This paper proposes a four-degrees-of-freedom (4-DOF) modulation scheme to mitigate conduction and switching loss in dual-active-bridge (DAB) series-resonant converter. Under wide-range variations in voltage gain and output current, increased reactive power and root-mean-square (rms) tank current can contribute significantly to conduction loss in DAB converter while the occurrence of hard switching leads to switching loss and increased device stress. The proposed modulation scheme utilizes internal, external phase shifts, and switching frequency as modulation parameters to achieve zero reactive power, minimum-tank-current and complete soft-switching operation. Analysis of the proposed modulation scheme is given for both buck and boost-mode operation. The proposed modulation scheme is validated by means of a 1-kW experimental prototype of DAB series-resonant converter operating at 100 kHz designed to interface a super-capacitor with a rated output voltage of 48 V to a 250-V dc bus. The effectiveness of the proposed topology for charging/discharging a super-capacitor under wide-range variations in voltage gain and output current is verified by simulations and experimental results. A maximum efficiency of 97.6 % is recorded from experimental prototype.

LEDs must be externally driven by power sources to emit light. One problem associated with driving LEDs is its inherent nonlinear relation between the emission intensity and the forward current. Thus, the light output obtained from an LED is strongly dependent on the actual current waveforms employed to drive it. It is found that driving an LED with dc produces light output that surpasses all other techniques including the commonly used pulse-width modulation (PWM) technique. On the other hand, for dimming function, it is found that the PWM technique offers greater dimming flexibility in comparison to dc technique. In this paper, a generalized methodology for driving LEDs inheriting the features of both of these techniques is proposed. It employs a pulsating current switching between two discrete current levels, where the current levels and their durations can be concurrently varied for a more precise mapping of the driving conditions to the light output. The existing dc and PWM techniques can be viewed alternatively as being special cases of this more general approach.

The significant improvements recently achieved in LED technology in terms of lifetime, luminous efficacy, power rating, and color property render LED one of the most promising candidates to replace conventional light sources in various residential and industrial applications. The rapid advancement in the device characteristics has simultaneously stimulated interests in developing efficient LED drivers with optimized control circuitries. The two conventional techniques currently employed in most LED drivers, namely the amplitude-mode and pulsewidth modulation (PWM) mode driving techniques, suffer from the disadvantage that high luminous efficacy in the amplitude mode has to be traded for control flexibility in the PWM mode and vice versa . In this paper, a method is proposed to improve the luminous efficacy of conventional PWM-mode driving technique while retaining their control flexibility by introducing a dc-offset component into the PWM current. Two LEDs were used in the experimental verifications. Improvements of 17.6% and 18.1% on average were measured by maintaining a dc offset of 100 and 200 mA, respectively, in the LED current. Further improvement can be achieved by increasing the dc-offset current. The main tradeoff is the reduction of the dynamic range over which the average LED current can be controlled. For a given set of performance criteria, the proposed method offers designers of LED drivers the flexibility of balancing between luminous efficacy and dynamic range for control.

Hard-switching-induced switching loss can contribute significantly to the power loss of an isolated bidirectional dual-active-bridge (DAB) dc-dc converter operating at high frequency. An LC-type series resonant DAB converter based on a switch-controlled-inductor (SCI) is proposed to mitigate the loss arising from hard switching under wide-range variations in output voltage and current. Zero-voltage switching is achieved at the primary side (high voltage), while at the secondary side (low voltage), zero-current switching is preferred to reduce excessive ringing due to circulating current and switching loss. In order to achieve reduced conduction loss, a nominal operating point is chosen where the root-mean-square resonant tank current is the minimum. To validate the proposed topology and modulation scheme, an LC-type series resonant DAB converter based on SCI operating at 100 kHz is designed to interface a 400-V dc bus to a supercapacitor-based energy storage. Simulation and experimental results validate the effectiveness of the proposed topology for charging/discharging a supercapacitor with an output voltage variation of between 10 and 48 V and a maximum rated power of 480 W. A maximum efficiency of 94.6% is achieved using the proposed topology and modulation scheme.

Both conduction loss and switching loss can contribute significantly to the overall power loss of an isolated bidirectional dual-active-bridge series-resonant dc-dc converter (DABSRC) operating at high frequency. To achieve soft switching and minimum-tank-current operation under wide-range variations in output voltage and current, a switched-impedance-based DABSRC is proposed. Minimum-tank-current operation aims to reduce conduction loss arising from circulating current at the low voltage, high-current side of DABSRC. Full-range soft switching is achieved in all switches, thus, switching loss is significantly reduced. With this new topology, power control is achieved by controlling a switch-controlled capacitor in the series resonant tank while ensuring minimum-tank-current operation and soft switching in all switches. The proposed topology and modulation scheme are validated by means of a 1-kW experimental prototype of DABSRC operating at 100 kHz designed to interface a 250-V dc bus to a supercapacitor with a rated output voltage of 48 V. The effectiveness of the proposed topology for charging/discharging a supercapacitor at a maximum rated power of 1 kW is verified by simulations and experimental results with a maximum efficiency of 97.5%.