ISO5852S-Q1: DESAT protection

Rupesh Gelani

Part Number: ISO5852S-Q1

Hi,

We are designing gate driver for IGBT(IKW40N120CS6) which short circuit handling time is very low (3us), for such short time DESAT protection if we calculate blanking capacitor using current source of 0.5mA, it will come around 15pF. If we are placing such small capacitor DESAT protection getting triggered frequently, I think it is due to charging current during turn-off through blocking diode parasitic capacitor.

1. Am I thinking in right direction?

2. Is there any way we can increase detection time without reducing blanking capacitor?

over 2 years ago

+1 dimitri james over 2 years ago

TI__Genius 10652 points

Rupesh,

3us withstand time is quite short for an IGBT, comparable to SiC. Thank you for sharing the P/N of the IGBT it helps a lot.

Rupesh Gelani said:
1. Am I thinking in right direction?

This actually makes sense, because with such a small blanking cap the capacitor divider effect will give a larger voltage when smaller blanking cap is used.

But such a small cap, the blanking time is miniscule as well. You might as well not even have a capacitor there almost.

15pF is very very small so capacitive effects of driver and diode impact much more. 15pF is at least an order of magnitude less than the minimum blanking cap that should be used.

Rupesh Gelani said:
2. Is there any way we can increase detection time without reducing blanking capacitor?

We actually have an FAQ on this exact topic. Link Below. I would also advise you to increase the blanking cap to 220pF minimum.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1037256/faq-ucc21750-how-can-we-increase-desat-charging-current-for-faster-short-circuit-detection-time-in-ucc217xx-and-iso5x5x

I would also point you to this FAQ below. You can significantly speed up the DESAT detection time by adding a series zener, which would probably be required for this device.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1011100/faq-ucc21750-how-can-we-adjust-the-desat-detection-threshold-in-ucc217xx-iso5x5x

I would recommend to use one page of the UCC217xx DESAT calculator page to help you decide component values. If you add a series zener, just add its VZ to the VFW field specific field for zener.

Note that other pages of the calculator should not be used for ISO5x5x as they may use different parameters. the page is the lat page in the calculator (called DESAT calculator).

https://www.ti.com/lit/zip/sluc695

Also note that this page cannot consider the extra resistor from the first FAQ link, so if you need to use that, this calculator can't be used.

Rupesh Gelani said:
short circuit handling time is very low (3us)

If you can't achieve sufficiently short detection time with the above methods, you may need to look at a part like UCC21710. This has an OC pin which can be configured as DESAT without the timing disadvantage for power switches with very low withstand time (leading edge blanking time).

If you have any further questions, please let me know. Please press the green button to let me know if ive answers your question.

Best

Dimitri

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ISO5852S-Q1: DESAT protection