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1. It is important to note that the energy efficiency standard demands an input power measurement uncertainty of less than 2 for power measurements of 0 5 W or greater and 10 mW for power measurements less than 0 5 W Wattmeters from several leading instrument manufacturers can be configured to meet these requirements Since a wattmeter features both a current and voltage sensing element the voltage sensing element can be configured either before or after the input current sense element Typically information on how to configure the instrument can be found in its user manual For low or no load measurements you can achieve better accuracy by configuring the voltage sense before the current sense element This prevents the current from the voltage sense element from being measured by the current sense element Given that the current consumed by the voltage sense element is typically greater than the allowable 10 mW at 230 Vac this configuration is critical to meeting the standard s low measurement uncertainty requirements AC Source Fig 3 Wattmeter connections Connect voltage sense before current sense for low or no load measurements High Power Applications Higher power designs present different issues In these applications the power loss in the voltage sense element is so small the voltage sense element can be connected after the current sense to locate it close to the power supply input This approach prevents the voltage drop2. across the current sense element and the internal 2012 How2Power All rights reserved Page 2 of 6 Exclusive Technology Feature wiring of the wattmeter from being incorrectly included in the power measurement That in turn can lead to lower efficiency calculations By setting the meter to average over 32 or 64 samples you can generate more Stable results You will need two multimeters to measure output power one to measure output voltage and the other to measure current Use the higher resolution meter to measure current Since output power is purely dc it is calculated by multiplying output voltage times output current To simplify the task of determining whether the specifications of your power supply comply with worldwide regulations Power Integrations has developed a helpful Energy Compliance Calculator The targets for no load and active mode efficiency are derived from the nameplate rating of each power supply That rating is simply the rated output marked on the power supply case and represents the minimum rated output power of the supply at room temperature and nominal line As an example a constant voltage constant current charger with a nameplate rating of 5 V 350 mA will deliver 5 V 350 mA minimum Once you enter the nameplate output power rating for your power supply the calculator will automatically tell you the target values for compliance to the energy standards relevant to your design Designs that accept universal
3. input voltages require measurements made at both 115 Vac 60 Hz and 230 Vac 50 Hz Measurements for single input designs should be made at the nominal line voltage of either 115 Vac or 230 Vac To demonstrate this process let s look at the test procedure for a 5 V 350 mA cell phone charger rated for universal input We begin with a series of tests to measure active mode efficiency at 115 Vac 60 Hz Active mode efficiency is the average of efficiencies measured at 25 50 75 and 100 of the nameplate rating at a nominal line input voltage and nominal line frequency Since the nameplate load rating of this charger is 350 mA efficiency must be measured at the following currents full load 350 mA 75 load 262 mA 50 load 175 mA and 25 load 88 mA Begin the test process by warming up the power supply for 30 minutes To make the first measurement at 100 load connect the power supply to an ac source and apply a 60 Hz 115 Vac input Gradually increase the load on the power supply to 100 and allow at least 30 minutes for the circuit to reach thermal equilibrium and the input power reading to stabilize Before recording any measurements make sure no oscilloscope probe or other meter is connected to the circuit In some cases you may need to manually set the voltage and or current range of the wattmeter to prevent it from auto ranging and giving fluctuating results The same measuring instrument in two different ranges will typically have
4. recorded as 0 35 A and the output voltage is 6 124 V from the multimeters giving an output power of 2 14 W Efficiency is calculated as Efficiency at full load Pout Pin 2 14 W 3 14 W 68 2 To make the next measurement adjust the load to 75 or 262 mA When using the averaging feature on the wattmeter remember to allow at least one minute for the reading to stabilize Then record the input power from the wattmeter Wait five minutes record again and use the lt 5 difference rule to decide whether to use the value or calculate the integral input power In this example the output current is recorded as 0 262 A and the output voltage is 6 502 V giving an output power of 1 704 W Efficiency is then calculated as Efficiency at 75 load 1 704 W 2 42 W 70 4 This procedure should then be repeated for 50 and 25 load levels Measuring No Load Input Power TO measure no load input power disconnect the output load and all output multimeters from your power supply Next turn off the ac input and configure your wattmeter with the voltage sense element before the current sense element Take an initial power reading from the wattmeter and wait five minutes before taking a second reading As we have discussed in previous test procedures record the second reading if the difference between the two readings is less than 5 If not you must integrate the input power and divide by the time period of integration using the process we describ
