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UCC27211: Sometimes the HO has got a delay with respect to the Input, why???

PowerSystemsEngineer
Prodigy 90 points
Part Number: UCC27211

Hello dear Texas Instruments Team,

I want to use you halb Bridge Driver UCC27211. Sometimes, it happens that there is a massive delay of the HO with respect to the HI. Please see the scope Image:

CH1=Yellow=Drain-Source-Voltage of Lowside-MOSFET

CH2=Red=Driver Input voltage for highside-MOSFET with respect to Ground ("HIF", pin 5 of the UCC27211)

CH3=Blue=Driver Input voltage for highside-MOSFET with respect to Ground ("HO", pin 3 of the UCC27211)

CH4=Green=Current of Inductor

You can clearly see that the blue line ("HO", pin 3 of the UCC27211) has got a massive delay of 700ns to the red line ("HIF", pin 5 of the UCC27211).

Why is this delay? The Driver is correctly supplied with 12V, the input signals are 0V/3.3V. And the UCC27211 is used, which should work with 0V/3.3V Input signals. Help is greatly appreciated. Kind regards´, Martin

  • 0
    TI__Mastermind 20950 points

    Hello Patrick,

    Thank you for the interest in the UCC27211 driver. I think what you are seeing is the effect of the UVLO delay of the HB-HS bias. In a typical half bridge configuration, the HB-HS bias is charged on the initial LO high pulse which turns on the low side MOSFET providing a path to charge the HB-HS capacitor through the boot diode from VDD. There is a typical HB-HS UVLO delay of ~2us max which means even if HI is high the HO output will not respond until after the rising edge of HB-HS above the UVLO threshold and a delay of up to 2us. It looks like in your scope plots that from the time the blue trace goes low, which is when HB to HS can start charging to the time HO goes high is ~1.6us. This looks like on this cycle the HB-HS bias was not charged above the HB UVLO when the switch node switched to ground.

    You can confirm this by recording the scope plot of HB-HS during this behavior. If you have a differential probe that is the most convenient, or you can plot HB and HS with the same voltage scale and ground reference, this will highlight the HB to HS voltage difference.

    Confirm if this addresses your questions, or you can post additional questions on this thread.

    Regards,