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september 2012 doc id 023315 rev 1 1/41 AN4130 application note steval-ill045v1: 120 v a19 dimmable high power factor 9 w led driver using the hvled815pf by thomas stamm introduction the steval-ill045v1 demonstration board showcases st's new hvled815pf led driver chip. it solves the problem of low-cost drive circuitry for led replacements for 40 w incandescent or equivalent compact-fluorescent lamps. the hvled815pf is a new integrated power controller using primary-side control to achieve led current regulation within +/-5%. (it also has primary-side voltage regulation, used here for open load protection.) the device incorporates an 800 v avalanche-rated fet and fits in a standard so-16 package. an internal startup circuit eliminates the need for external rapid-start circuitry. the pfc-flyback power converter operates in transition mode for highest efficiency and best use of components. with the addition of a few extra components the hvled815pf draws near-sinusoidal input current from the ac line. the circuit regulates led current over a wide range of line voltage and led string voltage, and is dimmable with standard triac-based dimmers. figure 1. images top and bottom www.st.com
contents AN4130 2/41 doc id 023315 rev 1 contents 1 features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 transition-mode flyback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 pfc-flyback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 non-isolated flyback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 primary-side control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5 using the hvled815pf current limit for power factor correction . . . . . . . 8 2.5.1 average current regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.5.2 adding an ac component to the current regulator . . . . . . . . . . . . . . . . . . 8 2.6 diode clamp to limit input current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.7 diode clamp effects on dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3 power converter performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.1 led current vs. line voltage and load voltage . . . . . . . . . . . . . . . . . . . . . 14 3.2 efficiency and power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.3 power factor and total harmonic distortion . . . . . . . . . . . . . . . . . . . . . . . . 15 3.4 dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.5 conducted emi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.6 startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.7 component stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.7.1 thermal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.7.2 electrical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.8 summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4 design guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.1 the load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2 preload resistor (r9) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.3 output filter capacitor (c11) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.3.1 led ripple current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.3.2 allowable ripple current in leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.4 diode selection (d3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 AN4130 contents doc id 023315 rev 1 3/41 4.4.1 speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.4.2 reverse voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.4.3 current rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.5 snubber capacitor selection (c10) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.6 transformer design (t1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.6.1 operating frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.6.2 primary inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.6.3 primary peak current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.6.4 reflected voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.6.5 leakage inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.6.6 auxiliary winding turns ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.6.7 final transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.7 dmg pin (r6, r7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.8 filter capacitor for vcc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.9 comp pin capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.10 current sense resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.11 ac injection divider (r3, r4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.12 emi filter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4.12.1 supporting the flyback input current . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.12.2 shunting the hf noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.12.3 limiting the noise current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.13 emi filter and dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.13.1 damping the input filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 6 transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 7 extensions and modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 7.1 lower output voltage, higher current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 7.2 higher line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 8 pc layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 9 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 10 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 list of figures AN4130 4/41 doc id 023315 rev 1 list of figures figure 1. images top and bottom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. physical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 3. fet drain voltage waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 4. distortion of input current with sinewave reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 5. current distortion with sinewave input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 6. voltage and current waveforms with ac injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 7. waveforms with 90 v input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 8. waveforms with 110 v input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 9. waveforms with 130 v input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 10. power loss vs. sinusoidal input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 11. output current vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 12. thd vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 13. 70 vrms input, no diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 14. bat48, ~0.3 v drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 15. 40 vrms dimmed input, no diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 16. 