Walnut Creek, CA--In designing medical equipment, Wayne Merryman, chief electronic engineer at Fresenius USA, is always looking to enhance the medical imperative of increasing reliability. As an example, a dialysis machine being developed to cleanse blood must precisely move fluids through a set of tubes surrounded by semi-permeable membranes--and over 30 valves may be needed in a given unit. But two problems are often encountered in electromechanical control of such fluid flow and other functions, he notes. "Heat from the actuator and power driver challenges reliability and the size and weight of the power supply add bulk."
To counter these concerns, recently developed DRV101 stepped pulse-width drivers from Burr-Brown (Tucson, AZ) may be incorporated in Fresenius' next generation Hemo-dialysis system. The advantage in using this single-chip, pulse-width modulation (PWM) power switch to energize the solenoid pinch valves is, Merryman says, "The steady-state or holding current can be appropriately set to a reduced level while still allowing full power (100% duty cycle) for device activation. Thus, heat in the areas of both the actuator and driver is reduced by a factor of between two and eight." Less heat boosts reliability and permits greater density of the power drivers without the bulk and cost of heat sinks. Moving components such as fans may also be eliminated.
These benefits arise from "the actuators themselves remaining cooler because the average current is reduced by a percentage equal to the duty cycle, which can vary from 10 to 90%," he adds. Similarly, the drivers run cooler, which is also aided by low-saturation on-voltages. And current reductions mean a proportionate decrease in power-supply size and weight, as well as lower energy requirements and power-consumption costs.
Driver PWM output functions include an internal 25-kHz oscillator, digital control input, and external-delay and duty-cycle adjustments. The latter is set by a resistor or controlled by an analog voltage, directly or via a D/A converter. A flag output indicates thermal shutdown or over/under current conditions. In addition to its medical use, the DRV101 can drive positioners, relays, and actuators in applications including industrial and motor control, and automation.
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