Are there any integrated thermal protection PMICs / FETs? - Page 1
Forum section: electronics / general

author: jusaca
author_rank: Unknown
timestamp: June 18, 2021, 07:28:04 am
content: I'm looking for a compact IC that switches off my supply rail above a certain temperature, bascially like a bimetallic switch. But I can't really find such a thing?The problem is overheating in the device at a fault condition. The standby current is in the order of 10mA, with short bursts to about 5A for something like 2s. The supply voltage varies between 5.5V and 8.5V and switches off at a current above 5.5A. So a hard short circuit is not my concern. But with a fault condition and a continuous current of something like 4A the device would completely overheat and possibly start to burn, so I definitly need to implement some kind of temperature protection.In theory I could use a temperature sensor, a comparator and a FET. But building this with descrete components will become too large, I only have a few mm^2 of PCB space.I assumed there would be ready to use temperature protection ICs to do exactly that job, but at least with the keywords I used to search I could not find anything.Thermal protection PMICresults in fan control ICs.Thermal protection FETactually results in FETs with integrated thermal shutdown, but only with the typical fixed junction protection threshold of something like 150°C and nothing I could set to a reasonable case-temperature of 75°C or so.Does anyone know how I can find the desired IC? Any keywords that might be helpful? Maybe even a specific chip suggestion?

author: jusaca
author_rank: Unknown
timestamp: June 21, 2021, 08:28:12 am
content: Ah, these keywords actually were quite helpful already! Unfortunatly it seems like there is no such IC with an easily settable shutdown temperature.Regarding your second post: Yes, I am only looking for a protection against long overloads. Currents up to 5A are normal use case, but only for something like 2s. But when a connected device draws 5A for a whole minute, my board would overheat (due to limited trace width).

author: Siwastaja
author_rank: Super Contributor
timestamp: June 22, 2021, 01:28:34 pm
content: Any device (power switch IC, upstream regulator UVLO/ENABLE pin) which has an enable pin with under-voltage lockout capability - i.e., properly specified turn-on/turn-off voltages + a hysteresis - can be used if you replace one of the resistor divider resistors with an NTC or a PTC, those thermistors are available in tiny SMD packages.So maybe you can find a voltage supervisor than a temperature supervisor more easily. In the end, a thermistor in a divider converts temperature into voltage.PTC thermistors have such steep curves they can work directly with even a poorly specified enable pin (just any logic enable withsomespecifications like TTL turn-on at 0.6V). Basically, a tiny PTC in series with the enable signal + pulldown.Best case is, you find a way to control the problematic device itself without need to add a separate power switch. Think about it.

author: jusaca
author_rank: Unknown
timestamp: June 23, 2021, 07:22:04 am
content: That is a lot of very helpful input, thanks for all the ideas. I espescially like the idea to just use an enable input in combination with a resistive divider.Just the explanation why the temperature protection is desired: The heating effect at overload does not (only) happen in my pass element, but mainly in the PCB traces (which are very limited in size due to small PCB size of only 6mm width, while the power traces have to go back and forth several times. We could use 70µ copper and more layers, but that is actually not needed if we just turn the power off in overload condition).

author: sandalcandal
author_rank: Unknown
timestamp: June 23, 2021, 07:44:57 am
content: Are these small enough?https://www.littelfuse.com/products/battery-mini-breakers/metal-hybrid-protection/mhp-tac.aspx

author: Siwastaja
author_rank: Super Contributor
timestamp: June 23, 2021, 07:50:42 am
content: Yes, secondary, slow, board-level thermal protection is often necessary, the approach is fine.Just design the semiconductor side (current sensing, limitation, part rating, etc.) so that in no instances heating is so fast the board-level sensor can't react to it. But part ratings need to be based in some ambient temperature and this means local ambient near the board, and board level sensing and protection will limit this Ta to some sensible value like 60-70 degC leaving some 30-50 degC over RthJ-A.

author: jusaca
author_rank: Unknown
timestamp: June 29, 2021, 07:50:06 am
content: Quote from: sandalcandal on June 23, 2021, 07:44:57 amThat was actually a pretty cool suggestion, I wasn't aware that there are bimetall switches as small as these! We ordered a few to test them, they are really nice.Only problem is with soldering - according to the datasheet we can't solder these, but have to weld the contacts, to avoid heating of the contact materials. We tested careful and quick soldering with a few of the units and it worked quite well... If the cutoff-temperature is shifting by a few degrees due to soldering, that would be no big deal. So I guess we will carefully try to use these with handsoldering.

author: sandalcandal
author_rank: Unknown
timestamp: June 29, 2021, 11:27:00 am
content: Quote from: jusaca on June 29, 2021, 07:50:06 amGlad to know it helped! The soldering compatibility is an unfortunate pain. "Good enough" spot welders meant for assembling lithium ion battery packs are pretty affordable. I'd recommend the K-weld which was posted and developed partially on this forum. The chinese buzz box, transformer-based welders aren't particularly nice but are cheap and can be made to work (mostly).

author: sandalcandal
author_rank: Unknown
timestamp: June 30, 2021, 04:43:41 pm
content: Just came across these:https://www.bourns.com/docs/Product-Datasheets/SC.pdfA similar size but solder-able.