Design of single-stage high power factor dimming fluorescent lamp electronic ballast

1 Introduction In recent years, electronic ballasts for high-frequency fluorescent lamps have gradually been accepted by people for their advantages such as high efficiency, small size, light weight, no flicker, and long lamp life. The research and development of electronic ballasts in our country was from the late 1980s to the early 1990s. In the early days, in order to save costs, many manufacturers chose simple topologies, whose performance indicators often failed to meet national standards and were easily damaged. This undoubtedly caused more obstacles to the popularization of electronic ballasts. At present, some people directly apply foreign advanced circuit topologies, resulting in complicated design methods, and some are even not suitable for working under the 220V/50Hz power grid. As energy saving issues attract more and more attention, high-performance fluorescent lamp electronic ballasts need to add dimming functions. When full power output is not necessary, reducing the output power not only saves energy and extends the service life of the lamp, but also can to transform visual effects. Therefore, it is urgent to develop electronic ballasts with high performance, closer to lamp characteristics, and full functions. 2 Design Points 2.1 Overview The dimming function actually refers to the function of adjusting the output power of the lamp. When the lighting device does not require full power output, studies have shown that the application of dimming systems can save 50% of energy. In the traditional ballast design without dimming system, since the output power of the lamp is constant at high frequency and stable operation, the lamp can be approximately considered to be a steady resistor. When the grid voltage fluctuates, or the lamp current and lamp voltage change due to other reasons, that is, when the lamp voltage, lamp current RMS value and lamp power change, the lamp can be stably operated near the rated point through closed-loop control. The resistance will not change significantly. However, the design becomes complicated in the dimming operating mode. If the lamp is still equivalent to a purely resistive load, considerable deviations will occur, because at different dimming levels, the negative resistance characteristics of fluorescent lamps are different. Therefore, when designing a dimming electronic ballast, a simple resistive load cannot be used to equivalent a lamp. In recent years, the use of computer-aided design has greatly simplified the design process of power electronic devices, and more circuit working information can be obtained. Commonly used simulation software includes PSPICE, MATLAB, etc., and PSPICE is mostly used in the design of power electronic devices. Therefore, establishing a PSPICE model of fluorescent lamps has become an urgent problem that needs to be solved. 2.2 Modeling of fluorescent lamps There are two main methods for modeling fluorescent lamps. One is physical modeling, which is based on the physical discharge phenomenon of the lamp. However, this modeling method involves more complex equations and many variables. It is not suitable for circuit simulation; the other method is to use curve fitting, which uses the V-I characteristic curve of the lamp to model, and approximates it with a curve equation containing undetermined coefficients based on the experimental results. Some of them use cubic curve equations. , and also use exponential curve equations, parabolic curve equations, and even linear equations to fit. PSPICE models can be static models or dynamic models. The static model needs to calculate the impedance value of the lamp at different operating points, and then perform distribution simulation. Usually, this type of model is relatively simple to establish, but it is very inconvenient to apply. The dynamic model needs to directly reflect the impedance value presented by the lamp when the operating point changes, including its startup process. Such a model is usually called a dimming model. This model is very suitable for dimming electronics. Ballast design. Figure 1 is a PSPICE dynamic model of a fluorescent lamp. It is based on exponential curve fitting, and this model is established for 32W-T8 lamps.    

 Figure 1 Fluorescent lamp PSPICE model 

 2.3 Dimming method Dimming refers to adjusting the energy transmitted to the lamp, thereby changing the lamp power. In a dimming control system, the purpose of dimming is generally achieved by controlling four parameters, namely 1) frequency modulation, 2) adjusting the duty cycle, 3) adjusting the DC bus voltage, and 4) adjusting the resonant impedance value. Frequency control refers to changing the switching frequency fs to make the operating frequency far away from the natural resonant frequency of the resonant network and reduce the lamp power. At this time, the duty cycle D is kept constant. Duty cycle modulation refers to changing the on-time of the switch when fs is constant. The reduction in on-time reduces the energy transferred to the lamp and thus reduces the power on the lamp. The duty cycle modulation range is changed from 0 to 0.5, therefore, the dimming range is limited. Adjusting the DC bus voltage refers to changing the amplitude of the DC bus voltage while keeping fs and D unchanged. This control method can only be used in a two-stage topology. Impedance control refers to changing the parameter values ​​of Ls and Cr of the resonant network. This control method is more complicated to implement. Among them, the circuit structure using frequency modulation is relatively simple and easy to control, so it has the most practical applications. However, it has the disadvantages that it is difficult to achieve soft switching in the entire dimming range; the device stress is great at light load; and hard turn-on and hard turn-off cause serious electromagnetic disturbance problems. In order to expand the dimming range, the frequency range needs to be expanded, and the frequency range is limited by electromagnetic components and gate drive circuits. The lamp current is approximately inversely proportional to the inverter frequency, so this aspect must be considered when designing electromagnetic components such as inductors. Impact. 2.4 Verification of the model Figure 2 uses a simple circuit to verify the lamp model. The topology consists of only one CLASS-D inverter. The parameters are Ls=1.56mH, Cr=5.6nF, fs=45kHz, D=0.45.