5. 58 EC Ecodesign EuP Tier 1 0 671 at 75 0 704 0 5838 EC Ecodesign EuP Tier 2 0 0 671 A at 100 0 633 0 687 China USB Charger Avg iden 702 ou in sa For mobile phone chargers lt 8 W no load power s 0 25 Enter percentage as decimal EC Integrated Product Policy IPP 2 4 Four Stars 0 03 to 0 15 W Note This calculator is for power supplies of lt 250 W only While every effort will be made to keep source data current Power Integrations is not liable for inaccuracies To obtain most recent data please refer to published standards of specific agency Fig 4 Data entered into PI Efficiency Compliance Calculator Power Integration s calculator computes active mode efficiency as the equally weighted average of efficiency across all load levels The fields on the right side of the screenshot above show various efficiency standards The calculator automatically calculates compliance requirements for active mode efficiency and no load input power and compares your test results with the standards requirements To simplify analysis of results it highlights those results that meet requirements in green and those that do not in red For those not using Power Integration s calculator to perform efficiency calculations see the test tip Calculating By Hand in the appendix Conclusion As leading standards organizations focus on eliminating wasted power they are developing increasingly stringent efficiency req
6. Exclusive Technology Feature ISSUE August 2012 Testing Power Supply Compliance To Energy Efficiency Standards by Silvestro Fimiani Power Integrations San Jose Calif Over the last several years leading worldwide standards organizations including the Environmental Protection Agency s Energy Star program the European Commission s Code of Conduct and the California Energy Commission CEC have defined new efficiency requirements for external power supplies These standards demand that designers test their products for active mode efficiency and no load power consumption to high levels of accuracy To help simplify that process and accelerate product development this article will describe a relatively straightforward way to measure compliance to these evolving energy efficiency standards and offer a few testing tips Basic Requirements We ll begin with the equipment needed to perform these tests You will need four components to accurately measure your product s efficiency 1 A wattmeter 2 A programmable ac source 3 An electronic load 4 Two digital multimeters one must be a high resolution current meter Next a few general guidelines will help you achieve accurate results First since the energy efficiency compliance measurement of a power supply is a system level test that includes losses in the input and output cables you must ensure that the same cables used in this testing process are used in the final product
7. Second it is important to note that these tests require long temperature stabilization periods between output line and input line voltage changes Therefore expect the tests to take several hours to complete Finally every time you alter your product design these tests must be repeated to ensure accurate results Maximizing Accuracy This test requires measurement of both no load input power and active mode efficiency of the supply To calculate efficiency one must measure both input and output power When measuring input power the selection of the proper power source is crucial As the graph in Fig 1 illustrates the use of raw ac power from a wall outlet and Variac introduces inaccuracy into the measurement To ensure the testing is performed at a precise input voltage use a programmable ac source instead Wall Outlet Voltage Waveform AC Source Voltage Waveform Fig 1 Comparison of raw mains ac quality vs a programmable ac power source 2012 How2Power All rights reserved Page 1 of 6 Exclusive Technology Feature The graph in Fig 2 depicts the output from a programmable ac source Note the incoming waveform is a pure sine wave P V x I x COS Voltage Current Fig 2 Power factor cosine of phase angle between voltage and current waveform To make this measurement we use a wattmeter because it factors in the effect of power factor cosine of the angle between the voltage and current waveforms cos
