Same as the FCCM device, it also can be divided into two conditions: V_bias > V_target and V_bias < V_target.

Because ZC detection exists in PSM mode device. If the bias output voltage is higher than target, high side will be off state and the lower side will turn off if inductor current is closed to zero. So, V_in will be clamped by the V_bias through the high-side body diode, as shown in Equation 1.

If bias output voltage is lower than target, the minimum off time will be triggered. Because the logic priority of minimum off time is higher than ZC and there is no load in V_in side, after the inductor current reaching the zero level, the low side MOSFET will not turn off through ZC logic. Only after high side MOSFET waiting a minimum off time, low side MOSFET will turn off and high side MOSFET will turn on, which cause a negative current same as the FCCM device, as shown in Figure 2-3. If bias output voltage is lower than target, PSM device will operate as FCCM device with the minimum off time. V_in is same as that in Equation 4.