Im working on an "off the line" switcher from 277 VAC to 24 volts DC using the UC1844A in a double ended forward converter topology. I've followed the application note U-100A as closely as possible but see some behavior that differs from published values.
1) Start Up Current. If the start-up current is limited to 1 mA by using series resistors the 5 volt reference (pin 8) is never established and the chip hiccups, pulling the supply voltage down below the UVLO limit and then oscillates indefintely. I find that the 5 volt reference (pin 8) is only established if the chip can draw at least 40 ma from the supply. Once the 5 volt ref is established the oscillator runs normally and the current requirement is reduced to around 20 ma, still a far cry from the 0.5 mA in the data sheet. The chip has this behavior with or without a tranformer connected (removing the transformer is a handy way to check the maximum duty cycle and frequency).
2) Loss of Control. A second observation is that occationally the output (pin 6) enters a mode where the duty cycle pattern changes and rather than limiting at 45% as designed it starts firing at about a 90% duty cycle. Once you enter this mode power has to be removed from the part in order to restore it.
Operating Conditions for the part are:
Freq = 100 KHZ
Error Op Amp Gain = 10
Any Ideas?