40 vrms dimmed input, bat48 diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 17. power loss vs. dimmed rms line voltage (120 v line) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 18. output current vs. dimmed rms line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 19. led current vs. line voltage with 18 series led load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 20. led current vs. led voltage at 120 v in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 figure 21. efficiency vs. line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 22. power loss vs. line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 23. power factor vs. line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 24. thd vs. line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 25. relative dimmed output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 26. conducted emi, line 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 27. conducted emi, line 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 28. unit startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 29. component electrical stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 30. schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 31. led dynamic resistance vs. current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 32. simplified lisn diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 33. conducted emi limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 34. flyback converter input current waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 figure 35. undamped input filter waveforms with triac dimmer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 figure 36. properly damped waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 figure 37. final input filter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 figure 38. input transient at 200 ma/div, 2.5 ms/div . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 figure 39. input transient at 500 ma/div, 500 s/div . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 figure 40. input transient at 1 a/div, 50 s/div. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 41. transformer specification for 18-led load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 figure 42. transformer specification for 9-led load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 7 figure 43. top placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 figure 44. top copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 figure 45. bottom placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 figure 46. bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 AN4130 features doc id 023315 rev 1 5/41 1 features the demonstration board features are: +/- 5% primary-side current regulation, no optocoupler low component count - 25 parts, including the emi filter only 1 tight-tolerance component high efficiency, >86% high power factor >0.98 and low thd, <20% over 90 v to 132 v range fits in 28 mm tubing, 52 mm overall length 9 w output, for light equal to 40-60 w incandescent dimmable with common triac dimmer. figure 2. physical theory of operation AN4130 6/41 doc id 023315 rev 1 2 theory of operation 2.1 transition-mode flyback flyback power converters operate by storing energy from the primary side in an inductor's air gap, and discharging the energy into a load on the secondary side. the converter can run in two modes: 1. discontinuous conduction, where there is a deadtime between discharge and charge cycles. 2. continuous conduction, where the discharge cycle is ended by starting the charge cycle before all the stored energy is delivered to the load. neither mode fully utilizes the magnetic structure of the inductor. however, if the recharge cycle is started just after the discharge cycle ends, the natural ringing of the inductor and stray capacitance can be used to reduce turn-on voltage stress on the switch. transition mode converters can be very efficient as a result, having greatly reduced turn-on loss - the switch does not have to discharge its own and stray capacitance from a high voltage. figure 3. fet drain voltage waveforms operating frequency is a function of source and load voltages, and load current. if the source voltage varies, the operating frequency varies. this makes the transition mode converter very popular in low-cost commercial applications, where the varying frequency, due to input voltage ripple, spreads noise over a wide spectrum, reducing the noise at any one frequency. conducted emi tests can be easier to pass. AN4130 theory of operation doc id 023315 rev 1 7/41 2.2 pfc-flyback in the pfc-flyback converter the input voltage is the rectified line voltage, with almost no filtering. converter input voltage goes to zero when the line voltage crosses zero. it's common practice to use the rectified line voltage as a reference for the peak current in the flyback converter switch. this does not result in sinusoidal input current, but it's close enough. the duty cycle change with input voltage still distorts the waveform. this is discussed in detail in st's an1059 application note. figure 4. distortion of input current with sinewave reference 2.3 non-isolated flyback in this circuit the transformer secondary shares part of the primary winding. the lack of isolation means that the burden of electrical safety rests on the led thermal mounting. the leds must be isolated from human contact. electrical stress on the power conversion components is lower than a corresponding isolated flyback converter. leakage inductance in the transformer is lower, because the secondary occupies the same space as part of the primary, and the remaining primary is very close to the secondary - interwinding insulation is not required. reduced leakage inductance results in less overshoot in the fet drain voltage, so a lower voltage fet can be used safely. the hveld815pf used here has an 800 v fet - too much for the us 120 v application but ideal for european line voltage. 2.4 primary-side control a pfc-flyback converter usually uses a pfc controller chip such as st's l6562at with an external fet and a feedback loop. the secondary side voltage and/or current are monitored, compared to a reference on the secondary side, and a control signal sent to the primary side with an opto-isolator. this signal is multiplied by a reference waveform (the rectified line voltage) and used to control peak switch current. st has developed a primary-side control circuit that eliminates the need for the secondary- side components. voltage is monitored on the housekeeping winding at the end of the flyback converter discharge cycle, just as the secondary current reaches zero. secondary current is set by measuring duty cycle and adjusting peak primary current, to provide a calculated secondary average current. but the circuit cannot work with a multiplier, so another method of shaping the peak switch current waveform must be found. 