Electronic metal halide ballast circuit design Buck

0 introduction

Metal halide lamp (MHL) as a green lighting source in the field of indoor and outdoor lighting has been widely used, however, when using the green lighting, and accompanying electronic metal halide ballast is the realization of green metal halide lighting.

Buck electronic ballasts for metal halide lamp circuit is an important component, mainly used in metal halide lamp for steady-state of the constant power supply. Hence, circuit design Buck on the performance of the entire electronic ballasts have very important implications.

1 metal halide lamp electronic ballast

Metal halide lamp is hid the most superior in performance of an electric light source with high efficiency, long life, excellent color rendering and other advantages, which has a broad development prospects.

Metal halide lamp of volt-ampere characteristic negative resistance, because of the negative resistance characteristics of gas-discharge lamps are not able to directly access the constant voltage power grid, so it must be equipped with ballast to work properly. As a result of working in the high frequency metal halide lamp is easy for low-frequency acoustic resonance, and square wave lighting scenarios can effectively eliminate metal halide lamps (MHL) Acoustic Resonance, so this article adopts low-frequency square wave lighting scenarios. The traditional low-frequency square wave lighting scenario is typical of low-frequency square-wave circuit, circuit diagram shown in Figure 1. In General, the first level of PFC circuit usually Boost PFC circuit, the main function is to improve the power factor, 50 Hz/220 V alternating current into direct current (in this article to 400 V DC); the second level of DC/DC buck buck circuit, the main function is to control the output voltage and current, to 400 V DC voltage down to 85 V; third-level to low-frequency full bridge inverter that converts direct current to the metal halide lamp power with low frequency square wave to avoid metal halide lamp work in DC State when the lamp life and avoiding metal in high frequency acoustic resonance phenomena occur. This three-level low frequency square-wave circuit ignition circuits are used to complete the metal halide lamp start and normal power, control and protection circuits you can achieve the coordination of the entire circuit, including metal halide lamp for startup control, current, power control and metal halide lamp protection control, etc. This article primarily discusses 70 W metal halide lamp electronic ballast circuit design for a new Buck.

2 Buck circuit design

Buck electronic ballasts for metal halide lamp circuit is the middle section, this article uses the current peak control chip UC3843 for PWM driver control, it does this by testing Buck loop current, which is tested sample resistance R both sides voltage for control of the MOS.

Before the lamp breakdown, Buck circuit output will maintain a fixed voltage values, this voltage is called no-load output voltage, its value is mainly composed of APFC output voltage. In General, no-load output voltage has two roles: first, to the ignition circuit supply input voltage and high voltage can reduce the ignition circuit transformer turns ratio; second, because no load voltage may determine the output capacitor in the metal halide lamp started before the stored energy and metal halide lamps in the breakdown, Buck converter speed of response is not sufficient to provide energy for metal halide lamp, therefore, output capacitor energy storage in this short time metal halide lamp provides energy. Buck circuit's main function is to achieve steady-state control in the metal halide lamp, constant power control. The so-called constant power control is to ensure the metal halide lamp Typ, maintain its output unchanged, in order to achieve good lighting effects.

As shown in Figure 2 is a traditional and improved Buck circuit schematics.

In the tradition of Buck circuit, MOS of the gate and the source is located in the high potential side, UC3843 output cannot directly driven MOS, and the need to implement by isolating circuit to switch tube driver, isolated circuit usually uses the optical coupling isolation and transformer isolation methods, optocoupler need to introduce another group isolated power supply, this will increase the complexity of the circuit design, and the transformer isolation will introduce electromagnetic interference, can also cause driver waveform distortion, while at the same time will significantly increase the volume of the circuit.

　　

Based on traditional Buck circuit of these shortcomings.

This article presents a new and improved the Buck circuit circuit shown in Figure 2 (b) the circuit is Buck traditional circuit of deformation. Although the circuit components location has changed, but the circuit topology unchanged, so the input and output voltage relationship changed. Figure 2 (b) the MOS transistors and inductance is placed near the end of the DC power supply, so you can direct drive UC3843 MOS, not need driver isolation circuit, thereby reducing driving waveform distortion and improve the reliability of the driver circuit, simplifies circuit, reduces circuit size, reduce costs, while, due to the Buck circuit output high potential side and Buck circuit input high voltage side is equipotential, thus reducing disturbance.

This design allows to work in Buck circuit interrupted mode, because of the intermittent mode inductance required than current continuous working mode inductance requirements, but also reduces the continued flow diode for reverse recovery loss arising.

70 w metal halide lamp for the steady-state operation, if their Buck output voltage VO 85 V, output current IO 0.824 A (85 70 W/V), Buck input voltage VIN for 400 V, MOS of pseudo-50 kHz operating frequency, MOS work cycle T 20 μ s. Then, the circuit in the critical state of inductance:

Generation data calculations available to 812 μ H L.

In order to ensure that the work in Buck circuit always intermittent mode, design time to select a value of L 650 μ H.

But MOS breakover's duty to:

Calculation, when L-650 μ H, D 0.194.

If the work in the Buck circuit 50 kHz switching frequencies,

Well, the switch breakover time surge current and the switch separates when surge voltage Buck circuit will form strong electromagnetic interference. Experimental discovery, MOSFET in switching process Burr is large, this is by switching instants circuit current rate of change of relatively large, the Burr on Buck's other circuit will have serious interference. Design time by MOS ends plus RC snubber circuit to reduce the Burr, at the same time in the design of printed circuit board, you should try to reduce the power and ground impedance. The ground wire for power cables, and other printed line with impedance changes when the supply current is large, can produce large voltage drop, and ground voltage drop is formed public impedance interference, so should be as short as possible, and should, as far as possible, the ground wire for widening the power and ground wire.

3 experimental results

These ballasts overall topology and analysis on BUCK circuit, I designed a three-level 70 W low-frequency square-wave electronic ballast, the circuit's PFC output voltage Buck for 400 V, 50 kHz operating frequency, output load 70 W metal halide lamp.

Steady when metal halide lamp, the measurement of the output voltage Buck is approximately 85 V, output voltage waveform as shown in Figure 3. Figure, the voltage waveform has smaller ripple, it and the output voltage requirements are basically the same. Figure 4 drivers for Buck circuit voltage waveform, from the diagram, its duty to 0.2, the data and the calculation of the basic agreement.

　　

　　

4 closing

This article proposes a three-wave frequency metal halide lamp electronic ballast design optimization of Buck circuit scheme that is used to direct drive UC3843 MOS, thus contributing to the metal halide lamp ignition starts with no high voltage, and ultimately the steady work of metal halide lamp constant power supply.

Unlike traditional Buck Buck than the new circuit circuit with simple structure, less used devices, low cost, high reliability.