by Dave Pereles, Tektronix
Most people and certainly most engineers appreciate the importance of minimizing the power that products use in standby mode. The stakes are high for engineers because standby performance may determine whether you get to claim ENERGY STAR compliance or compliance with European norms. Designing a product with low standby power is hard enough, but then you still have to prove your design actually works as intended.
Before I looked more closely at this subject, like many engineers I believed that measuring the standby power of a device was simply a matter of measuring low current and doing a little bit of math. I knew that because of the low currents involved, these measurements are generally done with digital AC power analyzers to get the required current and voltage resolution and accuracy. These are essentially sampling systems that produce a stream of accurate power measurements. So far, so good. But making simple power measurements is just the beginning.
I recently helped a colleague with a presentation on this topic and I quickly realized that real-life designs don’t draw power in tidy straight lines. Since real designs have constantly changing current draw all sorts of questions arise:
- How often do you have to sample?
- How long do you have to test? Should you average the results?
- Standby currents are usually highly distorted – capacitors within the power supplies can often provide most of the power needed, but they need to be recharged in small “gulps.” Current and voltage are out of phase. How do you deal with high crest factor and low power factor?
- How do you determine if the power draw is stable enough to draw conclusions?
- Under what conditions should you test? Line voltage distortion? Ambient temperature? Air flow?
The standby power dashboard, included as part of Tektronix PWRVIEW PC software, uses logic determined by IEC62301 to address measurement challenges.
While working with my colleague the applications engineer, I learned that these questions were thought through by the engineers who designed IEC62301 Ed.2:2011 and its derivative, EN50564:2011. The IEC62301 standard is referenced by both ENERGY STAR and 1275/2008/IEC. We have a Standby Power Primer available for download and it goes into much more detail.
The standard covers test conditions like line voltage stability and uncertainty with crest factor and power factor baked in. It describes a sampling approach that sets rules for reading rate, test duration, and stability.
High crest factor and low power factor make maintaining measurement accuracy challenging. The designers of the standard recognized this so they came up with a figure called Maximum Current Ratio (MCR) which is defined as the ratio of Crest Factor to Power Factor. A high crest factor will drive a high MCR as will a low power factor. If the MCR is 10 or more, the standard allows a bit higher measurement uncertainty.
Stability per the standard is determined by a least-squares linear regression though all the power measurements. For devices that use more than 1 W, the measurements are deemed stable if the resulting slope is less than 1% per hour. For devices that use 1 W or less, the measurements are deemed stable if the slope is less than 10 mW/h.
While these constructs help answer some of the measurement questions posed earlier, they also complicate the measurements and analysis that goes into standby power testing. PC software that works with power analyzers can automate this process and make these determinations in compliance with the standard. Be sure to download the Standby Power Primer to get more detail on testing procedures using a Tektronix power analyzer and for insight into how our PWRVIEW PC software works.