A practical, 92%-efficient LCD-backlight design for cold-cathode fluorescent lamps (CCFLs) is a classic study of compromise in a transduced electronic system. Every aspect of the design is interrelated, and its physical embodiment is an integral part of the electrical circuit. The choice and location of the lamp, wires, display housing, and other items have a major effect on electrical characteristics. To achieve a practical, highly efficient LCD backlight, you must take the greatest care in every detail. Getting the lamp to light is just the beginning. (Note: The circuits and design principles that this article presents improve the efficiency of the author's initial LCD backlight design, which appeared in the October 29, 1992, issue of EDN (Ref 1).

CCFLs are complex transducers, and many variables affect their ability to convert electrical current to light. Factors influencing conversion efficiency include the lamp's current, temperature, drive-waveform characteristics, length, width, gas constituents, and proximity to nearby conductors. These and other factors are interdependent, resulting in a complex overall response. Fig 1 shows two curves of typical lamp characteristics: current (Fig 1a) and temperature (Fig 1b) vs emissivity. These curves don't correspond to any particular lamp but show general characteristics averaged over a dozen lamps from different manufacturers. These curves hint at the difficulty in predicting lamp behavior as operating conditions—including current and temperature as well as lamp voltage and length—vary.

The lamp's current and temperature are clearly critical to emission, although high electrical efficiency may not necessarily correspond to the highest optical efficiency. It is possible, for example, to construct a CCFL circuit with 94% electrical efficiency that produces less light than an approach with 80% electrical efficiency. Because of this fact, you must often perform both electrical and photometric evaluation of a circuit.

Furthermore, a lossy display enclosure or excessive high-voltage-wire lengths can severely degrade the performance of a very well matched lamp-circuit combination. Display enclosures with too much conducting material near the lamp have huge losses due to capacitive coupling. A poorly designed display enclosure can easily degrade efficiency by 20%. High-voltage wire runs typically cause 1% loss/in. of wire.

Because the LCD backlight can take as much as 50% of a computer's power, any efficiency improvement is noteworthy. The circuit in Fig 2 improves the efficiency of the original circuit (Ref 1 and Ref 2) by 6 to 88%. However, if you don't have a low-voltage supply available in the display area to drive the LT1172's V_IN pin, this efficiency figure degrades by about 3%. The efficiency improvement is primarily due to the transistor's higher gain and lower saturation voltage. You must carefully select the base-drive resistor's value—nominally 1 kOhms—to provide full V_CE saturation without inducing base overdrive or beta starvation.

As in the original circuit, the lamp intensity is continuously and smoothly variable from zero to full intensity, and the input voltage range remains the same at 4.5 to 20V.

The details of the circuit's operation are the same as described in Ref 1 and Ref 2. In essence, the circuit is a current-sink-driven resonant Royer converter (Ref 3). The LT1172 and L₁ form a switch-mode current sink. Q₁, Q₂, T₁, and associated components comprise the resonant Royer converter. Rectified feedback from the Royer converter's output via D2 closes constant-current a loop back to the LT1172's V_FB pin.

In addition to improving efficiency, this general circuit approach has other advantages: tight line regulation and extended dimming range. Additionally, the circuit enhances the lamp's operating life because current can't increase as the lamp ages. The circuit's 0.1% line regulation is notably better than that of some other approaches. Tight regulation prevents the lamp intensity from varying when abrupt line changes occur, which typically happens when you connect a battery-powered apparatus to an ac-powered charger. The circuit's excellent line regulation stems from the fact that T1's drive waveform never changes shape as input voltage varies. This characteristic permits the simple 10-kV to 1-µF RC network to produce a consistent response. The RC averaging characteristic has serious error compared to a true rms conversion, but the error is constant and the 562 Ohms shunt's value calibrates out the error.

The circuit in Fig 3 is similar to the one in Fig 2 but uses a transformer with lower copper and core losses to increase efficiency to 92%. The tradeoff is slightly larger transformer size. Value shifts in C₁, L₁, and the base-drive resistor reflect different transformer characteristics. This circuit also features a dc or pulse-width-controlled dimming input and shutdown via Q₃.

