Design of New Electronic Ballast

1 Electronic ballast and related issues

The load of the electronic ballast is a special load, which requires the electronic ballast to be able to provide a stable current for the load when it is working. Because it has the advantages of energy saving, high efficiency, saving metal materials (copper, silicon steel), and has high economic and social benefits, it has attracted widespread attention from all walks of life.

Electronic ballasts use high-frequency switching conversion technology and can be made very small. However, due to high-frequency switching and rectification operation, they have disadvantages such as large source-side harmonic distortion and severe electromagnetic radiation interference. When a large number of electronic ballasts work at the same time, high-frequency harmonics will cause the power supply neutral line to deviate seriously from zero potential, while introducing extremely high peak power supply current, seriously interfering with the power supply quality of the power supply system, and even causing significant economic losses.

2 Development of electronic ballast

From a time perspective, electronic ballasts have gone through the following stages:

The first stage was from the mid-1980s to the early 1990s. During this period, power electronics technology developed from low frequency to high frequency, APFC (active power factor correction) also started, and the advantages and disadvantages of electronic ballasts began to emerge. The main characteristics of electronic ballasts in this stage are:

(1) The input end of the ballast uses uncontrolled rectification and large capacitor (or no capacitor) filtering, and the input current waveform is severely distorted. When used in large quantities, it will cause the neutral line current to increase. In severe cases, it will cause a large amount of damage to the ballast and even cause a fire.

(2) The "current-following" passive filtering technology is used to make PF>0.9 and THD<30%. However, the crest factor of the 9th harmonic Cf≈2, which exceeds the standard. Some people call the electronic ballast that adopts this "current-following" passive filtering technology the second-generation electronic ballast.

The second stage was from the early 1990s to the mid-1990s. During this period, APFC technology had matured and relevant dedicated integrated chips were introduced. The electronic ballast circuit mainly adopts two-stage power conversion, the first stage adopts APFC (commonly used BOOST type PFC circuit), and the second stage adopts power DC/AC inverter. People often call electronic ballasts using this technology third-generation electronic ballasts. Because it uses PFC technology, the PF can reach 0.99, and the THD and harmonic indicators can meet the requirements. However, this electronic ballast adopts two-stage high-frequency power conversion, so the overall efficiency is 80% to 90%, or even lower. At the same time, the circuit is complex and the cost is high, so it is difficult to promote it on a large scale for a while.

The third stage is the typical circuit using single-stage multifunctional electronic ballast, the main development directions are:

(1) The high-frequency energy feedback charge pump circuit proposed by VEPC in the United States has the following main indicators: PF>0.995, THD<5%, and Cf<1.6.

(2) Single-tube electronic ballast proposed by CUK et al.

This article mainly discusses two types of electronic ballasts using high-frequency energy feedback technology and their working principles.

3 Main parameters of electronic ballast

There are many indicators for evaluating electronic ballasts. Here are some commonly used indicators:

(1) The power factor on the input side is generally required to be PF>0.9;

(2) The total harmonic distortion THD of the input current is less than 20% to 30%, and the third harmonic component and the fundamental component (I3/I1) are required to be less than 17%;

(3) Transient overvoltage protection: Sometimes, transient pulse voltages with high amplitude (e.g., about 1 kV) may appear in the power grid. However, due to cost constraints, the parameter margin of power conversion devices of electronic ballasts is not large. In order to ensure reliable operation of electronic ballasts, such high-amplitude transient interference should be suppressed;

(4) Soft start function When the electronic ballast is turned on, the starting current is much larger than the normal working current, which reduces the working life of the load. Therefore, the electronic ballast should be equipped with a soft start circuit to reduce the starting current;

(5) No-load voltage When the electronic ballast starts working, the load is not working yet. At this time, the output voltage of the electronic ballast cannot be too high or too low. If it is too low, the lamp tube cannot ignite; if it is too high, the service life of the lamp tube will be reduced.

(6) Load-triggered starting methods There are two commonly used load-triggered starting methods:

①Use the positive temperature coefficient thermistor method. Due to the thermal inertia of the thermistor, the load working characteristics will be adversely affected when the load is working in a steady state;

② Instant trigger start: Since the load has no preheating process, directly applying a higher trigger voltage will reduce the service life of the lamp cathode;

(7) The current crest factor Cf of the electronic ballast is required to be Cf<1.7. Too large a factor will reduce the load life.

(8) Efficiency η: Since the advantage of electronic ballasts over ordinary ballasts is their high efficiency, improving efficiency will achieve energy saving effects;

(9) Temperature rise: Since magnetic components are used in self-oscillating electronic ballasts or electronic ballasts using APFC technology, the magnetic permeability of magnetic components is related to temperature. In practice, magnetic materials with high Bs values ​​should be used, and magnetic components with low temperature coefficients should be selected, especially oscillation coil magnetic materials;

(10) Stress parameters Due to the existence of parasitic parameters of high-frequency power converters, high-frequency oscillation occurs when power switching devices are working. This generates large dv/dt and di/dt, which are called dynamic voltage and current stress. Excessive dynamic stress can cause damage to power conversion devices. Soft switching technology can be used to reduce the dynamic stress value.

