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Comparison of Critical Conduction Mode, Triangular Conduction Mode and Fixed Frequency Modulation Schemes for Single-Phase H-Bridge Boost PFC

Fig. 1. H-Bridge PFC and its equivalent circuit for vin >0.
To implement a power conversion system (PCS) or energy control center (ECC) for medium voltage medium power aerospace application with a single phase Hbridge PFC, three control methods were implemented and their efficiency was calculated based on the converter model with consid-eration of the common source inductance. The most common modulation scheme used is the critical conduction mode (CRM), in which the on-time of the main switch is kept constant, the inductor current reaches a peak value, then the inductor current is brought back to zero and then the main switch is turned on again.

The structure of the H-Bridge PFC and its equivalent circuit when input voltage is positive is shown in Fig.1. The two switches of one leg operate at the mains frequency and the switches of the other leg operate at high frequency.

The current in the inductor is brought to zero as shown in CRM in Fig 2. However, this allows ZVS turn-on of the main switch only when vindc/2. While in triangular conduction mode (TCM), the current is brought to a negative value, which makes ZVS possible for the entire mains cycle, as shown in Figs. 2 and 3.

Thus TCM reduces the turn on losses of the main switch but it results in higher turn off losses for the auxiliary switch. Both CRM and TCM have the drawback of variable switching frequency. However, TCM can be modified to obtain a fixed-frequency PWM switching by choosing a particular duty ratio.

Fig. 2. Inductor current in CRM and TCM.
Fig3. Switching waveforms for CRM and TCM when vin>Vdc/2.