Expanding the dynamic range of current measurement
By E Lange, LEM USA Inc
Electricity+Control February 2014 (pages 22 – 24)
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In power electronics, the accuracy with which electrical current is measured is not based solely on the selection of a suitable transducer. Level-shifting and intermediate scaling in the circuit between the microcontroller and the transducer can introduce offset and gain whilst, in closed-loop designs, the use of burden resistors can affect the accuracy of the current measurement. The conversion of analogue current measurement into a digitised format is another area in which accuracy can be compromised.
These factors often mean that the measuring device needs to operate over an expanded range. This range typically combines the normal operating range, of between 80% to 125% of the nominal current, and an additional 300% of the nominal current to provide protection against over current events. The result of measuring this expanded range is that a limit is placed on the dynamic range which can be used for control.
The challenge of expanding the dynamic range can be addressed by using open-loop Hall-effect current transducers that use an Application Specific Integrated Circuit (ASIC)-based approach to over current detection (OCD). This construction enables techniques such as Hall-cell spinning, which is used to reduce offset. In addition, it provides programmable sensitivity and over current detection as a separate, programmable output. By externalising over current detection, the transducer is freed from measuring the peaks that are required for protection and enables its full measuring range to be used to measure the application’s dynamic operating range.
The benefits offered by Application Specific Integrated Circuit (ASIC)-based transducers with over current detection are not limited to enabling an expanded dynamic range. Transducers such as the HO series - based on the Open Loop Hall - can help to reduce component count and, as a result, deliver higher reliability. They can also enable higher resolution from the analogue-to-digital converter and improve the signal-to-noise ratio as well as providing higher sensitivity which increases the gain as the volts-per-amp increase.
Figure 1: Open-loop, Hall-effect current transducers using an application specific integrated circuit in the gap of the core.
- By externalising over current detection, the transducer is freed from measuring the peaks that are required for protection.
- Application specific integrated circuit-based transducers with over current detection are not limited to enabling an expanded dynamic range.
- Transducers based on the Open Loop Hall can help to reduce component count to deliver higher reliability.