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CCFL Transformer Application Note
Cold Cathode Fluorescent Lamps (CCFLs) are used to illuminate Liquid Crystal Displays (LCDs). The LCD display is used in laptop computers, gas pumps, automobiles, test equipment, PDAs and medical instruments. CCFLs are small, efficient and inexpensive. The lamp must be driven by a specialized power supply. High sinusoidal AC voltage is needed to start the lamps, but once started, the voltage drops to a lower level. CCFL circuits are usually powered from a low voltage DC source of 5-12V. The DC to AC power supply needs a transformer to change low DC input voltage to high sinusoidal AC voltage. The Cooper Bussmann(R) Coiltronics(R) brand CCFL transformers are designed to work with inexpensive Royer class self-oscillating circuits. The Royer circuit works with input voltage from 2.5 to 20Vdc and is capable of producing 90% efficiency above 5Vdc input.
Royer Diagram
Transformer Selection
The CCFL lamp manufacturer supplies the following lamp characteristics: 1. Strike voltage (Vstrike) 2. Running voltage (Vrun) 3. Frequency of operation (Fres) 4. Power (W) 5. Current (Ilamp) The first step is to select the transformer according to the power requirement from the catalog. The second step is to decide the termination. A current-fed, push-pull topology is commonly used to power the CCFL transformer. This topology accommodates a wide input voltage and consists of a resonant push-pull stage, a Pulse-Width-Modulated (PWM) buck-derived control stage and a high-voltage secondary stage. The pushpull stage consists of transistors Q2 and Q3 to drive the center-tapped transformer T1. The transistors are driven 180 out of phase at 50% duty cycle with an auxiliary winding on the transformer. A resonant tank is formed between the primary inductance of the transformer and a lowloss, external resonant bulk capacitor C1. The resonant tank provides a sinusoidal voltage to the transformer's primary winding and sets the system's operating frequency. The high voltage at the secondary of transformer is used to ignite and operate the lamp. Since the ignition or "strike voltage" is higher than the operating voltage, a high voltage ballast capacitor C2 is required to allow a voltage difference between the transformer secondary and the lamp. To minimize lamp stress and improve efficiency, the striking voltage waveforms should be sinusoidal. Use this formula to find the turn ratio needed to obtain the strike voltage of the lamp.
Vstrike =
x Vin min
2
x TR
TR = Turns ratio Vin min = Battery voltage
The operating frequency of the system is determined by the inductance of the primary and the bulk capacitor across the primary at resonance.
Coiltronics(R) Transformer Features:
1. Supply high voltage. 2. Operation frequency range from 40 to 80kHz. 3. Deliver output power from 2.5 to 14 watts. 4. Slim or low profile type easily built into your design. 5. Available in through-hole and SMT recess or gull wing type. 6. Operate in Royer and direct IC drive. 7. 1500 volt primary to secondary isolation. 8. Ferrite core material. 9. Designed for floating and non-floating applications. 10. Transformer secondary is machine wound on sections to increase leakage inductance and reduce voltage gradient between layers.
FRe s =
1 2 x LPr i x C Bulk
Fres = Resonance Frequency Determine the ballast capacitor value using the equation below.
Cballast =
I lamp 2 x x Fres (Vrun - Vsec )
2
Layout and Circuit Considerations
Vsec = Transformer secondary voltage Vrun = Lamp running voltage Ilamp = Lamp current Fres = Resonance frequency Cballast = Ballast capacitor The capacitor voltage is 90 degrees out of phase with the lamp running voltage. From this equation, determine the value of the ballast capacitor. * The high voltage traces must be separated from low voltage traces. * The ballast capacitor must be placed closer to the transformer secondary pin. * Avoid long wire connections from the transformer to the lamp. Stray capacitance between wire and ground will reduce efficiency. * Incorporate open lamp and overload protection in the circuit design. Open lamp will cause full voltage in the transformer output and will burn the transformer. Most of the CCFL IC has protection built in the circuit for open and overload condition.