8. different levels of accuracy Typically the operator s manual will indicate the range that will generate the highest accuracy Once the circuit has reached thermal equilibrium take the initial power reading from the wattmeter Wait five minutes and then take a second reading If you find less than 5 difference between the two readings record the second reading If the difference is larger than 5 you should wait another five minutes and continue to do so until two successive readings are within 5 of one another Alternatively you can use the integration mode available on most wattmeters and described in the following section Calculating Integral Input Power To measure the input power of a design where the input power varies over time take the following steps 1 Set your wattmeter to integration mode 2 Set the interval of integration for your wattmeter to capture approximately one full cycle of variable input power The longer the duration the higher the accuracy of results We recommend integrating over a period of 1 minute for most applications 3 Read the input energy from your wattmeter as W hr 4 Divide this number by the interval of integration Make sure to adjust the time units so they cancel For example Input Energy W hr Measurement Interval min 60 mins 1 hr Input Power W 2012 How2Power All rights reserved Page 3 of 6 Exclusive Technology Feature In this example the output current is
9. ed earlier This completes testing at 115 Vac Next you must reattach your load and output multimeters and increase the input voltage to 230 Vac Repeat all the previous tests at the new line voltage Remember to set up your wattmeter appropriately for your power range Once testing is complete you should have a complete set of data listing the results for all four load levels at both line voltages To ensure accurate measurements when changing line voltage see the tip Changing Input Voltage in the appendix At this point you can open the Efficiency Compliance Calculator and enter the nameplate specification in the top input fields and the newly generated test data in the columns on the left side Fig 4 2012 How2Power All rights reserved Page 4 of 6 OW2POWER c3 Your Power Design Newsletter Exclusive Technology Feature 4 External Power Supply Efficiency Compliance rent A Calculator by E E A POWER INTEGRATIONS Enter the specifications no load input power and measured active Nameplate Power W 1 75 mode efficiency data for your power supply to see if it complies with current worldwide energy efficiency regulations Enter Power Supply Specification 11S5S VAC 230 VAC No Load Active No load Input w 0 096 0 12788 ao oe ft ENERGY STAR v2 0 66 Enter Input Efficiency ai oez 11S VAC 230 vac EISA 2007 0 671 at 25 0 705 0 6368 EC Code of Conduct v4 0 671 at SO 0 7178 0 67
10. uirements for external power supplies At the same time efforts to synchronize these standards around the globe promise to bring conformity from region to region By following the test methods outlined above designers can ensure their power supplies meet these emerging regulations and take the first step to accessing worldwide markets Appendix Test Tip Calculating by Hand If you decide to calculate values by hand it is important to note that standards requirements are rounded to two digits for both no load requirements and active mode efficiency This seemingly minor point can play a major role in standards compliance Take the sample calculation below for minimum efficiency using the Energy Star formula for a 12 V 1 1 A supply Nameplate power 12 Vx 1 1 A 13 2 W V Nmin 2 0 0626 x In x 13 2 gt 0 784 or 78 4 gt 0 78 or 78 rounded to two digits Now let s look at an example that shows the effect of rounding using measured results for the same 12 V 1 1 A Supply 2012 How2Power All rights reserved Page 5 of 6 ee ONS Exclusive Technology Feature Your Power Design Newsletter Navg N25 Nso N75 Nioo 4 79 2 78 4 75 9 80 5 4 0 785 or 78 5 0 79 or 79 rounded to two digits As the measured average is 79 the power supply meets the standard s requirement of 78 efficiency Test Tip Changing Input Voltage Are you measuring no load power consumption and planning to change input
11. voltage during testing Always apply full load to the output during this transition Why Under no load the input bulk capacitor takes a long time to discharge If the output is left with no load connected when the input voltage is dropped from high line to low line the capacitor will support the dc bus for a long period of time before drawing power from the ac input That in turn causes the power supply to have a no load input power of 0 W To avoid this problem begin with the power supply fully loaded and only then remove all output loads before beginning the measurement About The Author Silvestro Fimiani is product marketing manager of appliance and industrial applications at Power Integrations PI Prior to joining PI in 2005 Silvestro served as director of engineering of high power products at International Rectifier He holds a Bachelor of Arts in Physics from the University of Naples Italy For further reading on power supply efficiency issues see the How2Power Design Guide select the Advanced Search option go to Search by Design Guide Category and select Efficiency in the Design Area category 2012 How2Power All rights reserved Page 6 of 6
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