Matching inverters to CCFT backlights (1992)

Most backlit liquid-crystal displays (LCDs) are transmissive and therefore unreadable without a light source. That source is usually a cold-cathode fluorescent tube (CCFT) because CCFTs are compact, durable, low in cost, and high in luminous efficiency. An effective backlighting system depends on fine tuning the relationships between the CCFT light source and other essential components, notably the liquid-crystal screen and the dc-ac inverter that powers the lamp. Four basic parameters define the operation of any cold-cathode fluorescent tube: minimum discharge voltage operating voltage frequency tube current The discharge voltage is the minimum voltage required to ignite an aged tube—one at the end of its useful life—at cold ambient. This definition is very important because the required starting voltage increases as the lube ages and the operating temperature decreases. If the power supply—the inverter—fails to produce the minimum required discharge voltage, the tube will not light. The operating voltage is the voltage across the tube when the tube is functioning normally and drawing its nominal current. This voltage—usually expressed as an rms voltage in most LCD applications—is determined by the physical characteristics of the tube and is fairly constant over time and temperature changes. The frequency of the ac voltage across the tube does not affect brightness or tube life in most CCFT-backlit monochrome LCD modules. The designers of these modules generally specify frequencies in the 30-kHz range, but they occasionally call for higher frequencies. The situation is more complicated in many color LCDs, where certain frequencies can interfere with the multiplexing of the LCD module. In general, these critical frequencies are clearly described in the LCD-module specification. The CCFT current is the nominal rms current passing through the tube. This is the most critical parameter as it determines the brightness, power consumption, and life of the CCFT backlight and, therefore, of the entire LCD module. As a rule, the useful life of a CCFT-backlit LCD module ranges from 5,000 to 20,000 hours, and is determined by the current passing through the fluorescent tube. Lifetime can be traded off against brightness since tube brightness varies linearly with tube current. But playing this game has its limits. In addition to shortening tube life, exceeding the nominal lube current generates additional heat that can damage the display. A CCFT backlight must be powered by a property selected inverter for optimal results. This is clear when we realize that many different LCD modules use the same fluorescent tube even though different drive conditions and a different rated life are specified for each application. One display manufacturer may choose to sacrifice life for the impact of an initially brighter display. Another may opt for lower initial brightness and longer life. The difference is the inverter, which must be properly matched to the CCFT (or CCFTs) as well as to the driving conditions each manufacturer selects for his display. Despite its importance to the success of the system, display-system designers often pay remarkably little attention to the correct manner of driving a fluorescent-tube backlight. Most specifications for CCFT-backlit LCDs do identify the tube current, frequency, and voltages that will deliver the backlight's specified lifetime. An inverter that does not produce the specified values compromises the integrity of the system design, as well as the display's readability and/or the backlight's lifetime. A mismatched inverter can also reduce the system's efficiency and battery life—a matter of intense concern to designers of laptop computers. Balanced against these considerations are some cold hard business realities. It is clearly less expensive for an inverter manufacturer to make and stock a few models instead of many. Indeed, some manufacturers sell one general-purpose inverter for many different backlighting applications. This inverter design may fit one CCFT application very well, but it is then a compromise for others. These compromised backlight systems—which are commonly seen in the real world—are being over- or under-driven because the inverter is not properly matched to the fluorescent tubes. Under-driving the CCFT can produce an unreadably dim display over time. In addition to shortening the life of the tube and generating extra heat, over-driving can adversely affect the uniformity of the lighting. The classic current-fed two-transistor dc-to-ac inverter circuit offers excellent power transfer and high operating efficiency. A CCFT is powered at hundreds of volts in an environment where millivolts make a difference. To start the tube(s), a dc-ac inverter generates even more: between 1000 and 1200 V. With such high voltages, encapsulation is desirable for proper dielectric protection. Most CCFT dc-to-ac inverters are tuned switchers designed to produce a specific voltage, frequency, and output current when a specified tube is connected to the output. The classic current-fed two-transistor inverter has a tuned resonating output, tuned resonating input, and inductive dc input that allow excellent power transfer and high operating efficiency [Fig. 1]. This type of circuit inherently produces a pure sine-wave output, but the voltage and current waveforms are both distorted when they are applied to a CCFT, which is a highly nonlinear device. And here is where the matching of the inverter to the CCFT