Design of Energy-saving Electronic Ballast for High-pressure Sodium Lamp

In today's global energy shortage environment, energy conservation has become a general trend. At the same time, the country also vigorously advocates energy conservation and emission reduction, and builds a resource-saving and environmentally friendly society. This has brought a huge historical opportunity to the development of high-pressure sodium lamp energy-saving electronic ballasts. At present, high-pressure sodium lamps are the main light source for road lighting in various countries around the world, and the electricity consumption accounts for more than 80% of the total road lighting electricity. Although the application of high-power LEDs in road lighting has developed from the trial stage to the application and promotion stage, the use rate of more than 80% of high-pressure sodium lamps is unlikely to be replaced by similar products in the near future and withdraw from the historical stage. Its historical mission will continue for a long time and continue to firmly occupy the dominant position of road lighting sources. Therefore, how to achieve effective power saving of high-pressure sodium lamps is the focus of road lighting energy saving. In the government's energy-saving work, energy saving of government agencies themselves and public administrative institutions is the primary task of the work. Street lighting with large electricity consumption and serious waste of electricity has obviously become the top priority of the government's energy-saving and emission reduction work. Street lighting has great potential for energy saving and consumption reduction. Choosing suitable street lighting energy-saving technology will bring considerable social and economic benefits. High-pressure sodium lamps have high luminous efficiency, uniformity, fog-breaking properties, and color rendering properties that are unmatched by other light sources. However, due to the characteristics of grid voltage changes and the time period characteristics of road lighting, high-pressure sodium lamps waste luminous efficiency and electricity, shorten the life of lamps, and thus increase huge maintenance costs. How to maximize the benefits of high-pressure sodium lamps, an excellent road lighting source, to achieve energy saving, reduce operating costs, increase lighting rates, reduce maintenance, and achieve safety, reliability, advanced technology, economic rationality, energy saving, environmental protection, easy maintenance, and beautify the environment are the goals that this energy-saving product transformation project ensures to achieve.

 

　　High-pressure sodium lamp (HPSL) is a high-intensity gas discharge lamp (HIDL) with excellent performance. It emits golden white light when used. Its advantages are luminous efficiency up to 140 lm/W, life up to 32,000h, and good light color. Therefore, it is widely used in roads, highways, airports, docks, docks, stations, squares, street intersections, industrial and mining enterprises, parks, garden lighting and plant cultivation. Like all gas discharge electric light sources, high-pressure sodium lamps have negative V-I characteristics, and require ballasts to suppress lamp current, and require a gas breakdown voltage of 5 to 20kV when starting. Traditional inductive ballasts are large in size, low in power factor, and have poor adaptability to grid voltage fluctuations. Therefore, it is a general trend to develop high-frequency electronic ballasts to replace inductive ballasts. Most of the electronic ballasts for high-pressure sodium lamps that have been developed use high-frequency electronic ballasts. Under high-frequency conditions, high-pressure sodium lamps are prone to arc extinguishing and have acoustic resonance problems. To avoid acoustic resonance, the 250W high-pressure sodium lamp energy-saving electronic ballast developed now uses frequency jitter technology to make the current operating frequency of the high-pressure sodium lamp fluctuate around the center frequency at all times. The starting part uses an LC series resonant circuit to generate high voltage, which is simple and reliable. In the design of street lighting, according to the characteristics of the time period, a timer is added to reduce the power of the street lamp at a certain time in the middle of the night to achieve energy saving.

Main circuit design

　　The main circuit of the electronic ballast of the high-pressure sodium lamp adopts a two-stage combination of active power factor correction (APFC) and DC/AC inverter, which has good dynamic response, solves the harmonic pollution problem of the power grid, and makes the electronic ballast more green and environmentally friendly. The active power factor correction circuit works in DCM mode, with small harmonic current and small voltage and current stress of the switch tube. The DC/AC inverter part usually adopts a half-bridge inverter or a full-bridge inverter circuit. The output voltage of the half-bridge is half of that of the full-bridge. Under the same output power conditions, the power tube current used is twice that of the full-bridge. Under the condition of equal power tube current, 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 trigger voltage of the 250W high-pressure sodium lamp is 150~190V, and the APFC output voltage is 400V, 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 and the full-bridge circuit is the same, but the former is cheaper than the latter, and the half-bridge circuit fully meets its needs. Compared with the symmetrical half bridge, the asymmetric half bridge (AHB) uses two fewer capacitors and has a simpler structure. The main circuit of the electronic ballast for high-pressure sodium lamps is shown in Figure 1. It can be seen that the electronic ballast is actually a typical AC/DC/AC conversion circuit.

