DRIVER FOR HIGH EFFICIENCY LED BASED ON
`FLYBACK STAGE WITH CURRENT MODE
`CONTROL FOR EMERGENCY LIGHTING SYSTEM
`
`
` M. Rico-Secades, A.J. Calleja, J. Cardesín, J. Ribas, E.L. Corominas, J.M. Alonso, J. García.
`
`
`
`Universidad de Oviedo
`UNIOVI-GEI Group
`Campus de Viesques - Edificio 3
`ES-33204 GIJON-SPAIN
`Phone +34-985182087
`Fax: +34-985182138
`mail: manuel@ate.uniovi.es
`
`
`
`Abstract—Nowadays, Permanent Emergency Lighting
`Systems (PELS) are widely used in many applications:
`Emergency exit indication, lighting in critical or strategic
`points. Limitation in operation hours in classical lamps
`(10.000-20.000 hours for fluorescent lamps) implies short
`lamp replacement times and, therefore, high maintenance
`costs.
`
`This paper shows a driver based on a flyback circuit for a
`PELS system. Control in current mode operation assure
`current constant in LED´s in any battery condition. The
`long operation life (above 100.000 hours) of high efficiency
`LED´s with a very simple electronics circuitry implies an
`interesting solution for this type of applications. A 30
`lumens and 1 hour PELS has been built and tested.
`
`Keyword: Lighting, Emergency lighting systems, LED
`
`I.
`
`INTRODUCTION
`
`
`LED´s are widely used in low light level applications. The
`introduction of new materials and manufacturing technologies
`allow us to use high efficiency diodes for lighting applications
`with efficacies (Lm/W) above incandescent lamp and growing
`to fluorescent efficacy levels. For instance, in this paper a
`White LED of 18 Lm/W from LUMILEDS has been used.
`
`Operation life of high efficiency LED´s is one of the main
`advantages in lighting applications, for instance LUMILEDS
`LED used in this paper after 100.000 hour of operation
`reduces its efficiency in a 40%.
`
`Additional advantages are the wide range of temperature
`operation for -20oC to 120oC and the simplicity of the supply
`system: No starting circuit is required and work with low and
`safe voltages. Prototypes built in this way are suitable for
`operation in open areas and freezing units.
`
`MAINS FAILURE
`MAINS FAILURE
`
`MAINS
`MAINS
`
`+
`++
`
`BATTERY
`BATTERY
`CHARGER
`CHARGER
`
`ON
`ON
`LAMP
`LAMP
`SUPPLY
`SUPPLY
`(FROM
`(FROM
`BATTERY)
`BATTERY)
`
`LAMP SUPPLY
`LAMP SUPPLY
`(FROM MAINS )
`(FROM MAINS )
`
`LAMP
`LAMP
`
`
`
`
`
`Figure 1.- Basic structure of a permanent Emergency
`Lighting System (PELS)
`
`In a previous work a very dissipative driver has been built a
`tested and limitation of this solution has been obtained [1].
`This work present an alternative driver based in a Flyback
`circuit with a very simple control.
`
`The proposed circuit shows a very high efficiency and allows
`us to maintain constant current thorough the LED assembly.
`
`The use of a flyback topology implies the series connection of
`all the LED´s without no dissipative elements. A final part of
`this work shows a complete overview of adopted solution and
`assembly details of the final prototype.
`
`
`II. HIGH EFFICIENCY LED: DESCRIPTION AND SELECTION
`
`
`The prototype of PELS is designed in order to supply light
`levels above 30 Lm during mains operation. After a main
`failure the light level must remain above 30 Lm during 1 hour.
`In figure 1 a basic structure of a PELS is shown.
`
`LUMILEDS emitters type LXHL-PW01 has been used in this
`work. This diode is based in a blue LED (InGaN) chip covered
`with a phosphor that absorbs some of the blue light and
`
`IAS 2004
`
`1655
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`
`IPR PAGE 1
`
`Acuity v. Lynk
`Acuity Ex.
`
`1028
`
`

`
`fluoresces with a broad spectral output ranging from mid-
`green to mid-red (See Figure 2). This diode is a 1W LED with
`a nominal voltage of 3.42 V at nominal current of 350 mA.