comes in: the amount of distortion depends on the closeness of the matching. A well designed and matched inverter-CCFT system should have minimal distortion [Fig. 2(a)]. A poorly matched system generates a badly distorted voltage or current waveform [Fig. 2(b)], which can produce electromagnetic interference and may also interfere with the multiplexing of the LCD. A designer matches the inverter to the CCFT by adjusting the output capacitor and transformer inductance to provide a resonating output. The transition from starting voltage to operating voltage in these systems is implemented by a small (27 pF) internal series capacitor, which serves as the ballast—dropping impedance—for the high voltage after the tube has ignited. This high series impedance is responsible for a dc-to-ac inverter that looks like a constant-current source while producing a high-voltage sine-wave output. As a result, the inverter produces its nominal design output current under almost any load—from a very high impedance to a “dead” short circuit. This ac constant-current source must meet the voltage and current requirements of the CCFT used in the LCD module. An over-current condition through the tube produces the excess heat we've already discussed. An under-current condition can produce very distorted current and voltage waveforms with their consequent interference. The unwanted high-frequency energy generated by the inverter under these circumstances can reflect back into the source power line or radiate, thus interfering with nearby devices. This electromagnetic interference is particularly troublesome for notebook computers, point-of-sale terminals, and any device that takes precise measurements. Airline-passenger users of offending laptops, for example, have been forced to turn off their computers while flying. Trying to measure the operating conditions of a functioning cold-cathode fluorescent tube can be an interesting experience.1 The combination of high voltage, low current, and high frequency makes conventional test equipment and methods almost useless. Any external resistance, impedance, or ground loop can dramatically alter the operation of the dc-to-ac inverter and distort the measurements being observed. Let's look at some specifics. The impedance of a typical operating 2.5-W 6-in. CCFT is 50,000–70,000 Ω. The small internal capacitor usually found in series with each output of the inverter offers an impedance that is typically three times that of the CCFT to which it is connected. Even a voltmeter with a 10-MΩ input impedance can load the output enough to render the reading useless. An oscilloscope can be even worse. The ~20 pF of probe capacitance presents a relatively low 250 kΩ of impedance at 30 kHz. The classic inverter produces a pure sine-wave output, but the sine wave is distorted when applied to the highly nonlinear CCFT. (a) When the inverter is well matched to the CCFT, the distortion is minimized. (b) When it is not well matched, the distortion is greater. This can produce electromagnetic interference, as well as interference with the LCD's multiplexing. Measuring the voltage, frequency, and current of an operating CCFT requires great care and special techniques. But there are methods for measuring the voltage, frequency and current of an operating CCFT. In one of them, the true rms tube current is measured with a broadband current probe—such as the Tektronix CT-2—that magnetically detects the ac current without appreciable circuit interaction [Fig. 3]. The probe's output can be fed into an appropriate true-rms voltmeter or digital oscilloscope capable of calculating true rms. The output voltage to the tube can be measured with a dual-channel oscilloscope and two low-capacitance (< 2.5 pF) probes. The oscilloscope should be connected differentially, with the probe grounds connected and floating. Channel A should be added to the inverse of channel B to produce the complete waveform on the oscilloscope. The rms voltage value can be calculated by the digital oscilloscope or measured with a true-rms voltmeter connected to the oscilloscope's vertical-signal output jack (if available). Although this method loads the CCFT and inverter minimally, there may still be enough loading to change the operating voltage slightly. Even a slight change in the brightness of the tube—or display—is an indication of some interaction between the inverter or tube and the measuring equipment. The no-load output voltage of the inverter is the most difficult parameter to measure accurately. The slightest external impedance can reduce the inverter's no-load output voltage. Nonetheless, the setup used to measure the operating voltage can be used to measure the no-load output voltage fairly accurately. The length of test leads and wires should be minimized to reduce stray capacitance. Monitoring differences in dc input current with and without the measuring equipment connected indicates the magnitude of external equipment effects. The capacitance of operational lead wires and connectors between the inverter and the LCD is a more critical issue than the capacitance of test leads. These capacitances can reduce the inverter's no-load output voltage—the starting voltage—which can, in turn, affect the useful life and/or cold-temperature operation of the tube. Designers of LCD modules should take this voltage loss into account when they select an inverter for a particular module and application. 1 I would like to thank John Peterson, chief engineer at ERG, for supplying technical data for this article.