With the rapid development of science and technology, the world's natural resources are also being consumed at the same rapid rate. Energy saving has become an important project in the scientific research plans of various countries. Lighting energy saving has attracted more and more attention and has gradually risen to the level of protecting resources and the environment. In recent years, thanks to the development of power electronics, energy-saving electric light source supporting equipment has also developed rapidly. Compared with traditional inductive ballasts, electronic ballasts have greatly reduced their own energy consumption, greatly improved performance parameters, greatly reduced volume and weight, extended service life, and improved lighting quality.
As a new type of energy-saving high-intensity discharge lamp, metal halide lamps have good color rendering, light color close to sunlight, and high luminous efficiency. They are widely used in indoor and outdoor lighting such as squares, docks, and workshops. Therefore, the electronic ballasts that match them have also become a hot topic of research in recent years. Reducing the cost of metal halide lamps and electronic ballasts and reducing their size have become the key to research.
1 Hardware circuit design
1.1 Technical indicators of metal halide lamp electronic ballast
Power factor greater than or equal to 95%; current distortion coefficient less than or equal to 10%; current crest factor less than or equal to 1.7; service life greater than or equal to 10,000 hours; start-up time less than or equal to 1 minute; overvoltage, overcurrent and abnormal state protection functions.
1.2 Working principle of electronic ballast
The basic structural block diagram of the electronic ballast is shown in Figure 1. It is mainly composed of rectification and power factor correction circuit, constant power control circuit, high-frequency inverter circuit, ignition circuit and control circuit. The input of the circuit is 50/60 Hz, 220 V industrial frequency AC. When the power is turned on and the lamp is not ignited, the LC resonant circuit can generate a high voltage of more than 3 kV to light the lamp; and after the lamp works stably, the voltage across the lamp is about 85 V, which maintains the normal ignition of the metal halide lamp.
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1.2.1 Rectification and power factor correction circuit
The rectification and filtering of this circuit is realized by using a bridge rectifier capacitor filter circuit. The 310 V DC voltage is obtained through the EMI filter and sent to the APFC circuit. The APFC is completed by the power factor control chip L6561. Its circuit diagram is shown in Figure 2.
The basic working principle of APFC is as follows: the output voltage Vo of the main circuit is compared with the reference voltage VREF and then input to the voltage error amplifier EA. The rectified voltage detection value VM1 and the output voltage signal Verr of the error amplifier are added to the input of the multiplier M. The output of the multiplier M is used as the reference signal for current feedback control. The output of the current error amplifier CA and the output of the inductor zero current detector (ZCD) are used as the input signal of the switch tube VG driver to control the on and off of the switch tube, ensuring that the peak value of the inductor current tracks the rectified voltage, so that the input current (the average value of the inductor current) is basically consistent with the waveform of the input voltage, improving the input power factor and reducing the degree of current distortion.
1.2.2 Constant power control circuit
During the service life of the metal halide lamp, the lamp voltage will increase as the lamp tube continues to age, while the traditional inductor ballast has a constant current characteristic, which causes the lamp power to increase continuously, thereby accelerating the aging of the lamp tube. This paper adopts an approximate constant power control scheme with current and voltage limiting functions, which has the characteristics of low cost and stable performance. When the voltage of the lamp changes due to factors such as grid voltage fluctuations and lamp aging, it can automatically achieve constant power output. The circuit schematic is shown in Figure 3.
This circuit can increase the power factor λ of the system to above 0.99, effectively suppress the waveform distortion of the input power current iin, reach the distortion index lower than L level required by GB15144, and ensure that the current crest factor of the HID lamp is less than 1.7.
The detection resistor Rs is connected in series at the input end of the Buck circuit, and then filtered through the resistor R1 to obtain the average voltage signal. The voltage signal of the lamp is detected from the L2 end. The two are amplified by EA. The output voltage is used as a feedback signal and input to the PWM controller through a relatively strong input. The PWM controller outputs a control signal Vf according to the input feedback voltage signal to control the duty cycle of the switch S2 of the Buck circuit. By properly selecting R1, R2, and R3, the output power Po can be well controlled.
1.2.3 Full-bridge inverter
circuit The low-frequency square wave output in the ballast is realized by a full-bridge inverter circuit, as shown in Figure 4. The full bridge is composed of 4 MOSFETs. Under the control of the full-bridge controller, two pairs of MOS tubes are alternately turned on and output a 400 Hz square wave under stable working conditions.
1.2.4 Starting circuit
The resonant starting circuit is composed of LC. Before starting, the lamp is equivalent to an open circuit, and L and C form a series resonant circuit. When the lamp is lit, a small amount of high-frequency components when the full-bridge output square wave voltage jumps positively and negatively is used to make L and C resonate, and a high voltage is generated at both ends of C to ignite the lamp. The position of the ignition voltage of this starting circuit is conducive to the stability of the arc after the lamp is started, and when the lamp works in a steady state, the ignition circuit has no effect on the lamp.
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2 Software Design
The MCU mainly completes the control of the lamp startup process, constant power control in steady state, and abnormal state protection such as overvoltage, undervoltage, open circuit, short circuit, etc. Its main program flow is shown in Figure 5.
3 Experimental results
The circuit working waveform is shown in Figure 6.
4 Conclusion
The electronic ballast for metal halide lamp based on single chip microcomputer control can automatically complete fault detection, high voltage ignition, abnormal state protection and constant power control in steady state. The designed electronic ballast has passed the high temperature 80℃, low temperature -40℃, abnormal state protection and working time tests. From the research results, it can be seen that the prototype development is successful, which undoubtedly lays a good foundation for the early launch of digital control electronic ballast to the market.
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Professor at Beihang University, dedicated to promoting microcontrollers and embedded systems for over 20 years.
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