In high-power applications, parallel operation is required to improve both efficiency and reliability. In an LLC converter, it is difficult to achieve parallel operation due to the gain sensitivity to component tolerance. This paper proposes a new structure of a common resonant capacitor for a multiphase LLC converter, which can achieve very good current-sharing performance with no additional components or control. To evaluate the current-sharing performance, a new first harmonic approximation (FHA) model is built in this paper to analyze the LLC converter with coupled resonant tanks. Both FHA analysis and powersim (PSIM) simulation results demonstrate the load-sharing improvement of the proposed common capacitor LLC resonant converter over the conventional two-phase LLC converter. The current-sharing performance from the FHA analysis and the PSIM simulation are compared, and the error of the FHA analysis is evaluated and explained. A 600 W two-phase LLC converter prototype based on the proposed method is also built to verify the feasibility. Excellent current-sharing performance (6% resonant current sharing error at a wide load range) has been achieved.

A conventional offline single-stage light-emitting diode (LED) driver with a high power factor usually produces a significant twice-line-frequency ripple LED current, where the ripple LED current is presented as flickering to human eye. This paper introduces a ripple cancellation method to remove the twice-line-frequency voltage ripple for an offline single-stage LED driver with a power factor correction. Consequently, a dc LED current can be produced to achieve flicker-free LED driving performance. At the same time, the required storage capacitor for the proposed LED driver can be greatly reduced, and the circuit implementation to achieve ripple cancellation is simple. Thus, the overall cost of the proposed LED driver is low. The proposed LED driver also features high efficiency because of its power structure. A 35-W Flyback experimental prototype and a 10-W Buck-Boost experimental prototype have been built to validate the proposed design and demonstrate its optimal performance.

A conventional single stage ac-dc LED driver usually produces significant twice-line-frequency ripple current when a high power factor is achieved. The ripple current causes a lighting flicker of the LEDs. In this paper, an energy channeling ripple cancellation LED driver is proposed. It achieves true single stage power conversion, maintains a high power factor while significantly removing the twice-line-frequency ripple current - flicker free LED driving. An 8.5 W, 50 V/0.17 A Buck-Boost experimental prototype has been built to verify the proposed method.

Electrolytic capacitors are often needed to provide the high-density energy storage required by ac-powered LED drivers; however, they are also well known for their short lifespans. Eliminating electrolytic capacitors in ac-dc LED driver design has become a very important target to improve LED driver technology. In this paper, a cycle-by-cycle energy buffering LED driver has been proposed to achieve electrolytic capacitor-less flicker-free operation. An energy buffering unit, with high-voltage film capacitors being the energy storage device, is introduced in the design to buffer the imbalanced energy in every switching cycle. The switching current can be controlled to meet high power factor correction requirement, while maintaining dc LED output current at the same time. Compared to previous electrolytic capacitor-less designs, this technology can reduce circulating power and, therefore, power conversion loss. A 15-W experimental prototype had been built and tested to verify the proposed LED driving method.

AC-connected LED drivers experience imbalanced energy between input and output in a half line cycle. To achieve flicker-free operation the imbalanced energy needs to be buffered, often by energy-dense electrolytic capacitors. However, electrolytic capacitors are also well-known for their short lifespan. These capacitors are the limiting factor of an LED drivers' lifespan. High voltage film capacitors and a Buck converter have been used in the proposed LED driver to buffer the imbalanced energy. When Pin $\gt$ PLED the extra energy is transferred from the AC input directly to the high voltage film capacitors. When Pin $\lt$ PLED energy is transferred from the high voltage film capacitors to the output by the Buck converter. The imbalanced energy goes through two power conversion steps in the proposed LED driver, which is one time less than other comparable electrolytic capacitor-less designs, enabling higher efficiency to be achieved. A 28W Flyback topology based experimental prototype had been built and tested to verify the proposed design.

This paper proposed a new half-bridge (HB) LLC resonant converter family with pulse-width modulated (PWM) auxiliary switch (sLLC converter) for hold-up mode operation. The proposed sLLC converters could operate with synchronous rectifier, which is suitable for low-output voltage applications. In the proposed converters, an auxiliary switch is added at the primary side to provide charging path for the series resonant inductor. For nominal 400-V input, sLLC achieves same performance as conventional LLC converter, and all the good features such as soft switching are naturally retained. For slight input voltage fluctuation, frequency modulation is used to regulate the output voltage. When the input voltage reduces further, HB switches will operate at constant minimum frequency, and the auxiliary switch will operate in the PWM mode to energize the resonant inductor with the bus voltage directly during hold-up period. Thus, the converter achieves higher voltage gain, and the output voltage can be maintained at desired level. To verify the effectiveness of the proposed sLLC converter, operational principle and equivalent circuits will be carefully explained and analyzed in this paper. A 300-W prototype is built for 250-400-V input 12-V output application.