Development of electronic ballast for high pressure sodium lamp based on current tracking control
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Abstract: Aiming at the working characteristics of high-pressure sodium lamps and their defects in high-frequency operation, a low-frequency high-pressure sodium lamp electronic ballast is designed by using current tracking technology, and a reliable logic control starting circuit is designed. Finally, the experimental results are given. Keywords: high pressure sodium lamp; electronic ballast; closed loop; current tracking 0 Introduction High-pressure sodium lamp (HPS lamp) is a high-intensity gas discharge lamp (HID lamp) with excellent performance. Its advantages are high luminous efficiency, long life and good light color, so it is widely used. Like all gas discharge electric light sources, high-pressure sodium lamps also have negative V-I characteristics, requiring ballasts to suppress lamp current, and a gas breakdown voltage of 3 to 4 kV is required when starting. Traditional inductive ballasts are large in size, low in power factor (can only reach 0.3 to 0.4), and have poor adaptability to grid voltage fluctuations. Therefore, it is a general trend to develop cost-effective electronic ballasts to replace inductive ballasts. Most of the electronic ballasts for high-pressure sodium lamps that have been developed are high-frequency electronic ballasts. Under high-frequency conditions, high-pressure sodium lamps are prone to arc extinguishing and have acoustic resonance problems. In order to avoid acoustic resonance, the operating frequency needs to change up and down around the center frequency at all times, but this causes considerable difficulty in control. For this reason, this paper proposes a low-frequency electronic ballast based on current tracking control. 1 Control Principle and Circuit Analysis The circuit block diagram is shown in Figure 1. The main circuit is divided into two stages, the first stage is rectification and APFC (active power factor correction circuit), and the second stage is the inverter circuit. It can be seen that the electronic ballast is essentially a typical AC/DC/AC conversion circuit. 
Figure 1 Schematic diagram of electronic ballast 1.1 Rectification and APFC Although the input voltage of the diode uncontrolled rectifier is sinusoidal, the input current is seriously distorted. Large-scale use will cause serious harm to the power grid. At the same time, the noise generated by the input current harmonics will also affect the operation of the circuit. APFC can increase the input power factor of the circuit to above 0.95; the input current is basically a sine wave, and the harmonic content is greatly reduced. Here, the Boost circuit controlled by UC3854 is used as the APFC circuit (Figure 2). UC3854 is a high power factor correction chip produced by Unitrode, USA. This chip adopts voltage and current dual closed-loop control, and the current inner loop uses average current mode control. The voltage detection signal and the synchronization signal are multiplied as the current given, and Rs is the current detection resistor. The output voltage can be controlled within a large range. According to the needs of the next stage, it is controlled at 380V here. The power supply of UC3854 and the control circuit comes from the auxiliary power supply. The auxiliary power supply is composed of a flyback converter controlled by a pulse width modulator UC3844, which can provide multiple outputs. 
Figure 2 Active power factor correction (APFC) circuit 1.2 Inverter part and current feedback control The inverter circuit is the most important part of the electronic ballast, and usually adopts a half-bridge inverter or full-bridge inverter circuit. The output voltage of the half-bridge inverter circuit is half of that of the full-bridge. When the power tube current is equal, the output power of the full-bridge circuit is twice that of the half-bridge, but two more power tubes are used. Considering that the secondary triggering voltage of the 400W high-pressure sodium lamp is 150-190V, and the APFC output voltage is 380V, the output voltage of the half-bridge circuit can fully meet the needs of secondary triggering, and the voltage stress of the power tube of the half-bridge circuit is the same as that of the full-bridge circuit, but the former is cheaper than the latter. Therefore, the half-bridge inverter circuit is used here (Figure 3). 
Figure 3 Half-bridge inverter main circuit topology The essence of electronic ballast is to limit the current flowing through the lamp. According to the feedback control law, if you want to control a certain quantity, you can introduce negative feedback of this quantity. The inverter circuit topology shown in Figure 3 is actually still a high-frequency converter structure. In order to make the current flowing through the high-pressure sodium lamp a low-frequency current, a hysteresis comparison current tracking type PWM control is adopted here. Its principle is shown in the dotted box in Figure 3, which is composed of a hysteresis comparator. The difference between the given current signal i g and the current feedback signal if i g - if is used as the input of the hysteresis comparator, and its output is used to control the on and off of S 2 and S 3. Assume that the direction of the lamp current iL is as shown in Figure 3. When S 2 (or D 2 ) is turned on, i L increases, and when S 3 (or D 3 ) is turned on, i L decreases. Assuming the loop width of the hysteresis comparator is ΔI , if the current feedback coefficient is k (= if / iL ), the current iL tracks the given current in a sawtooth shape within the range of ( ig - ΔI )/ k and ( ig +ΔI ) / k , as shown in Figure 4. For simplicity, the current given signal is taken from the grid voltage sinusoidal wave signal. 
