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UCC21520: Missing or delayed gate drive signal

Bene N
Prodigy 20 points
Part Number: UCC21520
Other Parts Discussed in Thread: UCC21540

Dear all,

I am using the UCC21520 to drive a full bridge consiting of 4 Infineon IPD60R180C7 MOSFETs with SiC diodes in parallel, see the schematic below.

The other half of the bridge is designed accordingly. Not shown in the schametic above are low impedance film capacitors supporting the +400 V rail.

The bridge shall drive a (impedance matched) coil of <1 uH with a 2 MHz pulse train for creating a magnetic field. The bridge voltage (=coil driving voltage) shall be in the range 200...800 Vpp.

I am experiencing a strange problem:

When I rise the voltage at the "+400V" supply rail, starting at a specific voltage, the last pulses start to trigger later or go missing at all. This happens already at a supply voltage of appr. 55 V, depeding on the load connected to the bridge. When no load is connected, missing pulses start already at 50 V. With a coil connected, it is 55...60 V. With a load resistor, I get similar results.

Please have a look at the scope screenshots below, depicting the missing of the last pulses of a pulse train. In the first picture, the pulses look ok.

However, with increasing voltage, the last pulses (at least of one half bridge) go missing:

As you can see, there is some ringing. My layout is probably not the very best, as the distance between driver and MOSFET are ca. 20...25 mm, but I considered all layout examples from TI. The gate resistors are close to the gate. The missing pulses start at one half bridge and with increasing voltage at the other bridge, too.

I also tried to insert ferrite beads at the gates, but the ringing at the gate got much worse. Without the snubber network, ringing gets worse, too.

The signals at the UCC21520 inputs look fine. Of course, there is some noise, but it is well below 1 Vpp.

I don't know exactly where the ringing is coming from and why the pulses get delayed.

Do you have any ideas?

Thank you very much.

Best regards

Bene

  • 0
    TI__Mastermind 31610 points

    Hi Bene,

    I see two possible root causes:

    1. The CMTI rating of the driver could be exceeded as the supply voltage increases. Check the slew rate on the switch node, and ensure that it is <100V/ns. Use a small measurement loop. If the dv/dt exceeds the guaranteed ratings, you may want to consider a driver with higher average CMTI ratings such as UCC21540.
    2. The UCC21520 may not be able to drive this pulse train quickly enough due to internal regulator instability with rapid, repeated pulses. We have seen this behavior in the past with some fast pulse trains: as the bus voltage increases, the output current also increases due to the increased miller effect, which can cause oscillation and instability on internal regulators. Please see this E2E thread for more information and some suggestions.

    Regards,

  • 0 in reply to Derek Payne
    Prodigy 20 points

    Hello Derek,

    thank you for your evaluation of the issue.

    Due to the ringing, the slew rate on the switch node is close to 100 V/ns, measured with tip and barrel. However, the problem persists even when I use higher gate resistor values, thereby decreasing the slew rate to a safe level.

    The second point seems to hit the nail on the head. I can observe every issue mentioned in the thread you linked in my circuit, too. There is a dependence on the supply voltage of the gate driver: Lower voltages do allow higher bridge voltages before an instability occurs. In addition, the circuit works well at the full bridge supply voltage of 400 V when driving it with prolonged pulse widths.

    In summary, the UCC21520 seems to be unsuitable for my application. I have to say, I am a little disappointed by the quality of the datasheet - which are normally very good - in this case. The issue seems to be known at least for almost a year, but I have not found any mention of it in the datasheet. All operating parameters of the device (at least those I can measure with sufficient precision) seem to be within its maximum limits.

    Best regards

    Bene