Electronic – MC34063 with external FET. Why

mosfet-driverpower supplypush-pullswitch-mode-power-supply

Follow on from : earlier post

Below is an LTSpice simulation of an MC34063 step up (5v to 9v) circuit.
As you can see, this particular circuit is working correctly, and delivering about 300mA.

enter image description here

The N-Channel FET used is rated at 6A continuous
Raising the load to about 500mA the circuit still just manages 9V, but at 1A
I get this :

enter image description here

The inductor I'm using is rated at 15A.
What is stopping the circuit from delivering 1A at 9V?

below is the waveform
Green – output voltage
Blue – voltage source/drain

Best Answer

Apart from the problem of the PNP transistor being upside down (which I believe you have corrected), you might find that the newly corrected PNP will not switch off fast enough to deliver a decent square wave to the gate of the MOSFET. The input capacitance to the MOSFET (between gate and source) is 1.6 nF and this needs discharging probably a lot quicker than the circuit you have implemented.

If you look at the gate threshold voltage in the data sheet you will notice that it is 0.7 volts (quite low) and the MOSFET will not properly turn off until below that voltage. I suspect that trying to discharge the gate capacitor's voltage to that low level is the problem.