The output power of the proposed dimmable ballast topology is self-regulated and insensitive to the lamp characteristics. Accurate dimming operation with rather constant filament power can be realized simply by open loop control of the switching frequency of the converter. It can provide reliable operation during lamp ignition with fault conditions or even short circuit without the need of current sensing circuits. Prototype was built for experimental verifications. For completeness, a simple ultra low dimming method is also proposed together with the theoretical dimming limit considered. © 2005 IEEE.published_or_final_versio

Chan, YP

Liu, CP

Lu, DDC

Pong, MH

Poon, NK

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Title A low cost and reliable dimmable ballast topology with inherentpower regulation and insensitivity to lamp characteristicsAuthor(s) Liu, CP; Poon, NK; Pong, MH; Chan, YP; Lu, DDCCitation Conference Proceedings - Ieee Applied Power ElectronicsConference And Exposition - Apec, 2005, v. 1, p. 596-599Issued Date 2005URL http://hdl.handle.net/10722/45829Rights©2005 IEEE. Personal use of this material is permitted. However,permission to reprint/republish this material for advertising orpromotional purposes or for creating new collective works forresale or redistribution to servers or lists, or to reuse anycopyrighted component of this work in other works must beobtained from the IEEE.A Low Cost and Reliable Dimmable Ballast Topology with Inherent Power Regulation and Insensitivity to Lamp Characteristics C.P. Liu N.K.Poon M.H.Pong Y.P.Chan D.D.C.Lu Department of Electrical & Electronic Engineering, Power Electronics Laboratory, The University of Hong Kong, Pokhlam Road, Hong Kong, China Email: cpIiu(&eee.hku.hk, nknflollLu!eee.hku.hk, mhp(f3eec.hku.hk Abstract- The output power of the proposed dimmable ballast topology is self-regulated and insensitive to the lamp characteristics. Accurate dimming operation with rather constant filament power can be realized simply by open loop control of the switching frequency of the converter. It can provide reliable operation during lamp ignition with fault conditions or even short circuit without the need of current sensing circuits. Prototype was built for experimental verifications. For completeness, a simple ultra IOW dimming method i s  also proposed together with the theoretical dimming limit considered. 1. INTRODUCTION A common way to realize high frequency electronic ballast for fluorescent lamp is shown in Fig. la. The high frequency AC voltage source can be generated by a half-bridge circuit with a series capacitor and then drives the lamp through a LC network. The lamp exhibits resistive characteristic at high frequency after ignition and a commonly used simplified LCR model is shown in Fig. l b  with the filament resistors RI, RI, R3 m d R 4  inserted. TheLC network provides several important hc t ions .  During lamp ignition, the frequency of the AC voltage source is set to the resonant frequency of the LC network. Because lamp resistance R,,, is very high and can be treated as open circuit before ignition, the LC network exhibits very low impedance to the voltage source and absorbs the energy to build up high voltage across the capacitor to ignite the lamp. To guarantee reliable operation during the ignition period, fast response protection circuit is required to close monitoring the inductor current or the capacitor voltage because if the lamp fails to turn on, the current flowing through the LC network is nearly unlimited and can build up continuously to a dangerous level that will saturate the inductor and bum out the semiconductors that generating the voltage source. Another function of the LCnetwork is to facilitate the dimming operations. One of the most frequently used methods is to control the current delivered to the lamp by control the impedance of the inductor. This can be realized by varying the frequency of the AC voltage source in the range below the resonate frequency of the LC network. The drawback is that the determination of the inductance and actual output power to the lamp depends much on the lamp characteristics. Another dimming control method is to control the phase p between the AC voltage source and the inductor current proposed in [ 11. It provides a linear dimming curve between the lamp power and p but a special controller with phase detection is required. A topology with inherent output power regulated characteristic is proposed for electronic ballast realization. It can provide reliable operation during lamp ignition with fault conditions or even short circuit without the need of current sensing and fast acting protection circuits because of the inherent power limiting features and the lamp power can be adjusted easily and accurately by open control of the switching frequency oqjhe DC link voltage. m Fig. 1 a Practical electronic ballast configuwtion c1 R1 RZ I rrm c. I Fig.1b Simplified equivalent circuit model R I  RS 11. BASIC OPERATION IDEA The basic idea of the topology is proposed in [2][3]. The power regulated feature is based on the charging and discharging characteristics of capacitor with suitable voltage clamping. The basic circuit