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january 2011 doc id 16043 rev 1 1/37 AN3011 application note wide range input (90 - 265), single output (5 v-11 w) evlvip27h-12sb, viper27 demonstration board introduction in certain applications such as lcd or plasma tvs, desk top computers, etc., the power supply that converts the energy from the mains often includes two modules: the main power supply that provides most of the power which is off when the application is off or in standby mode, and the auxiliary power supply that only provides energy to sp ecific parts of the equipment, like the usb ports, remote receivers, or modems, but stays on when the application is in standby mode. in standby mode it is often required that the equipment input power is as low as possible, which means reducing the input po wer of the auxiliary power supp ly, in no-load or light-load conditions, as low as possible. this demonstration board meets the specifications of a wide range of auxiliary power supplies for the above mentioned applications. furthermore, it is optimized for very low standby consumption which helps to meet the most stringent energy saving requirements. using the viper27, which has a switching frequency of 115 khz, helps to reduce the transformer size. figure 1. demonstration board image !-v www.st.com
contents AN3011 2/37 doc id 16043 rev 1 contents 1 board descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 schematic and bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2 testing the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1 typical board waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2 precision of the regulation and output voltage ripple . . . . . . . . . . . . . . . . 11 3 efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.1 light load performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.1.1 no-load condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.1.2 low-load performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2 test equipment and measurement of efficiency and input power . . . . . . 23 3.2.1 measuring input power notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.3 overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.4 secondary winding short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . 27 3.5 output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.6 brown-out protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4 conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 5 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 6 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
AN3011 list of tables doc id 16043 rev 1 3/37 list of tables table 1. electrical specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 table 2. bom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 table 3. transformer characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 table 4. output voltage and v dd line-load regulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 table 5. efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 table 6. active-mode efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 table 7. line voltage average efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 table 8. energy efficiency criteria for standard models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 table 9. energy efficiency criteria for low voltage models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 table 10. no-load input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 table 11. energy consumption criteria for no-load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 table 12. low-load performance. p out = 30 mw (brown-out disabled) . . . . . . . . . . . . . . . . . . . . . . 19 table 13. low-load performance. p out = 30 mw (brown-out enabled) . . . . . . . . . . . . . . . . . . . . . . . 19 table 14. low-load performance. p out = 50 mw (brown-out disabled) . . . . . . . . . . . . . . . . . . . . . . 19 table 15. low-load performance. p out = 50 mw (brown-out enabled) . . . . . . . . . . . . . . . . . . . . . . . 20 table 16. low-load performance. p out = 100 mw (brown-out disabled) . . . . . . . . . . . . . . . . . . . . . 20 table 17. low-load performance. p out = 100 mw (brown-out enabled) . . . . . . . . . . . . . . . . . . . . . . 20 table 18. low-load performance. p out = 200 mw (brown-out disabled) . . . . . . . . . . . . . . . . . . . . . 21 table 19. output power when the input power is 1 w (br disabled) . . . . . . . . . . . . . . . . . . . . . . . . . 22 table 20. output power when the input power is 1 w (br enabled) . . . . . . . . . . . . . . . . . . . . . . . . . 22 table 21. overvoltage protection activation level test results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 table 22. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
list of figures AN3011 4/37 doc id 16043 rev 1 list of figures figure 1. demonstration board image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. transformer size - top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 4. transformer size - side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 5. pin placement diagram - bottom view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 6. pin placement diagram - electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 7. drain current and voltage at full-load 115 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 8. drain current and voltage at full-load 230 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 9. drain current and voltage at full-load 90 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 10. drain current and voltage at full-load 265 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 11. output voltage ripple 115 v inac full-load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 12. output voltage ripple 230 v inac full-load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 13. output voltage ripple 115 v inac no-load (burst mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 14. output voltage ripple 230 v inac 50 ma load (burst mode). . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 15. efficiency vs v in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 16. efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 17. active mode efficiency vs vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 18. input voltage average efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 19. energy star efficiency criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 20. converter input power vs vin_ac in light-load condition . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 21. converter efficiency vs vin_ac in light-load condition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 22. efficiency vs ac input voltage when the input power is 1 w . . . . . . . . . . . . . . . . . . . . . . . 23 figure 23. wattmeter possible connections with the u.u.t. (unit under test) . . . . . . . . . . . . . . . . . . . 24 figure 24. wattmeter connection scheme for low input current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 25. wattmeter connection scheme for high input current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 26. output short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 27. operation with output shorted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 28. converter power capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 29. second overcurrent protection - protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 figure 30. operating with the secondary winding shorted. restart mode . . . . . . . . . . . . . . . . . . . . . . 28 figure 31. ovp circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 figure 32. ovp protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 33. ovp protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 34. j7 jumper setting. brown-out disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 35. j7 jumper setting. brown-out enabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 36. brown-out protection, internal block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 37. input ac voltage steps from 90 v ac to 65 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 figure 38. input voltage steps from 90 v ac to 0 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
AN3011 board descriptions doc id 16043 rev 1 5/37 1 board descriptions 1.1 electrical specifications the electrical specifications of the demonstration board are listed in ta bl e 1 . 1.2 schematic and bill of materials the schematic of the board is shown in figure 2 , and the bill of materials is shown in ta bl e 2 . table 1. electrical specifications parameter symbol value input voltage range v in [90 v rms ; 265 v rms ] nameplate output voltage v outn 5 v max output current i out 2.2 a precision of output regulation 5 % high frequency output voltage ripple vout_hf 50 mv max ambient operating temperature t a 60 c outn outn out v v v ?
