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UCC21755-Q1: Fault shutdown

Shishir Trivedi
Prodigy 80 points
Part Number: UCC21755-Q1

I have designed a gate driver card to support a traction inverter which has a rating of (60V rms output, 60A rms output). I have designed a single driver card to support functioning of one single leg/ Half bridge of a 2-level VSI. The IGBT modules used (SKM300GB066D) has a gate charge of 2400nC. According to the design, after connecting an external gate resistance of 1.4 ohms, the peak gate current come to be 6.5A, which is supported by the selected IC. During testing of the inverter. The inverter works fine for 20A rms output currents (with a 1kW small EV motor).

However, when I connect my traction inverter after a step down dc-dc converter ( from 72V battery to 60Vdc), in order to operate an EV induction motor, I see that two of the legs of the inverter are not functional as soon as I turn ON the PWMs of the inverter. The Legs of the inverter that remain unfunctional ( i.e. shows fault at the IC input side) remain the same if I interchange my driver cards. Moreover, when I turn off the mains, and power the PWMs all the driver cards give PWM output. I have tried changing desat resistors (suspecting it draws a huge current during start) to very low values (for tripping at 500A peak), but the result was the same. 

Kindly help me out to figure out the issue/cases where ther driver IC would refuse to pass on the PWMs to its secondary side.

  • 0
    TI__Expert 8065 points

    Hi Shishir,

    Thanks for your detailed email and your interest with UCC21755 devices. Its hard to explain without observing your schematics and the waveforms? Can you please share your schematics and also is it possible to capture the Vgs, Vds, FLT and the DESAT voltage during the failure. In case of high side measurement of Vgs, Vds and DESAT, please use ISOVu differential probe - closest possible to the device. (with small twisted pair of cable coming out of the board to the probe tip)

    Is it FLT only/ RDY also getting triggered?

    Is it high side driver or low side driver which has the issue?

    Have you ensured that the dead time is enough so that the high and low side FETs are not turning on simultaneously.

    Based on your inputs, we could help you further.

    Thanks

    Sasi  

  • 0 in reply to Sasikala Thangam
    Prodigy 80 points

    Hi Sasikala,

    I shall be providing you with the said waveforms soon. The capturing of DESAT waveform is not possible in my case.

    I took the FLT and RDY voltage readings w.r.t grounfd (with Vcc = 3.3V) with and without the mains supply and the load connected. Unfortunately, as I have tied the FLT and RDY signals, both the high side driver and low side driver trigger fault. Following are the readings observed:

    case(1) Without mains supply & load connected: FLT=2.3V and RDY=3.3V

    case(2) With mains supply & load connected: FLT= 0V (R & B card), FLT= 2.3V (Y card) and RDY=3.3V 

    Note that in case(1) I get PWM outputs in all three gate driver cards (Naming it R, Y and B). However in case(2) Only one gate driver (Y) produces PWM outputs for higher and lower side, whereas the other two driver cards (R and B) fail to produce PWM outputs for both the higher and lower side.

    I have assigned dead time for each of the PWMs as 2us (switching frequency: 2kHz) and verified the same using a scope. 

    Can you please share your email id so that I could share the schematic of the driver card to you?

    Thanks,

    Shishir

  • 0 in reply to Shishir Trivedi
    TI__Expert 8065 points

    Hi  Shishir,

    I will close this ticket as we will continue further discussion in email thread.

    Thanks

    Sasi