configuration is shown in Fig. 2. Unlike conventional half-bridge circuit, CJ and C, are not intended to operate at constant voltage. I I I I I i c4 P O2 I 1 I I Fig.2a Circuit diagram with transformer parameters and output filter 0-7803-8975-11051820.00 02005 IEEE. 596 -1-1 I 1 I G4 02 Fig.Zb Power transfer when M1 is on added to increase the design freedom. The reflected equivalent model is shown in Fig. 3b. A design example is  given in this section which uses the proposed topology to drive a T8 TLD36W/54 fluorescent lamp with dimming range from 100% to 25% of full lamp power. Only switching frequency control is applied for dimming operation for simplification. From equation (l), the output power is proportional to the switching frequency when the DC link voltage and the capacitance of C, and C d  are fixed. F i g . 2 ~  Power transfer when M2 is on MI and M2 are driven on and off alternatively with duty cycle of 50%. Negligible dead time is inserted during the on and off transitions ofM, and M2 to prevent cross conduction. In Fig. 2b, MI is driven on and M2 is off, with the assumption that the initial voltage of C, and C, are Yi and zero respectively, the energy stored in CJ discharges to 2, through M, while C, will be charged up by the input voltage Vi through 2,. When C, is charged up to Vi, the voltage across C3 is zero and all the stored energy has been released to Z,. Further discharging and charging of C, and Cd is ceased even when Z, is inductive because of the present of clamping diodes D, and D,. At this time, the total energy E, delivered to 2, is equal to CF2 if C3 = C4 = C. If Z, is purely resistive, E, dissipates simultaneously to 2, just after the discharging and charging of C, and C4. If Z ,  is not purely resistive, energy will circulate among Z, C, D, and M, for a certain time t, until the energy equal to E, dissipates to the resistive part oTZ,. Similarly, when M2 is on and Ml is off which is shown in Fig.2c, the same amount of energy Ec will dissipate to 2, and completed one period of operations. If the switching frequency isfI, the total power Po delivered to Z, is given by equation ( I )  provided that the on time of M1 and M2 is greater than tc plus the charging time of CJ and C'd, Equation (1) shows that the output power is insensitive to& and can be controlled by the switching frequencyf;, the supply voltage V, or the capacitance of C, and C, and the maximum power that can delivery to the LC network during resonate startup is limited. 111. DETAILED OPERATION AND DESIGN PROCEDURES OF Dimmable ballast can be realized by replacing 2, with the LCR circuit described in section I. ' The circuit configuration is shown in Fig. 3a with a transformer T,  THE PROPOSED DIMMABLE BALLAST I 1 1 I Fig.3a Proposed dimmable ballast configuration i c 4 ?  Fig.3b Equivalent reflected circuit proposed in Fig.3a To avoid the switching frequency from entering the audible range, the switching frequency at full output power is set to be 1 10 kHz. This implies at 25% dimming operation, the switching frequency is 27.5 kHz. With Vi chosen to be 300VDC, C3 and C4 is then calculated by equation (1). 2nF is selected for C3 and C4 and the expected output power is 39.6W which is  a little bit higher than the required power for TS TLD36W/54 to account for the power loss o f  the ballast. The resonate frequency of the LC network formed by L1 and CZ' is also set to 110 kHz for resonate lamp ignition. Cz is chosen to provide sufficient heater current and if possible to set at a value such that the LCR network formed by L1, C2 and RI,, at full output power is nearly critically damped during the energy circulating period described in I1 to reduce voltage ringing. In this design example, the turn ratio of TI is 1 : 1.67 and C2 is chosen to be 2.35nF formed by two 4.7nF standard capacitor in series connection. The reflected capacitance C2' is 6.55nF and Ll is then calculated to be 300uH with resonant frequency = 113.5 kHz. Fig. 3c shows the expected waveforms of the proposed ballast at full output power 597 Fig .3~  The expected operating waveforms of the designed ballast prototype at full output power From to to tI (Ml is on and M2 is off): During this period, V, = Vi and C4 is being charged up and C, is discharging. The reflected Lamp voltage V'/,,, is given by the equation (2). The inductor current is given by equation (3). and Yc4 is given by equation (4) The turns ratio of transformer TI should be chosen such that the reflected capacitance C2' and lamp resistance R is large enough arid small enough respectively to provide enough 1, to charge up C4 to Vi well within the on period of MI.  