A new control method for metal halide lamp electronic ballast

Nowadays, high-intensity gas discharge lamps have been widely used in squares, road lighting and other occasions due to their high luminous efficiency, good color temperature, long life and other advantages. Among them, metal halide lamps are considered to be one of the best artificial light sources due to their many advantages. However, due to the negative resistance characteristics and special starting requirements of metal halide lamps, they must be used together with matching ballasts. Compared with traditional inductive ballasts, electronic ballasts have many advantages, and their research and development is also a hot spot in the power electronics industry.

In order to ensure that metal halide lamps do not have acoustic resonance, electronic ballasts generally operate in a low-frequency square wave state. Traditional low-frequency square wave electronic ballasts include a three-level structure: power factor correction circuit, step-down circuit

and full-bridge inverter circuit. This structure is very complex and makes the ballast expensive. Simplifying the circuit and reducing the cost have become the focus of research today. One method is to combine the first two stages of power factor correction and buck circuit into one stage. This method can reduce the size of the ballast, but the problems that may be brought about are reduced performance of the power factor correction circuit, increased stress on the switching device and other unfavorable factors. Another method is to combine the buck circuit and the inverter circuit together. This method can reduce the size and cost of the ballast, but the control method may be relatively complicated.

The solution of combining a buck circuit and a half-bridge inverter circuit is a popular and feasible method now. Because the circuit of this method is relatively simple and the cost is the lowest. In order to promote the widespread application of this solution, simplifying its control method, overcoming the shortcomings of this circuit, and improving its reliability have become the focus of research.

This paper proposes a new control method for this circuit, and proposes a simple and effective method to overcome the current overshoot problem that occurs when the output current is commutated, thereby reducing the crest factor.

1 Circuit Description

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

1.1 Analysis of circuit working mode

The gate signals of S1 and S2 are shown in Figure 2. When S1 works in high frequency state, S2 is disconnected. At this time, the circuit is equivalent to a Buck circuit, S1 is equivalent to the main power switch, and the body diode of S2 is equivalent to the Buck diode. The working state of the two switches will change alternately at a frequency of 400Hz. In this way, a low-frequency square wave current output can be obtained to supply the lamp.

There is a reverse recovery problem when the diode is turned off, which will greatly increase the switching loss of the device. To avoid this, we make the circuit work in the current discontinuous state. Figure 3 is a state diagram of the inverter stage Buck inductor current.

When S1 works in high frequency state and S2 is disconnected, the circuit works in three working modes.

1.ll Mode 1

S1 is turned on, and the current flows through C1, S1, Rs, lamp, ignition inductor and Lc. Figure 4 is the equivalent circuit diagram of this mode. The inductance of the ignition inductor is much smaller than the inductance of the inductor L. Assuming that the voltage on the capacitor C remains unchanged in each switching cycle, the voltage VL on the inductor L is

l.1.2 Mode 2

S1 is disconnected. Since the inductor current cannot change suddenly, the current flows through Rs, lamp, ignition inductor, L, C2 and D2o. Figure 5 is the equivalent circuit diagram of this mode. The voltage of inductor L is

1.1.3 Mode 3

S1 is still in the disconnected state. However, the current of the inductor drops to zero. At this time, the capacitor C provides energy to the lamp, and the current flows through C, Rs, the lamp and the ignition inductor. Figure 6 is the equivalent circuit diagram of this working mode. Assuming that the capacitor is large enough and the voltage on the capacitor remains unchanged, we can get the output lamp voltage from equations (1) and (2):

Where: The coefficient k1 is related to the inductor current fall time, which is clearly shown in Figure 3.

When S1 is disconnected and S2 works in a high-frequency switching state, the working process of the circuit is basically consistent with the above analysis.

According to the above analysis, the half-bridge inverter circuit can be equivalent to a Buck step-down circuit in each half low-frequency cycle. Therefore, a half-bridge inverter circuit can provide a suitable T operating voltage to the lamp and enable the lamp to operate in a low-frequency square wave state.

1.2 Closed-loop control

Because of the negative resistance of metal halide lamps, we control the output of the ballast to present the characteristics of a current source to ensure the stable operation of the lamp. Figure 7 is a block diagram of the control method of the half-bridge circuit. The lamp current is sampled through Rs. The voltage on the sampling resistor Rs is a low-frequency square wave signal. First, the signal is amplified by the operational amplifier l, and then the AC square wave signal is rectified into DC through a rectifier circuit, and then PI regulation is performed after RC filtering, and the regulated output is a constant current source. It can be seen that this control method is equivalent to the DC/DC control method, which is simple and effective.

1.3 Current Overshoot

However, this circuit has a serious problem that there will be a large current overshoot at each output current switching moment. Such a peak current will greatly reduce the service life of the lamp. This article proposes a simple and effective method to overcome this problem.

Figure 8 is a circuit for generating a half-bridge drive gate signal, in which a D flip-flop, two AND gates and two NOR gates are combined to reduce the duty cycle of the drive signal. This function is controlled by the control pin (CP). When CP is high, the duty cycle of the drive can be reduced to about half of the original. In this way, we can give CP a high level at the moment of each output current commutation, reduce the duty cycle of the drive, and greatly reduce or even eliminate the overshoot current. Figure 9 is a simple CP control signal generation circuit. Figure 10 is a timing diagram of the CP pin signal and the low-frequency inverter signal.

2 Experimental results

A prototype of a 70W metal halide lamp electronic ballast using the above control method has been completed. The main parameters of the circuit are as follows.

L="380" μH;
C=1μF;
Vo="200V";
fh="100" kHz;
f1=400Hz.

Where: fh is the high and low frequencies driven by the half-bridge circuit;

f1 is the low frequency driven by the half-bridge circuit.

Figure 11 shows the change of the drive signal when the output current is commutated. Figure 12 shows the change of the inductor current when the output current is commutated. It can be seen that the current overshoot is quite small. Figure 13 shows the ignition pulse. Since the ignition is carried out at a low frequency, the ignition success rate is very high. Figure 14 shows the current and voltage waveforms when the lamp is working normally. It can be seen that there is basically no current overshoot in the lamp current. The overall efficiency is about 90%, and the crest factor is 1.1.

3 Conclusion

This paper proposes a new control method for two-level low-frequency square wave metal halide lamp electronic ballast. It adjusts the output to a constant current source to ensure the stable operation of the lamp. At the same time, it overcomes the current overshoot problem in the circuit. Compared with the traditional three-level low-frequency square wave electronic ballast, this circuit and control method are simple and effective.