POWER SUPPLY SPECIAL - bearpwr.com

1 POWER SUPPLY SPECIALPOWER SUPPLY SPECIALIn ac/dc POWER convertersabove a few watts, a largeinrush current flowswhen the input capacitorsare suddenly charged duringthe initial application ofpower. If unrestricted, theinrush current can easily ex-ceed 50 A at the peak of theac cycle and severely stress theconverter s fuse and input rec-tifiers significantly reducingthe reliability and life expectancyof the modules. POWER - SUPPLY susceptibilityUniversal POWER supplies are particu-larly subject to high inrush currentsince their input capacitors must belarge enough to handle line voltagesas low as 110 Vac, as well as voltagesas high as 240 Vac at turn-on.

2 Design-ers of commercial, industrial, andmedical systems need to pay specialattention to inrush current. In theseenvironments, a POWER - SUPPLY failureor a tripped circuit breaker can be in-convenient at best, and expensive ordangerous at worst. Supplies for outdoor use such as security systems aretypically in remote hard-to-reach locations. Accessing thedevice to replace a POWER supplyis usually inconvenient and ex-pensive. Industrial facilities such as manufacturing plants often have multiple supplies ona line, and the combined inrushcurrent can trip a circuit inrush-current-limiting techniques increase the reliability and life expectancy of POWER supplies63 ELECTRONIC PRODUCTS MARCH 2006 Understanding POWER supplies and inrush currenthundreds of microfarads andis usually sized to meet aminimum holdup time andripple-current rating.

3 This figure shows that if theswitch is closed near the topof the ac cycle, a large inputcurrent can occur, limitedonly by R1, making it quitepossible to get an inrush-cur-rent spike of 50 A or more ona nominal 120-Vac line (170V peak). In countries wherethe nominal line voltage is240 Vac, the inrush currentcan exceed 100 large inrush currentdegrades the performanceand lifetime of the powersupply in a number of ways: The sparking of the switch contactsleads to premature switch failure. Itcan also cause the line circuit break-er to trip, especially if there are mul-tiple POWER supplies on the samecircuit. The current can thermally overstressthe input rectifiers, causing imme-diate POWER SUPPLY failure.

4 High currents on the fuse causeheating, which can slowly degradethe fuse over time. POWER - SUPPLY manufacturers useone of several inrush-current-limitingNew inrush-current-limitingtechniques can protect pow-er supplies from early ALLENBear POWER SuppliesPhelps, 1. Inrush currents of 50 A or more are possi-ble on a 120-Vac line if the switch is closed nearthe top of the ac result is unplanned downtimethat can be extremely expensive upto 30% to 40% of profits, according toa recent study. Large inrush currentsA typical POWER SUPPLY (see Fig. 1) haslumped equivalent impedance R1from the common-mode choke,bridge rectifier, line cord, and is typically a few ohms.

5 C1is used for EMI filtering and isusually around F. It does nothold enough energy to present muchof an inrush-current problem. C2, thebulk storage capacitor, can be many example, a 25-Vresistor would beneeded to limit the current to 15 Apeak at a high universal line of 265 Vac (375 V peak). The instantaneous POWER in this re-sistor would be over 5 kW. A larger-value resistor would have less peakpower, but would take longer tocharge C2, resulting in the same dissi-pated energy. These high-wattage re-sistors are large and expensive, addingbulk and cost to the POWER module. New current-limiting techniqueA new patent-pending technique forinrush-current limiting consists of anelectronic switch in series with C2.

6 Acontrol circuit uses zero-crossing tech-niques to monitor the ac line andturn on the POWER SUPPLY only whenthe input line is low. C2will thencharge in a quarter cycle, limited bythe rise in voltage of the ac line. This technique effectively limits theinrush current to about 8 A. It doesnot require high-wattage resistors, en-abling smaller POWER modules. It is ef-fective over a wide operating tempera-ture range ( 40 to 70 C ) since it doesnot employ an NTC resistor. This new technique is most effectivein supplies less than 500 W. At higherpower, charging C2in a quarter cyclestill results in a fairly large inrush cur-rent. For these larger supplies, aswitch in parallel with a resistor is rec-ommended.

7 Inrush-current specsHigher inrush-current specificationsequate to greater stress on the rectifierand lower looking at inrush-currentspecs, consider the following: Is the inrush current rated as aver-age or peak? Many companies spec-ify the half-cycle average inrushcurrent. Peak current is 40% larger. Is the inrush current specified onlyat 120 Vac or 240 Vac? You mustconsider the inrush current overthe real line voltage, which can beas high as 265 Vac in universal sup-plies. Is the inrush-current-limiting tech-nique effective over your entire ex-pected operating temperaturerange? This can be problematicwhen using an NTC resistor for in-rush-current limiting.

8 SUPPLY SPECIAL - Understanding POWER supplies and inrush currenttechniques to avoid these used can tell the POWER - SUPPLY buyer a great deal about thequality and reliability of the SUPPLY . Resistor in series with lineFor very small POWER supplies a fewwatts at most adding a resistor in se-ries with the line is a simple and prac-tical solution to limit the inrush cur-rent. The large resistance required tolimit peak inrush current causes toogreat a loss in efficiency to be used inhigher-wattage POWER supplies. Series NTC resistorMany POWER - SUPPLY manufacturersuse a negative temperature coeffi-cient (NTC) resistor in series with theline.

9 An NTC resistor offers tens ofohms of resistance when cool, drop-ping to less than one ohm as its tem-perature increases. If the POWER SUPPLY is cool whenturned on, the NTC provides good in-rush-current limiting. Its effect on ef-ficiency is reduced as the POWER sup-ply warms up. However,thisapproach is not effective over largetemperature extremes. A POWER SUPPLY used outdoors inthe northern winter may never warmup enough for the NTC s resistance todrop. Conversely, a SUPPLY in the hotsummer sun will be very warm evenwith the POWER off, so that the warmNTC resistor will fail to provide ade-quate inrush-current protection onstartup.

10 An NTC resistor can also be prob-lematic when a user turns the systemoff and then immediately switches itback on. The capacitor voltage maydrop, but the NTC resistor will notcool quickly enough to provide in-rush-current protection. Parallel electronic switchPlacing a relay or electronic switch inparallel with either a resistor or NTCcan offer high impedance only atstartup. The SUPPLY manufacturermust ensure that the switch is turnedon even at low line conditions so thatthe resistor does not burn up duringbrownouts. Series electronic switchAll three of the above techniques re-quire a resistor with at least a 5-Wpower rating in order to charge C2ina reasonable time without failing.