I'm currently building a power supply using a MC34063 IC. I have used an external n-channel MOSFET for switching the high current used to drive an LED pair.
The switch operates on a fixed duty cycle of about 6:1, frequency 40kHz. I have modified the circuit to run current mode, using a ZXCT1109 current sense amplifier and sense resistor. Gain is controlled by R1: whenever voltage at the INPUT node exceeds 1.25V, the internal comparator outputs low and the emitter output (i.e. gate voltage) stays low until the sense voltage drops below this threshold. Current is maintained via C3 decoupling capacitors.
At low currents (i.e. when current gain is high), the circuit operates fine: the switch correctly shuts down whenever current is above threshold. Decreasing gain while the circuit is operating leads to an increasing in gain, and the output stays stable. However, whenever I turn on the circuit again at this higher current, current feedback no longer works correctly, and the switch always turns on during its on cycle. Naturally this leads to an overcurrent condition and if I left the circuit on for more than a few seconds the MOSFET would fry. The LEDs continue to light up in the overcurrent, but at reduced intensity. Oscilloscope readings show the comparator input node voltage just reaching 1.25V at its peak before rapidly falling off. Non-fault readings have this voltage exceeding ~2V before slowly falling off as expected.
I strongly suspect this is due to current latchup as described here. The input steady state current threshold above which this occurs is around 1A. My benchtop power supply has a maximum current limit of 2A. This overcurrent problem does not occur whenever I use Li-ion batteries to power it.
I can understand why this is occurring on a very rough basis: the current limiting is preventing effective IC startup. However, I don't understand the effect any further than this. Why does the voltage at the comparator input node never go above threshold even though the current is definitely high enough to trigger it? Why would the current limit affect the circuit this much and prevent the trigger threshold voltage ever being reached?
The article suggests using a soft start measure to fix this. I know this works (since I did it manually via changing current after the circuit was already online), but the MC34063 lacks an internal soft start function. Is there any way to implement this in a relatively simple fashion?
I have not included oscilloscope readings but I can provide them if requested.