

Only when I connect the N channel mosfets gates to the LO and HO output (without connecting to mosfet any further) the HO output goes to 0 and will not recover anymore even when it is not connected anymore. And when I try to build the circuit stated in the datasheet, using the circuit with only one UCC27211. The mosfets will conduct straight away doesnt matter what timing the HI and/or LI has. 

Does someone already experienced this or knows a solution, I will be very glad to hear it.

Kind regards,

Remco

  • 0
    TI__Mastermind 23215 points
    Hello Remco,
    Thank you for the interest in the UCC27211 driver. To be able to provide the best advice I would need to review a schematic of the driver, MOSFETs, and the load connected to the power train. One common issue we see with users of the high side driver in HB drivers, is the floating HB bias and the sequence required to charge the HB-HS capacitor, and maintain this capacitor charge.
    The HB and HO is referenced to the HS switch node. The HB-HS capacitor is charged typically when the LO turns on which forces the HS switch node near ground. At that time the HB capacitor is charged from VDD thru the bootstrap diode to HB.
    Can you confirm the HO, HB and HS waveforms in the condition you describe?
    If you still have concerns, provide a schematic and scope plots of the HI, HO, HB and HS waveforms so I can comment.

    Regards,
    Richard Herring
  • 0 in reply to Richard Herring
    Prodigy 30 points

    Thank you for your quick reply. I will perform those measurements with my next setup. I am quit busy at the moment with it, so I hope I can reply you as soon as possible.

    I have still one question for now (just to verify if I understand the required timing of HI and LI). If I want that only the high side N channel mosfet conducts  HI needs to be high and LI needs to be low. And vise versa. If I want that only the low side N channel mosfet conducts HI needs to be low and LI needs to be high. Am I correct with this statement?

  • 0 in reply to Remco Hardeveld
    TI__Mastermind 23215 points

    Hello Remco,

    You can reach out to us for further assistance when you have a chance to make some measurements.

    Regarding the question. Your statement is correct, for HO to be high, and LO low HI needs to be high and LI needs to be low. This driver allows both outputs to be high if both inputs are high, the outputs will follow the inputs. If you want HO and LO not to be on at the same time, LI and HI inputs need to not be high at the same time, preferably with some dead time to allow time for the HO and LO output to discharge the MOSFET gate charge.

    For any updates or questions, you can post on this thread.

  • 0 in reply to Richard Herring
    Prodigy 30 points
    I had some time to improve the design of the circuit. Instead on a breadboard it is now assembled on a perfboard, trying to follow the rules in the datasheet. It looks like that made an significant improvement because it appears to work at a frequency of 500kHz. It will only get hot really quickly when I try to get it working on 800kHz (driver drawing ~100mA). if I am still working on it, i will make a different question for this (if i cannot be helped with the excising posts that are available with UC27211 getting hot).
  • 0 in reply to Remco Hardeveld
    TI__Mastermind 23215 points
    Hello Remco,
    Thank you for the update. Yes I would recommend starting a new thread if you have additional questions going forward. It sounds like you are able to operate at high switching frequencies at this time.

    Regards
    Richard Herring