Heating up of Charger IC - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: March 04, 2025, 10:01:57 am
content: I'm using this charger IC - MCP73842T (https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/21823D.pdf). The IC is driving my PMOS (https://www.vishay.com/docs/63262/sis413dn.pdf) gate about 2V with input voltage Vdd = 11.5V. The datasheet suggests a higher voltage (4.5V to 7V). With my charging current of 0.5A, the MOSFET is getting very hot.What's the main reason my MOSFET is getting hot? Is the DRV pin of the Charger IC not driving my MOSFET properly?Since the MOSFET part (https://www.digikey.in/en/products/detail/onsemi/NDS8434/244237) mentioned in the evaluation guide (https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/UserGuides/51267b.pdf) is obsolete, I need to select another MOSFET and see if it helps.I have selected this alternative MOSFET (https://www.vishay.com/docs/63239/sis407adn.pdf). Any suggestions whether it would help?If not, please let me know the parameters which I need to check for selecting a new MOSFET to ensure it does not heat up.

author: tunk
author_rank: Super Contributor
timestamp: March 04, 2025, 11:20:27 am
content: Absolutely no expert on this, but datasheet says it's a"linear charge management controller". An uneducatedguess is that the excess power is bled off in the mosfet,maybe:(11.5-3)*0.5= 4.25W

author: Freshman
author_rank: Regular Contributor
timestamp: March 04, 2025, 12:01:29 pm
content: Thank you @tunk. Will my alternate MOSFET part suit? What should I look for in the new MOSFET part?

author: xvr
author_rank: Frequent Contributor
timestamp: March 04, 2025, 02:49:59 pm
content: QuoteIt will get very hot (110°C above ambient). If you can live with a MOSFET at 135°C, you'll be fine.QuoteOn the package on which the radiator can be installed.Or on different charger IC (not linear)

author: squadchannel
author_rank: Frequent Contributor
timestamp: March 04, 2025, 03:02:17 pm
content: You need to understand linear topology.Linear topology means that any difference between the input and output voltage is dissipated as heat.It is "no surprise" to get hot. changing the FET to something else will not change that.what you need is an IC that uses a switching topology.

author: BillyO
author_rank: Super Contributor
timestamp: March 04, 2025, 03:20:29 pm
content: What kind of heat sinking are you applying to this?Can you show a picture of your PCB layout and maybe one of the finished board too?

author: Freshman
author_rank: Regular Contributor
timestamp: March 04, 2025, 04:22:26 pm
content: I have not installed any heatsink.So, you mean to say, the difference between input and output, (11.5V - 8.4V)*0.5A = 1.55W, will be dissipated in my MOSFET?And regardless of whichever MOSFET I take, this is going to be the same issue?

author: tunk
author_rank: Super Contributor
timestamp: March 04, 2025, 04:28:30 pm
content: A fully discharged li-ion is 3V, so more like this:(11.5-6)*0.5= 2.75W

author: BillyO
author_rank: Super Contributor
timestamp: March 04, 2025, 06:14:15 pm
content: Quote from: Freshman on March 04, 2025, 04:22:26 pmYes, but more like 3W for fully discharged cells. The energy must go somewhere and that somewhere is the MOSFET. The heatsink can be a large area of copper on the PCB where the dissipation pins solder to. You can put this on both sides and connect with thermal vias to get extra cooling. You can also reduce the input voltage. Putting 2 rectifiers (1N4001) in series will drop the voltage down by 1.5V which will help, but you will still need some heatsinking. Google "using copper pours as heatsinks".

author: Freshman
author_rank: Regular Contributor
timestamp: March 05, 2025, 02:47:36 pm
content: Thank you all for the suggestions. So, I understand that changing the MOSFET isn't going to work.Proper thermal relief, heatsinks and better MOSFET package with good thermal properties might work.Changing MOSFET isn't going to help.Also, any idea if we have some charger ICs (probably switching charge controller ICs instead of linear) that I can try instead of this linear charge management controller (MCP73842)