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Constant-current driver saves power
March 6, 1995
6 Min Read
Tucson, AZ--Complex mechanical systems like those used on a 747 jetliner require a great many electrically powered actuators. These actuators impose significant demands on power-generation systems. The present design approach for switching power to such actuators creates a mismatch between initial operating power and delivered power.
Onboard a 747, which may have as many as 300 components that contain magnetic elements, a typical 17W-per-element mismatch produces 5.5 kW of power dissipation. The main cause of power loss in such components as actuators, relays, and valves is the standard practice of applying the 28V bus directly to the element. Usually this delivery occurs through a high- or low-side transistor.
"The solution is to modify a conventional switch-mode power-supply control loop by reversing the normal voltage- and current-feedback signals while adding circuitry to sense the output current," says F. Michael Barlage, a senior electronics specialist at AlliedSignal Equipment Systems. His new topology for a constant-current magnetic-driver element has several improved characteristics. For example, the control loop holds output current to a very precise value, regardless of variations in the output resistance or input-voltage bus.
Basically, engineers design the magnetic elements in aircraft components to operate at a specified minimum voltage. For instance, assume that Mil Standard 704 sets 16V as the minimum operating voltage for a component. If the magnetic-element designer must allow for an additional 2- to 3V drop caused by the switching circuitry, total voltage applied to the magnetic element can fall to a minimum of 13V.
Further complicating the designer's task, allowance must be made for variations in the element's resistance. That parameter can vary by a ratio of four to one across the -50 degrees to 250 degrees F operating range. The resistance of a typical valve might swing from 13 to 52e.
To ensure the magnetic element has enough amp-turns to activate the component, design engineers calculate the minimum available current. In the case cited above, this value would equal 13V/52e or 0.25A. The number of turns will be set high enough to ensure operation under all circumstances. In normal operation, however, maximum power required will be only 0.25A x 0.25A x52e or 3.25W. Given that the power supply will deliver 28V to the magnetic element, power dissipation will amount to 20.8W.
The circuit designed to deal with this problem turns out to have excellent fault tolerance, reports Barlage. "It can operate into a dead short for an indefinite time, while the input power actually goes down to a few tenths of a watt because the output power demand has gone down to about zero." If the load is removed, the voltage feedback network holds the output at a set maximum via the current-limiting inputs. Any single-point failures result in removal of the activating current from the magnetic element, shutting it off.
Improved fault tolerance means that not only can the circuit withstand a shorted load for an indefinite period, but it will automatically recover once the short is removed. In fact, because the circuit experiences less-than-normal operating stresses when the load shorts, field service technicians can deliberately short-out a load while troubleshooting system failures. "The usual approach is to apply voltage to the element to turn it on. This approach requires additional circuitry to sense failure and turn the component off," explains Barlage.
Aside from smooth operation during failures and faults, the design provides other benefits. For instance, the new control loop integrates the advantages of current-mode operation with the circuit's ability to output a high voltage, even on an 8 to 9V power bus. The result is a control-loop bandwidth three- to four-times greater than the conventional approach. According to Barlage, his circuit improves torque-motor responses significantly when shutting down on overspeed or during surge control.
"The improved fault isolation comes with no increase in component count, and reversing the normal on/off characteristics promotes additional safety," says Barlage. In the typical high- or low-side driver, once the switching element shorts, it applies the bus voltage to the magnetic element controlled by the switch. This action turns on the component that contains the magnetic element.
Should the component be a fuel valve on a jet engine, the result could be disastrous. So engineers must undertake additional efforts using the conventional circuit to ensure the component can be turned off. When the new circuit fails, it automatically falls-for example-into "valve off" mode, removing the need for additional hardware- or software-shutdown backup methods.
Currently just about every aircraft employs magnetic elements to control fuel delivery to jet engines or cabin-pressure control. Barlage estimates that production costs, and PC board area occupied by the new circuit, are about the same as for the old circuit. Therefore customers will realize an immediate benefit from the new topology without a need to redesign for bulkier components and additional control circuitry.
Wider dynamic operating bandwidth also results from the topology's ability to drive the element to the maximum operating voltage on every transient, if need be, to maintain the current at the set level. "The older approach was always limited by the available bus voltage," says Barlage.
The first commercial application for the new constant current topology is in the TFE 731 engine. It has also been implemented on the FSX Fighter, F15, C130 and Fokker 70/100. In almost every case, users identified a power savings of 23W per circuit. The design engineer's work has not gone unnoticed. Barlage recently won the prestigious Technical Achievement Award at AlliedSignal-few have been awarded during the company's history.
Additional details...Contact F. Michael Barlage, Senior Electronics Specialist, AlliedSignal Equipment Systems, Box 38001, Tucson, AZ, 85740-8001, (602) 469-6056.
F. Michael Barlage holds four patents and currently has several patent applications pending. Employed at AlliedSignal in Tucson, Arizona since 1984, he has spent approximately 30 years in various engineering fields. Barlage has worked with Hall Effect devices, lasers, gas turbine and diesel fuel controls, power supplies and power drivers. In 1976, he undertook Ph.D. studies in laser-plasma interactions at Colorado State University. He received his B.S.E.E. and M.S.E.E. from Ohio State University.
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