Square waves from UPS into transformer increase heating?

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.

"Joel Kolstad" wrote:

jk

On Sat, 24 Jan 2004 22:40:34 -0800, jk Gave us:

If the first thing it hits is a full wave bridge, then even DC can be applied to its input terminals! (for the non PF units, of course)

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 ???