Chandler, AZ--A compact inverter design based on a road not taken by the digital-electronics industry may provide the answer to the question of how to illuminate the instrument panels, keypads, and LCDs of today's portable electronics. The design eliminates a problem that has plagued electroluminescent (EL) lamps since their invention 60 years ago.
Sandwiches of dielectric and phosphor particles between two flat electrodes, EL lamps emit a cool, soft glow when supplied with ac voltage. Unfortunately, the inverter circuits needed to change dc power into ac traditionally relied on relatively large wire-wrapped transformers.
According to Rob Kimball, a staff engineer at Durel Corp., "It was a chronic roadblock in our applications." The company overcame the traditional EL lamp problem of moisture contamination by a proprietary process for microencapsulation of phosphor crystals. Its thin, rugged, Durel 3 lamps offered longer life than conventional designs. "But people want to use EL because they're out of room," he says. "That huge driver circuit defeated the purpose."
Durel is a joint venture of 3M Corp., St. Paul, MN, and Rogers Corp., Rogers, CT. A project for backlighting a specialized compass brought Kimball, a mechanical engineer by training, in contact with Rogers' electrical engineers. Together, they saw a need for a switching circuit for the lamp driver. The driver's size could be reduced by charging and discharging an inductor and rectifying its output. Compared to a transformer circuit, the energy delivered per cycle would be small. But by operating at high frequency, says Kimball, "charging the capacitor-like EL lamps would be like filling a swimming pool with a spoon that moves very fast."
Their original design was equivocal. Says Kimball, "It sucked a ton of power, but we could fit the circuit inside the compass envelope." The limited success spurred a patent search and further development.
EL lamps have special requirements. They're most efficient on high-voltage, 400-Hz power. The CMOS-technology switching-mode power supplies now common in digital electronics could be made to work, but their junction-isolation design requires parasitic diodes that prevent negative-going voltage. Thus, lamp-driver circuits with CMOS supplies needed differential outputs, complicating interconnection to the lamp and EMI suppression (both lamp electrodes are hot).
His search led Kimball to the largely ignored dielectric-isolation technique. "It's a neat process; high voltage, high power, high yield, and symmetrical to ground," he explains.
After several iterations, and help from chip-foundry engineers Grady Wood and Andy Sabol, Durel now offers two quasi-integrated EL lamp drivers, the D310 and the D350 for 160 to 250V ac and 120V ac output, respectively, from 0.9 to 6V dc in. The chips' non-integrated portion is the timing capacitors, which allows users to easily optimize the drivers for their designs. A designer's kit, available from Durel, simplifies that optimization.
Additional details...Contact Customer Service, Durel Corp., 2225 W. Chandler Blvd., Chandler, AZ 85224, (602) 917-6201.