Schematic Relation to Part Number
Schematic A CTX110652-R CTX210652-R
Schematic B CTX110655-R CTX110657-R CTX110659-R CTX210655-R CTX210657-R CTX210659-R
Schematic C CTX210403-R CTX110603-R CTX210407-R CTX110605-R CTX210409-R CTX110607-R CTX210411-R CTX110609-R CTX310403-R CTX110611-R CTX310405-R CTX210603-R CTX310407-R CTX210605-R CTX310409-R CTX210607-R CTX310411-R CTX210609-R CTX210611-R
Schematic D CTX110600-R CTX210600-R
Schematic E CTX410805-R CTX410807-R CTX410809-R
Part Number CTX110652-R CTX110655-R CTX110657-R CTX110659-R CTX210652-R CTX210655-R CTX210657-R CTX210659-R CTX210403-R CTX210407-R CTX210409-R CTX210411-R CTX310403-R CTX310407-R CTX310409-R CTX310411-R CTX110600-R CTX110603-R CTX110605-R CTX110607-R CTX110609-R CTX110611-R CTX210600-R CTX210603-R CTX210605-R CTX210607-R CTX210609-R CTX210611-R CTX410805-R CTX410807-R CTX410809-R
Power Watts 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 14.00 14.00 14.00
TR1 Ns/Np 67 67 86 100 67 67 86 100 50 86 100 125 50 86 100 125 67 50 67 86 100 125 67 50 67 86 100 125 67 86 100
TR2 Np/FB 6 6 2 4 6 6 2 4 6 4.7 4 4 6 4.7 4 4 6 6 6 4.7 6 4 6 6 6 4.7 6 4 5 4 4
Lpri H 43 43 26 19 43 43 26 19 44 27 20 20 44 27 20 20 44 44 44 27 20 20 44 44 44 27 20 20 24 16 16
Vpri Volts 20 20 15 13 20 20 15 13 26 15 13 10 26 15 13 10 20 26 20 15 13 13 20 26 20 15 13 13 30 23 23
Isec DCRpri mA max max 5 5 5 5 5 5 5 5 7 7 7 7 7 7 7 7 12 12 12 12 12 12 12 12 12 12 12 12 30 30 30 0.220 0.220 0.212 0.190 0.220 0.220 0.212 0.190 0.220 0.160 0.160 0.160 0.220 0.160 0.160 0.160 0.160 0.160 0.160 0.132 0.132 0.132 0.160 0.160 0.160 0.132 0.132 0.132 0.030 0.024 0.024
DCRsec max 285 285 285 285 285 285 285 285 165 220 220 330 165 220 220 330 176 132 176 176 176 291 176 132 176 176 176 291 262 272 314
Schematic Reference A B B B A B B B C C C C C C C C D C C C C C D C C C C C F F F
Mechanical Reference A A A A C C C C B B B B D D D D E E E E E E F F F F F F G G G
Full primary turns used in turns ratio calculation. Ns/Np= Turns Secondary/Turns Primary. Np/FB= Turns Primary/FeedBack Winding.
Mechanical References
A
B
Mechanical References
C
D
E
F
G
The Cooper Bussmann(R) Coiltronics(R) brand of magnetics specializes in standard and custom solutions, offering the latest in state-of-the-art low-profile high power density magnetic components. We remain at the forefront of innovation and new technology to deliver the optimal mix of packaging, high efficiency and unbeatable reliability. Our designs utilize high frequency, low core loss materials, and new and custom core shapes in combination with innovative construction and packaging to provide designers with the highest performance parts available on the market. The Coiltronics Brand product line of power magnetics continually expands to satisfy shifts in technology and related market needs. Standard Product Categories include: * Shielded Drum Inductors * Toroidal Inductors * Unshielded Drum Inductors * Specialty Magnetics * High Current Inductors * Custom Magnetics
Please visit http://www.cooperbussmann.com/datasheets/elx to see the wide variety of inductor solutions we have to offer.
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(c) 2008 Cooper Bussmann St. Louis, MO 63178 636-394-2877 www.cooperbussmann.com
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