Real-time energy consumption measurement, including input real power, input RMS voltage and input RMS current measurement for off-line power supplies, is becoming more important nowadays. These measurements could be used to adjust power delivery and optimize energy usage. Traditionally the input power and current are measured by a dedicated power metering chip, however, it adds extra cost and design effort.
For an analog-controlled power supply, using a dedicated meter IC is the only choice if power metering is a requirement. However, with a digital power controller, since it already has analog to digital converters (ADC) and CPU, it could be used for input power and current measurements as well. The implementation of a power metering function in a digital power controller seems straight forward, but there are several implications that make it really challenging.
First, the digital controller is primarily a power controller; its main goal is to control a power supply. Any extra functions of the controller should not deteriorate power supply performance, such as power factor (PF), total harmonic distortion (THD), etc. Therefore, the firmware needs to be carefully organized such that only spare CPU time will be used for power metering function.
Second, the digital controller is optimized for power supply control, not optimized for power metering purposes. For example, the dedicated meter IC may have up to 22 bits ADC, while most of the digital power controllers on the market have ADC of only 10 or 12 bits. How to use a not so good ADC to achieve the same measurement accuracy is challenging.
Last, the power metering accuracy is very strict. For example, the input real power measurement needs to be 1% accurate. It is probably not so difficult to implement power metering function in a digital controller, but it is very difficult to meet the accuracy requirement. Special actions need to be taken such as measuring voltage and current at the same time, EMI filter reactive current compensation, etc.
Don’t be scared of these challenges, although it is difficult, it is definitely doable. TI has successfully developed a low cost, but accurate power metering solution based on digital power controller UCD3138, it has the following features:
For PFC applications where the input power measurement is required, using a digital PFC controller and implementing a power meter function in the controller is an effective way to reduce cost. Now it is not only a power supply, it is also a power meter. This will place your power supply in an advanced position among your competitors.
For more information please visit www.ti.com/digitalpower
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