4Electronic ballasts using passive filtering technology and active filtering technology

4.1 Electronic ballast circuit using charge pump passive filtering technology

The working block diagram of the circuit is shown in Figure 1.

Figure 1 Electronic ballast circuit using passive filtering

The output and input characteristics of the circuit are shown in Table 1 and Table 2 respectively.

Table 1 Output characteristics

Table 2 Input characteristics

The voltage and current waveforms on the power supply side are shown in Figure 2.

Figure 2 Voltage and current waveforms on the power supply side

Figure 3 Electronic ballast circuit using passive PFC

Figure 4 Voltage and current waveforms on the power supply side

As can be seen from Figures 1 and 2 and Tables 1 and 2, the performance indicators of the circuit have been taken into account and improved. The PF value is greater than 0.95, and the THD value is about 25%, but the current conduction angle still has a certain dead zone. The crest factor Cf has been greatly improved, ranging from 1.70 to 1.80. Because this circuit is simple and low in cost, it is very practical.

4.2 Electronic ballast with passive power factor correction using energy feedback

The working principle of the circuit is shown in Figure 3.

The voltage and current waveforms on the power supply side are shown in Figure 4.

The circuit output characteristics are shown in Table 3.

Table 3 Output characteristics of energy feedback electronic ballast

The circuit input characteristics are shown in Table 4.

From the data in Figures 3 and 4 and Tables 3 and 4, it can be seen that its input current conduction angle is 180°, there is no dead zone, and the voltage and current are basically in phase. PF>0.97, current harmonics are around 18%, and Cf is around 1.6. Since the circuit is simple and the operation is relatively reliable, it is more suitable for electronic ballast circuits.

Figure 5 Dual-pump electronic ballast circuit

Table 4 Input characteristics of energy feedback electronic ballast

Figure 6 High frequency pump electronic ballast circuit

Table 5 Comprehensive comparison of electronic ballasts with different filter circuits

5. Comparison of dual pump and high frequency pump electronic ballast circuits and experimental results

The following introduces the passive filtering electronic ballast circuit and experimental results using dual pump type (bidirectional self-supplied auxiliary power supply type) and high frequency pump (high frequency energy feedback).

Figure 5 is a working principle diagram of a dual-pump electronic ballast. Capacitors C1, C2, electrolytic capacitors C3, C4 and diodes V1-V4 form a positive and negative bidirectional auxiliary charge pump. Within a short period of time after the electronic ballast is powered on, V1-V4 and C3, C4 form a parasitic diode and capacitor filter loop to make the circuit work. After the lamp is lit, part of the high-frequency current returns to the power supply through C1 and C2, and the other part is rectified by V1 and V2 and filtered by C3 and C4 to form positive and negative auxiliary voltages ±△U, which are superimposed on the 100Hz pulsating DC voltage after bridge rectification through V3 and V4 to form a power supply with a relatively small peak ratio Cf to power the power conversion stage. By properly selecting the value of the capacitor, the peak ratio Cf of the current can be <1.7, while keeping the PF and THD values ​​in a relatively ideal range. After using this method for a typical 220V/36W electronic ballast, we can get: PF="0.945," THD="32％", third harmonic content HD3="0.28," Cf="1.61," η="0.93," f="24.5kHz." Since the high-frequency current of this electronic ballast is recycled, the circuit has a high operating efficiency. Since the lamp current Cf value is small, the luminous efficiency is also relatively high. The source voltage range is 160～270VAC. The entire circuit has no special requirements for components.

Figure 6 is a schematic diagram of the high-frequency pump electronic ballast circuit using passive filtering technology. Capacitors C1, C2, diodes V1, V2 form a high-frequency pump feedback loop. When the electronic ballast is working, the high-frequency current passes through capacitors C1, C2 and V1, V2 to form positive and negative loops. In the positive half cycle, V2 charges C0 to fill the valley, the peak-to-peak ratio becomes smaller, and the negative peak wave returns to the power supply from V1. For the rectifier, a negative resistance state is generated at a high level, thereby increasing the PF value. Appropriate selection of component parameters can improve the efficiency of the circuit. The typical parameters of a 220V/36W electronic ballast using this method are: PF=0.994, THD=0.068, HD3=0.051, Cf=1.62, η=0.89, f=25.3kHz.

6 Conclusion

The comprehensive comparison results of electronic ballasts using different filtering circuits are shown in Table 5.

As can be seen from Table 5, the electronic ballast using APFC has the best overall index, but its cost is relatively high, and the circuit is relatively complex, so it is difficult to popularize for a while. The high-frequency pump and double-pump electronic ballast circuits have better overall indexes, and the third harmonic content HD3: the double-pump type is 0.2-0.3, while the high-frequency pump type is less than 0.1, and the active filter type is less than 0.1. Since the use of double-pump and high-frequency pump energy feedback circuits only adds a few passive components, the cost is low, so it is more practical. Of course, to improve the overall performance, working reliability and other indicators of the electronic ballast, it is also related to the power conversion circuit form (such as resonant soft switch) and the quality of components.