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UCC21530-Q1: Boostrap capacitor voltage

Part Number: UCC21530-Q1

In my current application a three phase 2 level inverter is used to control a PMSM.

I'm using UCC21530-Q1 to drive SIC mosfet. Swtiching frequency is 20 kHz and switching speed in the range of 50 ns.

min and max duty cycle is 4 / 96 %

We use boostrap capacitor (1uF) to supply high side mosfet with a 10 ohm resistor + 1200V diode

We also use negative Bias with Single Power Supply and Zener Diode in Gate Drive Path as shown in figure 9.4 in component datasheet

The misunderstood we are facing:

In a first test, PWM was set at 50 % (minus DT), fixed value. DC bus voltage was 60V.

As you can see, the voltage at boostrap capacitor is quite stable with high frequency “oscillation”. Everything works as expected. After 15s we shut down the gate driver enabling to see how many time is needed to deplete boostrap capacitor.

Measure of boostrap capacitor voltage

In a second test, we applied same condition, except one thing. Someone was turning back and forth PMSM rotorshaft by hand. Again duty cycle was fixed at 50%

Measure of boostrap capacitor voltage

I’m wondering the different value i'm seeing. In the first 1.5s  the ramp up and ramp down are very slow. Then bettween 1.6 to 2.3s voltage stay constant but below the nominal value. Finally from 2.5 to 5s, ramp up / ramp down are faster and the lowest value is near 0V. Do you have any idea explaining this behavior ?

• Hello Sebastien,

During the experiment with rotating the motor with hand, did you measure the low-side voltage as well? If not, I suggest measuring that voltage as well to check if that's stable. Please let me know.

Also, if the input of the HS channel is on when the motor is rotated by hand, the gate will try to turn-on. Combined with the back-EMF generated by the motor, that might cause erratic bootstrap voltage by the gate driver.

Vivian

• When you mention low side, do you mean low voltage side or low side mosfet ?

Also, if the input of the HS channel is on when the motor is rotated by hand, the gate will try to turn-on. Combined with the back-EMF generated by the motor, that might cause erratic bootstrap voltage by the gate driver.

HS channel will be ON during some amount of time, but i can't explain "low frequency" you could see in first 1.5 second of the capture. Can't find a logical explanation.

• Hello Sebastien,

Our expert is currently Out of Office. We'll get back to you by EOD Monday Mar 4.

Best,

Pratik

• Hi Sebastien,

I meant the VDD used to drive the low-side MOSFET - since if that VDD is unstable, bootstrap voltage that comes from that voltage might be unstable as well.

At the same time, can you also measure the input/output of the channels? I want to make sure the gate driver functions normally under the interference.

Thanks,

Vivian

• Hello Everyone,

We just repeated the measure. Both boostrap voltage and VDD has been recorded and voltage are perfectly stable. I believe  we had a  mass issue leading to improper measurement.

Sorry and thanks for the support.

Sébastien

• Hi Sebastien,

Good to know and thank you for confirming! Closing this thread as resolved now.

Vivian