 

　　Figure 1 Schematic diagram of the main circuit of the electronic ballast

　　The 220V AC power is rectified by a diode. Although the input voltage is sinusoidal, the input current is severely distorted and the efficiency is very low. 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, making the input current basically a sine wave and greatly reducing the harmonic content. The power factor correction circuit is implemented using L6562A, which has a simple structure, high PF value, low THD value, and works in variable frequency control mode. The minimum switching frequency is 25kHz, and the BOOST boost inductor works in critical current mode. The secondary side of the coupled boost inductor L1 has two functions, one is to power the chip, and the other is to serve as a detection signal for current zero crossing. This chip adopts voltage and current dual closed-loop control, and the inner loop uses current intermittent indefinite frequency mode control. The voltage detection signal (pin 1) and the synchronization signal (pin 3) are multiplied as the current given, and R1 is the current detection resistor. The output voltage is controlled at 400V. As shown in Figure 2, the rectifier bridge, energy storage inductor L1, power switch device S1, boost diode D1, output filter capacitor and current sampling resistor R1 constitute the APFC main circuit.

　　Figure 2 Schematic diagram of the front-stage power factor correction circuit

　　The post-stage DC/AC inverter circuit uses a voltage-type PWM controller KA3525A to control an asymmetric half-bridge. It includes two complementary controlled power tube MOSFETs. S1 and S2 are two MOS tubes with anti-parallel diodes D1 and D2. The load circuit is composed of a ballast circuit composed of a capacitor Cb and an inductor L2 and a lamp (steady-state resistance is R) in series. As shown in Figure 3. The chip is simple and reliable, and is convenient and flexible to use. Through appropriate external circuits, it can not only realize frequency conversion control, but also complete multiple functions such as input soft start, overload current limiting, overvoltage protection, and dead zone adjustment. Its working principle is to send the two complementary PWM drive pulses output by the trigger pulse transformer T1 to the gates of the switch tubes S2 and S3 to control the two power tubes to work alternately with a fixed duty cycle. The oscillator in KA3525A is externally connected to the time-stamp capacitor CT, which provides a discharge path through the resistor Rd. Changing the value of RT can change the discharge time of Cd and the dead zone time at the same time. The charging current of CT is determined by the current source specified by RT, and the pin 8 of KA3525A plays the role of input soft start. The oscillation frequency of KA3525A is f=1/[CT×(0.7RT＋Rd)].

　　Due to the nonlinear impedance of HPSL, it is in high impedance before the lamp is lit. Once the external high voltage triggers the lamp to light up, the lamp will turn on, the voltage across it will drop rapidly, the lamp current will increase, and it will show a negative resistance characteristic. If the normal voltage is still applied to the lamp, the lamp will burn out. The impedance of HPSL in the cold state just started and the hot state after long-term operation is very different. Therefore, the HPSL controller must be a constant power output under current mode control. In this scheme, an asymmetric half-bridge circuit is used to reduce the 400V voltage output by APFC to the working voltage required by HPSL under constant current. Its working process is that the inductor L2 and Ca first reach series resonance with a frequency of 200kHz, generating a 5kV high voltage to ignite the high-pressure sodium lamp. After ignition, the high-pressure sodium lamp is turned on and Ca does not work. Ca is a small-capacity high-voltage capacitor. Since the DC isolation capacitor Cb>>Ca, Cb and the inductor will not resonate, and it plays a ballast role. At this time, the frequency fluctuates up and down within a range of 2kHz with 35kHz as the center. After 1 minute, the high-pressure sodium lamp reaches normal operation at constant power. The asymmetric half-bridge outputs a square wave, which is converted into high-frequency AC after being ballasted by Cb and L2.

　　

　　Figure 3 Schematic diagram of the asymmetric half-bridge circuit in the rear stage

　　The auxiliary power supply is composed of a single-ended flyback power supply composed of TOP221 with built-in MOSFET, and the output voltage +15V is used to power the control chip, as shown in Figure 4.