High pressure sodium (HPSL) is a performance of high-intensity gas discharge lamps (HIDL led lights), its high luminous efficiency, long life, light color, so widely used. As with all gas discharge electric light, high pressure sodium lamp was negative the V-I characteristics of the ballast to suppress the lamp current, and 5 to 20kV the gas breakdown voltage necessary to start. Traditional magnetic ballast volume, low power factor (only 0.3 to 0.4), and ability to adapt to the grid voltage fluctuation is not strong, so the development of the higher cost of electronic ballasts to replace inductive ballast the general trend. Have been developed to high pressure sodium lamp electronic ballasts are high frequency electronic ballast, at high frequencies, high pressure sodium lamp is easy to put out the arc, and the existence of acoustic resonance. In order to avoid acoustic resonance, have been developed to high-frequency electronic ballasts for 600W high pressure sodium lamp with a frequency modulation technique, the current operating frequency of the high-pressure sodium lamp moment around the center frequency up and down changes. Start part of the LC series resonant circuit generates a high voltage, simple and reliable.
An overall control strategy
The circuit block diagram shown in Figure 1. The main circuit is divided into two, the first level for the rectifier and active power factor correction circuit (APFC), the second stage inverter circuit. It can be seen, the electronic ballast is essentially a typical AC / DC / AC conversion circuit. Auxiliary power by the UC3844 single-ended flyback power structure, the output voltage of 18V to the chip supply.
Overall structure of the map in Figure 1 600W electronic ballasts
Two rectifier and APFC part
Alternating current through the diode rectifier input voltage is sinusoidal, but the input current distortion, inefficient, large-scale use will cause serious harm to grid, while the input current harmonics generated noise will also affect the circuit operation. The APFC enable circuit input power factor to 0.95, so that the input current of the basic sine wave, harmonic content is greatly reduced. In this paper, FAN7527B control Boost Circuit as the APFC circuit shown in Figure 2. FAN7527B Fairchild Semiconductor’s production is simple and efficient power factor correction, contains a R / C filter, so the external circuit does not need to access the R / C filter. Transformer T1 secondary has two roles: as a current zero detection signal to the chip power supply at the same time. This chip uses the voltage and current double closed loop control, and the inner use of discontinuous current uncertain frequency mode control. Voltage detection signal and synchronous signal multiplied by the given as a current, R5 is the current sense resistor. The output voltage can be controlled in a wide range According to the latter, a need here to control 400V.
Three high-frequency half-bridge inverter part
High frequency inverter is an important part of the high pressure sodium lamp electronic ballasts, the half-bridge inverter variant form, the less the two capacitors, a simple structure, and cost savings. Circuit output voltage is half of the full-bridge, in the same output power conditions, the half-bridge power tube current than the full-bridge twice, taking into account the normal operating voltage of the high pressure sodium lamp, half-bridge circuit to fully meet their needs. Shown in Figure 3, the course of their work for the inductor L1 and C9 the first of the series resonant frequency of 200kHz, 7kV high pressure, high pressure sodium lamp ignition, after ignition, high pressure sodium conduction, C9 does not work, due to C8, >> C9, C8 and inductors are not resonant, played the role of the ballast, then the fluctuations of the frequency to 35kHz to 2kHz range, high pressure sodium to the normal operation of the constant power for 1 min. Half-bridge output square wave, C8 and L1 ballast into high frequency alternating current.