Full Bridge Single Stage Electronic Ballast for a 250 W High Pressure Sodium Lamp

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1 Full Bridge Single Stage Electronic Ballast for a 50 W High Pressure Sodium am Abstract In this aer will be reorted the study and imlementation of a single stage High Power Factor (HPF) electronic ballast for High Pressure Sodium (HPS) lams using a CC filter. In the recent years many authors are working to obtain single stage HPF electronic ballast for fluorescent lams [][]. Normally to obtain HPF in electronic ballast for high ressure sodium lams a Power Factor Preregulator (PFP) is used between the mains and the electronic ballast [3]. The main idea in this work is to resent a simle electronic ballast with HPF for HPS lams. Design criteria and exerimental results will be also resented in the final aer. KEYWODS Electronic Ballast, HID ams, High Power Factor. I. INTODUCTION Nowadays, an imortant toic of awareness is the imortance of environment reservation. In this direction, imortant efforts have been made in the diverse areas of knowledge. In electrical engineering field, this henomenon has reflected in searching for alternatives energy systems, higher efficiency on available resources utilization, losses reduction in equiments and to increase electric energy quality. In the last few years the market was flooded by a great number of electronic ballasts for fluorescent lams oerating in high frequency, esecially by comact fluorescent lams. Its utilization was widely stimulated by Brazilian media for energy economy, due the fact that luminous efficiency increases with the frequency for this kind of lam. Brazil faced a serious energy crisis in 00. Many corrective actions were taken to mitigate this serious roblem. One of them was the energy rationing which consisted in overtaxing or even cutting energy suly from consumers which exceeds the refixed energy quotes. Also many electric energy concessionaires had distributed gratuitously comact fluorescent lams for residential consumers, showing the imortance of illumination s segment inside the global energy consumtion, estimated to be about thirty ercent of total consumtion of electrical energy in the country. Because of these, innumerable research grous around the world, like [], [], [3] and [4], have dedicated their efforts to the develoment of new toologies and new control techniques for different kinds of discharge lams. Most of magnetic ballast manufacturers had to develo electronic ballasts for discharge lams to guarantee their survival in business because the consumers started to demand more and more this tye of roduct. It also simlifies the roduction line, which has exressive hysical reduction and roductivity increase in relation the line that roduces the conventional ballasts. Now, the challenges for industries are the reduction of roduction costs, the reduction of converter size, unitary ower factor and null harmonic distortion which imlies in a substantial imrovement of energy quality consumed by ballasts. Here in Brazil, the develoment of electronic ballasts for HID lams is being made by a few grous of researchers. However in a close future, these ballasts will be in the roduction lines of main national manufacturers. The oroise of this aer is to reort the

2 develoment of a low cost single stage HPF electronic ballast for HPS lams. The design criteria will be resented in this work for the roosed circuit. There are many kind of high-ressure lams; however, this work will focus only the high-ressure sodium lams (HPS), widely used in ublic illumination. The HPS lams radiate energy on a great art of the visible sectrum [5]. These lams rovide a reasonable color reroduction (it has IC 3 color reroduction index). They are available u to 30 lm/w of luminous efficiency and color temerature of 00 K, aroximately. The HPS lams, as any other HID lams, need ballast to oerate correctly. The ballast is additional equiment connected between the ower line and the discharge lam. The ballast has two main functions: to guarantee lams ignition through the alication of a high voltage ulse between the lam electrodes and to limit the current that will circulate through it. The lam would be quickly destroyed without current limitation, due the negative resistance characteristic of the lam, as can be observed in figure. The HPS lams have many articularities when they oerate in high frequency, such as: Can be modeled by a resistance in steady state; Can have luminous intensity controlled; The sectrum color reroduction can be modified; Presents the acoustic resonance henomenon, which can result in the arc extinguishing until the lam destruction; am Current Positive esistance Negative esistance am oltage Breakdown oltage Fig.. - Tyical voltage x current curve for HID lams. In order to obtain low cost electronic ballast for HPS lams with HPF a single stage converter was conceived. The idea is very simle: Once, in high frequency, the HPS lams have a resistive behavior why the electronic ballast (full-bridge inverter and CC filter) can not be connected directly to a full bridge rectifier? This idea will be studied in this aer. II. STUDIED EECTONIC BAAST The studied single stage high ower factor electronic ballast for high ressure sodium lams structure incororates a bridge rectifier and an inut C filter to minimize the EMI generated by the electronic ballast. Figure shows an electrical diagram of the roosed circuit. Similar circuits have been roosed by other authors using fluorescent lams. but any aer using this toology for HPS lams was not found. The caacitor C F in this figure has two main functions first of all is to receive the reactive current from the electronic ballast and work as line filter with the inductor. This arrangement rovides high ower factor to the electronic ballast because in this case the caacitor C F is not a bulk caacitor. Actually this is a small caacitor in the range of nano Faradays.

