Abstract
The LLC series resonant converter has emerged as a solution to applications requiring power conversion with isolation, reduced volume and high efficiency, such as PV systems and EV chargers. However, the LLC resonant converter is limited in power, so it requires a multi-phase configuration in order to provide higher currents. This configuration connects the outputs of two or more LLC converters in parallel, increasing the output current but introducing imbalance and circulating currents due to the mismatch and tolerance values of components in each resonant tank. This paper proposes a simple PI control scheme to compensate the current imbalance and eliminate circulating currents generated when several LLC resonant converters are connected in parallel. Unlike reported current sharing methods, the proposed control scheme is based on multiple current control loops operating independently, using the switching frequency of each parallel-connected unit as a degree of freedom of the overall converter. The proposed control scheme has been successfully validated under simulations and experimental assessment, implementing two resonant tanks with ±5% tolerance of parameters, providing excellent steady-state and transient performance.
Highlights
High frequency isolated DC-DC converters offer high power density due to small transformers, inductors and capacitors, and have been successfully implemented in trending applications where isolation and high power density is relevant, such as photovoltaics [1]–[4], DC distribution grid [5]–[7] and EV chargers [8]–[10]
The converter achieves zero-voltage switching (ZVS) on the primary side inverter and zero-current switching (ZCS) on the secondary side rectifier when it operates around the resonant frequency of the LLC resonant tank
EXPERIMENTAL ASSESSMENT To validate the proposed multi-current control loop for multi-phase LLC converters, and to support the output and circulating current analysis presented in section II, an experimental setup with two LLC resonant converters was implemented using two resonant tanks designed with ±5% mismatch on its parameters
Summary
High frequency isolated DC-DC converters offer high power density due to small transformers, inductors and capacitors, and have been successfully implemented in trending applications where isolation and high power density is relevant, such as photovoltaics [1]–[4], DC distribution grid [5]–[7] and EV chargers [8]–[10]. Closed-loop control schemes have been proposed as an effective alternative to achieve current sharing in multi-phase LLC resonant converters These methods use modulation strategies to control each primary side inverter independently and balance the output currents. The master-slave strategy has been proposed to interleave the LLC converters and tracking the master-unit frequency, improving the steady-state error of current sharing, but the control reliability is reduced due to complete dependence on the master control and the communication bus [15], [32]–[34] Both control schemes use master or centralized controllers, which reduce the modularity and increase the complexity due to their coupled behavior, resulting in complicated tuning processes every time a parallel converter is added [8]. OPERATION OF MULTI-PHASE LLC RESONANT CONVERTER This section first presents a model of the LLC converter and analyzes the circulating current according to three modulation techniques, excluding the control
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