Figure 4 Current tracking waveform using hysteresis comparison method There are two switching modes for S 2 and S 3 , namely bipolar switching and unipolar switching. In bipolar switching, regardless of whether the given current i g is in the positive half cycle or the negative half cycle, S 2 and S 3 are complementary on and off. In unipolar switching, when i g is in the positive half cycle, S 3 is always off and S 2 is chopped; in the negative half cycle, S 2 is off and S 3 is chopped. The analysis of the unipolar switching principle is shown in Figure 4, that is, in the period t 0 ~ t 1 , S 2 is turned on and the current i L increases; at t 1 , i L increases to a value slightly larger than [ i g ( t 1 ) + Δ I ] / k , the hysteresis comparator is activated, S 2 is turned off, the inductor L discharges, and i L continues to flow through the capacitor C 2 and the diode D 3 ; until t 2 , it drops to a value slightly smaller than [ i g ( t 2 ) - Δ I ] / k , S 2 is turned on again, and i L will increase again. The same analysis can be done when i g is in the negative half cycle. Compared with the bipolar switching mode, unipolar switching has the following advantages: 1) Only one power tube is in operation, and the switching loss is half of that of bipolar switching; 2) The dynamic stress of each physical quantity of the main circuit, such as dv / dt and di / dt , is smaller than that of the bipolar switching mode. Therefore, the interference to the control circuit is smaller than that of the bipolar circuit. However, the control circuit of unipolar mode needs to add some simple logic control circuits. The size of the inductor L and the hysteresis width 2Δ I determine the switching frequency. When other conditions are constant, the switching frequency is inversely proportional to the product of the inductor L and the hysteresis width. Because the switching frequency of the power MOSFET is very high, a smaller inductor can meet the requirements. 2 Design of starting circuit The startup circuit adopts logic control, and no high-voltage isolation measures are required, which simplifies the main circuit and can start instantly. The working principle is shown in Figure 5. It is easy to generate high-frequency pulses using the LC oscillation principle. The freewheeling inductor L in the main circuit serves as the secondary side of the LC oscillator, and the primary-secondary turns ratio is set to 1:20. The primary 300V voltage comes from the control auxiliary power supply, and the secondary side can theoretically generate a 6kV pulse. Because the thyristor SCR is connected in series in the LC oscillation circuit, only positive trigger pulses can be generated at both ends of L when triggered . M1 , M2 , and M3 are three logic control signals. Only when all three signals are high level, a high-voltage trigger pulse will be generated. 
Figure 5 Trigger start circuit The voltage of M1 on C3 is charged to a certain value and becomes a high level . M2 is used to determine whether there is lamp current. When there is lamp current, it is at a low level and startup is prohibited. The signal on M2 has a delay function to avoid hot start after the lamp is turned off. After the lamp returns to a cold state, M2 becomes a high level and startup is allowed. M3 is synchronized with the driving signal of the power tube S2 , so that the trigger voltage can be generated only when S2 is turned on. In this way, when a high voltage is generated at both ends of L , S2 is in the on state, avoiding the damage of the power tube by the trigger voltage. 3 Experimental Results The sodium lamp model used in the experiment is NG400, the mains voltage is 230V, and the dynamic storage oscilloscope model is TDS3012. The voltage and current waveforms of the incoming line are shown in Figure 6. The power factor of the electronic ballast can reach above 0.97, and the harmonic distortion is also effectively suppressed, indicating that the APFC composed of UC3854 has good performance. Figure 7 is the lamp current waveform during stable operation, which is basically the required sine wave, with a current amplitude of 6.5A and an effective value of 4.6A. The measured incoming line power is 412W, the lamp power is 387W, and the efficiency of the designed electronic ballast reaches 0.94. 
Figure 6 Voltage and current waveforms at the incoming line 
Figure 7 Lamp current waveform 4 Conclusion The experimental results of the 400W high-pressure sodium lamp electronic ballast show that the circuit works stably, there is no acoustic resonance, and the constant power requirement is basically achieved. This shows that this design is more reasonable. The designed trigger circuit starts quickly and no isolation measures are required.
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