by Tom Novitsky and Bill Abbott
Endicott Research Group, Inc.
www.ergpower.com
Just a few years ago, LEDs were the future of medical display backlights. Today, the future is here as LEDs have displaced CCFLs as the BLU technology of choice in new display designs. LED drivers have evolved to meet the demand.
Medical diagnostic devices have become powerful state-of-the-art instruments for instantaneous display of vital patient information. With the development of digital signal processing (DSP) technology, medical monitors have evolved enormously from the old analog versions. Current models are digital multi-parameter, thin-film transistor (TFT) LCD monitors that can track many different vital signs at once, with the added advantages of miniaturization and portability. Devices such as defibrillators, cardiac monitors, vital signs monitors, and full-clinical-parameter bedside and portable monitors require many capabilities to be integrated into a single display and demand that the displayed information is clear, crisp, and easily readable.
Fig. 1: Example of 12.1” diagonal TFT LCD patient monitor with 16:9 wide format display.
The New Age of Displays
Today’s medical monitor LCDs are thinner, more lightweight and brighter than ever before, and while CCFL (cold cathode fluorescent lamp) backlights powered by DC-AC inverters are still in use (and supported by ERG), those are primarily legacy applications. New designs are based on LED backlights powered by LED driver modules. LED backlighting units (BLUs) have enabled medical device displays and monitors to provide increased brightness – up to 1,500 cd/m2 (or “nits”) – with higher contrast ratios, better legibility, greater image clarity, and power savings as well.
Fig. 2. CCFLs (top) have been the dominant backlighting technology for medical displays for years, but LED backlights (bottom) are taking over.
Medical devices present a special set of requirements for the LCD, the BLU, and the BLU driver. The display needs to provide high-contrast, high-resolution images and data. The display may also need to be resistant to glare in high-ambient-light environments where surface reflections can degrade contrast by reflecting ambient light. This would be the case in ambulatory surgical units, emergency rooms, operating rooms and intensive care units. Portable medical devices must provide a bright, long lasting image when powered by batteries whereas displays incorporated into devices used in emergency vehicles must be able to withstand extreme temperatures, shock, and vibration.
Evolution of the Backlight
LCDs need to be backlit and the quality of the displayed image is heavily dependent on the BLU and the power supply driving it. In the last few years, more and more TFT LCDs are using LED-based BLUs powered by LED driver circuits.
LED backlights offer numerous advantages over CCFL BLUs including no warm-up time at cold temperatures, lower power consumption, higher dimming ratios and, because they do not require high-voltage and high-frequency inverter circuits, significantly reduced electro-magnetic interference (EMI). Since LED backlights contain no mercury, these displays also offer a more environmentally friendly or “green” option. LEDs are less fragile and more reliable than CCFLs and won’t degrade if operated for long periods of time such as the 24/7 operation demanded of a bedside patient monitor. Average lifetime is 60,000 hours, and the wide range dimming capability of LED drivers (in excess of 1,000:1 dimming steps) offers a valuable advantage for medical displays, which must be legible in bright daylight as well as in the subdued lighting environment of nighttime operation.
Cost of LED versus CCFL
The higher cost of LEDs versus CCFLs has been a stumbling block to the widespread adaption of LED backlights. However, the cost ratio has come down from > 20% several years ago to > 5-10% today. LED manufacturers have also ramped up production considerably which has, in turn, contributed to driving the cost of LED sdown. The real difference in cost is in the total cost of ownership – i.e., the overall cost for panel, backlight, driver, controller board, OSD (on screen display) controller board, interconnect harness or flat flex cable, etc.
What’s behind the screen? Fig. 3 shows the LCD panel, BLU, and backlight driver and associated components.
Powering the LED Backlight
An LED BLU is generally driven by a constant current and does not require an alternating current at high voltage; therefore, an inverter is not required. However, LED BLUs do require appropriately designed drivers to maintain constant brightness and to provide dimming. An LED’s efficiency depends on the amount of current flowing through it. The LED driver must be designed to compensate for supply voltage swings that can be caused by a variety of factors, including operating on a true battery voltage, which could, in turn, affect the power to the backlight. A properly designed driver should compensate for such power swings to maintain brightness consistency in the design.
A wide variety of LED driver ICs (integrated circuits) are available for use in LED drivers. Most of these driver ICs employ switching and/or linear circuits to control the LED current. Many times, however, the voltage available is not usable in it’s present state to drive the wide array of string voltages that are required from display to display, so it is not possible to use the standard +5V or +12V available with a driver IC chip to drive the backlight. Depending on the configuration and number of strings, there is generally an optimum topology for a given display backlight. Drivers must have the ability to adapt to changes in LED voltage to compensate for changes in the voltage of an LED string that may occur due to changing temperature or may vary with tolerances across a large number of LED backlights. This requires a full-function constant-current driver board.
Many LED driver boards employ boost or buck circuits to either increase or decrease the supply voltage and they also employ a secondary circuit that will directly drive the LEDs to a constant current. High-power backlights are often driven by a different topology than a lower power backlight though, as one topology may be more optimized than another for a given power level.
