“Metal halide lamp (MHL), as a typical representative of high-pressure gas discharge lamp (HID), is known as one of the most ideal light sources for its advantages of high luminous efficiency, high color rendering and long life. As a gas discharge lamp, the metal halide lamp has negative resistance characteristics, so a ballast must be used to ensure its stable operation. At present, the Electronic ballast is the most widely studied in related industries. Compared with the traditional inductive ballast, the electronic ballast is smaller in size, lighter in weight, higher in efficiency and can effectively eliminate working noise. The main problem that restricts the development of electronic ballasts is that their control is complicated.Control electronics based on analog devices
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introduction
Metal halide lamp (MHL), as a typical representative of high-pressure gas discharge lamp (HID), is known as one of the most ideal light sources for its advantages of high luminous efficiency, high color rendering and long life. As a gas discharge lamp, the metal halide lamp has negative resistance characteristics, so a ballast must be used to ensure its stable operation. At present, the electronic ballast is the most widely studied in related industries. Compared with the traditional inductive ballast, the electronic ballast is smaller in size, lighter in weight, higher in efficiency and can effectively eliminate working noise. The main problem that restricts the development of electronic ballasts is that their control is complicated. The control circuit structure based on the analog device will be very complicated, the cost is high and the stability cannot be guaranteed. And because the single-chip control circuit can simplify the structure of the electronic ballast and improve its performance significantly. This paper introduces a three-stage constant power metal halide lamp electronic ballast controlled by STC microcontroller, analyzes its control strategy in detail, and designs the corresponding software flow and hardware control circuit.
1 Basic topology of electronic ballast
At present, a variety of control ideas for electronic ballasts have been proposed in related fields. The common point is to seek to avoid the occurrence of acoustic resonance while ensuring the stable operation of the halogen lamp. Acoustic resonance phenomenon refers to the unstable arc of the HID lamp when it works at high frequency, which will seriously affect the lighting effect of the lamp, and may even lead to the damage of the lamp. In order to avoid acoustic resonance, the operating frequency of the lamp must be adjusted away from the acoustic resonance frequency band. Practice has shown that the scheme of driving the metal halide lamp with a low frequency square wave is the most effective solution to eliminate the acoustic resonance phenomenon.
The most widely used solution of low frequency square wave drive metal halide lamp is three-stage electronic ballast, its basic composition is shown in Figure 1. The first stage power factor correction circuit (namely APFC circuit) can reduce the current harmonics and improve the power factor while providing constant DC bus voltage for the subsequent power control stage circuits. This design uses the Boost circuit structure to provide a higher bus voltage to facilitate the generation of high-voltage ignition pulses for driving metal halide lamps. The second-level power control circuit is a Buck circuit, which is used to realize the steady-state control of the metal halide lamp, mainly referring to constant power control. The so-called constant power control is to ensure that the output power of the halogen lamp remains unchanged during normal operation. Under constant power, the light output and correlated color temperature index of the metal halide lamp will be very stable, which can not only ensure the lighting quality, but also prolong the service life of the metal halide lamp. During steady state operation, the Buck circuit will adjust the working current of the metal halide lamp to a stable value, and at the same time, the APFC circuit will output a constant DC bus voltage, thereby realizing the constant power control of the metal halide lamp. The third-stage circuit is a full-bridge inverter circuit, which can invert the output voltage of the Buck circuit into a low-frequency square wave to drive the HID lamp, thereby avoiding the occurrence of acoustic resonance. The starting of the metal halide lamp needs to be realized by high-voltage pulse ignition. The ignition circuit in Figure 1 mainly uses the LC resonance method to generate high-voltage pulses up to several thousand volts to complete the starting of the metal halide lamp. The function of the single-chip control and protection circuit in Fig. 1 is mainly to complete the start-up control of the metal halide lamp and the monitoring and protection of the metal halide lamp when it works stably to ensure the normal operation of the entire system.
2 Metal halide lamp control strategy and software program flow
The starting process of metal halide lamps is usually complicated, and the volt-ampere characteristic curve during the starting process is shown in Figure 2. In order to simplify the control, a linear control strategy that matches the load characteristics of the metal halide lamp can be used in the design. The program flow of the strategy is shown in Figure 3.
In the high-voltage triggering stage, the Buck circuit will maintain a high stable no-load output voltage, so that the ignition circuit can generate high-voltage pulses to start the metal halide lamp. Since it takes a period of time from the system power-on to the establishment of the Buck no-load output voltage, the control circuit needs to detect the Buck output voltage to avoid premature ignition. When the detected voltage is lower than a limited low voltage value, the control circuit will consider it Circuit failure, and command the system to enter the standby state; and when it is detected that it is greater than a set high voltage value, the single-chip microcomputer controls the full bridge to output a low-frequency square wave to ignite the metal halide lamp.
It can be seen from the characteristic curve of the metal halide lamp that it will enter a transitional stage after the ignition is successful. The transition stage can be divided into two stages, the first stage is the low-voltage maintenance stage, the voltage across the lamp will drop sharply and remain at a low level; and in the second stage, the voltage across the lamp will gradually increase until its steady-state operating voltage. Therefore, after starting to generate the full-bridge drive signal, the control circuit will continue to detect the Buck output voltage, and when the detected voltage is less than a certain set value, the metal halide lamp is considered to be ignited successfully, and the ballast can change the full-bridge operating frequency by changing the to reduce the pulse spike interference in its output voltage. When the control circuit detects that the Buck output voltage begins to rise to the corresponding set value, the system will set the full-bridge operating frequency to a stable operating frequency again.
