Tool/software:
I am suddenly experiencing an issue with a buck converter the historically used the UC1525AJ or the SG1525AJ PWM IC as a substitute when required due to availability issues. While the SG1525AJ IC continues to operate normally in the circuit, the UC1525AJ IC now causes the circuit to break into no load oscillation. Converter seems to overshoot into rail, shut down, and re-start only to repeat over and over. This doesn't occur with the SG1525AJ IC.
Reducing the capacitance in the compensation network slightly from the comp pin to the -in pin stops the oscillation until several minutes into operation when the circuit breaks into oscillation again. Although to IC does not become noticeably hot to the touch during operation, slightly cooling the IC brings the converter back to normal operation. As stated before, the SG1525AJ exhibits none of these issues and continues to operate normally in this buck converter.
Due to special surge and sag line requirements of the power supply design, a 115VAC input is first boosted to 200VDC in a power factor correction circuit which then feeds a buck converter to produce 120 VDC. This Buck Converter is where the UC1525AJ is used. The output of the Buck converter is used to feed a fixed duty cycle driver circuit that operates at 50KHZ to switch power through a 50KHz transformer to produce lower output voltages. The transformer outputs are then regulated for the necessary output voltage. While this is costly efficiency wise, it is required to meet certain customer specifications.
Since the capacitance on the output of the buck converter is high, loop response is slow when switching from high to low load currents on the power supply outputs.
This is only a recent development using the UC1525AJ IC it this circuit. Given that the SG1525AJ IC continues to operate normally, I am beginning to suspect a batch problem with the UC1525AJ. since cooling the IC seems to bring the power supply back to normal operation.. Nothing else is cooled in the power supply except the IC which returns the power supply back to normal operation instantly.
Are you aware of any problems with the internal workings of the error amplifier that may occur with temperature? I'm out of other Ideas myself.