`Luminous flux in this point is around 14 Lm (max 18 Lm).
`
`Temperature has an important influence in the light emission,
`the nominal luminous flux of 14 Lm at 25 ºC reduces in a 10%
`with operation around 60ºC. The current across the LED has
`been reduced from 350 mA to 250 mA in order to achieve the
`temperature limitations.
`
`
`RELATIVE
`EMISSION
`
`The assembly of this holder is compatible with the previous
`plastic cases used
`in
`the present fluorescent product
`commercialized for the Spanish company participant in this
`project. The figure 7 shows assembly details of the custom
`made heatsink-holder designed. This assembly of 5 LED`s
`behaves like a power load of 5 x 3.6 V = 18 V and 250 mA
`(series connection).
`
`
`THERMAL
`ADHESIVE
`
`LUMILED
`EMITTER
`
`ALUMINUM
`BASE
`
`PCB BOARD
`
`
`
`HEATSINK
`
`
`Figure 3.- Assembly and details of the custom made heatsink-
`holder designed.
`
`
`III. LED DRIVER PROPOSED: BASIC OPERATION AND DESIGN
`
`The circuit used in this PELS prototype is supplied for a set
`of batteries of NiCd (3 cells of 1.2 V with 1.5 AH). Then, input
`voltage in this application fluctuates from 3 V to 5 V with a
`nominal value of 3.6 V).
`
`The proposed Flyback topology is shown in figure 4. Only one
`switch (bipolar transistor) and a magnetic element (Flyback
`transformer) is enough to supply the series connection of 5
`LED.
`The circuit operation is very simple.
`
`
`
`Figure 4.- Basic topology of the proposed Flyback circuit
`
`During ON stage, the magnetizing transformer inductance
`is charge from the battery, during this time there are no current
`across the LED´s.
`
`
`WAVELENGTH [nm]
`Figure 2.- White LED spectrum.
`
`
`
`
`Considering voltage and current nominal operation and
`operation temperature below 60 ºC the luminous flux obtained
`is:
`
`
`Initial: 5 x 14 Lm/LED x 250 mA/350 mA = 50 Lm
`
`
`For a heatsink temperature of 60ºC a light depreciation is
`estimated from LED datasheet:
`
`Temperature corrected: 0.9 x 50 Lm = 45 Lm
`
`
`Assuming a life of 50.000 hours a light depreciation of 20% is
`estimated:
`
`
`
`
`
`Life corrected: 0.8 x 45 Lm = 36 Lm
`
`
`In the PELS prototype the plastic case cover introduce a
`luminous flux reduction of around the 15% from estimations
`obtained in previous works [1]:
`
`
`
`Plastic cover corrected: 0.85 x 36 Lm = 30.6 Lm
`
`
`Then, a set of 5 LED with an average current of 250 mA is
`enough to satisfy all the product requirements (1 hour of
`autonomy with 30 Lm) and all the limitations (Temperature,
`Life, and losses due to plastic case).
`
`In order to achieve the right operation temperature a custom
`made heatsink-holder has been built and tested at ambient
`temperature around 30 ºC.
`
`
`IAS 2004
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`
`IPR PAGE 2
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`

`
`The bipolar power switch in maintained in ON state until the
`current through it reaches a specified value (ISMAX value). The
`current through the bipolar is sensed using a simple resistor
`is
`(RSHUNT) and
`the maximum current switch (ISMAX)
`established using an external reference. A simple open
`collector comparator is used for this purpose.
`
`
`Flyback
`Transformer
`
`IS
`R shunt
`
`ISMAX
`CONTROL
`
`- +
`
`IC1
`
`+UB
`
`TOFF
`CONTROL
`
`+UB
`
`+-
`
`IC2
`
`+UB
`
`COFF
`
`+ -
`
`uC
`
`OPEN COLLECTOR COMPARATORS
`Figure 6.- Proposed control circuitry (IMAX, TOFF)
`
`
`
`
`
`The OFF stage in the bipolar starts at this point and
`simultaneously a capacitor (COFF) is discharged using the
`above mentioned comparator.
