Abstract
In this paper, a systematic overview of PLL-based closed-loop control methods for interleaved DCM/CCM boundary boost PFC converters is presented. It is shown that the PLL-based closed-loop methods always provide stable operation, unlike the open-loop control methods, where the only method which results in stable operation is the slave synchronization to the turn-on instant of the master with current-mode control. It is also shown that the dynamic response of the PLL-based closed-loop methods with master-slave approach and democratic approach is almost identical. Experimental results obtained on a 300-W, universal input, 400-V output, interleaved DCM/CCM boundary boost PFC prototype circuit with a dedicated controller IC utilizing a democratic, PLL-based closed-loop method is also provided.
I. INTRODUCTION
In off-line power supplies that require active power factor correction (PFC), a boost converter operating at the boundary of discontinuous conduction mode (DCM) and continuous conduction mode (CCM) is a widely employed topology at low power levels (up to 200-300 W) [1]-[4]. The major benefit of the DCM/CCM boundary boost PFC converter, compared to the CCM boost PFC converter, is that the reverse-recovery losses of the boost diode are eliminated [5]. In addition, turn-on with zero-voltage switching (ZVS) of the boost switch or near ZVS (also called valley switching) can be easily achieved. Other benefits of the DCM/CCM boundary boost PFC converter compared to the constant-switching-frequency DCM boost PFC converter are a lower total-harmonic distortion (THD) of the line current, and a smaller peak inductor current resulting in lower turn-off switching losses and lower conduction losses [6]. Although the DCM/CCM boundary boost PFC converter exhibits a smaller peak inductor current than the DCM boost PFC converter, its peak inductor current is still twice its average current, which often necessitates a large differential mode (DM) electromagnetic interference (EMI) filter [9]. Another drawback is that its switching frequency, which changes with the instantaneous line voltage and the output power, varies over a wide range
Generally, the input current ripple and, consequently, the input DM-EMI filter can be significantly reduced by interleaving two or more boost PFC converters as shown in Fig. 1 [7]-[21]. In addition, the output current ripple can also be significantly reduced, resulting in a reduced equivalent-series-resistance (esr) loss of the output capacitor, and possibly a reduction in capacitor volume. Another benefit of interleaving is that the efficiency at lighter loads can be increased by employing phase shedding, i.e., by progressively turning off converters as the load is decreased.
By interleaving two or more DCM/CCM boundary boost converters, the benefits of DCM/CCM boundary boost PFC converters mentioned above can be extended to higher power levels. However, since the switching frequency is variable, the synchronization of interleaved DCM/CCM boundary boost PFC converters presents a challenging task.
Very few implementations of the interleaved DCM/CCM boundary boost PFC converters have been published in the literature [9]-[21]. All of the previously published implementations except one [20] are based on a master-slave relationship, where the master converter operates as a stand-alone converter, whereas, the slave converter is partially controlled by the master in order to achieve proper interleaving, i.e., a proper phase shift with respect to the master. It has been shown that the slave converter can be synchronized to the master converter with an open-loop method [9]-[14], i.e., by generating a time delay equal to half the switching period of the master determined from its previous switching cycle, or with a closed-loop method [15]-[17], [21], i.e., by measuring the phase difference between the converters and adjusting the phase of the slave based on the phase error. The slave converter with open-loop synchronization can be synchronized to the turn-on instant of the master converter [10]-[13] or to the turn-off instant of the master converter


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