It was time to change the testing strategy and look at lessons learned from previous experience. When issues occur, users are often instructed to turn a device off for 10 seconds or more before powering back on because powering back on too quickly sometimes causes circuits to misbehave. Taking enough time to fully reset to a known state is necessary to ensure a predictable power-on sequence. Lightning can induce surges on the ac-power line. If these surges are large enough, they can destroy electronic devices. However, it is more common for short interruptions of ac power to occur when protective devices in substations of the power company disconnect momentarily, resulting in a corresponding interruption of ac power to customers.
I set up a test by interrupting ac power for short durations to find out whether our power supply had such a problem. Everything was fine for interruptions lasting more than a second. As I continued to test with interruptions of less than a second, I found that, at approximately 750 msec of power-line interruption, our power supply would fail; heat was destroying many components around the main MOSFET’s switch-mode transistor. It was difficult to analyze what had happened after the PCBs and their components had burned or melted. When I set the interruption time to less than 300 msec, however, the problem disappeared. It seemed that our product was susceptible to failures with ac-power-line interruptions of 300 to 750 msec.
The oscilloscope showed that a circuit associated with ensuring an orderly shutdown after loss of ac power was operating in unexpected ways. The ac-power-line interruptions of the critical time allowed the main switch-mode transistor to enter a linear operating region, which caused enough dissipation to destroy it. The ensuing rush of current caused a general meltdown of surrounding circuitry, including the PCB.
Luckily the fix was simple. We soldered a discrete silicon diode to the back of the PCB at the factory, eliminating any future problems.
Jim Sylivant is a professional engineer in Apex, NC.