`
`Once the capacitor is discharged and the switch current falls to
`zero, the state of IC1 change again, and COFF is charged again.
`
`The OFF time (TOFF) of the bipolar can be easily adjusted in
`order to limit the voltage across the COFF capacitor. A second
`external reference and a second open collector comparator is
`used for this purpose (IC1).
`
` A
`
` IMAX and TOFF control is easily implemented in this way.
`
`
`Note that, an intrinsic short circuit protection is present in the
`circuit.
`
`All the design equations have been obtained and a design
`method has been established. For instance, average current
`across the LED´s can be easily obtained from the equation:
`
`
`Vs
`⋅
`Ln
`1
`⋅
`Ve
`n
`
`⋅
`
`T
`OFF
`+
`
`Vs
`
`)
`
`⋅
`
`Ve
`
`)
`
`
`
`2(
`
`⋅
`
`I
`
`M
`
`+
`
`⋅
`(2
`
`O
`
`−⋅
`1(
`
`d
`
`)
`
`=
`
`nI
`⋅+
`
`M
`2
`
`I
`
`I
`
`LED
`
`=
`
`
`
`During the OFF stage, the current flows across the LED´s.
`From power topology point of view, LED voltage can be
`considered constant (18 V), then relationship between duty
`cycle and transformation ratio (n) is well established.
`
`In nominal conditions:
`
`
`⋅=
`n
`
`d
`−
`
`1
`
`d
`
`=
`
`18
`6.3
`
`
`
`ES
`
`UU
`
`
`In order to assure low current peaks across the LED a low duty
`cycle is interesting from this purpose. Figure 5 shows the
`typical current waveforms across the LED´s with a d = 0.3 and
`a ripple current of 100 mA. Taken into account that previous
`consideration a transformation ratio of 11.6 has been obtained.
`
`The magnetizing inductance of the Flyback transformer can be
`easily obtained form current ripple of 100 mA. From equations
`this values has been obtained: Lm = 31 µH. Switching
`frequency has been fixed in 30 KHz for nominal conditions.
`
`Figure 5.- Current LED waveforms.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`From all the previous conditions the set of design
`parameters can be easily obtained:
`d = 0.3
` n = 11.6
`(ILED)AVG = 250 mA
`(ILED)MAX = 407 mA
`(ILED)MIN = 307 mA
`L1 = 31 µH
`A very simple current mode control has been easily
`implemented using a low cost comparator (ML393), this circuit
`is shown in figure 6.
`This circuit allows maintaining constant average current across
`the LED with independence of the battery fluctuations. An
`additional advantage is the low switching frequency excursion
`(28.5 KHz with low battery voltage in front of 30 KHz in
`nominal condition).
`
`IAS 2004
`
`1657
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`0-7803-8486-5/04/$20.00 © 2004 IEEE
`
`IPR PAGE 3
`
`

`
`in the C capacitor reaches a low voltage the DIAC turn-off,
`the COFF capacitor goes to normal operation. This situation is
`repeated
`indefinitely
`in no LED situation (Open
`load
`operation).
`
`
`
`
`Figure 7.- No LED protection circuitry.
`
`IV. EXPERIMENTAL RESULTS AND CONCLUSIONS.
`
` A
`
`tested
` complete prototype has been built and
`satisfactorily. Electrical, thermal and optical characteristics
`have been obtained from prototype. The figure 8 shows a photo
`with the final version of the circuit including plastic cases and
`assembly facilities.
`
`V. ACKNOWLEDGES
`This work has been supported for the Spanish company GSSA
`that also supply us with plastic cases for the final version of
`prototypes. Authors would like to thank FICYT (1), CICYT (2)
`and FEDER (3) for partially support this work.
`
`Authors are actually involved in European action COST(4)
`529: ”Efficient Lighting for the 21st Century”
`
`(1) Fundación para la Investigación Científica Y la Tecnología
`(Regional organism).
`(2) Comisión Interministerial de Ciencia Y Tecnología
`(National organism)
`(3) Fondo Europeo de DEsarrollo Regional (European
`organism).
