STM32F722 temp going to 85C when pin is set to ON - Page 1
Forum section: electronics / general

author: nepalien
author_rank: Newbie
timestamp: April 30, 2020, 12:40:10 am
content: Hi everyone first time posting here. I have a flight controller with STM32F722. It has one of the pins set to power on a Video transmitter. My tests showed that the temp of the MCU goes up to 85C when this pin is enabled. I have retraced the part of the VTX switch circuit and wanted to find out what could be causing the MCU to overheat. Looking at the circuit my thought was ot having a base resistor on Q1 was drawing too much current but I don't know a whole lot about STM32's. Thank you for the help

author: nuclearcat
author_rank: Unknown
timestamp: April 30, 2020, 03:30:31 am
content: No current limiting resistor between pin and Q1?It's NPN transistor, not MOSFET, so i guess resistor is required.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 30, 2020, 06:24:47 am
content: Since Vbe for Q1 is limited to 0.7V (the base-emitter junction looks like a diode), the current drawn from the output of the STM32 is limited only by the output resistance of the driver. It's no wonder the CPU gets hot.Is this a commercial product, or a hobby project?Are you absolutely sure Q1 is an NPN transistor and not a MOSFET? The design, with no series resistor, could be correct if Q1 were a MOSFET. In that case I'd check Q1 to see if it has a short from the gate to source, which is a common failure mode.Also, for the design to work, Q2 would have to be PNP, not NPN.

author: dtodorov
author_rank: Contributor
timestamp: April 30, 2020, 11:56:35 am
content: What AndyC_772 says seems right - directly connecting a base of a BJT to a push-pull output of a MCU without current limiting resistor is not a good practice.Furthermore, the pull-down resistor R1 is a typical approach to make sure a MOSFET stays disabled when the MCU pin is not initialized in HiZ state.So could it be that indeed the Q1 is a MOSFET? In that case, if the gate oxide has been damaged it could be shorted to the source terminal and high current can be drawn from the MCU?

author: nepalien
author_rank: Newbie
timestamp: April 30, 2020, 02:31:52 pm
content: Thank you,AndyC_722 this is a commercial Flight controller for Hobby drone racing (Talon F7). This is a popular one a lot of drone racers use currently because of the features(specially built in VTX switch). A lot of us were getting the CPU temp warnings while getting ready to race so i decided to troubleshoot the issue. I contacted the company and designer and was told that upto 100C on the MCU should be no issue.The marking on the Q1 transistor was "VJ" which i found was a ON semi NPN transistorhttps://www.onsemi.com/pub/Collateral/NSS60201L-D.PDFFor Q2 I think you are right it should be a PNP. I could not find datasheet for 099Q2.@dtodorov this issue was on multiple boards. there were only few complaints because no one stayed on the ground long enough to reach 85C and airflow from flight cooled it down.(Still bugged me that this was and issue) Do you think replacing with a equivalent mosfet should solve this issue?

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 30, 2020, 03:04:33 pm
content: Ouch.Pretty bad design.As said above - from the MCU's POV, it's like shorting the GPIO to ground through a diode. You bet it's going to get hot. Lucky you didn't fry the GPIO in question.Problem if you replace Q1 with a MOSFET is that (unless I missed something), in the OFF state, Q2's base will basically be left floating, and in ON state, Q2 will just be not conducting as well. Sorry though if I missed something, I may have!

author: thinkfat
author_rank: Unknown
timestamp: April 30, 2020, 03:10:23 pm
content: I'd definitely replace Q1 with a n-channel mosfet. Look for a type with low Vgs threshold so that it reaches minimal Rds_on with 3.3V. Make sure that the footprint/pinout matches. Apart from that it can be any small signal mosfet, it will not have to drain a lot of current.Quote from: SiliconWizard on April 30, 2020, 03:04:33 pmAdd a resistor (50k or so) from the base of Q2 to +5V.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 30, 2020, 03:55:45 pm
content: Quote from: thinkfat on April 30, 2020, 03:10:23 pmYes of course. I suppose the idea the OP had to use a MOSFET instead was to be able to use the exact same PCB and not have to otherwise do some nasty mod on each and every one.

author: thinkfat
author_rank: Unknown
timestamp: April 30, 2020, 05:14:54 pm
content: Quote from: SiliconWizard on April 30, 2020, 03:55:45 pmUsing a mosfet would still be the preferred modification as it does not require any cutting of traces. Just one additional resistor that can be easily soldered on.

author: langwadt
author_rank: Super Contributor
timestamp: April 30, 2020, 06:06:44 pm
content: Quote from: SiliconWizard on April 30, 2020, 03:04:33 pmdoes it make much difference if the base of Q2 is floating on the collector of an npn or the drain of a mosfet ?

author: AndyC_772
author_rank: Super Contributor
timestamp: April 30, 2020, 06:35:11 pm
content: Quote from: nepalien on April 30, 2020, 02:31:52 pmOh, jeezEither there's a lot more to that story, or you've identified a hugely embarrassing mistake.At best it's wasting current. They might get away with it because most CPUs are designed to withstand a short to ground on an I/O because it's a common enough fault condition - at least for short durations.It's not "no issue", though, and I'd be ashamed if I ever made a mistake like that which found its way through testing and into production.Quote from: SiliconWizard on April 30, 2020, 03:04:33 pmIf Q2 is a silicon transistor (and I do think it needs to be PNP, not NPN as shown), then leaving the base floating is fine. It's a current controlled device, so a floating base means the base current equals zero, whch is exactly what's needed to turn it off.

author: langwadt
author_rank: Super Contributor
timestamp: April 30, 2020, 07:16:26 pm
content: replace Q1,https://uk.rs-online.com/web/p/digital-transistors/0287673P/

author: nepalien
author_rank: Newbie
timestamp: April 30, 2020, 08:07:49 pm
content: This is great Thanks everyone. I will give that a shot.