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UCC21756-Q1: Soft turn-off problem when using an external push-pull current buffer

Yuze ZHENG
Prodigy 10 points
Part Number: UCC21756-Q1

Tool/software:

The recommended soft turn-off circuit in UCC21756 datasheet, in my design VDD-VEE=25V (+20/-5V) and I want a 600ns STO process, so Csto is about 22nF according to the formula in datasheet, and Rsto is 10 ohm.

Now the problem is:

When there is no short-circuit fault detected, the normal turn-on and turn-off process become as slow as STO process.

Here is the experimental waveforms, ch1 is VGS, ch2 is input PWM signal, ch3 is out put voltage of UCC21756 (OUTH/OUTL pin), the switching process become extremely slow due to the 22nF Csto.

I'm not sure if the datasheet recommends a faulty circuit or if my design is wrong, and would like to know how to achieve soft turn-off while maintaining normal switching speeds.

  • 0
    TI__Expert 8205 points

    Hi Yuze Zheng,

    Thanks for your interest with UCC217xx family of devices.

    The datasheet highlights about how to incorporate the STO for external BJT, as the STO of the gate driver is bypassed when using external BJT. However now the normal current return path for OUTL is as well through this RC. 

    The gate driver's pull down strength for normal operation (10A) vs STO (400mA) should still be differentiated, unless the RC path is loading the BJT heavily. 

    We have the following reference design for the external BJT implementation with actual circuit implemented with waveforms showing the performance. It is for a different driver, but the concept is same. Please check if you could update the RC values and circuit similar to this one to have differentiated pull down strength between  normal turn off and STO during DESAT event.

    Reference user manual:

    Hope it helps

    Thanks

    Sasi