Voltage-Controlled Variable Inductor for Fixed-Frequency Critical Conduction Mode Operation

This article proposes a novel voltage-controlled variable inductor (VI) for fixing the switching frequency of dc-dc converters in the critical conduction mode (CRM). It overcomes the limitations of conventional current-controlled VIs such as complex winding structures, difficulties in control, and high sensitivity to temperature. In the proposed voltage-controlled VI, the primary side inductance of a coupled inductor is regulated by an auxiliary voltage source on the secondary side. The auxiliary voltage source is implemented by an auxiliary dc-dc converter, which delivers power to the load in parallel with the main converter. The operating principle of the converter system for employing the proposed VI is presented, and a control loop is designed to fix the switching frequency, considering the small-signal model of the auxiliary converter. The validity of the proposed VI is experimentally verified using a prototype CRM boost converter with an output voltage of 400 V and a maximum power of 400 W. The switching frequency is fixed at 100 kHz and the soft switching of the main converter is maintained regardless of the input voltage and output power.