Adding thermal current regulation to DCDC current regulation ISP/ISN line. - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: September 20, 2025, 07:36:57 am
content: I've been pondering different ways to make this DCDC buck-boost chipTPS552882ramp down the output current limit to a lower current if the overall PCB temp gets to what I consider 'too hot'. Probably around 80C. but could be anywhere in the 70-100C range, the exact trigger point isn't super critical.The PCB is potted and, from testing, that does seem to help spread out the heat vs being unpotted in a box in still air.80% of PCB heat comes from the input/output protection diodes (65C PCB temp in 22C ambient) which would normally be totally fine until someone tries to use it in the Australia outback in a black box at 60C ambient, etc. Edge case, but I want to add some protection.I don't particularly want to swap the diodes for fets/ideal diodes due to cost and needing to replace one diode with 2x fets in series, however I am open to any ideas how to do this cheaply.There are some ideal diode chips with two fets built in that will do 6A+ but I need them to withstand up to 36V abs max. That's what the TVS in the design clamps too.So, assuming the diodes stay, I was thinking of doing this (SCH below) to ramp down the current to a lower setting by messing with the chips current regulation sense line to add some more mV of drop and reduce the current to a lower level once a NTC turns on a NPN at 0.6V.I pondered messing with COMP using a NTC but that's a bit risky as it might mess with stability so decided against that.I also pondered just lowering the output voltage, which would actually work ok in my application, but since heat is coming from the current it seems better to ramp down the current at high temps.Anyone have any comments or suggestions?Thanks.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 21, 2025, 12:13:50 pm
content: That circuit might work (you could also have the circuit affect the voltage feedback path as well, likely with similar effect), but I don't advise doing it.Even if it does prevent excessive temperature rise, in general it's a bad idea to handle over temperature by throttling the output voltage/current. In most cases the load requires a tight range of voltage, outside of which behavior is undefined or unpredictable. Depending on what the load is, this may have nasty consequences.For example consider how your PC's motherboard handles excessive temperature of the CPU. It doesn't respond by reducing the CPU voltage (doing so would likely crash the system), it instead reduces clock frequency, or turns off cores, etc. This reduces power consumption while still providing the CPU with what it needs to operate relialbly (though with reduced performance).In general the best way to react to OTP is to completely power down the system (or put it in reset/suspend/etc). Heavy-handed, but robust. Whether there's a better option depends on the specifics of your design.

author: Psi
author_rank: Super Contributor
timestamp: September 21, 2025, 12:20:43 pm
content: The output is a battery charger, so it really doesnt care much if you drop the charge current to keep PCB temps in a safe range.But yeah, if it was powering electronics it wouldn't be a good idea.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 21, 2025, 12:42:41 pm
content: Quote from: Psi on September 21, 2025, 12:20:43 pmYou sure about that? Most battery chargers are made with "electronics".For example, if the charger is a SMPS, then it will just demand more current as supply voltage drops. Overall power (and thus temperature) won't decrease until it's unable to satisfy its feedback loop. Unless it's got some sort of MPPT function, I would expect it to continue to overload the TPS552882 until its Vout collapses. What happens then depend on the details of the charger.

author: Psi
author_rank: Super Contributor
timestamp: September 21, 2025, 10:59:27 pm
content: na, I mean the output load for the TPS552882 is a battery that is is charging.If the current regulation is reduced down to 2A the output voltage will fall to whatever is needed to flow 2A(assuming the battery can take that charge current before reaching the TPS552882's voltage regulation setting)

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 21, 2025, 11:14:45 pm
content: Ah, I misunderstood. In that case you should be fine, so long as there's no bizarre quirks to the TPS552882 which cause it to malfunction with the "artificial" current sense. Nothing comes to my mind though.

author: Psi
author_rank: Super Contributor
timestamp: September 21, 2025, 11:22:28 pm
content: Yeah, I cant see why it would care. I'm only pulling ISN towards ground by a few mV to expand the ISP/ISN difference