UCC21710-Q1: UCC21710-Q1

Hi Shubham,

Thanks for reaching out. DESAT is the most common overcurrent protection circuit and is the default choice for many applications because it is simple to implement. However, there are inherent differences between IGBTs and SiC MOSFETs (their I-V curve) that make DESAT protection better suited for IGBTs than SiC MOSFETs. 

For UCC21710-Q1, please refer to section 9.2.2.6.3 (page 39) of the datasheet which talks about how the OC pin can be used for both IGBT or SiC based on shunt resistor. In the section just before that (9.2.2.6.2) the datasheet also talks about using OC pin as DESAT for both IGBT & SiC modules. You can also refer to page 33 of the app note mentioned below:

SiC Gate Driver Fundamentals e-book (ti.com)

Please let me know if you have further question or clarifications on the same. 

Best,

Pratik

Hello Pratik, I have already gone through Section 9.2.2.6.3.

I would like to know calculations and selection criteria for for RFLT and CFLT. 

how i can make sure that my MOSFET is getting turn off in right time. 

do you have any application note with calculations?

can we have a short call to discussion?

Regards

Shubham Bajpeyee

Hi Shubham,

Please refer to the typical values below:

The value selection for Rshunt highly depends on the SiC MOSFET that is being used and it's I-V curve. If you can provide those details, we can help guide you more. 

Best,

Pratik

Hello Pratik, 

We are planning to use sct018h65g3ag SiC MOSFET for our application.

Datasheet Link : 

https://www.st.com/en/power-transistors/sct018h65g3ag.html

You can suggest the values now, but We would like to understand detail calculations.

Call for detail discussion will be helpful for us.

Regards

Shubham Bajpeyee

+91 8484 860 263

we would like to do overcurrent detection at 20A. and we would like to know how MOSFET will be turn off ? 

is it internally pull down and Fault will be communicated? or only fault will be communicated and we have to cut down the PWM lines and Power supply?

Hi Shubham,

Thanks for the follow up question and the details.

Rs = (0.7V-0V)/20A = 35 mΩ. This is how Rshunt is designed based on current that needs to detected for over current protection.

The RFLT & CFLT are just R & C filter and the value recommended above should be fine. One thing to be aware of is the C_FLT value, the reason we have provided a range is, if you use 100pF cap, the response time will be faster, but noise performance wouldn't be as good as the 10nF cap but using the 10nF cap will add the RC time constant to the OC pin detection.

As shown below, that part is affected by the RC filter hence you need to choose the capacitor value based on how fast you want the response and based on how much noise you want to filter at the OC pin.

To answer your second part of the question, we have deglitch filter time "tOCFIL" - (120ns typ) after which the OUTH goes to High Z mode after OC has been detected. The time after which the OC is detected and the FLT pin in the IC goes low is given by "tOCFLT" - (530ns) which is way shorter than the short circuit withstand time of SiC MOSFETs and then we have a soft turn off mechanism which starts when the GATE signal starts to go low until the GATE voltage hits VCLAMPTH and then when OUTL goes low. 

You don't have to cut down PWM lines or power signals since once the FLT has been communicated and the nFLT pin is pulled low and the gate driver OUT is pulled low.

We believe and hope this answers your question. Please let me know if you still have further questions.

Best,

Pratik

Hello Pratik, 

I have already selected Rshunt values. I am more interested in Rflt and Cflt. 

Because this timing can damage my MOSFET.

Can you please guide us how to select right values using I_V characteristic of MOSFET?]

Regards

Shubham