AN3011 board descriptions doc id 16043 rev 1 6/37 figure 2. schematic !-v , 1 3 8 7 > @ 9 d f (3&26 %60 ' %$ 7 & x) 9 5 5 n / x+ & x) 9 & q) & q) ' 6 7 36 & x) 9 95 7 6 5 n 5 n 5 n 5 n %5 ') 0 & q) & x) 9 ' 6 77 + / & 1 0 ) $)8 6( 7 %85%/ w 17& 2kp & q) ; & q ' 1 5 n 5 n & < q) 5 n 2372 3& 7 :( 2 8 7 9 $ ' . ( %5 &217 '5 $,1 6285&( &21752/ 9' ' )% 8 9 , 3 ( 5 + 1 5 0hj & x) 9 & q) ; ' 9] 5 + 6 + ($ 7 6 , 1 . 5 0hj 5 0hj - 6(/( &725 5 n
AN3011 board descriptions doc id 16043 rev 1 7/37 table 2. bom part reference description part name manufacturer br1 bridge diodes df06m fairchild/ vishay c1,c13 100 nf x2 capacitor c3 33 f 450 v electrolytic cap. c4 22 f 35 v electrolytic cap. c5 n.m c6 1.8 nf ceramic cap c7 15 nf c8 2.2 nf y1 capacitor c9, c14 zl 1000 f 16 v electrolytic cap. rubycon c10 yxf 47 f 25 v electrolytic cap. yxf 47 f 25 v rubycon c11 22 nf ceramic cap 22 nf c12 10 nf ceramic cap 10 nf d1 100 v small signal schottky diode bat46 stmicroelectronics d2 100 v small signal fast diode 1n4148 d3 600 v 1 a ultra-fast diode stth1l06 stmicroelectronics d4 power schottky diode stps745 stmicroelectronics d5 250 v transil 1.5ke250 stmicroelectronics d6 18 v zener f1 1 a fuse hs1 heat sink j7 selector l1 3.3 h 3 a inductor ntc1 15 epcos opto1 opto-coupler pc817 sharp r1 3.3 resistor r3 33 k 1% precision resistor r6 1 2k 1% precision resistor r8 120 k 1% precision resistor r9 39 k 1% precision resistor r10 270 k r12 47 k r13 1.5 k r14 180 k 1% precision resistor r15 3.3 meg 1% precision resistor
board descriptions AN3011 8/37 doc id 16043 rev 1 1.3 transformer transformer characteristics are listed in ta bl e 3 : figure 3 , 4 , 5 , and 6 show the size and pin distances (inches and [mm]) of the transformer. r16, r17 2.7 meg 1% precision resistor r18 47 k 1% precision resistor r19 220 t1 switch mode transformer we - 750871012 wrth elektronik t2 common mode line filter bu15-4530r4bl coilcraft u1 offline switching regulato r viper27hn stmicroelectronics vr1 voltage reference ts431 stmicroelectronics table 2. bom (continued) part reference description part name manufacturer table 3. transformer characteristics properties value test condition manufacturer wrth elektronik part number 750871012 primary inductance 900 h 10 % measured at 10 khz 0.1 v leakage inductance 25 h max measured at 100 khz 0.1 v (primary and secondary windings shorted) primary to secondary turn ratio (4 - 5) / (6, 7 ? 8, 9) 14.75 1 % measured at 10 khz 0.1 v primary to auxiliary turn ratio (6 - 4) / (3 - 1) 5.36 1 % measured at 10 khz 0.1 v insulation 4 kv primary to secondary
AN3011 board descriptions doc id 16043 rev 1 9/37 figure 3. transformer size - top view figure 4. transformer size - side view !-v !-v figure 5. pin placement diagram - bottom view figure 6. pin placement diagram - electrical diagram !-v !-v
testing the board AN3011 10/37 doc id 16043 rev 1 2 testing the board 2.1 typical board waveforms figure 7 and 8 show the drain current and the drain voltage waveforms at the nominal input voltages, which are 115 v ac and 230 v ac when at maximum load (2.2 a). figure 9 and 10 show the same waveforms for the same load cond ition, but with the input voltages at the minimum 90 v ac and the maximum 265 v ac . the converter is designed to operate in continuous conduction mode (in full-load condition) at low-line. ccm (continuous conduction mode) allows the reducing of the root mean square currents value, at the primary side, in the power switch inside the viper, and in the primary winding of the transformer; at the secondary si de in the output diode (d4) and in the output capacitors (c9 and c14). reducing rms curr ents means reducing the power dissipation (mainly in the viper) and the stress on the above mentioned components. figure 7. drain current and voltage at full- load 115 v ac figure 8. drain current and voltage at full- load 230 v ac figure 9. drain current and voltage at full- load 90 v ac figure 10. drain current and voltage at full- load 265 v ac !-v !-v !-v !-v
AN3011 testing the board doc id 16043 rev 1 11/37 2.2 precision of the regulati on and output voltage ripple the output voltage of the board was measur ed in different line and load conditions. the results are given in ta b l e 4 . the output voltage is practically not affected by the line condition and only slightly affe cted by load condition (a diff erence of 10 mv between max and minimum v out , see ta b l e 4 ). the v dd voltage was also measured. in a two-output flyback converter, when just on e output is regulated, the unregulated output does not rigorously respect the turn ratio. th e unregulated output voltage value depends not only by the turn ratio but also, approximately, fr om the output currents ratio (output current at the regulated output divided by output current of the unregulated output). as confirmed from the results reported in ta b l e 4 , the v dd voltage (unregulated auxiliary output) increases as the load on the regulated output increases. in order to avoid the v dd voltage exceeding the viper27 operating range, an external clamp was used (d6, r19, see