What is the PEAK-TO-PEAK source current ripple in amperes for the chopper?
If the converter is in continuous mode,

\$ V_L = L \dfrac{\Delta i}{\Delta t}\$

\$ \Delta t = \dfrac{0.4}{250kHz} = 1.6 \mu s\$

\$ \Delta i = \dfrac{V \Delta t}{L} = \dfrac{12V \cdot 1.6 \mu s}{100 \mu H} = 192mA \$

If you simulate this circuit in LTspice (or some other Spice simulator), you get very similar results. I got 196mA, but I didn't use an ideal MOSFET.