A high efficiency critical mode boost PFC using a variable inductor

In low-to-mid power applications, a critical conduction mode (CRM) boost power factor corrector (PFC) is widely used due to its simple control and a reduced switching loss. One of the important characteristics of CRM boost PFC is that the switching frequency is not constant and the range of variation is wide. Due to this variability of the switching frequency, there is a design guideline, which must be satisfied. That is, the minimum switching frequency must be higher than the audible frequency. In that reason, a small boost inductance is required to increase the switching frequency. However, this increases the switching loss resulting in a reduced efficiency. In this paper, a new method of improving the efficiency using a variable inductor is proposed. A variable inductor can be implemented by a simple auxiliary circuit. At nominal, where a high efficiency is required, the proposed method operates with a large inductance, which results in the improved efficiency with a decreased switching frequency. On the other hand, when the switching frequency decreases to the audible frequency, the inductance is changed to a smaller value, keeping the frequency higher than the audible frequency over all the range of input line voltage.