schematic). the ripple at the switching frequency su perimposed at the output voltage was also measured. the board is provided with an lc f ilter for cleaner output voltage. the high frequency voltage ripple acro ss capacitors c9 and c14 (vou t_fly), that is the output capacitors of the flyback converter bef ore the lc filter (see schematic in figure 2 ), was also measured to verify the effectiveness of the lc filter. the waveforms of the two voltages (vout and vout_fly) are reported in figure 11 and 12 . the output voltage ripple when the converter input voltage is 115 v ac is shown in figure 11 , and the output voltage ripple when the converter input voltage is 230 v ac is shown in figure 12 . table 4. output voltage and v dd line-load regulation v inac (v) full load half load no load v out (v) v dd (v) v out (v) v dd (v) v out (v) v dd (v) 90 5.073 21.1 5.078 20.00 5.083 9.98 115 5.073 20.98 5.078 20.02 5.083 9.83 230 5.073 20.94 5.077 20.08 5.083 9.30 265 5.073 20.98 5.077 20.04 5.083 9.17
testing the board AN3011 12/37 doc id 16043 rev 1 figure 11. output voltage ripple 115 v inac full-load the measured output voltage ripple is aroun d 20 mv, well below the maximum admitted value (50 mv, see electrical specification in ta bl e 1 ). figure 12. output voltage ripple 230 v inac full-load when the device is working in burst mode, a lower frequency ripple is present. in this operation mode the converter does not supply continuous power to its output. it alternates periods when the power mosfet is kept off, and no power is processed by the converter, and periods when the power mosfet is switching and power flows towards the converter output. even no-load is present at the output of the converter, during no switching periods the output capacitors are discharged by thei r leakage currents and by the currents needed to supply the circuitry of th e feedback loop present at the secondary side. during the switching period the output capacitance is recharged. figure 13 and 14 show the output voltage and the feedback voltage when the converter is no-loaded. in figure 13 the converter is supplied with 115 v ac , and with 230 v ac in figure 14 . !-v #h6/54?&,9 #h6/54 #h6$2!). !-v #h6/54?&,9 #h6/54 #h6$2!).
AN3011 testing the board doc id 16043 rev 1 13/37 figure 13. output voltage ripple 115 v inac no-load (burst mode) figure 14. output voltage ripple 230 v inac 50 ma load (burst mode) !-v #h6 /54 #h) $2!). !-v #h6 /54 #h) $2!).
efficiency AN3011 14/37 doc id 16043 rev 1 3 efficiency the efficiency of the converter was measured in different load and line voltage conditions. in accordance with the energy star ? active mode testing efficiency method, the measurements are done with different load valu es (full-load, 75%, 50%, and 25% of the full- load) for different input voltages. the results are given in ta bl e 5 below. for better visibility the results are plotted in th e diagrams below. in figure 15 , efficiency versus converter ac input voltage (v in ), for four different load values, is plotted. in figure 16 , the value of efficiency versus load for different input voltages is plotted. figure 15. efficiency vs v in table 5. efficiency v inac (vrms) efficiency (%) full load (2.2 a) 75 % load (1.65 a) 50 % load (1.1 a) 25 % load (0.55 a) 90 73.0 75.1 76.9 77.9 115 75.3 76.5 77.9 78.1 132 75.9 76.9 77.8 77.7 175 76.8 77.3 77.6 76.4 230 77.4 77.6 77.3 75.4 265 76.8 76.9 76.3 74.2 !-v (iilflhqf\9v9 ,1$& 9 ,1$& 9506 (iilflhqf\
AN3011 efficiency doc id 16043 rev 1 15/37 figure 16. efficiency vs load the active mode efficiency is defined as the average of the efficienci es measured at 25%, 50%, and 75% of maximum load and the maximum load itself. ta b l e 6 shows the active mode efficiency calculated from the measured value of ta bl e 5 . the values in ta b l e 6 are plotted in figure 17 . in figure 18 the average value of the efficiency versus load is shown (the average was obtained considering efficiency at different input voltages). table 6. active-mode efficiency active mode efficiency v inac (v rms ) efficiency (%) 90 75.8 115 77.0 230 76.9 265 76.1 !-v (iilflhqf\9v3 287 3 287 : (iilflhqf\
efficiency AN3011 16/37 doc id 16043 rev 1 figure 17. active mode efficiency vs v in figure 18. input voltage average efficiency vs load in version 2.0 of the energy star ? program requirement for single voltage external ac/dc power supplies (see references 2 ), the power supplies are divided into two categories: low voltage power supplies and st andard power supply, with respect to the nameplate output voltage and current. an exter nal power supply, in order to be considered a table 7. line voltage average efficiency vs load load (% of full load) efficiency (%) 100 75.7 75 76.5 50 77.1 25 76.4 !-v $fwlyh0rg h(iilflhqf\9v9 ,1b$& 9 ,1b$& 9 506 (iilflh qf\ !-v (iilflhqf\$yhudjhuhvshfw9lq9v3rxw /2$' (iilflhqf\
