An industry standard PWM chip in this particular switchmode supply was driving a type 4013 D-type flip-flop using that PWM chip's clock (CLK) output. The 4013's response to this drive was sporadic. The 4013 would toggle properly on each rising edge of CLK, but would often toggle improperly on the falling edges of CLK which would mess things up quite badly.
The reason turned out to be that the falling edge of CLK was very much slower that the rising edge and with just a little bit of noise on that falling edge, would upwardly cross the threshold voltage of the 4013 clock input "C" and cause the unintended toggling. Note that the 4013 has no input Schmitt trigger which might have prevented this trouble.
Continue reading "PWM Chip Slow CLK Fall Time - John Dunn, Consultant, Ambertec, P.E., P.C." »