Estimates of the typical household energy wasted by electronic equipment in standby mode range from 5% to 15% of total household power consumption. There is now International awareness of the financial and environmental cost of this wasted energy. This recognition has resulted in standards that force manufacturers of electronic products to reduce the power that their devices consume when not in normal operation.
|You can download this AppNote here (2.2Mb pdf)|
There are an increasing number of domestic standards that specify the power limit associated with particular product groups or categories within a product group. Domestic standards include: Energy Star, Blue Angel, EcoDesign, Top Runner and Nordic Swan.
However, the internationally recognised standard for the measurement technique and measurement accuracy for standby power is IEC62301. The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). Any regulatory body that wishes to use a different technique to that defined by the IEC must separately define the differences, so in most cases, regulatory bodies have chosen to adopt the IEC 62301 standard.
In the following table there are example limits for power adapters, in the following document we will explore what challenges power analyzer designers are met with when undertaking the measurement of standby power and how we overcome these challenges.
Many power analyzer manufacturers claim to have a ‘solution’ to IEC 62301 testing but generally, these products have weaknesses.
The two most common weaknesses are:
1. Long integration required to obtain a stable power reading
If the measured power is not stable, the IEC permits long measurement periods in order to provide a stable reading. However the instability is due to the power analyzer not the DUT. High performance power analyzers can achieve measurement stability with a short measurement time; therefore minimum test time can be achieved.
2. External shunts are required to measure low current
For measurement applications with current down to 1mA, external current shunts can be helpful but this should NOT be necessary for IEC 62301 testing. External shunts add complication and add error stages to the system. The best power analyzers can test to IEC62301 using only the internal current shunts. High quality power analyzers with a good dynamic range do not require an external shunt to measure standby power. Here, a PPA5500 power analyzer with 300Apk and 30Arms direct inputs easily measures a low duty cycle standby power waveform with 24mApk and 8.5mArms.
As described previously, the Energy Star program along with all major standards that are associated with standby power now recognize IEC 62301 as the reference for measurement techniques and accuracy.
“It is also desirable for measurement instruments to be able to average power accurately over any user selected time...” and “As an alternative, the measurement instrument would have to be capable of integrating energy over any user selected time interval with an energy resolution of less than or equal to 0.1 mWh and integrating time displayed with a resolution of 1 second or less.”
Under low power conditions, it is clearly important that the measurement instrument being used has a current measurement channel with sufficient sensitivity to measure the minimum expected current.
With a sinusoidal current waveform, this may be relatively easy but with peaky current demand such as that pictured above that produces a high crest factor, this becomes more difficult.
The problem is complicated further with a DUT that exhibits low duty cycle current pulses.
A common mistake made by many instrument vendors is the assumption that a standby power profile is periodic and therefore can be accurately quantified with gaps between measurements by integration over a long period of time, but this not true.
The above waveform is closer to what would be encountered by a power analyzer which is measuring standby power.
In practice, low duty cycle standby modes are usually not symmetrical and in fact, this is also true of more traditional power supply designs with continuous cycle by cycle power consumption. To obtain the true standby power, an ideal power analyzer would measure continuously so that no event is missed. However, most power analyzers have a gap between measurement windows and therefore the greatest cause of instability in measured power is often the power measurement equipment, not the DUT.
Most power analyzers have gaps between measurement windows, with non periodic current demand or multiple stage standby, such techniques may miss events and provide only an ‘average approximation’ rather than a ‘true’ measurement.
If an instrument with no measurement gap is selected, the consumed power measurement can include all events. Given the peaky nature of most standby current profiles, it is common to think only about the current pulses but it is very important accurately measure the residual current between peaks because this often represents a significant proportion of the total standby power.
The ideal solution would have continuously variable measurement windows that automatically fit to the changing current pulse period. In this way, the power measurement will quickly reflect the true standby power.
Here and in the following pages, we illustrate the measurement of three different low duty cycle standby modes using direct connection to the standard Voltage and Current inputs of a PPA series power analyzer.
|Duty cycle 1 - 5 Standby period 10Hz||Note: 23.82mApk / 8.5597mArms = 2.78 CF|
|Duty cycle 1 - 20 Standby period 2.5Hz||Note: 23.48mApk / 4.9461mArms = 4.75 CF|
|Duty cycle 1 - 50 Standby period 1Hz||Note: 23.47mApk / 3.7884mArms = 6.19 CF|
Class leading frequency range, sample rate and crest factor combined with unique current shunt technology and no-gap analysis, the PPA series provides the best possible measurements for standby power to IEC62301.
Compliance to IEC62301 requires the ability to maintain defined measurement accuracy when measuring any DUT in standby mode. PPA series power analyzers provide complete assurance by being well within the required accuracy.
Required Watts accuracy @ > 0.5W = 2.0%
PPA2530 Measured accuracy is within 0.2%
Required Watts accuracy @ < 0.5W = 0.01W
PPA2530 Measured accuracy within 0.001W
The standard states that approved meters will include a “Power resolution of 1mW or better” and also that “Measurements of power of less than 0.5 W shall be made with an uncertainty of less than or equal to 0.01 W at the 95% confidence level”. The ideal measurement solution will therefore provide a resolution of 0.0001W.
Note: IEC62301 also specifies test conditions under which power measurements should be made. Total Harmonic Content of the supply voltage (up to and including the13th harmonic) must be less than 2%. Voltage Crest Factor should be between 1.34 and 1.49.
Due to the complex nature of standby power, it is common for statements of accuracy to be made with little supporting evidence. However, in common with other areas of metrology, power accuracy can be proven by comparison of measurement results with a known or calculable reference.
In this case, three controllable elements are required:
When each signal is constant, measurement of the respective power at upper and lower signal levels is quite straight forward. Deriving the correct power for a composite signal of defined duty cycle is then a simple ratio computation.
|External shunt (0.47mO 3Arms 30Apk)|
|Internal shunt (0.01mO 30Arms 300Apk)|
From the power measurements of pulse on and pulse off periods, we can calculate standby power simulations as follows:
Continuous Power = 2.75W
Off Power = 0.121W
= 1/5 on + 4/5 off
= 0.55W + 0.097
= 1/20 on + 19/20 off
= 0.1375W + 0.115
= 1/50 on + 49/50 off
= 0.055W + 0.119
1 in 5 Standby Power Real Test
We will now use the same power supply measure in the previous pages and set it to 1 in 5 standby power mode.
Previously calculated 1 in 5 standby power from On and Off periods
The above pictures show the instrument measured very accurately the 1 in 5 standby power.
|Internal Shunt||External Shunt|
|Internal Shunt||External Shunt|
The ‘Standby POWER’ program makes testing that is compliant to IEC62301 a simple 4 step process.
Step 1: Enter details of DUT and Test Environment: Date, Time and Measurement Instrument details are entered buy a button click.
Step 2: Nominal test conditions are tested by clicking on a ‘read data’ button. All values will be measured against the required limits.
Step 3: Start a test with either manual ‘start’ – ‘stop’ buttons or set a test period, then ‘run’ and the standby power test will start, count down the requested time and then stop.
Step 4: At the end of a manual or automatic test, click on the ‘Export to .xls’ button and a spreadsheet will open with all test details, test conditions and test results automatically entered. The spreadsheet can be saved to any file and is pre-formatted for direct printing.
Example Standby Power Test report in accordance with IEC62301