AN3011 efficiency doc id 16043 rev 1 17/37 low voltage power supply, must have a nameplate output voltage lower than 6 v and a nameplate output current great er than or equal to 550 ma. ta bl e 8 and 9 show the epa energy efficiency criter ia for ac/dc power supplies in active mode for standard models and for low voltage models respectively. figure 19. energy star efficiency criteria the criteria are plotted in figure 19 above where the red line is the criteria for the standard model and the blue line is the criteria for the low voltage model. the pno axe is in the logarithmic scale. the presented power supply belongs to the low vo ltage power supply category and, in order to be compliant with energy star requirement s, must have an efficiency higher than 74.1 % when the converter input voltage is at the nominal value (115 v ac or 230 v ac in this case). for all the considered input voltages the efficiency (see ta bl e 6 ) results are higher than the required value. table 8. energy efficiency criteria for standard models nameplate output power (p no ) minimum average efficiency in active mode (expressed as a decimal) 0 to = 1 w = 0.48 *pno+0.140 > 1 to = 49 w = [0.0626 * in (p no )] + 0.622 > 49 w = 0.870 table 9. energy efficiency criteria for low voltage models nameplate output power (p no ) minimum average efficiency in active mode (expressed as a decimal) 0 to = 1 w = 0.497 *pno+0.067 > 1 to = 49 w = [0.075 * in (p no )] + 0.561 > 49 w = 0.860 !-v 1dphsodwhrxwsxw3rzhu (iilflhqf\ vw 3qr oy 3qr : 3qr
efficiency AN3011 18/37 doc id 16043 rev 1 3.1 light load performance 3.1.1 no-load condition the input power of the converter was measured in no-load condition, with brown-out protection disabled (see relevant section) and brown-out protection enabled for different applied input voltages (see ta bl e 1 0 ). the converter in no-load condition always works in burst mode so that the average switching frequency is strongly reduced. the average switching frequency values were also measured. the presence of the resistor dividers (r16, r17 and r18, see schematic of figure 2 ) to sense the flyback input voltage, when brown- out protection is enabled, does not affect the average switching frequency, but obviously affects the input power due to the power dissipated in the resistor divider itself. in the energy star program version, the po wer consumption of the power supply when it is no-loaded is also considered. th e compliance criteria is shown in ta b l e 1 1 : the performance of the demonstration board is far better then required, but it is worth noting that often the ac/dc adapter or battery char ger manufacturer have stricter requirements regarding no-load consumption, compared to energy star requirements, due also to other standards or recommendations which they want to be compliant with. in cases where the converter is used as the standby power supp ly for lcd tvs, pdps or other applications, the line filter is often the big line filter of the main power supply which heavily contributes to the standby consumpt ion, even though the power needed to the auxiliary power supply is very low. the energy star program does not have other requirements regarding light-load performance, however the input power and efficiency of the demonstration board, also in other low load cases, is given in orde r to supply more complete information. table 10. no-load input power vin ac (v rms ) pin (mw) (br enabled) pin (mw) (no br) f sw_avg (khz) 90 19.20 16.80 1.0816 115 22.90 17.50 0.9706 132 25.00 18.60 0.9139 175 33.00 23.00 0.7552 230 48.00 29.00 0.6923 265 62.00 37.00 0.6561 table 11. energy consumption criteria for no-load nameplate output power (p no ) maximum power in no-load for ac/dc eps 0 to = 50 w < 0.3 w > 50 watts < 250 w < 0.5 w
AN3011 efficiency doc id 16043 rev 1 19/37 3.1.2 low-load performance the demonstration board was tested not only in no-load condition but also with a low-load applied. the tests were performed with 30 mw, 50 mw, 100 mw and 200 mw with brown- out protection enabled and with brown-out protection disabled p out = 30 mw p out = 50 mw table 12. low-load performance. p out = 30 mw (brown-out disabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) f sw_avg (khz) 90 29.48 51.60 57.13 22.12 3.731 115 29.48 54.40 54.19 24.92 3.375 132 29.48 55.00 53.60 25.52 3.155 175 29.48 59.60 49.47 30.12 2.814 230 29.48 69.00 42.73 39.52 2.876 265 29.48 74.00 39.84 44.52 2.534 table 13. low-load performance. p out = 30 mw (brown-out enabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) 90 29.48 54.80 53.80 25.32 115 29.48 57.90 50.92 28.42 132 29.48 62.30 47.32 32.82 175 29.48 69.80 42.24 40.32 230 29.48 87.00 33.89 57.52 265 29.48 101.00 29.19 71.52 table 14. low-load performance. p out = 50 mw (brown-out disabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) f sw_avg (khz) 90 54.39 85.90 63.32 31.51 6.248 115 54.39 87.40 62.23 33.01 5.663 132 54.39 88.20 61.66 33.81 5.314 175 54.39 94.80 57.37 40.41 5.259 230 54.39 104.00 52.30 49.61 4.845 265 54.39 111.00 49.00 56.61 4.299
