In an effort to improve vision systems in helicopters, designers are replacing bulky cathode-ray tube (CRT) monitors with much thinner and space-efficient liquid crystal displays (LCDs).
For new applications, some are specifying LCDs as small as 6,4 cm across to make the most of the cockpit space. These changes make new demands on power and the need to manage electromagnetic interference (EMI).
UK-based Highlander, one of the companies active in this market, has shipped more than 500 LCD monitors to police and military organisations in the UK, France, Germany, Spain, the US and other countries around the world.
Complex mixture of output voltages needed
As a company, Highlander started in 1998 assembling display systems for avionics but was soon being asked to develop the displays as well. A key issue with designing LCD monitors for airborne use lies in the power supply. Helicopters generally provide a single supply voltage of 28 V. LCDs however, need a complex mixture of voltages and control frequencies to generate the display and provide illumination through an inverter-driven fluorescent backlight. As customers demand the integration of compute functions into the monitors, the ability to support a complex mixture of output voltages will become even more important.
To provide the necessary power at the right voltages, the company has used DC-DC converters from Vicor.
Vicor's second-generation converters were designed to be user-customised. Almost any combination of input, output and power is available on each module. This reduces the need to mix and match multiple off-the-shelf modules within a system, which helps to save space. Customisation is made easy through the Vicor Design Assistance Computer (VDAC) tool, a patented system that enables Vicor's customers and partners to specify and check the configuration of its second-generation DC-DC converters. Using VDAC, Vicor was able to provide a converter tuned specifically to Highlander's latest generation of products.
Further space savings are made through the DC-DC converters' use of low-noise zero-current switching and zero-voltage switching, These techniques, coupled with advanced power semiconductor packaging, provide high density with low temperature gradients. The second-generation design tripled power density compared with previous DC-DC converters, vital for the designs in the space-constrained aerospace sector.
EMI becomes a problem
Many designers of electronic systems are faced with the dual challenge of trying to squeeze as much functionality as possible into a small space as well as dealing with the EMI from systems that operate at increasing frequencies. As the size of the system reduces, space constraints make it difficult to use conventional passive shielding techniques, which are often bulky. However, careful design can stop EMI problems close to the source.
By making use of active EMI filtering techniques, it is possible to meet the stringent requirements of defence standards such as the US Department of Defense DO160 standard. This standard demands that electronic systems pass stringent EMI tests to ensure that they will not interfere with other critical systems in an aircraft.
The problem of achieving small size with low EMI is particularly difficult in the cockpit of a helicopter. Helicopters equipped with daylight and night-vision cameras are being used around the world by law-enforcement agencies to help capture criminals, watch for people moving illegally across borders, and by the military to check on enemy movements. Those agencies want more sophisticated computer systems to be integrated into their vision systems, supporting functions such as GPS, to improve their efficiency. The problem for the companies supplying the vision systems to both defence and law-enforcement agencies is that space in the helicopter cockpit is limited.
The high frequencies that need to be used within the monitor subsystem present a challenge in terms of the EMI that can be transmitted through the power subsystem. With Highlander's products, a solid aluminium case means that there should be no emissions through the enclosure. A conductive coating of indium-tin oxide on the display itself prevents emissions through the front of the monitor. However, one of the most insidious EMI leakage paths is through the power supply and on through the inlet cables.
Without the use of filters, back-EMI will be transmitted through the power cable, causing radiation from the back of the monitor. For previous generations of monitor designs, Highlander used discrete filtering components on the main PCB to control EMI leakage back through the power supply. However, the components used for passive filtering are bulky, a problem in an environment where customers insist on the smallest unit possible for a given display size.
Active filtering the answer
A solution was suggested based on much smaller active filtering than would normally be necessary - a combination of Vicor's DC-DC converter modules with an active-filter module from Vicor's QuietPower range. Indeed, the specified QuietPower QPO-1 system-in-package (SiP) ouput filter provides significantly higher attenuation than passive filters in a much smaller space. The active filtering techniques used by the QuietPower range can provide output ripple and noise attenuation of more than 30 dB from 1 kHz to 500 kHz and more than 20 dB in the 50 Hz to 500 Hz range. A single module replaces at least five bulky passive components and provides better results.
Able to support load currents up to 10 A, the tiny filter module can handle a wide voltage range from 3 V to 30 V d.c. As well as providing better noise attenuation than a bulky network of external passive components, the QPO-1's closed-loop architecture improves transient response - vital for modern embedded computers and displays - and ensures quiet point-of-load attenuation. The module's efficiency can go as high as 99% and it offers power-limit protection.
The QPO-1 has a number of features that let the designer tune the filtering to each application, so that different LCD and computer combinations can easily be accommodated. For example, the QPO-1 includes a peak-detect function that adds the input peak of the ripple voltage to the headroom voltage. The QPO-1 will track the input ripple, adjusting the headroom within the dynamic range of the filter as the peak voltage of the ripple changes. Used in combination with a slope-adjust function, which is used to alter the filter's efficiency when ripple amplitude reduces with increasing load, the peak detect function lets the user optimise both the initial headroom voltage and overall efficiency needed for the application.
Conclusion
Highlander is now working on a monitor that includes GPS as well as the possibility of a camera source. Use of the QuietPower module will eliminate the interference between the computer, the camera and the display system. For Highlander, the space savings made possible by the QuietPower active filter modules and compact DC-DC converters means that the company can further enhance its range of monitors to enhance its position in the market for helicopter cockpit systems. The wide applicability of tuneable DC-DC converters and active-filter modules mean many other applications can achieve small size and minimal EMI emissions.
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version