AC Current Sensors Chip Away at Losses and Design Size

Aug 1, 2007 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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A family of IC-based ac current sensors from Silicon Laboratories provides a smaller, lower-loss alternative to conventional current-transformer (CT) circuits used to control and protect power systems. The Si8500 family of unidirectional ac current sensors integrates the equivalent of a 20-A CT circuit into a 4-mm × 4-mm × 1-mm QFN. In so doing, the sensors reduce pc-board space requirements up to 75%, versus the conventional discrete designs, while also improving the accuracy, quality and reliability of the circuit.

An Si8500 sensor takes a monolithic sensing circuit with on-chip inductor and co-packages that die with a metal-slug sense element (see the figure). When current flows through the slug, it induces a current in the on-chip inductor. That signal is then integrated and compensated for temperature and offsets with auto-calibration circuitry. This circuitry improves manufacturability and reliability by eliminating measurement offset and temperature sensitivities found in discrete CT implementations.

This chip-based sensor also features signal-conditioning circuitry, which combines with the sensor's low-parasitic inductance to deliver a clean current signal that is free of the ringing usually produced by conventional CT circuits. Other on-chip functions eliminate the need for a blocking diode and burden resistor, common in CT circuits.

To save further cost and space, the company offers the Si8511, a version with a “ping-pong” output that enables one device to replace two CT circuits in full-bridge designs. The Si8500 is suited to a range of applications, including ac-dc power supplies, isolated dc-dc converters, motor controls and electronic lighting ballasts.

The current measurement produced by this device is accurate to within a tolerance of ±5% with a 2-V_PP full-scale output; that compares with about a 15% tolerance on accuracy for a conventional CT circuit.

According to the vendor, the Si8500 architecture provides greater than five times lower series resistance and more than two times lower series inductance than traditional discrete implementations. The metal slug exhibits 1 mΩ of resistance versus 6 mΩ to 20 mΩ for existing CTs. The inductance of the slug is less than 2 nH. The lower resistance reduces power dissipation versus discrete CT circuits, while the lower inductance contributes to a cleaner sensor output as previously described.

Meanwhile, using the Si8500 sensor reduces the bill-of-materials from four parts (transformer, diode, resistor and capacitor) to two parts (IC and capacitor). In a typical 20-A current-sense circuit, this results in a reduction in pc-board footprint from about 60 mm² for the conventional CT circuit to 16 mm² for the Si8500 design.

The 20-A rating on the Si8500 corresponds to the measured current level that produces a full-scale 2-V output from the sensor. Given the slug's 1-mΩ resistance, that current level produces 0.4 W of dissipation in the slug, assumming a 100% duty cycle. For customers who wish to measure current levels of 30 A, the sensor can still be used if heatsinking and/or airflow can be provided. Without proper thermal management there is the potential for indirectly overheating the CMOS die. An overvoltage condition would be another potential failure mode.

Other limits on device operation include a maximum switching frequency of 1.2 MHz for the current-sense signal. In addition, there's a duty-cycle limitation imposed by the requirement that the chip's integrator be reset to 0 A for 250 ns every cycle. The company notes that these limits are much broader than those found in traditional solutions.

The Si8500 family is now sampling with production slated to begin in the fourth quarter. Pricing starts at $1.49 each in quantities of 1000. The company also offers an evaluation board with leads that can be soldered directly into the customer's application.

For more information, see www.silabs.com.

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