OK elt,
Darn was hoping you would run the simulations and show how it worked!.
Seriously:
Basic, inductors store energy = power * time.
P = iv = Li * di/dt current looks like the only factor. Not true.
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/indeng.html
Inductance is Henry = volt second/amp
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/induct.html#c1
Take a look at Faraday's Law:
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/farlaw.html#c1
Key is the Volt sec, need voltage across the inductor to increase the energy stored.
Going back to the spice model link:
http://chuck-wright.com/apppv/pv_spice.html
Look at the Power into a load plotted on panels output. (page or two down on the web page)
Goal is to Match load to panel to maximize power transferred.
Back to the boost converter and FET voltage. Must determine on the fly the proper loading.
FET voltage while on is NOT panels voltage. FET is a resistive on/off switch.
The voltage developed across the FET when on is V = IR based on the current flowing through the FET.
Inductor connected across a voltage source:
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/indtra.html#c1
Notice how the current keeps going up until DC conditions are met. Little energy in the inductor at the end point, even with the largest current!
Back to solar panel, all that above to get to this point:
As the on time increased the current will increase to a point.
Once the energy absorbed by the inductor exceeds panels maximum power the current will stop rising and even decrease. Think of it as the transient conditions i = V/R, and the V has come crashing down! Thus by monitoring the FETs cycle by cycle voltage, you can determine the max power point. The FETs on time should be varied about this point to continuously determine if max power point has moved. Adjust to the edge of time for max current. If kept on for couple micro sec longer with same current then back off. (shorten on time).
A FETs resistance changes with temperature, so not considered a good Rshunt. Here looking for a relative maximum, absolute value is Not important. FET shunt should work fine.
Just to be clear measuring the FET voltage while the FET is ON and just before its turned OFF.
72Khz PWM rate snap shot at end of time is faster than most Micro ADC can handle. High speed sample and hold will be necessary. Protection from voltage generated while FET is off is needed.
Assumptions. PWM is generally done at a fixed frequency. Varying the on time, off time completes the fixed cycle time.
Yes, Off time can be fixed with varying on time and many other combinations.
Most inductors have a range of power they can handle, about 10x as a rule (not a steed fast rule).
Have fun,
Scott.