`(4) European CO-operation in the field of Scientific and
`Technical Research
`REFERENCES
`
`
`[1] M. Rico-Secades, A.J. Calleja, J. Ribas, E.L. Corominas, J.M. Alonso, J.
`Cardesín, J. García
`"Evaluation of a low cost permanent emergency lighting system based
`on high efficiency LEDs". IEEE-IAS-2003. 38th Annual Meeting,
`Industry Applications Conference. Salt Lake City, Utah, USA
`
`
`[2]
`
`
`
` Application notes from LUMILEDS (www.lumileds.com)
`-LUXEON assembly guide
`-LUXEON emitter
`-LUXEON Thermal design
`
`If L1 value is high in order to have a low ripple current in the
`magnetizing inductance, the expression of the average current
`across the LED can be simplified:
`
`
`I
`
`LED
`
`≅
`
`(
`
`n
`
`⋅
`I
`M
`⋅
`Ve
`
`Ve
`+
`Vs
`
`)
`
`
`
`
`Taking into account the battery voltage variation from 3 V to 5
`V, the average current across the LED´s varies from 237 mA
`to 275 mA.
`
`And in the same way, the period of the switching frequency
`can be obtained from the next equation:
`
`
`)
`
`nV
`
`s
`
`=
`
`T
`
`T
`OFF
`−
`d
`1
`
`T
`OFF
`
`⋅
`
`=
`
`+
`
`Ve
`(
`Ve
`
`
`
`
`With the same battery fluctuations, the switching frequency
`varies from 28.5 KHz to 32.7 KHz.
`
`In this particular design the following values for main
`elements of both circuit and control parameter have been used:
`
`Inductance (L1) = 31 µH
`Maximum current in the switch (iMAX) = 4.72 A
`Switch off time (TOFF) = 23.3 µS
`
`The designed circuit is really stable again magnetizing
`inductance variations:
`
`With L1= 25 µH and battery in nominal conditions 3.6 V the
`circuit operates with:
`
`
`Fs = 30 KHz
`d = 0.3
`ILED = 243 mA
`
`And with L1= 40 µH and battery in nominal conditions 3.6 V
`the circuit operates with:
`
`
`Fs = 30 KHz
`d = 0.3
`ILED = 257 mA
`
`
`The Flyback transformer has been implemented using a
`conventional E25 core with N27 material. Winding
`interleaving has been optimized using UOM2T Magnetic
`modelling Tool in order to reduce primary leakage inductance.
`
` A
`
` no LED presence protection has been easily implemented
`(Figure 7). An excessive voltage in the output voltage is
`detected by a 33 V DIAC device which activates a simple
`bipolar transistor. Then the voltage in the COFF capacitor goes
`to zero and the inverters stops its operation. When the voltage
`
`IAS 2004
`
`1658
`
`0-7803-8486-5/04/$20.00 © 2004 IEEE
`
`IPR PAGE 4
`
`

`
` Loctite 383 application notes.
`
`[3]
`
`[4] 4.- J. M. Alonso, P. Villegas, J. Díaz, C. Blanco, M. Rico-Secades
`"A microcontroller-based emergency ballast for fluorescent lamps"
`IEEE Transactions on Industrial Applications. Vol 44, Nº 2, pp. 207-
`2166 .Abril 1997.
`
`
`[5] Handbook of batteries, D. Linden, Ed. New York: Mc Graw Hill 1995.
`
`
`
`[6] 6.- A. J. Calleja, J. M. Alonso, E. L. Corominas, J. Ribas, J. A. Martinez,
`M. Rico-Secades
` “Analysis and Experimental Results of a Single-Stage High-Power-
`Factor Electronic Ballast Based on Flyback Converter”
`.IEEE
`Transactions on Power Electronics. Vol 14. Nº 6. Noviembre 1999
`
`
`
`(a).- LED Holder assembly details
`
`(b).- LED holder in operation
`
`(c).- Complete PELS under test
`Figure 8.- Photos with the final prototype.
`
`
`
`IAS 2004
`
`1659
`
`0-7803-8486-5/04/$20.00 © 2004 IEEE
`
`IPR PAGE 5