Unknown behaviour of MOSFET circuit - Page 1
Forum section: electronics / projects

author: DolceBanana
author_rank: Newbie
timestamp: April 20, 2023, 03:26:22 pm
content: Hello fellow electronics enthusiasts/guru’s,I'm having an issue with a circuit used to power a device which uses a MIMXRT1064 controller.Circuit DescirptionThe circuit uses three MOSFETS to enable/disable the main 3V3 power rail. These are needed as the controller has strict power-up and -down sequences that need to be respected.When the controller receives SNVS_3V3, it starts it power-up sequence and PMIC_ON_REQ becomes high. This enables T6 which pulls the gate of T5 low. As this happens, C59 slowly charges up until the Vgs of T5 is reached and closes, which enables the main 3V3 power rail.C59 is chosen large to introduce a soft start of the 3V3 to prevent to much current to be drawn into the bulk capacity of the main 3V3 and cause a voltage drop at the input of the buck/boost converter.If a voltage drop would occur on the SNVS_3V3, the supervisor IC (U8) triggers MOSFET T7 to short the gate-source voltage for 140ms. This causes T5 to open instantly, which turns off the main 3V3. After the fixed delay, T5 is closed again and the 3V3 is re-enabled. The supervision of the SNVS is done to respect to power-down sequence of the controller as otherwise it could cause an unstable/unknown state (source: hardware application manual). (See image 1)IssueThe circuit works as intended when the device is powered up initially. As the screen of the device is turned on, a large current is drawn which causes a voltage drop on the SNVS (this should be avoided of course). The supervisor is triggered, T5 is opened and the 3V3 rail turns off until T5 is closed again. (See image 2)The issue occurs after the device has been operational (screen is on) and sleep mode is enabled (screen off but all other functions on). As soon as the screen is turned back on, the same voltage drop on SNVS occurs, but this time the 3V3 rails stays 100ms on 1.9V before dropping to the same level. (See image 3)Sometimes it stays on 2V for the complete duration (140ms) of the reset delay (T7 closed, T5 open). This causes the controller to enter an unstable/unknown state which bricks the device until a complete shutdown is performed. The reason for this behaviour is still unknown. (See image 4)Determining the source of the unwanted voltageI suspect the voltage is coming from T5, so I measured the Vgs which is 0V when T7 is closed, which in turn should mean T5 is completely opened.. I also checked the drawn current by measuring over R87 (replaced by 0.25Ω) but those results where mixed. Differential probe reading showed no current flowing whereas measuring with a normal scope probe showed current was flowing…Important is that there are no other power sources in the circuit: everything is powered from this converter.Things I tried• I already tried to find another source in the circuit which could be providing the 1.9V on the 3V3 rail but haven’t found one.• I’ve also tried to add a load of 100Ω on the 3V3 to see how the it would react: it kept steady at 1.9V for the full 100ms… but then dropped quicker, showing the load was doing it’s job… (See image 5)Thank you in advance for anyone willing to dive into this with me. Also, I'm new to EEVblog so please tell/correct me if the way I'm asking my question is wrong or if I forgot something!Kind regards!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2023, 03:34:17 pm
content: What is the load? Can it be self-powered from a sneak path (don't forget ESD clamp diodes on bus or comm signals, etc.)? Is it reproducible with a resistor load? How else can you bisect the load, what is the minimum working/failing example? (May not be trivial to assess if 3V3 is wired via internal plane.)Tim

author: DolceBanana
author_rank: Newbie
timestamp: April 21, 2023, 02:22:22 pm
content: Hi Tim,Which load are you referring to? The load I added was a simple resistor, but the load that causes the drop on the SNVS is the boost converter used to power the screen. It happens as bulk capacitance is charged up without current limiting.I already checked all signals connected to the SNVS trough resistors but non of them goes directly to the 3V3. I thought the power could be coming through some signal going through the controller but I have no idea on how to test that.What do you mean with bisecting the load?Simone

author: temperance
author_rank: Super Contributor
timestamp: April 21, 2023, 03:19:48 pm
content: The way you arranged the switching delays by adding C59 and C77 will be unpredictable and load dependent. What you're looking for is a miller integrator which is formed by adding extra capacitance from drain to gate. You can now control the rise time by adjusting the gate resistors. (the thing will behave like a current source while switching on)Also, the rise and fall time can be made independent of each other by adding steering diodes in series with the capacitors.I know this doesn't answer your question. But now you have a means to control the display inrush current in a proper way.I also think C10 must be a lot larger than 10µF.As for your question: 1.9V is that de IMX core voltage?Groeten uit Antwerpen

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 21, 2023, 03:50:28 pm
content: Load, everything drawn from 3V3.Bisecting, remove loads (I assume there are many on 3V3) until the problem goes away, then back. Is it due to certain loads or their interaction? (The display boost I guess is the trigger, but surely not the cause of the ~1.9V thing.)What else does SNVS connect to?Tim

author: DolceBanana
author_rank: Newbie
timestamp: April 25, 2023, 07:32:27 am
content: Hello guysI've finally found the issue. It was indeed a sneak path coming from the SNVS which powered the 3V3.The device has an accelerometer which is powered by SNVS to be able to wake up the controller when in low power mode. This chip has two interrupts, one connected to the wakeup pin of the controller and one to a GPIO pin.This last one was connected directly and the interrupts signal was exactly 100ms, which matches the duration of the issue I had.By removing the trace or disabling the interrupt in the firmware the issue was solved!The MOSFET circuit was indeed doing it's job but unfortunately I overlooked the accelerometer.Thank you very much for your advice!Kind regardsSimone

author: dietert1
author_rank: Super Contributor
timestamp: April 25, 2023, 08:22:36 am
content: The power fault is caused by C96 and C69. When you turn on the p-channel switch T5 there will be 1.65 V on both of them. The voltage may be even smaller, if the application circuit includes more buffer caps,So the switch needs to be turned on slowly. As mentioned above a simple way of doing that is a miller capacitor. Simply connect C59 to T5 drain instead of source. Then you can turn on T5 without triggering the supervisor.Regards, Dieter

author: DolceBanana
author_rank: Newbie
timestamp: April 27, 2023, 08:03:17 am
content: Thank you @temperance and @dietert1 for informing me about the miller integrator.After fixing the issue with the interrupt line I noticed bad behaviour of the 3V3 rail when opening T5.By implementing the miller integrator and placing C59 between drain and gate and calculating the values for the desired inrush I also fixed that problem.Your advice saved me a lot of time and fixed that issue right away, so thanks again!Kind regardsSimone