From f., to t2 (MI is on and M2 is off) C, is charged up to Vi and DI start to conduct. The energy stored in L ,  and C2 continuously deliver to the Lamp. Since DI and MI are on, the LCR network changes to a parallel connected LCR network when the heater resislances are neglected. The energy delivery time r, depends on the Q factor of the LCR network. For Q I 0.5, almost all power can deliver to the lamp during the on time of M,. During dimming operation, the lamp resistance increases and hence increases the Q factor. This increases the energy delivery time but fortunately, the switching frequency is reduced during dimming operation, so there is much more time reserved. From f 2  to t3  (MI is on and M2 is off) All energy ha5 been delivered to the lamp. From t3  to t$ (MI is off and A42 is on) The operation is similar to the period from to t ,  and will not be discussed here. Iv. EXPERIMENTAL RESULTS A prototype was built according to the design parameters given in 111. The parameters shown in Fig. 3c were measured from the prototype. Fig. 4a, Fig. 4b and Fig. 4c shows the waveforms at full lamp power, dimmed to 50% and 25% respectively. CHI = Y,, CH2 = Vc4, CH3 = Vlomp and CH4 = I,. Table 1 shows the measured results of some important parameters. The calculated input power according to equation (1) matches with the results. In addition the filament power tends to increase and compensate the decrease in lamp current which favors the dimming operation. FigAa Maximum output power Fig.4b Dimmed to 50% Fig.4~ Dimmed to 25% 598 V. PRACTICAL CONSIDERATION Commercial dimmable ballasts may require very low dimming levels ( < I  0%). Our current demonstration shows that in order to achieve such low levels, the operating frequency or the DC link voltage of the ballast theoretically has to be very low, which causes audible problem and also the instability of the lamp. We therefore proposed a 2-dimensional dimming method which integrates with the topology described in section I1 [2]  [3] in order to achieve ultra low dimming levels to the theoretical limit of 0%. The basic idea is to introduce a duty cycle factor D, which is superimposed on the operating frequency as seen in Fig. 5. During the ON cycle period, normal switching operation continues. When OFF cycle begins, there is no switching operation, i.e. no power is delivering to the output. Input power 39.53W 19.60W 10.65W VI. CONCLUSION A topology with inherent output power regulation for implemcntation of dimining ballast is presented. The output power to the lamp can be controlled easily and accurately by controlling the switching frequency (verified experimentally), the DC link voltage or two capacitor values (verified mathematically) without the need of feedback control. The inherent power limiting features provides more reliable operation during resonant Iamp ignition even without current sensing and fast response protection circuit. The calculation of output power to lamp is simple and insensitive to the lamp characteristics. All these features can greatly simplify the design and hence lowering the cost. A ballast prototype was built and some design considerations are stated. Waveforms and important parameters are measured and matched with the cakulated results. The measured results also show that the filament power is rather constant during dimming operation which favors the lamp life. Dimming level consideration is discussed with a proposed solution to achieve ultra low dimming levels to the theoretical limit of 0%. Calculated Lamp power Lamp rms Filament Filament power Total output input power current power 39.6W 33w 0.35OArms 0.178Ams 1.301 W 34.3 02 W (fs = 1 IOKHZ) 19.8W 16W 0.160Arms 0.2 1 O A m  0.798 W 36.798W (fs = 55KHz) 9.9W 7.2W 0.065Arms D.244Arms 0.865W 8.065W (fs = 27.5KHz) i 1 = t s  1 '  I- T2 * 'f '-rI Fig3 Simplified model with corresponding switching waveform The output power is simply given by equation (5) The duty cycle of the switching circuit gives an extra dimension to control the power. A high enough frequency is selected to avoid the fluorescent lamp turns off unintentionally, i.e. the frequency of the ON period is high enough so that the lamp does not realized for some part of times there are no energy delivered to it. Input Voltage vi 300.0V 300.0V 300.0V REFERENCE Riborich, T.J.: Riburich. J.J.; "A n e w  control method for dimmable high-frequency e lectronic  ballasts'lndustry The 1998 lEEE ,Volume: 3 , 12-15 Oct. 1998 Pages2038 - 2043 vo1.3 Poon, N.K.; Pong, B.M.H.: Tse. C.K.; A c o n s t a n t - p o w e r  battery charger with inherent soft switching and p o w e r  Factor correction Power Electronics, IEEE  transaction^ on,  Volume: 18 , Issue: 6 , Nov. 2003 Pages: 1262 - 1269 M.H.Pong, W.C. Ho, N.K. Poon , "A Soft Switching Converter with Power Limiting Feature" IEE Proceedings - Electric Power Applications, January 1999, Vol.146, No.1, p.95-p.102. Terri, E.; Effect of cathode heat ing  on l a m p  life in dimming use Industry Applicalions Conference, 2001. Thirty-Sixth 1AS Annual Meeting. Conference Record of the 2001 IEEE , Volume: 2 , 3 0  Sept.4 Oct. 2001 Pages895 - 900 voL.2 A c o m p a r a t i v e  study on dimming control methods for electronic ballasts HQ, Y.K.E.; Lee, S.T.S.: Chung, H.S.-H.; Hui, S. Y.;Power Electronics, IEEE Transactions on,  Volume: 1 6 ,  Issue: 6 ,  Nov. 2001 Pages:828 - 836 Applications Conference, 1998. Thirty-Third IAS Annual Meeting. 599 

A low cost and reliable dimmable ballast topology with inherent power regulation and insensitivity to lamp characteristics

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A low cost and reliable dimmable ballast topology with inherent power regulation and insensitivity to lamp characteristics

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