MC34063 with external FET. Why?

Question

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.

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 :

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

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

Answer 1

Here's a CircuitLab model of the output stage of the MC34063, driving the push-pull pair of transistors you have implemented. The 1.6nF capacitor represents (somewhat simplistically, I admit) the input capacitance of the MOSFET. The simulation shows that the output cannot drop below 0.6V or so, which is not surprising for an emitter follower configuration:

^{simulate this circuit – Schematic created using CircuitLab}

I believe your MOSFET is not switching completely off. That particular model, the IRF7401 (datasheet), has an unusually low \$V_{GS(TH)}\$, meaning that you'll need an unusually low gate voltage to switch it off, and I don't think 0.6V is low enough.

The gate voltage is certainly slewing fast enough, though, and the high of 3.7V is more than enough to switch it on, as can be seen in figures 1 and 3 of the datasheet.

The obvious solution would be to use a MOSFET with a larger gate-source threshold voltage, but that seems more of a hack than a solution. Ideally you want the gate voltage to drop further. One thing to try is give the bottom transistor a helping hand, with a resistor R2:

^{simulate this circuit}

The downside to that idea is the wasted power in R2, due to the 50mA pulses (in the case where R2 is 100Ω).

It's all I have for a quick fix, that I can think of right now, but you should give it a whirl, and see if it helps. At the very least, you should be able to see if the problem really is due to an insufficiently low \$V_{GS}\$.

Answer 2

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.

Answer 3

Your peak MOSFET current is likely supposed to be in the 10A+ region and your drive voltage is pretty light (2.4V you say vs. the 4.5V the MOSFET would like to see). If that was the only problem you could replace the MOSFET with one that will operate from 1.8V drive, but that will make the problem Andy noted much worse.

There are three Vbe drops between the 5V supply and the MOSFET gate.

Sounds like it might be (depending on the waveform) turning off barely well enough for that particular MOSFET , but keep Andy's warning in mind and note that MOSFET Vgs(th) drops with increasing temperature. A low value resistor from gate to source should improve the minimum voltage.

You're above the optimal operating frequency for that poor old SMPS chip, so you are probably seeing some pretty ugly waveforms (meaning poor efficiency).

Also make sure the inductor rated saturation current (not carrying current) is high enough. 15A sounds about okay even if the efficiency is horrible.