Application Guide Testing Power Supplies A general test set-up for power supplies is illustrated in Figure 1. This setup can be used for most critical parameters. Some general conditions apply: Equipment: 1. The accuracy of voltmeters and current meters should be approximate ly ten times the resolution required to measure a parameter. All meters should be 4½ digit. Figure 1 General Power Supply Test Set-Up 2. Any oscilloscopes used should have Set-Up a resolution approximately ten times the parameter being 1. The input voltage is set to “nominal” value. This is typmeasured. Bandwidth of the oscilloscope should be 20 ically 110 VAC or 220 VAC for AC/DC power supplies; MHz to 100 MHz. or 5 VDC, 12 VDC, 24 VDC or 48 VDC for DC/DC 3. The input power source used should be sufficient to converters. Nominal input values should be included supply the maximum input power required by the Unit on the product data sheet. Under Test (UUT), plus an adequate guard band. 2. The output load is set to the full rated value. Example Product 3. The ambient temperature should be maintained at 25°C. For our examples, we have used the specifications of the A512RW, a 5W DC/DC converter available from MPD. 4. All connections to the power supply should be made with great care to avoid inducing errors in the measurement. Specifications Input Conditions Nominal Voltage Specified Measured Units 24.0 VDC 18 to 36 VDC No Load Current 5 mA Full Load Current 251 Reflected Ripple 15 Voltage Range Ouput mA mA Specified Measured Units Nom. Input, Full Load 5.00 5.02 VDC 1.0 1.0 A Accuracy Nom. Input, Full Load ±1.0 ±0.4 % Line Regulation Low Line to High Line ±0.3 ±0.2 % Load Regulation Nom. Input, Full Load ±1.0 ±0.6 Voltage Conditions 250 Current Ripple & Noise Transient Recovery Time General to 1% for IOUT change of 75% to 100% Conditions Efficiency Nom. Input, Full Load Isolation Voltage Input to Output % 75 mV P-P 300 μSec Specified Measured 83 84 1,500 Units % VDC Power supplies with a remote sensing option should be connected in local sensing mode (with the sense leads con nect ed to the appropriate output pin). All test equipment should be al lowed suficient time to warm-up and stabilize before any testing begins. To help illustrate some of the examples used in this ap pli ca tion note, we will use the A512RW, a 5W DIP compatible DC/DC con vert er avail able from MicroPower Direct. Summary specifications are given in Table 1 at left. A full data sheet is available on the MPD website. MicroPower Di rect Equipment/Test Set-Up Go Direct - For Product, Performance and Price As a complex electronic subassembly, a power supply has a high number of parameters that are routinely specified by manufacturers. It is important to accurately evaluate the product initially in engineering to insure it is fit for use in the application. It is equally important that production testing insures an appropriate performance and quality level is maintained by the vendor. MicroPower Direct Output Voltage Accuracy Output accuracy is the maximum deviation between the measured output of a power supply and its specified nominal output. Typically given as a percentage, output voltage accuracy is sometimes refered to as “output voltage tolerance” or “output voltage setpoint”. The measurement is made as follows: 1. Set the input voltage to the specified “nominal” level. Set the output load to 100% of rated value. 2. Measure the output voltage using the 4½ digit voltmeter. This is VOUT. 3. Output accuracy (as a %) equals: VOUT - VNOM VNOM 5.00 VDC regulation as a “%/% change in VIN”. Essentially, this means the percentage change in input line voltage is factored into the calculation to derive output regulation. For a unit specified in this manner, line regulation is derived as follows: X 100 = 0.4% VOUTN - VMDEV ( VOUTN ( 5.02 VDC - 3.75 VDC 5.02 VDC 24 VDC - 18 VDC 24 VDC 25% Line Regulation Line regulation specifies the change in output voltage caused by varying the input line over a specified range. Expressed as a percentage, the range is typically low line to high line. The measurement is made as follows: 1. Set the input voltage to the specified “nominal” level. Set the output load to 100% of rated value. 2. Measure the output voltage using a 4½ digit voltmeter. This is VOUTN 3. Change the input voltage level to low line (or the specified low value for the line regulation spec.). Measure the output voltage. This is VOUTL ) ) X 100 X 100 1. Set the input voltage to the specified “nominal” level. Set the output load to 100% of rated value. 3. Change the output load to the specified low value. Measure the output voltage. This is VOUTML 4. Load regulation (as a %) equals: VOUTFL - VOUTML For our unit, an output load change of 75% to 100% is specified. At 75% load, an output voltage measurement of 5.05 VDC yields: 5.02 VDC - 5.05 VDC 5.02 VDC The output is changing 1% for each 1% change in input voltage. Obviously, care must be taken when using units specified this way, that the input line is very stable or that the application can operate with potentialy wide output swings. Load Regulation Load regulation specifies the change in output voltage caused by varying the output load over a specified range. Expressed as a percentage, the range will vary somewhat, dependent upon the product design; but is typically given as a condition in the manufacturers data sheet. The measurement Also called Periodic And Random Deviation, (PARD), this is the noise and ripple voltage super imposed on the output of a power supply. Output ripple is the periodic AC component and noise refers to the high frequency spikes that are unrelated to the switching frequency of the power supply. Due to the high frequencies involved, care must be taken not to induce errors into the measurement. An oscilloscope with a minimum bandwidth of 20 MHz so that all significant harmonics of the ripple spikes are included. Figure 2 illustrates how to make this measurement. A standard oscilloscope probe is shown in Figure 2A with a ground clip. When making measurements 5. Line regulation (as a %) equals: VOUTN X 100 Where VMDEV is the output voltage measured (VOUTH or VOUTL) that causes the greatest deviation from VOUTN. For our example unit, VOUTL equals 5.015 VDC, while VOUTH equals 5.03 VDC. Using 5.03 VDC, we derive: 5.02 VDC - 5.03 VDC 5.02 VDC X 100 = 0.2% Well within the specified ±0.3%. Some unregulated power supplies specify line X 100 = 0.6% Output Ripple & Noise X 100 = 1.0% 4. Change the input voltage level to high line (or the specified high value for the line regulation spec.). Measure the output voltage. This is VOUTH VOUTN - VMDEV X 100 VOUTFL Well within the specified ±1%. This in turn equals: 0.25 VDC Well within the specified ±1%. X 100 Where ∆VIN % is the change in input line as a percentage and VMDEV is the output voltage measured that causes the greatest deviation from VOUTN. If our example unit was specified this way; and an output of 3.75 VDC was measured at an input of 18 VDC. We could we derive the line regulation as follows: ( is made as follows: 2. Measure the output voltage using a 4½ digit voltmeter. This is VOUTFL ) ∆VIN % X 100 Where VNOM is the nominal output voltage specified for the power supply. For our unit, a measurement of 5.02 VDC yields: 5.02 VDC - 5.00 VDC Testing Power Supplies Figure 2 Measuring Output Ripple & Noise in a field of radiated, high frequency energy, the ground clip acts as an antenna. Since this could inject unwanted noise into the measurement, the clip should be disconnected and not used. The injection of unwanted noise is eliminated by using the technique shown in Figure 2B. Here the scope probe is placed directly across the output terminals of the power supply. The ground band on the probe is pressed against the output common terminal, while the probe tip is put in contact with the voltage output pin. This creates the shortest possible connection across the pow er supply output terminals. Another method for measuring true output noise and ripple is shown in Figure 2C. A 20 AWG “twisted pair” wire, approximately one foot in length, is attached between the power supply outputs and the load. An electrolytic capacitor (typicaly 10 μF to 50 μF) is connected across the load. The scope probe, with the ground clip disconnected, measure the the ripple at the connection of the twisted pair wires and the load.This technique will eliminate interference caused by common mode noise. Transient Recovery Time The time required for a power supply output to return to within specified limits following a step change in output load current. The specified load change and error band will vary with the product and/or manufacturer, but should be stated clearly in the product data sheet. Using the test set-up shown in Figure 1, measure transient response as follows: 4. Measure the transient response time as illustrated in Figure 3. If the power supply has multiple outputs, auxiliary outputs should be set to 100% load whiletransient recovery time is measured. Efficiency The Lowest Cost Power Converters! Don't bust your budget on expensive power conversion products. Go direct, MicroPower Direct, for our new "Budget Saver" product families. These high performance DC/DC converters offer a unique combination of economy, product selection, and field reliability. Standard features available include: • 1W to 10W Output Power A800ERW New 8W DIP! • Miniature Packaging • Regulated & Unregulated Models • Input/Output Isolation • Industry Standard Pin-Out The ratio of total output power to input power, expressed as a percentage. Efficiency has become an increasingly important specification as the power density of power supplies has increased and the relative size of electronic Full Info Online!! Got a problem? Tight budget? sytems has decreased. Efficiency is measured as follows: Check out our extensive line of standard 1. Set the input voltage to the specified “nominal” level. Set the output load to 100% of Nearly 2,000 standard models! rated value. 2. Using the test set-up shown in Figure 1, measure the output voltage (VOUT), output current (IOUT), input voltage (VIN) and input current (IIN). IOUT x VOUT TEL: (781) 344-8226 FAX: (781) 344-8481 WEB: www.micropowerdirect.com E-Mail: [email protected] X 100 IIN x VIN = = = = 5.02 VDC 1.00A 24 VDC 0.25A We can then derive the units efficieny as: 1.00 A x 5.05 VDC 0.25 A x 24.0 VDC Testing power supply isolation requires specialized equipment (Hipot tester) and can be destructive. This test should not be attempted without contacting the manufacturer. ATE Returning to our example unit, if we measure the following: VOUT IOUT VIN IIN = power converters then call for a surprisingly fast and low cost quotation from MPD. MicroPower Direct 3. Efficiency (as a %) equals: 1. Set the input voltage to the specified “nominal” level. 2. Program the the adjustable load for the step load change specified by the manuafacturer. In our case, this is 75% to 100% of rated load. GO DIRECT For 3. With the oscilloscope triggered externally, switch the load over the specified range. X 100 = 84% At 84%, this is over the typical specification of 83%. I/O Isolation The maximum voltage (ac or dc) that can be continuously applied from the input to output or input to case of an isolated power supply. Minimum isolation voltage levels must be maintained to meet most safety regulations. Typical isolation values specified by manufacturers are: AC/DC: I/O 3000 VAC I/C 1500 VAC DC/DC: I/O 1500 VDC (New Designs) 1000 VDC (Low power designs 500 VDC (Old Designs) Automatic test equipment (ATE) designed specifically for power supplies is available from a number of vendors. Systems range from highly sophisticated (and expensive) stand-alone equipment to lower cost PC based systems. In-circuit testers (ICT) that are not specifically designed for power supplies may also be used. Selecting the right ATE is dependent upon a number of factors including application complexity, production volume & flow, company procedures, and the availability of technical expertise. If problems arise in the automated testing of power supplies, the vendor should be contacted immediatly. The can help in correlating the data and verifying the source of any potential problem. In Summary Whether power supplies are being bench tested for an engineering evaluation; sample tested at incoming as part of standard QA screening; or tested in the application as part of volume production; great care must be taken to use the correct methods and end-point specifications. Use the expertise of the power supply manufacturer if you have any Figure 3 Transient Recovery Time Web: www.micropowerdirect.com ● Email: [email protected] ● Tel: (781) 344 - 8226 ● Fax: (781) 344 - 8481 Page 3