Metal halide lamp is a third generation of electric light, because of its high luminous efficiency, long life such as led lights, play an increasingly important role in the green lighting. As an energy-saving lighting appliances, electronic ballasts, has become one of the key elements of the implementation of green lighting. Compared with the traditional magnetic ballast, electronic ballast volume and weight are greatly reduced, to improve network power quality, improved lighting quality. However, the universal application on the market today is the inductance ballast, mainly due to the simple structure, high reliability, and inexpensive. Therefore, the development of metal halide electronic ballasts higher requirements must be met to reliably start metal halide lamp; provide constant rated power; the need to eliminate the acoustic resonance phenomenon; protection function. In this paper, design for indoor lighting, small power metal halide lamp electronic ballast topology, to study the control strategy.
Start-up phase and steady-state phases using the half-bridge parallel load resonant (ParallEL Load Resonant, referred to as PLR) circuit and the half bridge dual Buck circuit. Before the start of the metal halide lamp, the arc tube filled with inert gas is not ionizing, light in a high impedance state can be approximately considered to be open. In the startup phase in Figure 1 Relay disconnect switch VQ1 and VQ2 complementary high-frequency drive signal. Figure 2 shows the PLR circuit. Parallel resonant capacitor Cp to generate high voltage and resonant inductor L (L1 and L2) in the high-frequency AC voltage, driven by resonance of the Cp at both ends.
From high frequency to low frequency, fc the closer fz, the greater the amplitude of Cp on the resonant voltage ur. The input voltage of the PLR circuit is rich in odd harmonics of the square wave voltage. Fz, the fundamental frequency of the square wave voltage is higher than the circuit is designed only to consider the fundamental and ignore the impact of high harmonics; fz, if the fundamental frequency of square wave voltage is lower than the circuit must be considered high harmonic frequency, harmonic resonance may become a major part of ur. This article 3 times ur resonance, effectively reduces the series inductance in the startup current, improve the reliability of the power circuit.
Furthermore, using the right side of the resonant peak from high to low slip frequency, the resonant circuit can be realized in the emotional area, VQ1 VQ2 the zero-voltage, reducing MOSFET switching losses. Due to the discrete nature of the parameters, the design of the resonant peak (Figure 3, curve 2) may be to the left (Figure 3, curve 3) or right (Figure 3, curve 1) offset. If you rely on to set slip frequency cut-off point to control the boot process, when the resonant peak is shifted to the left, to the slip frequency cut-off point, ur may still be unable to lamp is lit; when the resonant peak is shifted to the right, the boot process may be slippery over the resonant peak, resulting in excessive ur damage to the power switch. Therefore, by setting start voltage ug limit amplitude control the slip frequency cut-off point. When the slip frequency of the high-pressure reaches the voltage limit amplitude, high pressure, automatic stop sliding frequency.
After the lamp is lit, the light from high-impedance characteristics into the nature of the low resistance. Until the lamp arc stability, lamp power reaches the set value, in Figure 1 Relay is closed, the drive signal into a low frequency square wave signal with frequency modulation. In this case, the circuit structure and operating mode has the obvious symmetry, Buck circuit characteristics in the low-frequency switching cycle, Figure 4 shows the structure of the half bridge dual Buck circuit. Lamp voltage u0 and lamp current io are low frequency square wave signal, can effectively inhibit the light acoustic resonance phenomenon. Because the square wave uo and io resistive when the volt-ampere characteristics of low power metal halide lamp, alternating positive and negative changes in lamp power AC component, which can not stimulate the resonator.
Design of electronic ballasts FREESCALE 8-bit microcontroller for the control of the core, the lamp power closed-loop control and protection functions by detecting the lamp terminal voltage uc and dc bus current, open circuit, short circuit and abnormal state when the light side ballast automatic protection. Figure 5 shows the control block diagram. Starts, the high-frequency square-wave signal control circuits output complementary control VQ1 off VQ2 the frequency from high to low slide, uc gradually increased, the limit set uc to control the slip frequency cut-off point. When when the uc suddenly landed, and to judge the value below the set, the ionized gas in the lamp arc tube, into a single frequency transient transition; when the lamp power reaches the set value, description of the lamp has been lit, to meet the switching conditions; Relay closed circuit from parallel load resonant switch to the half bridge dual Buck circuit structure, the control circuit output with high-frequency modulation of low frequency square wave signal, and the closed-loop control of steady state power. Half Bridge Dual Buck inputs of the circuit is of 400V Ubus, sampling will be able to link current approximate control lamp power. During the transition or steady state control stage light side open or short, you can automatically block the power circuit output, enter the state of the delay protection.