When the voltage across the metal halide lamp rises and stabilizes to its steady-state operating voltage, the circuit enters the constant power operation stage. At this stage, the control circuit will detect the system performance parameters to ensure the stable operation of the device, which is mainly to detect the two parameters of system temperature and Buck output voltage. The control circuit detects the system temperature through the thermistor. If the temperature is too high, the single-chip control ballast will stop working; only when the system temperature is lower than a certain temperature again, the device will resume work. Likewise, the control circuit will always keep checking the Buck output voltage during steady-state operation, and if the voltage is abnormal, it will stop the device and wait for inspection.
If the metal halide lamp is not connected or the lamp is broken during work, the system will keep igniting. In addition, when the metal halide lamp is overheated, its starting demand voltage will rise from the original 3 to 5 kV to more than 20,000 volts. At this time, the high-voltage pulse output by the ballast is not enough to light the metal halide lamp, and there will be continuous phenomenon of ignition. Prolonged high voltage pulses are dangerous to electronic ballasts and lamps. In order to ensure circuit safety, this situation must be avoided. Therefore, if the ignition is unsuccessful, it can be judged that the electronic ballast is open, the lamp is faulty, or the lamp is overheated. If the lamp is overheated, it can be delayed for a period of time, and then the next round of ignition can be performed after the lamp has cooled down. After the predetermined number of ignitions, if the ignition is still unsuccessful, it can be judged that the electronic ballast is in an open state or the lamp appears. If the fault occurs, the electronic ballast can be stopped to wait for maintenance by sending a control signal through the single-chip microcomputer at this time.
3 Hardware control circuit design
The functional block diagram of the hardware control circuit given by the analysis of the electronic ballast control strategy and the control flow chart is shown in Figure 4. The control chip selects the STC 12C 5410AD single-chip microcomputer of Hongjing to realize the functions of signal detection and circuit control.
The STC 12C 5410AD single-chip microcomputer uses +5 V power supply. In this design, the secondary auxiliary winding of the Inductor in the Boost APFC power circuit is used to provide the chip supply voltage of the single-chip microcomputer. When the ballast control circuit detects a malfunction of the system, it goes into a standby state. The control circuit detects the operating parameters of the ballast by converting it into a voltage sampling signal and then processing it through the A/D module inside the STC 12C 5410AD single-chip microcomputer for A/D conversion. The STC 12C 5410AD single-chip microcomputer has 8 channels of 10-bit high-speed A/D conversion channels, and the speed can reach 100 kHz (100,000 times/second), which can fully meet the sequence control requirements of the ballast. In this design, the detection of the operating parameters of the ballast is mainly to detect the working environment temperature and Buck output voltage. The ambient temperature detection signal is obtained by dividing the voltage of the thermistor, and the Buck output voltage detection signal can also be obtained by dividing the voltage of the constant value resistor. . In addition, since the reference voltage of STC 12C 5410AD single-chip microcomputer during A/D conversion is its chip power supply voltage, in order to avoid the influence of power supply voltage fluctuation on the system, the power supply voltage must be detected before A/D conversion. Make the conversion result of the A/D converter consistent with the actual signal voltage value.
The full-bridge circuit in the design uses two half-bridge driving chips? IR2103 to provide two complementary square-wave driving signals for the four MOSFETs in the full-bridge inverter circuit. IR2103 is a high-voltage, high-speed, high-power MOS Transistor and IGBT driver. It has independent high and low reference level output channels. The drive design logic is very simple, and it is easy to design software program flow. It is very suitable for low-cost electronic ballasts. Each piece of IR2103 can drive a pair of upper and lower bridge arms of the full bridge respectively, and its half-bridge drive circuit structure is shown in Figure 5. The PWM control signal required by IR2103 can be provided by STC 12C 5410AD microcontroller.
4 Experimental results and analysis
According to the above analysis, the author developed a 70 W three-stage constant power metal halide lamp electronic ballast, and tested it to verify its performance. The design requirements of this electronic ballast are as follows:
Input range: 170 ~ 250 VAC / 50 Hz, under the working voltage change, the output voltage change is less than ± 2%;
Power factor PF>0.95;
Current total harmonic (THD) value <15%;
Output power: 70 ~ 76W.
By using the FLUKFA34 turtle energy quality analyzer to measure the input end of the ballast (when entering the steady state), the obtained input voltage is 220 V (AC), the active power is 0.07 kW, and the reactive power is 0.01 kVA. The apparent power is 0.07KVAR, the power factor PF is 0.98, and the current total harmonic distortion THD is 11.1%. When the AC input voltage varies between 170VAC and 250VAC, the author’s test shows that the working voltage of the metal halide lamp is basically unchanged, which can meet the design requirements.
5 Conclusion
This paper introduces a design scheme of a constant power metal halide lamp electronic ballast based on single chip microcomputer control, analyzes its hardware structure and software program flow, and develops a 70 W electronic ballast sample accordingly. The test results show that this ballast can drive metal halide lamps safely and effectively, with low cost and high reliability, and has a good market prospect.
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