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CSD88599Q5DC: I want to eliminate the MOSFET ringing in the motor driver.

Part Number: CSD88599Q5DC
Other Parts Discussed in Thread: DRV8323,

Tool/software:

Hello,
I am developing a 3-phase BLDC motor driver using the DRV8323 and CSD88599Q5DC. However, I have noticed that the voltage regulator is experiencing fluctuations due to the ringing effect of the MOSFETs. The oscilloscope capture below shows the situation when operating the DRV8323 in mode 3, repeatedly switching between the (111) vector and the (000) vector, i.e., when phases a, b, and c are simultaneously set to high and then low. CH1 is phase a, and CH2 is the output voltage of the regulator. According to the CSD88599Q5DC datasheet, it suggests that adding a gate resistor to the high side and not to the low side reduces ringing. But doesn't that mean the ringing won't be reduced when the low side is on? I am a bit confused... Shouldn't I add a gate resistor to the low side as well?

(The PCB layout was made to be as similar as possible to the BOOSTXL)

  • JH,

    Thank you for looking at our device, our MOSFET apps person is away until Wednesday USA time and will provide any further answers to your specific questions in addition to the general information I will try to answer here, if you have further questions.

    Normally for any ring reduction technique there are few methods employed, common ones are:

    1. Gate resistors to slow the turn on and turn off of the MOSFET - the datasheet recommends the high side only as this faster dv/dt on the high side induces ringing. It is this fast switching high side FET that excites the body diode on the low side causing ringing. So adding a resistor to the low side will not help and may cause an issue with unwanted cdv/dt turn on. There is an explanation in this app note here in the first couple of pages: https://www.ti.com/lit/an/slpa010/slpa010.pdf
    2. External snubber applied at the switch node - details of suggested snubber in datasheet pages 15-17.
    3. PCB layout - details in datasheet and also in datasheet https://www.ti.com/lit/ds/symlink/csd88599q5dc.pdf

    Finally, it is important to make sure you are measuring the switch node correctly, the probe you use can have a big impact on what you see on the scope. It is best to use a differential probe or if a single ended probe used the you need to make sure it has as short a group loop as possible, again information on page 4 of this same application note here: https://www.ti.com/lit/an/slpa010/slpa010.pdf

    There is also another technical article on the type of device you are looking at here: https://www.ti.com/lit/ta/sszt955/sszt955.pdf , this also has some other tips on using this device.

    This application note and other MOSFET technical data , calculator tools and tips are all contained int his application note here: https://www.ti.com/lit/an/slvafg3f/slvafg3f.pdf where I hope you can find many other tips when using MOSFETs.

    Thanks again for considering our device

    Chris

  • "Thank you! Your response perfectly resolved my question. Additionally, I am glad that I was able to find a solution to the ringing problem by reading the reference paper you sent. Thank you very much."