As a way to power-on/-off a circuit load, a MCU is used to drive a relay through two MOSFETs. The problem is, when the MCU GPIO output is measured 0-volt, the relay could stay connected.
Can anybody demonstrate how this could have possibly happened?
More details that may or may not be relevant to the problem:
This happens very rarely. We have not got enough chance to troubleshoot the issue.
Usually the process to operate the circuit is: MCU set GPIO 0; human switch the input off; the load circuit is changed; human switch the the input on; MCU set GPIO 1. When the input switch is turned off, the voltage will drain slowly as the schematic can show there are big decoupling capacitors.
We suspected that the relay is bad. Changing to another piece of relay only marginally changed the chance of running into the problem. Multiple boards are showing the same problem, thus it seems to rule out a damaged relay or MOSFET or any other single component.