UCC21550: Part 2: Half bridge circuit with UCC21550 not working

Hi, 

I suspect the issue might be mentioned in page 31 of the UCC21550 datasheet. My configuration, does not include a zener diode to have negative ate drive bias. There is excessive ringing on the VGS. 

Page 31

"When parasitic inductances are introduced by non-ideal PCB layout and long package leads (for example,
TO-220 and TO-247 type packages), there could be ringing in the gate-source drive voltage of the power
transistor during high di/dt and dv/dt switching. If the ringing is over the threshold voltage, there is the risk of
unintended turn-on and even shoot-through. Applying a negative bias on the gate drive is a popular way to keep
such ringing below the threshold. Below are a few examples of implementing negative gate drive bias"

For DC link below 46V, it switches okay, current 0.015A, power dissipation 0.4W. But the circuit design and MOSFET is intended for application DC link 400V. But once the DC link increases to  48V DC, the power dissipation goes to 20W, with temperature spike and needs to be shut down immediately.  I have attached the testing waveforms and notes in the dropbox link:

https://www.dropbox.com/scl/fo/w42m39qjk8by2nzfwe8em/AKD_F3lE95tK83RB2KleE8w?rlkey=29rsk3qvfvg0nrhr9oocrld7j&dl=0

The word document explain the waveform images from 1-19, and also a video that captures the transition. 

Kind regards,

Bright

Hi Sean,

Is the zener diode required for the negative bias? to ensure that it is fully turned off?

Attached are the waveforms:

The word document explain the waveform images from 1-19, and also a video that captures the transition. 

Kind regarrds,

Bright

Waveform Notes.docx

HI,

Mosfet part number sihp24n80ae. VGS is 12VDD.

Kind regards,

Bright

ALso, how do you apply  UCC27511 right between the gate and source pin of the FET. Please a hand sketch diagram would be good. Thank you

Also,

"Finally, snubbers on the HV BUS will be shorted to the switch node when it is ringing. Electrolytic capacitors can add resistance and capacitance to the rail, and can be an effective snubber for switch node ringing since they are large and can handle the thermal load."

Please a sketch of the connection would be good. Thank you

"you can add some distance between the switch node and the low-side FET. This will add a tiny amount of inductance that can help absorb dVds/dt across the low-side, which reduces the injected Miller current."

What is the recommended distance between the High side and low side MOSFET (T0-220 package)?

Hi Bright,

The Zener diode is there to clamp the voltage charged in the capacitor Cz. The charge that is remained on Cz is the shifted voltage that creates the negative bias.

Taking a look at the waveforms, it looks like this noise is being excited by the power stage. We have seen that some MOSFETs that are designed for high voltage applications that are extremely noisy at lower voltages. Do you have any other MOSFETs that you can try with this setup to see if that has an impact on the noise?

The clean waveforms from the no load condition and the sudden noise increase from 44V -> 48V bus voltage makes me think it is related to the MOSFET.

Regards,

Hiroki

Hi Hiroki,

Thank you for the response. 

What is the recommended  value for the zener diode  and the Cz in the application?

The application is intended for DC link 330-390, so I thought that a 800V MOSFET should be okay. Moreover, I need to test the circuit from small voltage with 5V increment until I have reached the DC link voltage. The first test that used the full DC link voltage damaged the circuit, hence, the caution to do it by stages. 

Seeing that the application is 330-390V DC link, so what MOSFET VDS is recommended? 

Kind regards,

Bright

Also, I suspect that 800V MOSFET should work fine at low voltage.

Hi Bright,

Thank you for following up.

A 1uF capacitor should be sufficient with a ~5V Zener diode reverse voltage.

Having a large margin for the MOSFETs is great and should not be a problem. I am not too familiar with the MOSFET being used here, but just wanted to provide some input from past experience with high voltage MOSFETs acting up at lower voltages. I recently did testing with a 650V Si MOSFET that showed very high noise from 50-300V.

Regards,

Hiroki

Hi Hiroko,

Thank you for your response,

But if its noisy at 50-300V, How can it be prevented/reduced to avoid shoot through before reaching the desire voltage of about 330-390V?

Kind regards,

Bright

Hi Bright,

For my testing, I was able to completely mitigate the noise by swapping the MOSFET to a different one (SiC in my case). Most of the noise was mitigated and behaved consistently across all tested bus voltage levels 

From other experiments, I found that increasing the gate to source capacitance with an additional capacitor helped slow down the switching and mitigate a lot of the noise. A ferrite bead on the gate of the MOSFET also helped prevent noise from entering the gate driver which can cause logic glitches if extreme enough.

Regards,

Hiroki

Also, does the zener diode in this configuration provide negative bias during turn off? or is only clamps the gate voltage?

Hi Hiroki,

Thank you for the response,

The new MOSFET that I want to used is STF28N60DM2, 600V, 130m ohms.

1)The current gate capacitor that I have is 1000pF, is that enough? 

2)is the ferrite bead connected after the ROFF-diode and RON resistor or before? will it not effect slow down the switching performance?

Kind regards,

Bright

3) Also, what is the recommended value for the ferrite beads?

4) Just to mention, from the previous threads,  I am using 10uF capacitive divider, midpoint connected to the load. Literally, Half bridge configuration with two MOSFETS and a capacitive divider. I hope this is not introducing additional noise to the circuit?

Thank you for the detailed responses.

Does the zener diode in this configuration provide negative bias during turn off? or is only clamps the gate voltage?

Kind regards,

Bright

Also, is there any need for external diode and TVS diode across the drain source of the High and low side MOSFETS?

Thank you. I will make the changes and inform you later on the progress.