Design And Implementation Of Offline White LED Driver IC With PSR

White light-emitting diode (LED) with high brightness has attracted wide attention dueto its high efficiency, ease to drive, environmental friendliness and long lifetime. As the mostimportant green illumination source, it has been replacing the incandescent lamp andmercury-based fluorescent lamp with lower energy efficiency gradually for generalillumination. As an important part of LED lighting system, the LED driver regulating the LEDcurrent between LED and AC grid should have the features of high efficiency, highperformance, high reliability and long lifetime to match the characteristics of LED lightingsources. The white LED driver for common illumination markets also requires highintegration degree and low cost.The major research aim of this thesis is design and implementation of control integratedcircuit (IC) with primary side regulation (PSR) for the offline white LED driver, providingLED driver with high efficiency, high performance, high reliability and low cost forresidential lighting applications. The topology, control strategy, circuit design, design fortestability, process realization and reliability of driving IC are studied. Two PSR ICs havebeen designed, implemented and applied successfully in the low power (3~5W) and mediumpower (10~22W) offline white LED drivers. The main research content is described asfollows.1. Design and implementation of high power factor PSR single-stage flyback driving ICThe power factor correction (PFC) circuit with an analog multiplier has thedisadvantages of complex circuit and high cost. A single-stage critical conduction mode(CRM) flyback PFC driving IC is proposed, which removes the complex analog multiplierand increases the cost performance. A simple external resistor divider is adopted to produce acompensation voltage and improve the line regulation performance. As the input line currentis remarkably distorted around the zero-crossing of the line voltage, especially in theconditions of high input voltage and light load, a method for reducing line currentzero-crossing distortion by duty cycle limitation (DCL) is proposed. Furthermore, by adoptingthe proposed valley turn-on circuit with adaptive delay time (ADT), the power MOSFETturns on nearly at the valley voltage even if the primary winding inductance varies with theoutput power. The circuit has been designed and fabricated in a0.35μm BCD(Bipolar-CMOS-DMOS) process. Measured results show that the driver works under theuniversal input voltage (85~265Vrms) with an LED current variation within±0.5%, and theload regulation is within±1%. The average efficiency is around88.5%when the output poweris19W. The line regulation performance depends on the external resistor divider. When theinput voltage is265Vrmsand the output power is10W, the driver using DCL can achieve an increase of8%for power factor. All these indicate the effectiveness of the proposedcontrolling methods.2. Design and implementation of the simple PSR LED driver without auxiliary windingThe efficiency, lifetime and robustness are important to LED drivers. Besides, the lowcost, compact volume, and simplicity are key factors in low power applications. Therefore, asimple cost-effective PSR LED driver without auxiliary winding is proposed and developed.The output current zero cross detecting (ZCD) signal is sensed by the negative change ofpower VDMOS’s source voltage due to the resonance of the primary winding with theparasitic capacitor Cdssof VDMOS. By controlling the charging and discharging current of aninternal timing capacitor, the conduction time Ton_sof the secondary-side current is monitoredand the ratio of Ton_sto the switching period Tsis kept constant, resulting in a constant outputcurrent. This control circuit is simple and easy to realize. Based on a0.35μm5-30V CMOSprocess, the IC is designed and fabricated. The LED-load power ranges from about3to5W,its current variation is within±3%, and the efficiency exceeds80%under universal voltage.Although the system’s auxiliary winding and its relevant rectifier diode, divider resistors areeliminated, the current variation and the efficiency are kept at the same level, as comparedwith other low power LED drivers with auxiliary winding.3. Reliability investigation of the driving transistor in LED driving ICThe device characteristics and the process realization of the driving IC are studied bycombining designs of the system architecture and the circuit. The electrical safe operatingarea (E-SOA) performance of the driving transistors in driving IC is particularly investigatedin details. Based on a5-30V CMOS process, the energy capability of the isolated EDNMOS(Extended Drain NMOS) is improved by about30percent by adopting the different dopingsource of N-type buried layer and optimizing P-type epitaxial conditions. Based on a0.35μmBCD process, a planar active-gap LDMOS with sectional channel (S-LDMOS) is developed.Compared with the conventional LDMOS with single channel (C-LDMOS), the depletionlayer depth is enlarged, and then the maximum electric field in the N-drift under the poly-gateedge decreases by modulation, resulting in increased off-state breakdown voltage. The Kirkeffect can then be suppressed by improving N-drift concentration. Experimental results showthat the snapback current and voltage increase by about40%and20%, respectively, byadopting the optimized N-drift length.4. Study on design for testabilityThe test method of the essential characteristic parameters is introduced. In order toalleviate the LED output current deviations induced by the process parametric variations andpackage stress, a post-package trimming by blowing poly fuse links is performed. Specifiedvoltage signals are applied on the initial pins to bring the IC into the testing stage, and thebandgap reference which determines the output current can be measured. Based on the measured voltage value, a series of specified pulse signals are applied as programming signalsto burn out the poly fuses. As no extra pin is required, the size and cost of the driver arereduced. The LED output currents have been measured in two configurations before and afterpost-package trimming. The results show that after post-package trimming, the three-sigmal(3σ) deviations of LED output current are improved from38.4mA to11.4mA, and then theaccuracy and uniformity of LED output current are significantly developed.