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issue: July 2007 APPLIANCE Magazine

Electronics Report
Isolated Power

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Whether used for appliance motor control or current detection, a new integrated circuit (IC) transducer allows engineers to achieve ac and dc isolated current measurement up to 100 KHz.

Named Minisens for its small size, a new application specific integrated circuit (ASIC) transducer from electronics supplier LEM offers full isolation (no optocouplers required) and high sensitivity (from 20 mV to 200 mV per amp of primary current), as well as additional benefits such as energy savings and cost-effective manufacturing.

According to Bernard Richard, application manager for Geneva, Switzerland–based LEM, the key feature the transducer offers appliance engineers is the ability to measure a current with isolation and without heating using a product that is compatible with power electronics such as A/D converters, microcontrollers, and DSPs. “The Minisens offers unique possibilities in the measurement chain that were previously not feasible,” Richard says. “It features a 5-V power supply and the possibility to use the same reference voltage for the transducer and the power electronics. Another benefit is the ability to have only one reference for the transducer to measure current from 2 A up to 100 A or more only by changing the PCB design on the customer side.”

Many parameters of the ASIC can be configured by on-chip non-volatile memory, including adjustment of the transducer’s gain, offset, polarity, temperature drift, and gain algorithm. Two outputs are available—one filtered, to limit the noise bandwidth, and one unfiltered, to ensure a response time of less than 3 microseconds.

In addition, the degree of electrical isolation and output sensitivity can be increased by the PCB design. For example, a primary track on the opposite side of the IC gives the best isolation; a track on the same side gives the highest sensitivity. To minimize manufacturing costs, the device is mounted as part of a standard PCB assembly process.

With dimensions of an SO8 package (3.9 × 4.89 × 1.62 mm), the component’s small size is certainly another plus for designers. “We were able to achieve these dimensions due to the fact that we could develop an ASIC that includes all the electronics, the Hall element, and also integrates the magnetic material,” Richard says.

The transducer’s noncontact measurement enables an almost unlimited current level, eliminating the need for current flow through the device. The only limiting factor is the thermal capacity of the primary conductor; however, the current can be carried either by a track (or tracks) located on a PCB underneath the transducer or by a cable or bus bar under or above the IC.

The component can also enable enhanced motor control to produce energy savings where other methods, such as the shunt, have traditionally been used. Richard says this means the transducer is suitable for any appliance application using motor control and where performance or energy efficiency can be increased by using electronics. Specific examples include washing machines and HVAC systems with inverters. “For these two applications, Minisens enables engineers to develop a system with inverter control by measuring the current with isolation directly on the motor phases,” Richard explains. “This solution drastically reduces the complexity of the programming in comparison to a measurement without isolation on the dc link. The measurement directly on the motor phase of the motor allows reaching better performances in terms of torque (fewer ripples) or speed.”


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