Power measurements made with conventional meters can be drastically inaccurate. Here’s how to decide when to bring in a specialized power analyzer.
BOB ZOLLO | KEYSIGHT TECHNOLOGIES, INC.
IT’S IMPORTANT to measure energy efficiency accurately in applications running from limited power sources such as batteries. And an understanding of how a device consumes energy can lead to a better understanding of operational costs regardless of whether the source of energy is limited or not. Server farms, for example, consume enough energy to influence power consumption on a national scale, and the operational costs of the server farm are set to a great degree by the cost of the power to run the servers.
High accuracy is often a requirement in measurements of energy efficiency. For example, developers may have to detect small changes in power consumption to determine if a new design hits overall device efficiency goals. As efficiency approaches 90% to 100%, the measurement of overall efficiency becomes quite challenging as it becomes necessary to measure tiny differences in power-in and power-out to determine the energy lost.
First a few basics. Electrical power is usually calculated by measuring both voltage and current and then calculating the product of the two. If the power is pure dc, this calculation is fairly straightforward. The measurement becomes more complex with ac power because it involves phase relationships, and often engineers turn to power analyzers for these measurements. But regardless of the measurement tool (DMM, voltmeter, ammeter, scope, power analyzer), power measurements must take place under the right conditions.
Suppose we measure the power a light bulb consumes. We can measure the ac power in, and this entity will be static (assuming we are not measuring an electronically dimmable LED bulb). We can first measure RMS voltage on the bulb, then the RMS current into the bulb, then multiply the two entities together and arrive at the right power consumption, as the RMS voltage and RMS current consumption won’t change vs time.
This is not the case for the power consumed by a washing machine. The washer cycles between pumping, soaking, agitating, and spinning. Each stage draws a different amount of power. So the manufacturer will probably average or integrate the power consumed over a full wash cycle to come up with an average consumption figure. This practice is probably OK as this cycle is predictable and repeatable. But other devices, like electric vehicles or cell phones, consume power in ways that are unpredictable and based on the behavior of the individual user.
In the case of these dynamic power consumers, the usual approach to measuring power consumption is to decide on a specific power consumption profile and only then settle on a measurement method. The reason is the power profile of devices in the real world can be complicated enough to demand specialized measurement methods.
Suppose, for example, we use a DMM to measure RMS voltage and then switch over to ammeter mode and measure RMS current. The current will be measured at a different time than the voltage. We won’t get an accurate calculation of power if the voltage and current are changing. Even relatively simple devices increasingly require a simultaneous voltage and current measurement to calculate instantaneous power accurately. The instantaneous measurements are then integrated or averaged over the time interval of interest to determine the overall power consumption.
So much for dynamics on a macro level, where it is obvious the power is varying. Now consider power on a microscale. Most power converters today are switch-mode devices (or switching power supplies). The input voltage to the switch-mode converter can be constant (dc or ac RMS), but the current will not be. The switch-mode power supply draws current nonlinearly – it does not look like a resistive load. It draws current in pulses if running from dc or in the form of non-sinusoid waveforms if running from ac.
These nonsinusoidal waveforms make it difficult to calculate the instantaneous power accurately. To properly measure power, you must simultaneously capture voltage and current, point by point, multiply them together to get an instantaneous power, then integrate the resulting relationship to determine the real power consumption.
Effectively, a power analyzer goes through the above steps. That is why a power analyzer is the weapon of choice when measuring power. Regardless of the wave shape or skew of the voltage or current waveforms, the answer will be correct as long as you are within the bandwidth of the power analyzer (typically 10 kHz or higher).
Measuring static dc/dc converter efficiency
Under static conditions (basically, a constant load and supply of power), it is relatively easy to measure the efficiency of a dc/dc converter. The inputs and outputs are dc, and if they not changing, then a DMM as a voltmeter can measure the input voltage. You can also use the DMM as an ammeter to measure average input current and calculate average input power. Similarly, a DMM can measure output voltage and current. With all measurements in steady state and in the form of dc, there is no concern about time skew between the voltage and the current measurement or between input and output power measurement.
DMMs can offer outstanding measurement accuracy so the accuracy of efficiency measurements based on their readings will be high as well. For example, in a Keysight 34460A DMM, the basic measurement accuracy of 0.0025% for dc voltage and 0.007% for dc current. The efficiency calculations based on these readings can be on the order of 0.1%, depending on the voltage and current range.
Now consider the case where conditions on the dc/dc converter change, as with a variable input voltage or variable load current. Now you must treat the converter as an ac device. When inputs or outputs are time varying, the DMM method will not always work. Voltage and current will most likely not be in phase. An RMS voltmeter can probably measure ac input voltage accurately, as it is most likely a sine wave. The current waveform will most likely not be a sine wave. It may have a high crest factor or other non-sinusoidal qualities, thus making RMS current measurements challenging.
A four-channel oscilloscope can be a good tool for measuring efficiency. Most certainly, all four channels will simultaneously measure input voltage, input current, output voltage, and output current. However, oscilloscopes are typically ground referenced, which could pose a challenge for measuring some power converters. But the problem can be addressed through use of differential voltage probes and current probes.
However, the oscilloscope does have some limitations. In general, an oscilloscope has about
3% error in its vertical measurements due to both the oscilloscope gain error and the external probe error. This 3% error will cause the oscilloscope to struggle to accurately measure efficiencies over 90% or to see incremental changes of 5% or less in efficiency. For measuring lower-efficiency devices, or to get a general sense for the efficiency, an oscilloscope is a handy tool.
Use of a power analyzer is a preferred method to measure power converter efficiency. These instruments are designed specifically to make accurate, simultaneous measurements of voltage and current. Multi-channel analyzers can simultaneously measure both input and output power on a single-phase or multi-phase ac or dc signal.
Most power analyzers are floating, capable of measuring hundreds of volts without differential probes, and can measure current directly. Power analyzers such as the Keysight PA2200 Series IntegraVision Power Analyzer easily measure power conversion efficiency with high accuracy. With a power analyzer, you can expect to measure power conversion efficiency with 0.25% to 0.5% accuracy. Power analyzers also handle the complexity of multi-phase ac power measurement.