efficiency AN3011 20/37 doc id 16043 rev 1 p out = 100 mw table 15. low-load performance. p out = 50 mw (brown-out enabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) 90 54.39 87.20 62.37 32.81 115 54.39 93.80 57.98 39.41 132 54.39 94.00 57.86 39.61 175 54.39 104.20 52.20 49.81 230 54.39 125.00 43.51 70.61 265 54.39 139.00 39.13 84.61 table 16. low-load performance. p out = 100 mw (brown-out disabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) f sw_avg (khz) 90 106 152 69.5 46 11.3 115 106 157 67.3 51 10.2 132 106 157 67.3 51 9.6 175 106 162 65.3 56 8.5 230 106 177 59.7 71 8.7 265 106 181 58.4 75 7.8 table 17. low-load performance. p out = 100 mw (brown-out enabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) 90 106 155 68.2 49 115 106 159 66.5 53 132 106 166 63.7 60 175 106 174 60.8 68 230 106 195 54.2 89 265 106 206 51.3 100
AN3011 efficiency doc id 16043 rev 1 21/37 p out = 200 mw figure 20. converter input power vs vin_ac in light-load condition table 18. low-load performance. p out = 200 mw (brown-out disabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) f sw_avg (khz) 90 208.403 286 72.87 77.597 21.3115 115 208.403 293 71.13 84.597 19.2462 132 208.403 294 70.89 85.597 18.1681 175 208.403 296 70.41 87.597 16.0584 230 208.403 313 66.58 104.597 16.4671 265 208.403 328 63.54 119.597 14.7167 low-load performance. p out = 200 mw (brown-out enabled) v in_ac p out (mw) p in (mw) eff. (%) p in -p out (mw) 90 208.403 289 72.11 80.60 115 208.403 296 70.41 87.60 132 208.403 299 69.70 90.60 175 208.403 313 66.58 104.60 230 208.403 336 62.02 127.60 265 208.403 349 59.71 140.60 !-v 3 ,1 9v9 ,1b$& 1r%5 9 ,1b$& 9 506 3 ,1 p :
efficiency AN3011 22/37 doc id 16043 rev 1 figure 21. converter efficiency vs vin_ac in light-load condition depending on the equipment supplied, it?s possible to have several criteria to measure the standby or light-load performance of a converte r. one of these is the measurement of the output power when the input power is equal to 1 watt. in ta b l e 1 9 and 20 , the output power needed to have 1 w of input power in different line conditions is shown, with br disabled and with br enabled respectively. figure 22 shows the diagram of the efficiency (proportional to the output power) versus th e input voltage when the input power is 1 w. table 19. output power when the input power is 1 w (br disabled) v in (v rms )p in (mw) p out (mw) efficiency (%) pin-pout (mw) 90 1000 737 73.70 263 115 1000 752 75.23 248 132 1000 757 74.74 243 175 1000 717 71.67 283 230 1000 686 68.62 314 265 1000 666 66.59 334 table 20. output power when the input power is 1 w (br enabled) v in (v rms ) p in (mw) p out (mw) efficiency (%) pin-pout (mw) 90 1000 737 73.70 263 115 1000 752 75.23 248 132 1000 742 74.21 258 175 1000 712 71.16 288 230 1000 676 67.60 324 265 1000 656 65.57 344 !-v (iilflhqf\9v9 ,1b$& 1r%5 9 ,1b$& 9 506 (iilflhqf\ 3287
AN3011 efficiency doc id 16043 rev 1 23/37 figure 22. efficiency vs ac input voltage when the input power is 1 w 3.2 test equipment and measurem ent of efficiency and input power the converter input power was measured using a wattmeter. the wattmeter contemporaneously measures converted input current (using its internal ammeter) and voltage (using its internal voltmeter). the watt meter is a digital instrument, therefore, it samples the current and voltage and converts them into digital form. the digital samples are then multiplied giving the instantaneous meas ured power. the sampling frequency is in the range of 20 khz (or higher depending on the instrument used). the display provides the average measured power, averaging the instantaneous measured power. figure 23 shows how the wattmeter is connected to the uut (unit under test) and to the ac source and the wattmeter internal block diagram. an electronic load was connected to the outp ut of the power converter (uut) sinking the load current. the electronic load also measur es the load current. a voltmeter was used in order to measure the output voltage of the power converter. once the input power and the output power can be measured, the efficiency in different operating conditions can be calculated by prope rly setting the ac source output voltage and the current sourced by the electronic load. 3.2.1 measuring input power notes with reference to figure 23 , the uut input current causes a voltage drop across the ammeter internal shunt resistance (the ammeter is not ideal so it has an internal resistance higher than zero) and across the cables that connect the wattmeter to the uut. if the switch of figure 23 is in position 1 (see also the simplified scheme of figure 24 ) this voltage drop causes an input measured volt age higher than the input voltage at the uut input which, of course, affects the measured po wer. the voltage drop is generally negligible if the uut input current is low (for example, when measuring the input power of uut in low- load condition). in the case of high uut input current the voltage drop can be relevant (compared to the uut real input voltage) and ther efore, if this is the case, the switch in figure 23 can be changed to position 2 (see simplified scheme of figure 25 ) where the uut !-v (iilflhqf\zkhq,qs xwsr zhulv: 9 ,1b$& 9 506 (iilflh qf\
