COMPARISON BETWEEN LOW FREQUENCY SQUARE WAVE ELECTRONIC BALLASTS FOR HID LAMPS Márcio Almeida Có*, Márcio Brumatti*, Domingos S. L. Simonetti** and José Luiz F. Vieira** * CEFETES - Federal Technologic Education Center of Espírito Santo ** Electrical Engineering Department - Federal University of Espírito Santo Vitória – Brazil marcio.co@uol.com.br, marcio_brumatti@bol.com.br, d.simonetti@ele.ufes.br, j.vieira@ele.ufes.br Abstract – Electronic ballasts for high intensity discharge (HID) lamps that impose low frequency square waveform current seem to be a good solution to avoid the acoustic resonance phenomenon. This paper presents a comparison between four low frequency square wave electronic ballast topology. The first topology uses three converters stages: the power factor pre-regulator, the DC-DC and the low frequency inverter. The other topologies are obtained integrating theses stages. Experimental results of a three stage and a single power processing stage electronic ballast are shown for a 70W high-pressure sodium lamp. KEYWORDS HID lamps; microcontroller; ballast. I. INTRODUCTION The high intensity discharge (HID) lamps present features as high lighting efficiency, longer lifetime and good color rendering, which become them relevant in industrial light system. Due to the lamp negative dynamic impedance characteristic of the lamp, a device to limit its current should be used. Typically electromagnetic ballast is applied. However, it has high size and weight, low efficiency, poor power regulation, and presents high sensibility to voltage changes [1, 2, 3, 4]. Electronic ballast can overcome all these drawbacks. At a glance, high frequency operation (dozens up to hundreds of kHz) is the best choice for the electronic ballast inverter stage, due to the high frequency resistive behavior of lamps. Therefore, the pressure waves effect into the tube, called acoustic resonance, can happen. This phenomenon perturbs the discharge path, causing: arc bowing and snaking, flicker, changing in the color temperature, and in the worst case, the arc can be extinguished [1, 5, 6]. The acoustic resonance depends on the lamp tube geometry and dimensions, gas composition and thermodynamic conditions of the gas (temperature, pressure and density) [1, 7, 8]. Considering the presence of many manufactures in the lamp market, the tolerance in the production process and the changes in thermodynamic conditions of the lamp during lifetime, it is hard to predict the frequencies of resonance occurrence. Some solutions have been presented in the literature [1, 2, 3, 5-13], using electronic ballast to drive the HID lamps without acoustic resonance, including high frequency operation strategies. However, the low frequency operation driven the lamp with a square waveform seems to be a good solution due to its great simplicity, reliability and mainly due to the severe conditions of acoustic resonance that the low wattage HID lamps are submitted [4, 5, 13, 14]. Four topologies of low frequency square wave electronic ballast for HID lamps is discussed in the next section and are compared in this paper. II. THE LOW FREQUENCY SQUARE WAVE ELECTRONIC BALLASTS Topology A – Three Stage Solution: A low frequency square wave electronic ballast can be implemented by using three power-processing stages, as is illustrated in Fig. 1.a. The input one, called power factor pre-regulator (PFP) stage, is based on a boost converter operating in discontinuous conduction mode in order to obtain high power factor, maintaining the DC bus voltage constant. The intermediate stage is a DC-DC buck converter, operating at high frequency to control the lamp current and power. The output stage is a low frequency square wave inverter that drives the lamp [12,13,14,15]. The buck converter must be designed to operate with a ripple current less then 5% to prevent the acoustic resonance. Topology B – BIBRED: As the first alternative to reduce power stages, are mixed the PFP and DC-DC buck converter, forming the BIBRED (Boost Integrated with Buck Rectifier / Energy storage / Dc-dc converter) converter, that associated to the low frequency inverter completes the ballast circuit, as shown in Fig.1.b. The output power is controlled changing the duty cycle of S 1 switch. If the voltage on the capacitor C 0 operates in open loop, it can reaches high values during the lamp lifetime and utility line variations. Therefore, this voltage must be controlled acting on the switching frequency. The input current of the BIBRED converter must operate in discontinuous conduction mode and the output current in continuous conduction mode, as the previous topology. Topology C – DC-AC step-down converter: By combining the DC-DC buck converter with the inverter stage, and