3 and C is null just before the lam is turned on. This oeration mode will result in the MOSFET s and driver s destruction. e C Fig.. Studied HPF Electronic Ballast. Fig. 3. CC Ballast. III. BAAST DESIGN CITEIA To verify the erformance of the roosed system a CC electronic ballast (figure 3) for a 50 W HPS lam was designed. The nominal lam voltage ( lam ) was obtained from the lam s manufacturer datasheet and its value is 00 MS. To design the CC ballast it was added 0 % to consider loses effect. The electronic ballast inut ower voltage comes from the outut of an inut bridge rectifier; consequently, this inut voltage is mains deendent. In the resent design examle the mains voltage adoted was mains 0 MS. The switching frequency chosen was 68 khz. Assuming the resistive comortment of the lam, we can estimate the value of its resistance () after ignition using equation. lam 40Ω () P Where P is the lams ower. As it was indicated in [3], the best relationshi between the switching frequency and the resonance frequency before the lam turn on is ω 0 / ω s 3, guaranteeing the high voltage generation for the lam ignition and limiting the eak current at the MOSFET to accetable levels. If it was adoted to work at resonance ω 0 ω s in theory we would have the ossibility of an infinite voltage generation over the lam which could be good for a quickly lam turn on. On the other hand current would also rise to infinite because the imedance of the circuit formed by, C s For the circuit showed in figure 3, considering the voltage e a symmetrical wave (from k sin (ωt) to - k sin (ωt) ), a good simlification to study the system behavior comes from the frequency domain aroach. To use this aroach the first harmonic comonent for this wave must be knew. Bum & Hee [] resented the first harmonic eak amlitude for a half bridge inverter considering an ideal fixed DC bus voltage (E) the result for a full bridge is dislayed in equation. In the resent case, the first harmonic eak amlitude was obtained using the same exression but the DC bus voltage (E) was relaced by the mains MS voltage ( mains ) resulting in exression 3. The exerimental results validate this rocedure: stms stms 3 3π + mains 6 3π + mains () (3) In this study an exression was obtained to determine the eak voltage across the caacitor C. This exression, shown in equation 9, is valid before the lam start u. E c ES 4 F e (4) Where, ES is the circuit equivalent series resistance and F is the switching frequency. Preliminary tests demonstrated that necessary eak 3

4 voltage ( ok ) to guarantee the lam ignition is 3.8 k. A tyical ES value is 6.5 Ω. Maniulating equation (4) we can obtain the value of inductor in equation (5). ES 4 F 9.5µ H E ln ok (5) The resonance frequency may be calculated using equation (6). Fo π + C C s (6) Considering the fact that the switching frequency is estimated to be three times lesser then the resonance frequency and, usually, caacitor C is, at least, 0 times smaller then caacitor C s, equation 6 may be simlified into equation 7, because the effect of the caacitance C s is almost null. F (7) π C 6 Maniulating equation (7), it can be obtained the value for the caacitor C as it is shown in equation (8). C,767nF ( 6 π F) (8) To determinate the real value of the ES, an exerimental circuit using a 0 µh inductor and a,7 nf caacitor C was stimulated with a 60 eakto-eak square wave signal, which generated a 660 signal over the lam terminals, allowing the determination of ES using equation 4. This ES was obtained exerimentally and its value was 6.5 Ω. Before the lam startu a leakage current flows into the lam. To determine the equivalent lam resistance before the startu, the following measurement was made: a 0 Ω resistor was laced in series with the lam. The obtained equivalent lam resistance was 00 kω. If this resistance is taken to account a new ES 5.7 Ω could be easily obtained. The reference [] and our exerimental results allow us to consider that after lams ignition, the lam resistance is too low considering the C reactance. Therefore, it can be deduced the equation 9: // (9) ω C P Consequently, after lam ignition, the equivalent circuit is showed in figure 4. e Fig. 4. Ballast equivalent circuit after ignition. After lam ignition the ballast must guaranty that MS voltage over the lam do not overcome the nominal value. The MS lam voltage lam can be obtained using the well known voltage divider for the circuit shown in figure 4, the equation 0 resents this result: lam (0) m Z The modulus of the imedance of the circuit can be calculated with equation. To facilitate the design of the CC filter the arameterized C s circuit transfer function was obtained and the result is shown in figure 3. lam m ( κη, ) κτ τ + + η κη () Where η is the caacitor relationshi factor defined as η C s /C, κ the relationshi of switching frequency and resonance frequency of the circuit of figure 4, is the lam resistance after startu and τ is the arameterized time constant τ C Figure 5 resents the relationshi between the MS lam voltage and the MS first harmonic voltage,. 4

5 lam / stms, called as arameterized lam voltage, for different values of η as design arameter. Using the grahic of figure 5, a η 80 was adoted. This relationshi will allow us to achieve the desired relationshi in the startu frequency, when the lam is turned on. ( 0, κ) 80 (, κ) (, κ) 30 (, κ) (, κ) Jκ (, ) 0.4 i( κ) ( 80, κ) κ Fig. 5. Transfer Function varying κ for different values of η. With the η relationshi, the value of C s may be obtained using equation for a relationshi. η 80 η C 498nF C () 0 0 Fig. 6. Above, voltage in the lam and voltage and current in the mains below. I. SIMUATION ESUTS Fig. 7. oltage and current in the lam. To validate the roosed system, a full-bridge electronic ballast with the following secification: 50 W HPS lam, 0 AC grid connected and oeration frequency of 68 khz, was simulated using the software PSIM 6.0. Figure 6 shows the voltage and current in the mains. The voltage and current in lam is showed in figure 7. The crest factor was measured. Tests indicate that ballasts with higher crest factors may result in dereciation of lumen outut or reduced lam life. It was found a crest factor of using this ballast. HID lam recommendations suggest a maximum crest factor of.8 for HPS lams.. CONCUSION This aer described single stage high ower factor electronic ballast for high ressure sodium lams. This ballast resents a very low cost because it avoids an external PFP. The henomenon of the acoustic resonance was not observed. A very high ower factor was obtained. The crest factor found was not good enough and must be imroved. This could be solved changing the caacitor C s value. Increasing this caacitor is ossible to reduce the crest factor, on the other hand, the ower factor decreases. 5