efficiency AN3011 24/37 doc id 16043 rev 1 input voltage is measured directly to the uut input terminal, and the input current does not affect the measured input voltage. the voltage across the voltmeter causes a leaka ge current inside the voltmeter itself (which is not ideal and which doesn't have infini te input resistance). if the switch in figure 23 is in position 2 (see simplified scheme of figure 25 ) the voltmeter leakage current is measured by the ammeter, together with the uut inpu t current, causing a measurement error. the error is negligible in a case where the uut inpu t current is much high er than the voltmeter leakage. if the uut input current is low, and not much higher than the voltmeter leakage current, it is probably bett er to set the switch (in figure 23 ) to position 1. in a case where it is not certain which measur ement scheme least affects the results, it is possible to try with both and re gister the input power lower value. figure 23. wattmeter possible connections with the u.u.t. (unit under test) figure 24. wattmeter connection scheme for low input current !-v $ 9 ',63/$< ; $9* :$770(7(5 $& 6285&( 887 8qlw8qghuwhvw ,1387 0xowlsolhu 9rowphwhu $pphwhu 287387 6zlwfk !-v 9 $ a $& 6285&( 887 887 $& ,1387 9rowphwhu $pphwhu :dwwphwhu
AN3011 efficiency doc id 16043 rev 1 25/37 figure 25. wattmeter connection scheme for high input current as noted in iec 62301, instantaneous meas urements are appropriate when power readings are stable. the uut is to be operated at 100% of the nameplate output current for at least 30 minutes (warm-up period) immediately prior to conducting efficiency measurements. after this warm-up period, the ac input power is monitored for a period of 5 minutes, to assess the stability of the uut. if the power leve l does not drift by mo re than 5% from the maximum value observed, the uut can be considered stable and the measurements can be recorded at the end of the 5 minute period. if the ac input power is not stable over a 5 minute period, the average power or accumulated energy is measured over ti me for both ac input and dc output. some wattmeter models allow the integration of the measured input power in a time range, and then measures the energy absorbed by t he uut during the integration time. dividing this by the integration time itself gives the average input power. 3.3 overload protection viper27 is protected against overload or out put short-circuit. if the load power demand increases the output voltage decreases and the feedback loop reacts by increasing the voltage on the feedback pin. the pwm current set point is increased, leading to higher power delivered to the output until this power equals the load power demands. if the load power demand exceeds the converter power capability (fixed by the r lim value) the voltage on the feedback pin continuously rises, but the power delivered does not rise further. when the feedback pin voltage exceeds v fb_lin (3.3 v typ), viper27 logic assumes it is a warning for an overload event. before shutting-down the system, the device waits for a period, fixed by the capacitor present on the feedback pin. in fact, if the voltage on the feedback pin exceeds v fb_lin , the internal pull-up is disconnected and the pin starts sourcing a 3 a current that charges the capacitor connected to it. as the voltage on the feedback pin reaches the v fb_olp threshold (4.8 v typ.), viper27 st ops switching and is not allowed to switch again until the v dd voltage goes below v dd_restart (4.5 v typ.) and rises again up to v dd_on (14 v typ.). the following waveforms show the behavior of the converter when the output is shorted. !-v 9 a $& 6285&( 887 887 $& ,1387 9rowphwhu $pphwhu $ :dwwphwhu
efficiency AN3011 26/37 doc id 16043 rev 1 figure 26. output short-circuit if the short-circuit is not removed the system starts to work in auto-restart mode. the behavior, when a short-circuit is permanently applied on the output, is a short period of time where the mosfet is switching an d the converter tries to deliver as much power as it can to the output, and a longer period where the device is not switching and no power is processed. the duty cycle of power delivery is very low (around 1 %), therefore, the average power throughput is then very low (see figure 27 ). figure 27. operation with output shorted the power capability of the converter was also tested, verifying the minimum value of the output current needed to activate the overload protection (iol) and the maximum value of the output current that allows the system to restart (i_rest). results are given in figure 28 . !-v #h6 /54 #h) $2!). 1rupdor s hudwlrq 2yhu/rdg'hod \ 6wrsvzlwfklqj 2xwsxwvkruwhgkhuh #h6 &" 6 ). 6 !# &ullloadbefore theshort !-v #h6 $$ #h) $2!). #h6 &" 6 ). 6 !# av apv
