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april 2010 doc id 15494 rev 1 1/37 AN2950 application note evlviper28l-10w: 5 v/10 w, 60 khz isolated flyback with extra power management introduction this document describes a 5 v, 2 a application with 3.3 a peak current capability of 1.9 sec, using viper28, a new offline high-voltage converter from stmicroelectronics. in some applications, an smps sometimes undergoes load peaks that can be two or more times as much as the power it is supposed to deliver, though only for a short time interval compared to the thermal time constants of the power components. typical examples of such loads are printers and audio systems. in such cases, it is more cost-effective to thermally design the system for the maximum continuous power and not for the peak power demand, which is sustained only for a limited time window. such a design is possible thanks to the ept function of the viper28, which allows the designer to fix the maximum time window during which the converter is able to manage the peak power and still maintain ou tput voltage regulation. if the overload lasts for longer than this time window, the converter is automatically shut down and enters auto-restart mode until the overload is removed, so as to prevent damage to the power components. the device has many other features such as an 800 v avalanche rugged power section, pwm operation at 60 khz with frequency jittering for lower emi, a limiting current with adjustable setpoint, an on-board soft-start, a safe auto-restart after a fault condition, and a low standby power (< 50 mw at 265 v ac ). the available protections include adjustable and accurate overvoltage protection, thermal shutdown with hysteresis and delayed overload protection. figure 1. demonstration board www.st.com
contents AN2950 2/37 doc id 15494 rev 1 contents 1 adapter features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1 circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 testing the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1 typical board waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2 regulation precision and output voltage ripple . . . . . . . . . . . . . . . . . . . . 12 2.3 burst mode and output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4 efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.5 light load performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.6 overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.7 second ocp protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.8 output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.9 ept function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.10 thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.11 emi measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.12 board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3 conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 appendix a test equipment and measurem ent of efficiency and low load performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 a.1 notes on input power measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 AN2950 list of tables doc id 15494 rev 1 3/37 list of tables table 1. electrical specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 table 2. bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 table 3. 1338.0019 transformer characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 table 4. transformer pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 table 5. output voltage and vdd line-load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 table 6. output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 table 7. burst mode related output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 table 8. efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 table 9. active mode efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 table 10. input voltage averaged efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 table 11. energy efficiency criteria for standard models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 table 12. energy efficiency criteria for low-voltage models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 table 13. no load input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 table 14. energy consumption criteria for no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 table 15. low load performance - pout = 25 mw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 table 16. low load performance - pout = 50 mw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 table 17. output power when the input power is 1 w . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 table 18. output overvoltage threshold at different input/output conditions. . . . . . . . . . . . . . . . . . . . 25 table 19. temperature of key components at 115 vac full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 table 20. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 list of figures AN2950 4/37 doc id 15494 rev 1 list of figures figure 1. demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 3. transformer size and pin diagram - bottom view (pin side) . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 4. transformer size and pin diagram - electrical diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 5. transformer size - side view 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 6. transformer size - side view 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 7. drain current and voltage at maximum load 115 vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 8. drain current and voltage at maximum load 230 vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 9. drain current and voltage at maximum load 90 vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 10. drain current and voltage at maximum load 265 vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 11. drain current and voltage at peak load 90 vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 12. drain current and voltage at peak load 265 vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 13. output voltage ripple 115 vin_ac full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 14. output voltage ripple 230 vin_ac full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 15. output voltage ripple at 115 vin_ac no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 16. output voltage ripple at 230 vin_ac no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 17. output voltage ripple at 115 vin_ac 50 ma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 18. output voltage ripple at 230vin_ac 50 ma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 19. efficiency vs vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 20. efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 21. active mode efficiency vs vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 22. input voltage averaged efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 23. energy star? efficiency criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 24. pin vs vin at low load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 25. pout vs vin at pout = 1 w . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 26. olp: output short and protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 27. olp: output short and protection tripping (zoom on fb pin voltage) . . . . . . . . . . . . . . . . . 21 figure 28. olp: steady state (autorestart mode). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 29. olp: steady state (zoom on fb pin voltage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 30. olp: steady-state, short removal and restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 31. olp: restart after short removal (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 32. second ocp protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 33. operating with secondary winding shorted. restart mode . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 34. operating with secondary winding shorted. steady state . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 35. operating with secondary winding shorted. steady state (zoom). . . . . . . . . . . . . . . . . . . . 23 figure 36. ovp circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 37. output overvoltage protection at 115 vin_ac 0.2 a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 38. output voltage ripple at 230 vin_ac no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 39. auto-restart mode of the overvoltage protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 40. auto-restart mode of the overvoltage protection (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 41. normalized output ovp threshold vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 42. extra power applied and removed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 43. extra power applied indefinitely . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 44. ept thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 45. thermal measurements at 90 vac full load, tamb = 25 c . . . . . . . . . . . . . . . . . . . . . . . 27 figure 46. average measurement of background noise (board disconnected from mains). . . . . . . . . 28 figure 47. average measurement at 115 vac, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 figure 48. average measurements at 230 vac, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 AN2950 list of figures doc id 15494 rev 1 5/37 figure 49. top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 50. bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 51. connection of the uut to the wattmeter for power measurements . . . . . . . . . . . . . . . . . . 32 figure 52. suggested connection for low power measurements (switch in position 1) . . . . . . . . . . . . 33 figure 53. suggested connection for high power measurements (switch in position 2) . . . . . . . . . . . 33 adapter features AN2950 6/37 doc id 15494 rev 1 1 adapter features ta bl e 1 lists the electrical specifications of the demonstration board. 1.1 circuit description the power supply is set-up in a flybac k topology. its schematic is shown in figure 2 . the input section includes the protection elements (fuse and ntc for inrush current limiting), a filter for emc suppression (c1, t2, c13), a diode bridge (br1) and an electrolytic bulk capacitor (c3) as the front-end ac-dc converter. the transformer uses a standard e25 ferrite core. a transil clamp network is used to demagnetize the leakage inductance. at power-up, the drain pin supplies the internal hv start-up current generator that charges the c4 capacitor up to v ddon . at this point, the power mosfet starts switching, the generator is turned off and the ic is powered by the energy stored in c4 until the auxiliary winding voltage becomes high enough to sustain the operation through d1 and r1. the value of the resistor r3 between the cont and gnd pins is high enough that the viper28?s current limit does not change with respect to the datasheet?s default value i dlim . this resistor, in conjunction with d2, r14 a nd r15, is used to realize the overvoltage protection and the feedforward correction function, as described further in this document. the output rectifier d4 has been selected according to the calculated maximum reverse voltage, forward voltage drop and power dissipation, and is a power schottky type. the output voltage regulation is performed by a secondary feedback with a ts431 driving an optocoupler (in this case a pc817) ensuring the required insulation between the primary and secondary. the opto-transistor drives directly the fb pin of the viper28, which is connected to the compensation network made up by c6, c7 and r12. a small lc filter has been added at the output in order to filter the high-frequency ripple without increasing the size of the output capacitors, and a 100 nf capacitor has been placed very close to the solder points of the output connector to limit the spike amplitude. table 1. electrical specification parameter symbol value input voltage range v in [90 v rms ; 265 v rms ] output voltage v out 5 v maximum output current i outmax 2 a peak output current i outpk 3 a precision of output regulation vout_lf 5% high-frequency output voltage ripple vout_hf 50 mv maximum ambient operating temperature t a 60 c AN2950 adapter features doc id 15494 rev 1 7/37 figure 2. application schematic ! - v & |