For those that are interested, here is the schematic, pretty much "as built" by myself.
Q1/Q2 are BC327/337, although BC547/557 should work just as well.
Q3, N-ch mosfet. I used an IRL3705.
Q4, pre-regulator, optional. I used a TIP31.
D1, I used an MBR2535.
D2, optional (stops leakage back through the panel at night). I used an LTC3457 ideal diode.
R17 is apparently redundant, but is required for the add-on mppt controller, assuming I ever get it working to my satisfaction.
P1 should give enough adjustment range for 12 or 24 volt panels.
L1 is 50uH, as I described earlier. Adjust value to suit your exact application.
U3 can be 5 volt or 12 volt, depending on the gate voltage requirements of the main fet, Q3.
5 volts is also required by the current sensor I'm using on the output, for the mppt. So I kill 2 birds with one stone here.
E3 supplies the gate current at switch-on of the main fet. This is essential.
Please study the datasheet for the MC34063. Note that its output transistor is used to switch the fet off. So it operates in reverse to what you would normally expect.
All it does is maintain the input voltage constant.
As usual, free for personal use. Commercial rights reserved.
Looking at the circuit like this, it's almost too simple.
I hope the image comes out alright. I didn't want to scale it down any further, lest it become illegible.
Amanda.
EDIT: as Steve pointed out earlier, the input and output filter caps probably aren't big enough.