Study And Design Of The Constant Current LED Driver Based On Primary Side Control Scheme Without Auxiliary Winding

In recent years, LED has been widely used in residential lighting, commercial lighting and traffic lighting field, etc, due to its incomparable advantages, such as superior longevity, improved luminance efficiency and environmental friendliness. Hence, LED lighting has been developed as the next lighting generation modules by both academia and industry. Although LEDs are recognized as inevitable tendency in prospective energy efficient lighting (EEL), its promotion has been limited by cost and technical reasons. For example, the low output power (< 5W) LED drivers equipped with auxiliary winding, can output constant current with the precision of±2.5%, but the sysmatic conversion efficiency is below 80%. So there is great significance to research on LED drivers with high precision, low power consumption and low cost. According to the research of the LED driving technology, this paper proposes a constant current LED driver based on a primary side control flyback converter without auxiliary winding.After a detailed introduction to the operating principle of primary side control constant current LED driver with auxiliary winding, this paper theoretically proves the feasibility of eliminating the auxiliary winding. And a controlling chip is defined for the new constant current driving system, which is a constant current controller for flyback converters working in discontinuous current mode (DCM). The proposed chip is designed by using CSMC’s 0.8μm 700V BCD process and it can work properly within 85～265Vac wide input voltage range. The controller chip employs two cascaded power MOSFETS and such structure can lower the system power dissipation. The zero corssing sensing (ZCS) comparator can monitor the resonance between magnetizing inductor and parasitic capacitors, which starts when the demagnetization phase finishes. Thus the demagnetization information can be obtained with high precision without the help of auxiliary winding, which is commonly used in the conventional primary-side control scheme. Once the auxiliary winding is eliminated, the peripheral circuits are simplified. This can also reduce the volume and the cost of the LED driver. By adopting a novel constant current (CC) control scheme, the system can output precise constant current when the input voltage fluctuates or the load changes. In addition, the chip has a full protection mechanism, including over temperature protection, short protection of sampling resistor, over voltage protection and short protection of load.Simulation results show that the average output current precision is within±2.5%, and its line regulation and load regulation are within±4%, while the sysmatic conversion efficiency is above 80%. Besides, the reliability of the preotection function is verified. The simulation results also prove the feasibility of eliminating the auxiliary winding in the primary side control flyback converter.The layout design has also been finished and the area is 2.46× 1.02 mm2.