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Startup current and waveforms in the UC1844A

Other Parts Discussed in Thread: UC1844A

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?