Magnetically-controlled dimming techniquewith isolated output

M.S. Perdigao, J.M. Alonso and E.S. Saraiva

New advances on magnetically-controlled electronic ballasts can grantthe ability to dim the lamp and simultaneously guarantee output elec-trical isolation. By means of a variable transformer, the model of whichis presented, this new technique is developed and experimentally ver-ified. In addition, a simple method for the large signal characterisationof the transformer is described. An FH 14W/840 Osram lamp waschosen to validate this new technique and experimental resultsclearly show its adequate behaviour.

Introduction: The recent development of magnetically-controlled elec-tronic ballasts represents, for the authors, a scientific challenge whichresulted in some experimental outcomes for fluorescent lightingsystem applications [1, 2]. A new step forward in this type of magneticcontrol led to the inclusion of a variable transformer in the classic elec-tronic ballast circuit. The half-bridge inverter connected to a parallelloaded resonant tank, presented in Fig. 1, is the most common topologyused as a high-frequency inverter for electronic ballasts. The DC-block-ing capacitance CB must be high enough so that its AC voltage ripple isnegligible, avoiding DC current flowing through the lamp. The lamp isnow connected to the secondary side of the variable transformer.According to Fig. 1, this transformer presents a DC control windingand the secondary voltage can be adjusted according to the DC polaris-ing current level in this control winding. Also, this device allows controlof the equivalent series inductance of the transformer, thereby grantingthe ability to dim the lamp. The main purpose is to use this controlmethod in applications where electrical isolation is mandatory.

DC currentsource

controlwinding

N1 N2

lamp

C

CB LA

D2

D1

B

M2

M1

VDC

IDC

Fig. 1 Magnetically-controlled electronic ballast: half-bridge resonantinverter with variable transformer

Currently, CCFLs are intensively used in LCD backlighting systems.For safety purposes, these applications are always dependent on theinclusion of an isolation transformer between the inverter and thecircuit which directly supplies the lamp. The most common dimmingmethod is called burst dimming, where the CCFL is driven betweenno current and rated current, thereby causing surge currents and hencereducing lamp life [3]. Other methods are similar to the most commonones; duty-ratio control, frequency and voltage control. This applicationcan represent a breakthrough in this field of research since it presentssimilar advantages to those of the variable inductor control concept,additionally yielding electrical isolation. In this Letter, a detaileddescription of this technique is presented, as well as some experimentalresults for a T5–14W Osram fluorescent lamp.

Variable transformer: The physical structure of the variable transformeris illustrated in Fig. 2. This structure is inspired in the analysis presentedin [4] and [5] and consists of two E-E cores, side by side. The controlwindings are wound around one of the outer legs of each core separatelythen serially connected in opposite polarity and placed side by side. Theprimary winding is wound around the centre legs of both cores and thesecondary winding around the unused outer legs of both cores, whichincludes a small air gap. Connecting the inverter output voltage to theprimary terminals of the transformer, the AC flux generated by theprimary winding will flow through its corresponding legs and thendivide between the left and right paths according to the reluctance ofeach path. A DC current circulating through the control windingsvaries the DC flux density in the left half of the structure, therefore, con-sidering zero DC current, the path on the left will have a much lowerreluctance because of the air gap introduced in the right path, thuslittle AC voltage will be induced in the secondary winding. If the DC

ELECTRONICS LETTERS 2nd July 2009 Vol. 45 N

current is large enough to push the operating point towards saturation,the reluctance of the path to the left of the primary increases relativelyto the reluctance of the path of the right, thereby controlling theamount of AC flux coupled with the secondary winding.

left view right view

C1

C1

C2

C2

P1P1

P2P2

S1

S1

S2

S2

Fig. 2 Variable transformer structure

Variable transformer modelling: The theoretical analysis of the deviceis a non-trivial task, so in order to have prior notion of its behaviour anequivalent circuit model based upon the actual physical structure of thedevice represents a practical approach. In [4], the reluctance concepttogether with the duality theory is used to generate, first, the magneticequivalent circuit and then, the inductance circuit model which is pre-sented in Fig. 3a. The values of the inductances can be calculatedusing the typical equation which relates the number of turns with thereluctance of the magnetic path, Li ¼ N 2/<i. All the models’inductances are fixed except for Ll1, which is proportional to the per-meability of the control legs and thus will decrease withincreasing control current. An experimental prototype was built usingtwo EF25 cores, N1 ¼ N2 ¼ 100 turns, and for each control windingNc ¼ 80 turns. After some experimental observations, from the datameasured at the terminals of the device, Lm was considered to belarge enough and so could be neglected. The simplified equivalentmodel is now presented in Fig. 3b, where Leq represents the seriesequivalent transformer inductance referred to the primary side. Thisimplies that the addition of an external resonant inductor may beunnecessary.