LED Driver Topologies
In order to properly drive the wide variety of LED-backlit LCDs available on the market now, electrical design engineers need to successfully implement one of the many different driver topologies available. The most important consideration for any LED driver is the constant current source. Constant current source (LED driver) topologies currently in use include Linear Current Source, Hysteretic Current Source, PWM “Buck” Current Source and PWM “Boost” Current Source.
New Designs for Driving Multiple Long LED Strings
In order to adequately drive multiple LED strings that consist of more LEDs than can be powered by a 12V supply, ERG has incorporated the technology from the abovementioned LED driver topologies to create entirely new designs and new compact drivers able to power such configurations. These include:
PWM Voltage Boost with Multiple Linear Current Sources
This driver utilizes a slightly different version of the PWM boost current source mentioned above, followed by multiple linear current sources for each LED string. However, this change in configuration converts the PWM boost from a current source to a voltage source. Adding this in front of a linear current source allows the linear current source to drive LED strings with a voltage greater than the supply voltage. The addition of a second linear current source allows it to properly drive two LED strings, and there is no theoretical limit to the number of LED strings this driver can handle, as long as the power handling capability of all elements is adequate and there is a linear current source allocated to each string.
Adaptive PWM Boost with Multiple Linear Current Sources
This configuration is very similar to the PWM Voltage Boost with Multiple Linear Current Sources, but with a design change that significantly increases efficiency. This boost circuit utilizes a special feedback circuit to regulate its output based on the amount of headroom the linear current sources have. The circuit detects which LED has the highest forward voltage, then regulates the output voltage such that the linear current source has just enough headroom (typically 1 volt) to properly regulate the current flowing through that string. It is a good solution for powering multiple strings consisting of an identical number of LEDs.
The PWM Voltage Boost with Multiple Hysteretic Current Sources
This driver integrates a voltage boost converter with hysteretic current sources to drive multiple LED strings. It is ideally suited for higher power applications in which the linear and adaptive boost driver is not optimized. It will achieve high efficiencies and is capable of extremely fast transient response due to the hysteretic current control. It is capable of dimming steps in excess of 5,000:1, although the dimming range becomes non-linear at the lowest levels. This topology is also capable of driving multiple strings with different forward voltages, as the extra power that would be wasted in the linear sources is saved in the inductor.
Fig. 4: New topologies for driving multiple LED strings with examples with examples of associated driver modules.
Thermal Management
Along with making the correct topology choice, those developing a backlight driver need to be well versed in power supply layout for switching and linear circuits. The designer will also face EMI (smaller than CCFL) and thermal issues (greater than a CCFL backlight, although the CCFL inverter has heat challenges of its own) if the proper components are not chosen and the layout is not optimized.
Contrary to some misconceptions, LEDs do generate heat. You can have up to 24 or 36 LEDs in a single BLU. (The quantity varies depending on backlight design and the type of LED used; a BLU can use either a large number of low-power LEDs, or a smaller number of medium to high-power LEDs.) Efficient thermal management – keeping the LEDs cool – is the major design challenge for LED BLUs. The LED BLU rail shown in Fig. 2 incorporates the LEDs on a long, narrow PC board that is thermally bonded to a metal channel or “rail” that is similar to the channel used to house the CCFLs. The thermal management technology utilized in the rail assembly addresses the challenge of efficient thermal management, keeping the LEDs cool, by design with the typical LED junction temperature at or below the typical rated value. The design provides a technologically more efficient way to conduct heat away from the LEDs with respect to the LED junction temperature, which is extremely critical to long-life operation of the LED BLU.
Sourcing the Right Driver
The LCD manufacturer or distributor supplies the LCD panel to the medical device manufacturer with an LED BLU that meets the particular application requirements. The medical device manufacturer must add the other system components, such as an LED driver that will meet or exceed the design requirements: providing acceptable power that results in consistent brightness, backlight life, dimming features and dependable performance over the lifetime of the design. The LCD manufacturer may recommend an approved vendor for LED drivers to the medical device manufacturer for the OEM panel selected. Or, the medical device manufacturer may have a long-standing relationship with a power supply manufacturer or distributor who can recommend and design the right driver for any OEM panel and also provide value-added services such as a brightness upgrade or flat flex cable interconnection capability. For example, ERG can provide high-brightness LED rails with specially engineered thermal management as a value-added design-in or a drop-in replacement for devices already in the field.
Conclusion
LED BLUs are becoming the de facto standard in medical device displays. A critical factor in designing a high performance display is understanding that the LED driver needs to be a full-function power supply that maintains constant current, addresses thermal management, provides a wide dimming range and is resistant to EMI. Therefore, it is also critical to have a display driver designed by a power supply manufacturer who understands the application, the topology, and the electrical design of the specific OEM panel the medical device manufacturer has chosen.
Tom Novitsky is President and Bill Abbott is Corporate Distribution Manager, GMS atEndicott Research Group, Inc., 2601 Wayne St., Endicott, NY USA 137601-800-215-5866; 607-754-9187, www.ergpower.com They can be contacted atbacklight@ergpower.com