Keep several points in mind when observing the operation of these circuits. Only a wideband, high-voltage probe that's fully specified for this type of measurement can monitor T₁'s high-voltage secondary. Most scope probes break down and fail to make this measurement (Ref 4). To read T₁'s output, Tektronix probes types P-6007 and P-6009 are acceptable in some cases, but types P-6013A and P6015 are preferable.

Obtaining and verifying high efficiency takes diligence. The values in Fig 2 and Fig 3 for C₁ and C₂ are typical for achieving high efficiency and vary for specific types of lamps. An important realization is that the term "lamp" includes the total load seen by the transformer's secondary. This load, reflected back to the primary, sets the transformer's input impedance. The transformer's input impedance forms an integral part of the LC tank circuit that produces the high-voltage drive. Thus, you must optimize circuit efficiency by arranging the wiring, display housing, and physical layout exactly the same way they will be built in production. Deviations from this procedure result in lower efficiency than otherwise possible.

In practice, a first-cut efficiency optimization with best-guess lead lengths and the intended lamp in its display housing usually produces results within 5% of the achievable figure. When the production circuit's physical layout is set, you can fix the values for C₁ and C₂. C₁ sets the circuit's resonance point, which varies to some extent with the lamp's characteristics. C₂ ballasts the lamp, effectively buffering its negative-resistance characteristic. Small values of C₂ provide the most load isolation but require relatively large transformer output voltage for loop closure. Large C₂ values minimize transformer output voltage but degrade load buffering.

The best values for C₁ and C₂ depend on the specific lamp you use. Some interaction occurs between the two capacitors, but generalized selection guidelines are possible. Typical values for C₁ are 0.01 to 0.15 µF. C₂ usually ends up in the 10- to 47-pF range. C₁ must be a low-loss capacitor; substituting a poor-quality dielectric for C₁ can easily degrade efficiency by 10%. Before selecting a capacitor, set the value of the Q₁ to Q₂ base-drive resistor to a value, such as 470 Ohms, which ensures saturation. Next, select C₁ and C₂ by trying different values for each and iterating toward best efficiency. During this procedure, monitor IC₁'s feedback pin, which should be at 1.23V, to ensure that the loop is closed. Several trials usually produce the optimum C₁ and C₂ values. The highest efficiencies are not necessarily associated with the most esthetically pleasing waveshapes, particularly at Q₁, Q₂, and the output.

Now you should optimize the base-drive resistor's value. This resistor's value, which is nominally 1 kOhm, should provide full V_CE saturation without inducing base overdrive or beta starvation. Using full lamp power, the point at which the collector current peaks determines the optimum value. The resistor should equal the largest value that ensures saturation for worst-case transistor beta. You can verify this condition by varying the resistor about the ideal value and noting the small variations in input supply current. The minimum obtainable current corresponds to the best beta-vs-saturation tradeoff. In practice, supply current rises slightly on either side of this point. This double-value behavior is due to excessive base-drive or saturation losses, which degrade efficiency.

Other issues influencing efficiency include the lamp's wire length and energy leakage. The high-voltage side of the lamp should have the smallest practical lead length. Excessive length results in radiative losses, which can easily reach 3% for a 3-in. wire. Similarly, to prevent energy leakage, which can degrade efficiency by 10%, metal should neither contact nor be close to the lamp.

Finally, a custom-designed lamp affords the best possible results. A jointly tailored lamp-circuit combination permits precise optimization of circuit operation to yield the highest efficiency.

1. Williams, Jim, "Designing Supplies for Powering LCD Backlighting," EDN, October 29, 1992, pg 125.
2. Williams, Jim, "Techniques for 92% Efficient LCD Illumination," Application Note 55, Linear Technology Corp, 1993.
3. Bright, Pittman, and Royer, "Transistors as On-Off Switches in Saturable Core Circuits," Electrical Manufacturing, Technomic Publishing, Lancaster, PA, December, 1954.
4. "1968 Instrumentation/Electronic-Analytical-Medical.'' AC Voltage Measurement, Hewlett-Packard Co, pgs 197-198.