BQ24074: TVS diode for BQ24074

I previously posted this related support question (https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1269261/bq24074-safe-usb-c-input-protection-ptc-tvs-diode)

I made a prototype using the TVS1401 + PTC as shown in that schmatic. But unfortunately, when I raised VBUS to 19v, the TVS1401 conducted enough to self-heat to high temperatures without drawing enough current to trip the PTC. The TVS1401 drew around 140mA. And the PTC had a hold current of 160. After a while the PTC gets heated by conduction and eventually reduces the current. But this isn't a good enough solution.

I am not sure if most people just put a small 5v TVS diode on these VBUS lines which gets destroyed by the high voltage, but doesn't get so hot. (SMAJ5.0A for example)

I'm trying to find a simple solution which survives higher voltages without losing TVS protection.

Hi Josh, 

Thank you. I noticed the TVS2200 datasheet says "Take care to not violate the recommended VIN voltage range (0 V to 22 V) so that the device functions properly." 

I'm not sure if this means the TVS2200 will be damaged by over voltage, or if it means it just won't offer TVS protection when it's in an overvoltage condition. 

I was relying on the the TVS current in it's overvoltage state to trip the PTC, and in the case of the TVS1401, this doesn't work. I just tested it again placing my finger on the reverse of the PCB under the TVS1401 and it rapidly becomes too hot to touch and stays like that for over a minute until the adjacent PTC heats enough to begin restricting the current. At least I think the PTC is doing this. I assume the TVS2200 would have the same behaviour.

My lipo is in direct contact with the bottom of the PCB so high temperatures are not ok. I've considered placing a thermal cut off, but I'd prefer to avoid anything that generates heat in the first place.

Looking again at the BQ24074 datasheet. There's just a 1uF capacitor on the IN pin when connected to an Adaptor DC+

I cant find any surge protection recommendations in the datasheet, or appication notes.The BQ24074 evaluation board also doesn't have any TVS protection. At least nothing I can see on the schematic.

Could surge protection be built in to the BQ24074 already? Do I really just need a capacitor?

Can you tell me what happens to the BQ24074 if it is given DC voltages higher than 27v? Does it fail safely? 

In the case of the MCP73831, they show a "SMAJ5.0A" TVS on the IN pin. 

The SMAJ is large and has a high power rating so I expect it would also get hot.

So maybe one alternative is to use a small TVS which will break without causing significant heating. But obviously I need to test this thoroughly.

https://www.digikey.com/en/products/detail/micro-commercial-co/ESD12VD5-TP/1776997

Or perhaps something like this ? 

https://www.ti.com/product/TPD1S514

Kind regards,

-Jason

Hi Josh, thank you, I made a PCB with the TVS1401 placed after a Littelfuse 1206L 160mA hold current polyfuse, but at 19v it only drew 140mA. It got too hot to touch within 2 or 3 seconds and then stayed like this for over a minute before the current dropped to around 70mA. I assume this was when the PTC heated to over 125C and started to restrict the current. But I can't be certain

I looked on the TI product pages and found this.

https://www.ti.com/product/TPD1S514

It appears to be what I need.

Can you let me know what you think?

Hi Jason,

TPD1S514 is handled by another team here at TI, I will assign the thread to them so they can comment on this.

Best Regards,

Josh Prushing

Ok great, my schematic looks like this at the moment. I think this puts the TPD1S514 in "always on" mode.

Hi Jason, thanks for the schematic. Will review and reply by the end of the day today.

Hi Josh, I desoldered and tested a TVS1401 with my bench supply and I was surprised to see that at >16V it conducts with >500mA of current.

When I increased the current limit to 900mA I could feel it getting hot when I placed my finger on it. 

So I think the problems that I enountered were really due to the PTC restricting the current to 140mA, which may have also been where the heat was originating.

I've made a design with the TPD1S514 (and a standard TVS as a fall back)

Thanks for your help,

-Jason

Hi Jason,

I'm glad you were able to find a solution! Please let me know if you have any additional questions.

Best Regards,

Josh Prushing