Study on dynamic and steady state characteristics

Research on dynamic and steady-state characteristics of metal halide lamp electronic ballast

Abstract: a 250W metal halide lamp electronic ballast is introduced. The circuit is composed of PFC, buck converter and full bridge inverter. In order to avoid acoustic resonance, the full bridge inverter operates at 200Hz low-frequency square wave, and on this basis, the dynamic and steady-state characteristics of metal halide lamps are systematically studied

key words: electronic ballast; Metal halide lamp; Acoustic resonance

1 introduction

at present, metal halide lamps have been widely used in many lighting fields because of their high luminous efficiency, good color rendering, long service life and other advantages. However, due to its non-linear V-A characteristics, ballast is required to start and stabilize arc discharge

gas discharge lamps, especially metal halide lamps, have the problem of acoustic resonance when working at high frequency, which will lead to the instability of arc light, the change of color temperature, and even the cessation of arc. The main methods to overcome acoustic resonance are:

1) make the gas discharge lamp work at extremely high frequency [2], which is beyond the frequency range of acoustic resonance, but because the switching loss is proportional to the switching frequency, the problems of heat dissipation and efficiency will become extremely prominent

2) make the gas discharge lamp work in the frequency modulation state [3], making it impossible to form acoustic resonance, but the gas discharge lamps produced by various manufacturers are different in appearance, and the frequency band of silent resonance is also different, so it is also more difficult

3) make the gas discharge lamp work in low-frequency square wave, and the frequency band of acoustic resonance is generally a few kHz to 1MHz, so acoustic resonance will not generally form

this paper adopts a three-stage electronic ballast, which works in low-frequency square wave mode, and adopts constant power control to avoid the problem that the lamp voltage increases with the aging of the bulb caused by the constant current control method

2 circuit topology

the 250W electronic ballast introduced in this paper is shown in Figure 1. It is developed for the American electricity standard, so the input voltage is 120 V AC. In order to obtain a higher power factor, the first level power printing and other functional rate factor correction circuit (PFC) converts 120V AC to 240 V DC, the second level dc/dc converter is used as constant power control, and the third level dc/ac inverter converts DC to 200 Hz square wave voltage, which is then installed on the vehicle. Since all electrical components of metal halide lamp have reliable performance and can avoid mechanical vibration and voltage fluctuation, an ignition voltage of 2 ~ 3 kV is required, so a single-stage igniter is added to generate high-voltage pulse. The constant power control samples the output voltage and current of the buck converter (the second stage dc/dc) to control the PWM drive signal of the buck converter

Figure 1 general block diagram of electronic ballast

3 circuit working principle

the first level circuit is shown in Figure 2, which is composed of EMI filter and PFC circuit. The PFC circuit is controlled by MC34262 chip, which can obtain a power factor close to 1. It converts 120V AC to 240V DC as the input of the next dc/dc converter

Figure 2 main circuit of PFC at level 1

the main circuits of level 2 and level 3 are shown in Figure 3, which are composed of a buck converter and a full bridge inverter circuit respectively. Before the metal halide lamp lights up, due to the open circuit of the load, the current detection is zero, the maximum duty cycle of the buck control output, the output of the buck circuit is about 240V, and C3 is charged through R3. When the trigger voltage of the bidirectional trigger tube (Sidac) is charged, the trigger tube is turned on, C3 is discharged, the high voltage is applied to the primary side of the step-up transformer T1, and the secondary side outputs a 2 ~ 3KV high voltage pulse, which is applied to both ends of the metal halide lamp to provide its ignition voltage. If an ignition fails, C3 is charged again, and the above process is repeated until the metal halide lamp is lit. When the current increases after the metal halide lamp is turned on, the output of the buck converter will drop. The steady-state operating point is about 120V, which is lower than the trigger voltage of Sidac, and there will be no high-voltage pulse. When the bulb is broken, there is no need to turn off the power supply, As long as manager Yan changed again, he shared his views: "In the next five years, new bulbs can be lit up again.

Figure 3 main circuits at the second and third levels

4 experimental results and analysis

a 250W electronic ballast prototype has verified the above scheme. The component parameters are as follows: lboost=180 h, cboost=400 F, lbuck=120 h, cbuck=1 F, rsense=0.1, r1=200k, r2=256, r3=100k, c3=1 F, T1 turn ratio =1:20. Figure 4 shows the voltage waveform at both ends of C3. You can see Sida.Trigger frequency of C It is 3.6hz, that is, it takes about 280ms. When Sidac triggers conduction, a high-voltage pulse is induced at the secondary side. The waveform is shown in Figure 5, and the peak voltage reaches 2KV

Figure 4 voltage at both ends of C3

Figure 5 voltage at both ends of the lamp when ignition

1) dynamic characteristics

Figure 6 shows the change of voltage and current during the whole process of the lamp from start to steady state, from which we can see the control process of constant power. It takes about 150s for the lamp from start to steady state. Figure 7 and figure 8 respectively show the curve of lamp voltage and lamp current with time. The lamp voltage increases slowly with time, on the contrary, the lamp current decreases slowly with time. The starting process of the metal halide lamp has gone through four stages: gas ionization stage, glow discharge stage, transition stage from glow discharge to arc discharge, and arc discharge maintaining and boosting stage. Figure 9 shows the change curve of lamp equivalent resistance with time. Due to the nonlinearity of lamp resistance, constant power control should be used

Figure 6 lamp voltage and current dynamic process

Figure 7 lamp voltage time curve

figure 8 lamp current time curve

Figure 9 lamp resistance time curve

2) steady state characteristics

Figure 10 is the steady state voltage and current waveform of the lamp. It can be seen that the voltage and current are in the same phase, the voltage and current are linear, and the power factor is close to 1, so the metal halide lamp can be regarded as a resistor in the steady state, and the 250W metal halide lamp is about 62

Figure 10 steady state voltage and current waveforms at both ends of the lamp

5 conclusion

experimental research shows that the three-stage electronic ballast introduced in this paper has the characteristics of high reliability and good stability, and the starting circuit is simple and practical. At the same time, the steady-state and dynamic characteristics of 250W metal halide lamp are given