Upgrade Power-Supply Efficiency with Precise Current Measurement (.PDF Download)
Improving the efficiency of a power-supply design can yield many rewards. In a server farm, better efficiency translates into smaller size, reduced cooling costs, and a lower electricity bill. In a smartphone or tablet, it might mean shorter charging time or longer battery life.
For both applications, the precise measurement of current plays a key role in boosting efficiency. This article will discuss how current measurement can improve performance, save power, and reap other benefits for power designs.
Overview of Precision Current Measurement
How do we determine the current flowing in a wire or trace? A common approach is to measure the voltage induced by the current across a shunt resistor. For the highest efficiency, a low resistor value of a few milliohms minimizes the power wasted as heat, but requires accurately measuring a voltage in the millivolt range.
In a power supply that uses digital control, the voltage typically is input to an analog-to-digital converter (ADC). Voltage in an analog design can be used to close a feedback loop.
It’s possible to measure this voltage with a standard op amp, or integrate the function into another building-block device. However, a dedicated current-sense amplifier (also called a current-shunt monitor) provides the highest overall performance.
Let’s compare these three options. Integrating the current-measurement function into a power-management device, for example, can reduce the cost and component count versus either an op amp or a dedicated current-sense amplifier. The integrated solution might therefore be the preferred choice in a cost-sensitive, space-constrained portable or wearable application. On the other hand, the performance is likely to suffer, since the silicon process is optimized for efficient power delivery, not precision analog measurement.
Choosing a standalone op amp certainly offers the designer a multitude of devices at various price points. However, the performance of the lowest-cost option probably falls short in two important specifications that directly affect measurement accuracy: input offset voltage (VOS) and gain error.