AN3011 efficiency doc id 16043 rev 1 27/37 figure 28. converter power capability 3.4 secondary winding sh ort-circuit protection viper27 is provided with an adjustable first level of primary overcurrent limitation that switches off the power mosfet if this level is exceeded. this limitation acts cycle by cycle and its main purpose is to lim it the maximum deliverable output power. a second level of primary overcurrent protection is also present but in this case it is not adjustable, it is fixed to 1 a (typical value). if the drain current exceeds this second over current protection threshold, the device enters a warning state. in the next cycle the mosfet is switched on, and if the second level of overcurrent protection is exceeded again, the device assumes that a secondary winding short-circuit or a hard saturation of the transformer is happening and stops the operation. in order to re-enable the operation, v dd voltage must be recycled, that means: v dd must go down, up to v dd_restart , then rise up to v dd_on . when the viper27 is switched on again (v dd equals v dd_on ), the mosfet can start to switch again. if the cause of the second overcurrent protection activa tion is not removed, the device goes into auto-restart mode. this protection was tested on the demonstration board. the secondary winding of the transformer was shorted, in different operating conditions. figure 29 and 30 show the behavior of the system during these tests. !-v 2yhuordgsurwhfwlrqdfwlydwlrq 9 ,1b$& , 287 ,b 2/ ,5(67
efficiency AN3011 28/37 doc id 16043 rev 1 figure 29. second overcurrent protection - protection tripping in figure 29 , when the board is working in full-load condition with an input voltage of 115 v ac , the secondary wi nding is shorted. if the converte r runs with the secondary winding shorted, there is a very high current flowing through the transformer windings, the secondary diode, and the viper power section. the second over-current protection, which stops the converter operation after two switch ing cycles, prevents the flow of these high currents. figure 30 shows the situation when a permanent short-circuit is applied on the secondary winding. most of the time the power section of the viper is off, eliminating any risk of overheating. figure 30. operating with the secondary winding shorted. restart mode !-v 6hfrqgdu\zlqglqj6kruwhgkhuh 'hylfh/dwfkhvkhuh #h6 /54 #h) $2!). #h6 &" 6 ). 6 !# &ullloadbefore theshort 6 ). 6 !# &ullloadbefore theshort 4estcondition !-v #h) $2!). #h6 $$ 6 ). 6 !# 3econdary windingshorted 4estcondition
AN3011 efficiency doc id 16043 rev 1 29/37 3.5 output overvoltage protection output overvoltage prot ection is implemented, monitori ng the voltage across the auxiliary winding during the mosfet off time, through the d2 diode and the r3 and r14 resistor dividers (see schematic of figure 31 ) connected to the cont pin of the viper27. if the voltage on the cont pin exceeds the v ovp thresholds (3 v typ.), viper assumes an overvoltage event and the power section is no longer allowed to be switched-on. to re- enable operation, the v dd voltage has to be recycled. in order to provide high noise immunity and avoid spikes erroneously trippi ng the protection, a digital filter was implemented so the cont pin must sense a voltage higher than v ovp for four consecutive cycles before it stops operation (see device datasheet for details). figure 31. ovp circuit the value of the output voltage when the protection must be tripped can be properly fixed by selecting the r3 and r14 resistor dividers. w hen r3 is selected, co nsidering the maximum power that the converter must manage, r14 output must be selected, according to the formula: equation 1 the protection was tested by shorting the voltag e reference pin of the ts431 device (pin 3 of vr1 in the schematic) with pin 2 of the same device during converter operation. in this way the converter operates in open loop and delivers the maximum power possible to output. the excess of power, in respect to the maximum load, charges the output capacitance increasing the output voltage, as the ovp is tripped and the converter stops working. in figure 32 it is possible to see that the output voltage increases, and as it reaches the value of 5.58 v the converter stops switching. in the same figure the cont pin voltage (ch3, fuchsia waveform) and the drain current (ch4 green waveform) are shown (see figure 32 and 33 ). the crest value of the cont pin voltage tracks the output voltage. !-v &xuuhqw /lplw& rpsdudwru 293' (7(&7,21 /2*,& 5olp 'ry s' $x[loldu\ z lqglqj )urp 6hqvh)(7 7r2933urwhfw lrq 7r3:0/rjlf &xuu /lp %/2 &. 5 &2173,1 5ry s5 ? ? ? ? ? ? ? ? ? ? ? ? = v 3 v v n n v 3 r r ) 2 d _( ovp d _ drop ovp _ out s aux ) 2 r _( lim ) 14 r _( ovp
efficiency AN3011 30/37 doc id 16043 rev 1 figure 32. ovp protection figure 33. ovp protection the test was performed in different line and load conditions to check the dependence of the output voltage value, when protection is activa ted, from the converter input voltage and load. results are shown in ta b l e 2 1 . table 21. overvoltage protection activation level test results v out_ovp (v) v in (v rms ) load no-load 25% of load half-load (1.1 a) full-load 90 7.019 6.754 6.749 6.731 115 7.022 6.722 6.754 6.740 !-v #h) $2!). #h6 $$ 3horting4, referencetosignal'.$ 7orkingcondition )nputvoltage 6!# !ofload #h) $2!). !-v #h) $2!). #h6 $$ 3horting4, referencetosignal'.$ 7orkingcondition )nputvoltage 6!# .oload #h) $2!).
