In the steady state, the DC (average) voltage across the inductor must be zero so that after each cycle the inductor returns the same state, because voltage across the inductor is proportional to rate of change of current through it (explained in more detail below). Note in Figure 1 that the left hand side of "L" is at formula_4 and the right hand side of "L" sees the formula_5 voltage waveform from Figure 3. The average value of formula_5 is formula_7 where D is the duty cycle of the waveform driving the switch. From this we get the ideal transfer function: