Engineer DIY Big Reveal: Development of 600W High-Pressure Sodium Lamp Electronic Ballast

It is an inevitable trend to develop cost-effective electronic ballasts to replace inductive ballasts. This is because high-pressure sodium lamps (HPSL) are high-intensity gas discharge lamps (HIDL) with excellent performance. They have the advantages of high luminous efficiency, long life, and good light color, so they are widely used. Like all gas discharge electric light sources, high-pressure sodium lamps have negative VI characteristics, and require ballasts to suppress the lamp current. Moreover, a gas breakdown voltage of 5 to 20 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.

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. To avoid acoustic resonance, the 600W high-pressure sodium lamp high-frequency electronic ballasts that have been developed use frequency modulation technology to make the current operating frequency of the high-pressure sodium lamp always fluctuate around the center frequency. The starting part uses an LC series resonant circuit to generate high voltage, which is simple and reliable.

1 Overall control strategy

The circuit block diagram is shown in Figure 1. The main circuit is divided into two stages, the first stage is the rectifier and active power factor correction circuit (AP FC), 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. The auxiliary power supply is composed of a single-ended flyback power supply composed of UC3844, and the output voltage is 18V to power the chip.

Figure 1 Overall structure of 600W electronic ballast

2 Rectification and APFC Part of the AC power is rectified by the diode. Although the input voltage is sinusoidal, the input current is seriously 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 sinusoidal and greatly reducing the harmonic content. This article uses the Boost circuit controlled by FAN 7527B as the APFC circuit as shown in Figure 2. FAN7527B is a simple and efficient power factor corrector produced by Fairchild Semiconductor. It contains an R/C filter, so the peripheral circuit does not need to be connected to the R/C filter. The secondary side of the transformer T1 has two functions: to supply power to the chip and as a detection signal for the current zero crossing. This chip adopts voltage and current dual closed-loop control, and the inner loop uses current discontinuous frequency mode control. The voltage detection signal (pin 1) and the synchronization signal (pin 3) are multiplied as the current given, and R5 is the current detection resistor. The output voltage can be controlled within a wide range. According to the needs of the next stage, it is controlled at 400V here, as shown in Figure 2.

Figure 2 Rectification and APFC Principle Figure 3 High-frequency half-bridge inverter part High-frequency inverter is an important part of the electronic ballast of high-pressure sodium lamp. It adopts the variation of half-bridge inverter, uses two less capacitors, has a simple structure and saves costs. The only difference is that the output voltage of the circuit is half of that of the full bridge. Under the same output power condition, the power tube current of the half bridge is twice that of the full bridge. Considering the normal working voltage of the high-pressure sodium lamp, the half-bridge circuit can fully meet its needs. As shown in Figure 3, its working process is that the inductor L1 and C9 first reach series resonance with a frequency of 200kHz, generating 7kV high voltage, igniting the high-pressure sodium lamp. After ignition, the high-pressure sodium lamp is turned on, and C9 does not work. Since C8 >> C9, C8 and the inductor will not resonate, and play 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 half-bridge outputs a square wave, which is converted into high-frequency alternating current after being ballasted by C8 and L1.

As shown in Figure 3, the half-bridge driver chip uses the voltage-type PWM controller SG3525A produced by SiliconLink Corporation of the United States. It is a single-chip integrated PWM controller with excellent performance, complete functions and strong versatility. The chip is simple and reliable, and is easy and flexible to use. Through appropriate external circuits, it can not only realize PWM control, but also complete various 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 T3 to the gates of the switch tubes S2 and S3 to control the two power tubes to work alternately. The oscillator in the SG3525A is externally connected to the clock.

The standard capacitor CT provides a discharge path through the resistor RD. Changing the value of RD can change the discharge time of CT and the dead time. The charging current of CT is determined by the current source specified by RT. Pin 8 of SG3525A plays the role of input soft start. The oscillation frequency of SG3525A is f=1/[CT×(0.7RT+RD)].

Figure 3 Half-bridge high-frequency inverter circuit

4. Suppression of acoustic resonance

When a high-frequency power source is used to ignite the HPSL, the pulsation of the pressure wave in the tube is reflected from the inner wall of the tube. If it is in phase with the pulsation component of the high-frequency current of the lamp, a standing wave is formed, generating acoustic resonance. As long as the operating frequency of the ballast is the same as one of the acoustic oscillation frequencies, acoustic resonance may occur.

Research shows that acoustic resonance is easy to occur in the frequency range of 8kHz to 150kHz. For this reason, many methods have been proposed in foreign literature to suppress acoustic resonance, such as DC superposition method, periodic commutation method and variable frequency modulation method, but they cannot be put into mass production due to complex circuits and increased costs. Under appropriate high frequencies, frequency modulation drive technology is a simple and practical new technology, which is very effective in eliminating the acoustic resonance phenomenon in high-pressure sodium lamp electronic ballasts. As shown in Figure 4, this experiment uses a 555 timer to form a multivibrator, which emits a square wave with a frequency of 2kHz, so that S4 is continuously turned on and off, causing the level of pin 6 of the oscillator of SG3525A to change. As a result, the operating frequency of the ballast changes around the center frequency (35kHz) at all times. Sometimes, even if the audio oscillation frequency is encountered, the frequency changes because there is no time to form a standing wave, thus avoiding acoustic resonance.

Figure 4 Schematic diagram of acoustic resonance suppression

5 Experimental Results

According to the above design, the high-pressure sodium lamp model used in the experiment is NG400, and the digital storage oscilloscope model is DS-5000 series. As shown in Figure 5, its starting voltage is 7kV, and the pulse width is less than or equal to 0.2μs. Figure 6 is the voltage and current output waveform when the high-pressure sodium lamp is working normally, channel 1 is the voltage output waveform, and channel 2 is the current output waveform; the voltage effective value is 128V, the current maximum value is 9.8A, and the effective value is 4.7A.

Figure 5 High-pressure sodium lamp startup waveform

Figure 6 Voltage and current output waveforms of high pressure sodium lamps during normal operation

6 Conclusion

The experimental results of the 600W solar high-pressure sodium lamp electronic ballast show that its circuit works stably, the frequency modulation technology is used to eliminate acoustic resonance, and constant power control is basically achieved. The high-pressure sodium lamp works reliably and stably, indicating that the relevant technical requirements have been met, and the circuit starts quickly, is simple and reliable.