AN3011 efficiency doc id 16043 rev 1 31/37 except for the no-load condition, the variation wi th load and line is very low ([6.731 v; 6.786 v], v out_ovp = 55 mv less then 1% of variation); including the values in the no-load condition ([6.731 v; 7.035 v], v out_ovp = 304 mv less then 5% of variation). considering a 10% precision of the ovp threshold in the cont pin ([2.7 v; 3.3 v]), using as rovp and r lim 1% precision resistances and with a turn ratio between the secondary and auxiliary windings which has a precision of 5%, and considerin g a large production, it is possible to fix the output overvoltage at 25% over the nominal output voltage, making sure that the ovp protection is not erroneously activated. it is possible to not implement this protection, if it is not necessary, by not mounting the d2 diode and the r14 resistor. therefore if ovp protection is not required, the total number of components can be reduced. 3.6 brown-out protection brown-out protection is basically an unlatched device shutdown function with a typical use of sensing mains undervoltage or the main unplug. the viper27 has a dedicated pin (br, pin 5) for this function, which must be connecte d to the dc hv bus. if protection is not required, it can be disabled by connecting the pin to grou nd. in the conv erter presented here, brown-out protection is implemented but can be disabled by changing the j7 jumper setting (see schematic in figure 2 ). the settings of the j7 jumper are shown in figure 34 and 35 . the converter's shutdown is accomplish ed by means of an internal comparator, internally referenced to 450 mv (typ, vbrth), which disables the pwm if the voltage applied at the br pin is below the internal reference (as shown in figure 36 ). pwm operation is re- enabled as the br pin voltage is more than 4 50 mv plus 50 mv of voltage hysteresis that ensures noise immunity. the brown-out comparator is also provided with current hysteresis. an internal 10 a current generator is on as long as the voltage applied at the brown-out pin is below 450 mv and off if the voltage exceeds 450 mv plus the voltage hysteresis. 230 7.029 6.735 6.760 6.752 265 7.035 6.786 6.765 6.762 table 21. overvoltage protection activation level test results (continued) v out_ovp (v) v in (v rms ) load no-load 25% of load half-load (1.1 a) full-load
efficiency AN3011 32/37 doc id 16043 rev 1 figure 36. brown-out protection, internal block diagram the current hysteresis provides an additional degree of freedom. it is possible to set the on threshold and the off threshold for the fly back input voltage separately by properly choosing the external divider resistors. the following relationships can be established for the on (v in_on ) and off (v in_off ) thresholds of the input voltage: equation 2 equation 3 where i brhyst =10 a (typ.) is the current hysteresis, v brhyst =50 mv (typ.) is the voltage hysteresis and v brth =450 mv (typ.) is the brown-out comparator internal reference. one purpose of this protection is to stop op eration of the converter when the line voltage is too low, avoiding too high root mean square current value flowing inside the main switch an then its overheating. another purpose is to av oid a false restart of the converter and then figure 34. j7 jumper setting. brown-out disabled figure 35. j7 jumper setting. brown-out enabled !-v !-v !-v p97\s ,%5k\v w 9%5wk 9$&b2. 'lvdeo h 9%5k\v w ? ? ? ? ? ? ? ? + ? = l l h brth off _ in r r r v v () brhyst h l l h brhyst brth on _ in i r r r r v v v ? + ? ? ? ? ? ? ? ? + ? + =
AN3011 efficiency doc id 16043 rev 1 33/37 having a monotonically decay to zero of the output voltage when the converter itself is unplugged from the mains. a typical situation, in most cases for converters designed for the european range (230 v ac ), could be a converter that when unplugged shuts down due to the overload protection (due to the low input voltage the converter is not able to supply full power), but the voltage on the bulk capacitor is higher than v drain restart , so the device starts again and the output voltage rises again. this situation could be dangerous for some loads, and in many applications is best avoided. figures 37 and 38 show how brown-out protection works in the viper27 board when used. figure 37 shows the behavior of the board when the input voltage is changed from 90 v ac to 75 v ac with full-load applied. th e system stops switching and the output load, no longer supplied, decays monotonically to zero. figure 38 shows the system behavior when the input voltage changes from 75 v ac to 90 v ac . figure 37. input ac voltage steps from 90 v ac to 65 v ac figure 38. input voltage steps from 90 v ac to 0 v ac !-v #h6 "5,+ #h6 $$ #h6 /54 #h) $2!). ) /54 ! !-v #h6 "5,+ #h6 "2 #h6 /54 #h) $2!). ) /54 !
conclusions AN3011 34/37 doc id 16043 rev 1 4 conclusions the flyback converter is suitable for different applications and can be used as an external adapter or as an auxiliary power supply in consumer equipment. special focus was put on low-load performance and the bench results are good with a very low input power in light- load conditions. the efficiency performance was compared with the requirements of the energy star program for external ac/dc adapters with very good results, as the measured active mode efficiency is always hi gher with respect to the minimum required.
AN3011 references doc id 16043 rev 1 35/37 5 references 1. energy star ? program requirements for single voltage external ac/dc adapter (version 2.0) 2. viper27 datasheets
revision history AN3011 36/37 doc id 16043 rev 1 6 revision history table 22. document revision history date revision changes 13-jan-2011 1 initial release.
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