Study And Design Of Single-Phase Offline High-Brightness Led Driver For Lighting Application

With increasingly serious global energy crisis and the climate issue, energy conservation has become a common topic. Worldwide, about20%of the electric power is consumed by residential, commercial or industrial lighting. Due to the potential of high efficacy, long lifetime, environmental friendliness, and compact size over the conventional lighting devices, high-brightness light-emitting-diodes (HB-LEDs) have emerged as a promising lighting technology to replace the energy inefficiency incandescent lamps and mercury-based fluorescent lamps.The luminous flux of LEDs is in proportion to its forward average current. In order to ensure the constant luminous flux, LEDs have to be derived as constant current source. Presently, the power ratings of individual LED devices are a few watts, limited by the packaging technology and heat dissipation. To obtain sufficient luminance for lighting application, many LEDs have to be connected and arranged in parallel LED strings. The general photo-electro-thermal (PET) theory also indicates that a distributed LED system based on a plurality of relatively low-power LEDs has advantages over a concentrated system consisting of a small number of high-power LEDs for the same power level. Hence, paralleling LED strings has been a common practice. On the other aspect, LEDs typically require power electronic drivers to regulate the LED current and provide a high efficiency interface between LEDs and AC grid. Generally speaking, high power factor and low-input-current harmonics are becoming the mandatory criteria for this power electronics driver. In lighting equipment, the input current of ballasted lamps exceeding25-W are required to comply with stricter requirements as stated in IEC61000-3-2-Class C. Energy Star also requires the input power factor higher than0.9for commercial luminaries. Furthermore, to match the features of LED lighting source, low cost, high efficiency, and long lifespan will become the significant design requirements of LED drivers. In addition, PWM dimming control is often needed to regulate lighting levels for human needs as well as to achieve energy saving.Much research has been directed toward improving the power factor and controlling the LED current. Thus, how to implement LED divers with the feathers such as high efficiency, high power factor, long lifetime, PWM dimming and low cost is considered to be the key challenge lying ahead. This thesis is dedicated to the study and design of effective LED lighting drivers to overcome the inherent deficiencies and provide solutions for modern lighting applications.First of all, the traditional solution for driving multiple LED strings is the cascade configuration. This structure is a very good candidate with unity power factor (PF) and fast dynamic response. However, the complicated circuit, low efficiency and high cost are the main drawbacks. To increase the efficiency of LED lighting drivers, a non-cascade Twin-Bus structure is proposed in the third chapter. It is composed of a front-end isolated AC/DC converter with Twin-Bus output stage and post-stage current regulators with Twin-Bus input stage. Post-stage current regulators only handle the parts of the entire energy, thus, the total system efficiency can be increased significantly while maintaining the same cost. Meanwhile, a novel Twin-Bus buck converter is presented and employed as the post-stage current regulator with PWM dimming function. The Twin-Bus configuration and dimming control strategies is introduced in detail. To verify the validity of the studied Twin-Bus configuration and Twin-Bus Buck type current regulator, a100-W laboratory prototype is built and tested. The experimental results shows:(1) the independent string current can be regulated from zero to the desired350-mA for PWM dimming control;(2) for the individual stage efficiency, the current regulator employing the twin-bus buck converter has a peak efficiency of98.5%and maintains above98%for the output power range from10-W power to100-W under1MHz operation frequency.A review of literature shows many existing LED lighting drivers and solutions have to employ the electrolytic capacitor with large capacitance to obtain the smaller output ripple and to balance the difference between instantaneous input power and output power. Unfortunately, the lifetime of the high-quality electrolytic capacitor is typically10,000hours at105℃, which is much shorter than the lifetime of LEDs that is generally higher than50,000hours. Moreover, it is temperature-dependent due to the use liquid electrolyte and is reduced by half for every10℃rise in operating temperature. Thus electrolytic capacitors are the obstacle for prolonging the lifetime of the LED lighting driver. In order to prolong the overall lifetime of LED lighting products, a SEPIC-derived AC/DC converter topology is proposed as the front-end AC/DC stage. By allowing a relatively large voltage ripple in the AC/DC converters and operating in special discontinuous conduction mode (DCM), the proposed circuit is able to eliminate the electrolytic capacitor while maintaining high power factor and high efficiency. Furthermore, another novel AC/DC converter is also presented by inserting the valley fill circuit in the above SEPIC-derived circuit. Unlike the previous usage of the valley-fill circuit for improving the power factor and reducing the output voltage ripple, the major function of the valley fill circuit is to reduce the size of decoupling capacitors and decrease the voltage stress of output diodes. Under the electrolytic capacitor-less condition, considering the energy amount (CV2) as the capacitor sizing criterion, the proposed circuit requires only one quarter of the capacitor energy as compared to the origin circuit. Meanwhile, the voltage stress of storage capacitor and output diode under the same power factor condition can be reduced by half. The presented two SEPIC-derived AC/DC circuits are suitable for the traditional cascade and Twin-Bus structure, simultaneously. Two50-W laboratory prototypes are built and tested. The experimental results shows that the features of the proposed circuits.It is well known that the diode bridge hampers the further improvement of AC/DC converter efficiency. In an effort to maximize the AC/DC converter efficiency, considerable research efforts have been directed toward the development of bridgeless AC/DC circuit topologies. A bridgeless AC/DC converter allows the current to flow through a minimum number of switching devices compared to the conventional AC/DC circuit. Accordingly, the converter conduction losses can significantly be reduced, and high efficiency can be obtained, as well as cost savings. The sixth chapter reviewed and compared several bridgeless AC/DC converter topologies; based on which, the two bridgeless SEPIC-derived AC/DC converters are derived by inserting totem-pole bridgeless configuration in the SEPIC-derived and valley-filled SEPIC-derived converters to further improve the efficiency. Experimental results of two50-W laboratory prototypes are given to verify the validity of the studied two bridgeless AC/DC converters.Boost PFC plus LLC resonant converter in series post-stage current regulators has become the preferred solution for LED lighting application with power level above75-W. Although such a driver can help LEDs in achieving good operating performance, too many components, large size, low efficiency and relatively high cost are its main drawbacks. A single-stage bridgeless AC/DC converter with soft-switching feather is put forward and investigated. The totem-pole bridgeless boost PFC and half-bridge LLC resonant converter are integrated in together by sharing the switches. The bridgeless boost PFC cell works in discontinuous current mode (DCM) for achieving high power factor and low input current harmonics. Thus the resulted topology only needs one controller to regulate the output voltage, and that high efficiency can be obtained easily. The conventional resonant control chip, such as L6599and UCC25600can be employed to implement the controller. This circuit can be coupled with the Twin-Bus buck type current regulator for driving multiple LED strings, but also can drive the single LED string independently. The seventh chapter introduced the operation principle and design consideration in detail. The experimental efficiency is high as94%for100-W laboratory prototype, which shows that the features of the proposed circuits. It is noted that this circuit is suitable for the street lighting application especially.Finally, the contribution is summarized. Accordingly, the limits and future work are also pointed out.