Equal Loss Distribution in Duty-Cycle Controlled H-Bridge LLC Resonant Converters

Abstract

The duty-cycle control is a popular option for current limiting in LLC resonant converters. Such current limiting is required to protect the converter either during the start-up or during an overload. Although this method proved to be effective, the switching and conduction losses of the converter are increased because of the modified current waveforms in the resonant tank. Hence, for the safe operation, it is vital to distribute the losses between the switches symmetrically. However, the conventional duty-cycle or phase-shift modulation methods do not provide this naturally. This letter elaborates on a method that was proposed for a different soft-switching topology more than a decade ago to solve this problem, and is adaptable for the emerging dc transformers. Furthermore, a simple implementation of the method is shown and the equal distribution of losses is validated experimentally. The VHDL code with the implementation of the modulator is attached to this paper available openly for reuse.

Application-Oriented Review of the LLC-Based Resonant Converters

“…However, various components in the converter still contribute to overall losses [219][220][221][222][223]. Numerous approaches have been proposed to optimize the LLC-based resonant converters' performance [224][225][226][227][228][229][230] and [231]. The power losses associated with different components of the LLC resonant converter are illustrated in Fig.…”

Current Limiting in Overload Conditions of an LLC-Converter-Based DC Transformer

“…The implemented modulator uses a special asymmetric modulation technique studied in [20] that equalizes the losses in the particular switches of the H-bridge converter stage. This is vital in order to increase the capabilities of the dc transformer to deliver the current with reduced duty cycles, since these conditions lead to increased losses (as will be demonstrated later).…”

“…1) The steady-state output current waveform of Bridge 1 i r,1 is determined for the given operating conditions. 2) The current i r,1 is utilized to determine the conducted currents and the switched currents according to lossequalizing modulation method, as described in [20]. 3) The turn-off energies and conduction loss are determined according to switch and diode characteristics and the mean loss power is determined and saved into the lookup table.…”

“…Observing the current and voltage waveforms of the inverting stage i r1 and v r1 , it can be seen that by reducing the duty cycle during overload, the current waveforms are modified and significantly higher current is turned off by the IGBTs. Nevertheless, after a short analysis (similar as done in [20]), it can be recognized that the magnetizing current still ensures that the anti-parallel diode is conducting while the IGBT turns on. Hence, the zero-voltage switching is not affected by the reduced duty cycle (avoiding IGBT's turn-on losses and diode's reverse-recovery losses).…”

A Novel Center-Tapped Rectifier With Current-Balancing and Voltage-Clamped Capability for LLC Resonant Converter

“…Current imbalance problem should be solved because it increases the RMS current which causes low efficiency. Also, it leads to unequal loss distribution for primary switches and rectifier diodes which deteriorates the reliability and lifetime of the converter [27]- [28].…”