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UCC21750-Q1: Clarification on SC protection of SiC-MOSFET

Ravi Patel
Prodigy 130 points
Part Number: UCC21750-Q1


Hi,

This is in reference to gate driver IC: UCC21750-Q1. I want to clarify for below mentioned points.

The above mentioned gate driver is finalized to drive a SiC MOSFET with rating >1kV and current around 220 A in our application.

  1. In case of short circuit protection, there are various methods available such as D-SAT, shunt resistor and sense-fet. However, as per my knowledge, to implement sense-fet protection separate I-sense pin apart from kelvin-sense pin is required, right? And if that pin configuration is not available than can we implement current sense-fet by external means?
  2. I would like to know which is the most appropriate option for short circuit protection with above mentioned specifications of the SiC MOSFET? As, D-SAT is more preferable in IGBT. Besides the RMS current for the current application is around 79 Arms (just for your information).
  3. I have gone through relevant threads related to UCC21750-Q1 on E2E forum which are relating to D-SAT circuit calculation and how to add extra charge pump in the blanking capacitor as internal is only related to 500uA. Will share some more thoughts in subsequent conversation.
  4. There are different available options with the family of UCC217xx such as OC pin and DESAT pin for SC protection. Can you provide some information on which is the best option that can be used for SC protection of SiC-MOSFET? and how?
  5. If we have used bipolar supply for gate drive such as +18/-4V, still implementing miller clamp circuit can be advantageous? If not, than there is AMC in UCC21750-Q1, so should we connect it to the gate pin of should it be grounded?

Thanks, looking forward for answers.

  • +1
    TI__Expert 6730 points

    Hi Ravi, 

    Please see answers to your questions below - 

    1. Yes, in order to implement the OC pin with a SenseFET, the transistor should have a SenseFET connection. What the SenseFET does is that it scales down the transistor current, which makes it easier to measure with less power dissipation. You can also implement a current mirror instead of using SenseFET, as shown in the response here - https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1070001/ucc21736-q1-oc-protection---current-scale-down?tisearch=e2e-quicksearch&keymatch=sensefet

    2. The DESAT circuit can be configured to be used with SiC MOSFETs with a lower detection threshold, and we see many customers using DESAT with SiC MOSFETs. If you want a faster protection. you might want to look into parts with an OC pin instead of a DESAT pin. The DESAT pin comes with an internal leading edge blanking time of 200ns, which will increase the turn-off time, and the OC pin does not. Shunt resistor and SenseFET behave similarly, however as mentioned above, SenseFET current is smaller, easier to measure, and has lower power dissipation. 

    4. As mentioned above, if a faster protection is needed, OC pin might be a better choice. You can also implement the OC pin as a DESAT, shown here - https://www.ti.com/lit/ds/symlink/ucc21710-q1.pdf?ts=1656077086032&ref_url=https%253A%252F%252Fwww.google.com%252F#page=41

    5. We still highly recommend implementing the Miller Clamp, as it prevents the false turn-on caused by large dV/dt. Since you're using a SiC MOSFET, the dV/dt is likely to be larger, thus making the Miller Clamp more necessary. 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    Prodigy 130 points

    Hi Vivian,

    Thanks for detailed and satisfactory answers. However, I would like to ask you one question, this is in regard to the using of OC pin as DESAT where the calculations of peripheral components is provided in the datasheet. However, how to exactly size the resistors and is there any reference calculation pertaining to that circuitry would be helpful.

    Thanks

  • +1 in reply to Ravi Patel
    TI__Expert 6730 points

    Hi Ravi, 

    You can use the calculator here - https://www.ti.com/lit/zip/sluc695 - as a starting point. Under the "DESAT using OC Calculator" tab you can change the resistor values, blanking capacitor values, etc. We recommend keeping R1 in the kiloohms range. 

    Thanks, 

    Vivian

  • 0
    Prodigy 130 points

    Hi,

    I just wanted to confirm regarding the implementation of miller clamp in the circuit. As you have clearly mentioned that it is highly recommended to use miller clamp even though bipolar supply is used. But, does it have some significance in case if SiC ? Like will miller clamp be that much advantageous even with -4...-8V supply ? And is it the same case for IGBT?

  • 0 in reply to Ravi Patel
    TI__Expert 6730 points

    Hi Ravi, 

    As mentioned above, we recommend implementing the miller clamp even when you're using a bipolar supply; SiC MOSFETs have larger dv/dt which makes false turn-on more likely. However, the dv/dt induced turn-on is different for different systems; you can test your system with and without the Miller clamp and decide if it is necessary for you. 

    If the Miller clamp is not used, the CLMPI pin can be tied to VEE or left floating. 

    Thanks, 

    Vivian