Lm

LI1 LI2 Leq

N1

a b

N2 N1 N2

Fig. 3 Proposed inductance model and simplified equivalent circuit

The large signal characterisation of the transformer was done usingthe same resonant inverter, supplying a resistance, R ¼ 165 V, placedin the transformer secondary and employing a resonant circuitformed by VDC ¼ 150 V, f ¼ 0.8 KHz, L ¼ 0.8 mH and c ¼ 6.8 nF.Experimental observation showed that the voltage on the secondarylagged the voltage on the primary, which was not the case for theprimary and secondary currents, which remained in phase. Togetherwith the primary impedance, the phase angle allowed estimation ofLeq. The obtained results are presented in Fig. 4. It can be seen thatfor the specified DC control current range, the transformer voltageratio, rt, is approximately one.

0.55

l eq, m

H

volta

ge r

atio

(r t)

0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05IDC, A

equivalent seriesinductance

with resistance

transformer voltage ratio

1.20

1.40

1.60

1.80

2.00

2.20

2.40

0.568

0.757

0.946

1.136

Fig. 4 Experimental value of magnetic regulator against DC current

Experimental results: Fig. 5 shows the obtained experimental results forthe laboratory prototype, for the FH 14W/840 Osram lamp. The

o. 14

measurements performed were the average power in the arc, in the lamp,which means arc power plus electrode power, and at the input of theballast. The electrode power is obtained by subtracting arc power fromthe total lamp power. The efficiency was calculated as the ratiobetween the average lamp power and the input power. These measure-ments are presented in Fig. 5. High efficiency and low electrodepower are both guaranteed with this technique. Fig. 6 presents the wave-forms of the lamp voltage, lamp current and currents in the primary andsecondary sides. These last two waveforms confirm the 1:1 transformerratio.

18

02468

10121416

pow

er, W

IDC, A

Pin

Plamp

Parc

Pelectr

a

effic

ienc

y, %

0.6540

50

60

70

80

90

100

0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05IDC, A

b

Fig. 5 Experimental results

a Input power, average lamp power, arc power and electrode powerb Converter efficiency dimming

current in thesecondary

current in theprimary

=trace1=

CH1=200mVDC 1:1

CH2=200mVDC 10:1

5us/div

NORM:100MS/s

CH1=100VDC 100:1

CH2=500mVDC 10:1

Math1×2

5us/div

NRM:100MS/s

Rms 229.5mV Freq 84.03 kHz=trace2= Rms 211.3mV

=trace1= Rms 91.47V Freq 84.75 kHz

a

b

=trace2= Rms 143.4mV

lamp voltage lamp current

Fig. 6 Experimental waveforms at nominal power

a Lamp voltage, lamp current, voltage 100 V/div, current 0.5 A/divb Current in primary side (ch1) and in secondary side (ch2) (current 0.2 A/div.Horizontal scale: 5 ms/div)

ELECTRO

Conclusion: A new dimming technique using a variable transformer ispresented. This application is experimentally verified for an FH 14W/840 Osram lamp. The obtained results demonstrate clearly the viabilityof using this type of device in applications where isolated output require-ments are mandatory.

Acknowledgments: This work was supported by the Government of thePortuguese Republic, FCT-MCTES, under research grant numberPTDC/EEA-ENE/66859/2006, and by the Government of theSpanish Kingdom, Science and Innovation Office, under researchgrant number DPI-2007-61267.

# The Institution of Engineering and Technology 200912 January 2009doi: 10.1049/el.2009.0098

M.S. Perdigao and E.S. Saraiva (Instituto de Telecomunicacoes, DEE,University of Coimbra Pole II, Coimbra 3030-290, Portugal)

J.M. Alonso (Universidad de Oviedo, Electrical EngineeringDepartment, Campus de Viesques, Edificio 3, Gijon 33204, Spain)

E-mail: marcos@uniovi.es

References

1 Perdigao, M.S., Saraiva, E.S., Alonso, J.M., and Dalla Costa, M.A.:‘Comparative analysis and experiments of resonant tanks formagnetically-controlled electronic ballasts’, IEEE Trans. Ind. Electron.,2008, 55, (9), pp. 3201–3211

2 Perdigao, M.S., Alonso, J.M., Dalla Costa, M.A., and Saraiva, E.S.:‘Optimization of universal ballasts through magnetic regulators’, IEEETrans. Power Electron, 2008, 23, (6), pp. 1214–1220

3 Hwu, K.I., and Chen, Y.H.: ‘A novel dimming technique for cold cathodefluorescent lamp’. Int. Conf. on Power Electronics and Drive Systems(PEDS), 2007, pp. 1085–1090

4 Vollin, J., Dong Tang, F., and Cuk, S.M.: ‘Magnetic RegulatorModeling’. IEEE Applied Power Electronics Conf., (APEC), 1993,pp. 604–611

5 Washburn, R.D., and McClanahan, R.F.: ‘Non-saturating magneticamplifier controller’. U. S. Patent 4841428, June 1989

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