jk said:
Actually ANY standard cheap simple switchmode power supply will be
happiest when seeing a square wave input. A PF corrected one won't be,
but that is a different problem.
Actually, I ran into some "cheap" 100 watt switchmode power supplies
which interpreted modified squarewave 115VAC as 230VAC. There are
some inexpensive 115/230V voltage sense & "auto-select" the voltage
doubler (automatic version of the 115/230V manual slide switch we used
to see in 80's PC's) which uses the dV/dT (voltage slope). 115VAC
modified squarewave has as high (or higher) dV/dT as 230VAC sinewave -
therefore it did not engage the voltage doubler. Result was SMPS did
not work at all (since the rectifier output was only about 170VDC
whereas it should have been 340VDC if the voltage doubler would have
been turned ON as it should have). Problem with 170VDC rectified DC
in a "cheap" switchmode power supply is that some "real cheap" designs
may not even have Low Voltage Shutdown protection (to save money, of
course) and then to produce same watts output at half voltage, current
has to double. With double current through its switching power
transistors, diodes & transformers overheating can damage (smoke) them
in a few minutes. Let's caculated the heat generated by transistor at
double currrent. Since Rds(ON) remains same, P = R times I squared -
this means 4 times heat generation !!! Looking at the wimpy heat
sinks which barely keep temperature within SOA even when current is
lower than rated maximum, do you think they have a chance when
transistor is overheating by factor of 4 ???