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® Coiltronics® 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 = TR = Turns ratio Vin min = Battery voltage π × Vin min 2 × TR The operating frequency of the system is determined by the inductance of the primary and the bulk capacitor across the primary at resonance. FRe s = Coiltronics® Transformer Features: 1. Supply high voltage. 2. Operation frequency range from 40 to 80kHz. 1 2 × π LPr i × C Bulk 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. Fres = Resonance Frequency 6. Operate in Royer and direct IC drive. Determine the ballast capacitor value using the equation below. Cballast = 7. 1500 volt primary to secondary isolation. 8. Ferrite core material. I lamp 9. Designed for floating and non-floating applications. 2 2 × π × Fres (Vrun − Vsec ) 10. Transformer secondary is machine wound on sections to increase leakage inductance and reduce voltage gradient between layers. Layout and Circuit Considerations Vsec = Transformer secondary voltage • The high voltage traces must be separated from low voltage traces. Vrun = Lamp running voltage • The ballast capacitor must be placed closer to the transformer secondary pin. Ilamp = Lamp current • Avoid long wire connections from the transformer to the lamp. Stray capacitance between wire and ground will reduce efficiency. Fres = Resonance frequency Cballast = Ballast capacitor The capacitor voltage is 90 degrees out of phase with the lamp running voltage. • 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. From this equation, determine the value of the ballast capacitor. Schematic Relation to Part Number Schematic A Schematic B Schematic C Schematic D Schematic E CTX110652-R CTX110655-R CTX210403-R CTX110603-R CTX110600-R CTX410805-R CTX210652-R CTX110657-R CTX210407-R CTX110605-R CTX210600-R CTX410807-R CTX110659-R CTX210409-R CTX110607-R CTX210655-R CTX210411-R CTX110609-R CTX210657-R CTX310403-R CTX110611-R CTX210659-R CTX310405-R CTX210603-R CTX310407-R CTX210605-R CTX310409-R CTX210607-R CTX310411-R CTX210609-R CTX210611-R CTX410809-R Part Power TR1 TR2 Number Watts Ns/Np Np/FB Lpri μH Vpri Volts 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 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 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 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 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 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 Ω 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 Schematic Mechanical Ω max Reference Reference 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 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 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® Coiltronics® 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 • Unshielded Drum Inductors • Toroidal 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. Order samples online - www.cooperbussmann.com © 2008 Cooper Bussmann St. Louis, MO 63178 636-394-2877 www.cooperbussmann.com Reorder # 4035 PDF Only