2 e c t i f i e d l i n e v o l t a g e # o r r e s p o n d i n g i n p u t c u r r e n t ! - v theory of operation AN4130 8/41 doc id 023315 rev 1 2.5 using the hvled815pf current limit for power factor correction 2.5.1 average current regulation the hvled815pf does an excellent job of regulating output current in a dc input flyback supply. it calculates the peak current at which to shut off the driving fet by continuously looking at the duty cycle. the error between desired duty cycle and actual duty cycle appears as a current on the iled pin - a capacitor on this pin integrates the error to zero over time. since the voltage on this pin, divided by 2, directly sets the current at which the fet switch turns off, the output current is regulated. in dc-input flyback power supplies very small capacitors are used on the iled pin for quick response to changing loads or input voltage. the capacitor on this pin can be much larger if the load voltage does not change rapidly. led current can be regulated more slowly, averaging out the error over several cycles of input voltage. a 4.7 f low-voltage ceramic capacitor is used here. the average led current is kept constant even if the input voltage waveform is grossly distorted, such as a rectified sinewave, as occurs in the pfc-flyback topology. the input current waveform, however, is truly ugly. note the magenta trace in the figure below. figure 5. current distortion with sinewave input where: yellow = line voltage magenta = line current. the peak fet shut-off current remains at the same level throughout the ac half cycle, but the duty cycle of the converter changes. (fet on-time increases at lower input voltage - it takes longer to reach the same current if the converter input voltage is lower). the resulting input current waveform is very rich in harmonics (thd is in the range of 130%), though power factor is actually pretty good. 2.5.2 adding an ac component to the current regulator if an ac signal is injected into the iled pin, the instantaneous fet peak current can be controlled, while the average output current (a dc level) remains regulated. the figure below shows the injection of a small fraction of the line voltage into the bottom of the iled capacitor. the change in the input current waveform is dramatic. but it is best for only one AN4130 theory of operation doc id 023315 rev 1 9/41 line voltage, and is a compromise for all others. but it's ?good enough?. the small capacitor across the lower resistor is only there to keep switching noise out of the circuit. figure 6. voltage and current waveforms with ac injection the current waveform at the ?nominal line? above actually has the lowest harmonic content due to the input current distortion inherent in the pfc-flyback converter. the hvled815pf clamps the voltage on the iled pin between about 0.2 v on the low end, and at about 1.5 v on the high end. if the injected waveform wants to swing below 0.2 v, the peak current in the fet is set to zero, so no input current flows. the scope images in figure 7 , 8 , and 9 show how well the ac injection works. : e r o $ # l e v e l n e e d e d t o d e l i v e r u & n & + + ! p p x 6 , o w , i n e . o m i n a l , i n e ( i g h , i n e ) , % $ 0 ) . ! p p x 6 6 e r y , o w , i n e 2 e c t i f i e d , i n e : e r o : e r o : e r o ! p p x 6 ! p p x 6 3 i n e w a v e 2 e f e r e n c e ) n p u t # u r r e n t c o r r e c t l o a d c u r r e n t ( 6 , % $ 0 & ! - v figure 7. waveforms with 90 v input figure 8. waveforms with 110 v input trace colors: yellow = line voltage magenta = line current, 50 ma/div ref -3div blue = voltage at i led pin, ref -3 div green = led current, 50 ma/div ref -3div trace colors: yellow = line voltage magenta = line current, 50 ma/div ref -3div blue = voltage at i led pin, ref -3 div green = led current, 50 ma/div ref -3div theory of operation AN4130 10/41 doc id 023315 rev 1 figure 9. waveforms with 130 v input where: yellow = line voltage magenta = line current, 50 ma/div ref -3div blue = voltage at i led pin, ref -3 div green = led current, 50 ma/div ref -3div. power factor is excellent over the designed line voltage range of 90 v to 132 v, well above 0.98. total harmonic distortion reaches a minimum at one line voltage. over the range, thd is less than 20%. 2.6 diode clamp to limit input current since the peak fet current is directly controlled by the voltage on the iled pin, a diode clamp can be added to limit the voltage increase to reasonable levels. the graph below shows the results for two conditions - no diode, and a schottky diode having about 0.3 v forward drop, placed across the dc filter capacitor. the hvled815 attempts to provide regulated power even if low line voltage makes that task difficult. it raises the voltage on the iled pin to fairly high levels if the line voltage is reduced. losses in the unit rise dramatically with decreasing line voltage as the input current increases. AN4130 theory of operation doc id 023315 rev 1 11/41 figure 10. power loss vs. sinusoidal input voltage the input current increase can now be limited to a reasonable value. there are two consequences of this addition: line regulation is lost at low input voltages (the iled pin cannot rise to regulate current). figure 11. output current vs. input voltage harmonic distortion is greatly reduced at low line voltage. figure 12. thd vs. input voltage ! - v x ) q ) . . , / ( ' 3 , 1 5 h f w l i l h g / l q h + 9 / ( ' 3 ) ' , 2 ' ( 0 o w e r , o s s 7 a t t s ) n p u t 6 2 - 3 . o $ i o d e " ! 4 ! - v , % $ # u r r e n t m ! ) n p u t 6 r m s . o $ i o d e " ! 4 ! - v 4 o t a l ( a r m o n i c $ i s t o r t i o n ) n p u t 6 r m s . o $ i o d e " ! 4 theory of operation AN4130 12/41 doc id 023315 rev 1 the scope shots below show the result on the input current waveform. where: yellow = line voltage magenta = line current, 50 ma/div ref -3div blue = voltage at i led pin, ref -3 div green = voltage at bottom of 4.7 f cap on i led pin. 2.7 diode clamp effects on dimming the diode-improved waveform also helps when the circuit is dimmed with a triac, especially at low conduction angles. the iled pin voltage is not allowed to rise. note how high the iled pin voltage (green trace) has risen in the first image, compared to the second, as the chip attempts to regulate the average output current. the voltage on that pin directly controls the peak fet current. figure 13. 70 vrms input, no diode figure 14. bat48, ~0.3 v drop figure 15. 40 vrms dimmed input, no diode figure 16. 40 vrms dimmed input, bat48 diode AN4130 theory of operation doc id 023315 rev 1 13/41 where: yellow = line voltage magenta = line current, 50 ma/div ref -3div blue = voltage at i led pin, ref -3 div green = voltage at bottom of 4.7 f cap on i led pin. dissipated power is also reduced at low conduction angles due to the lower rms input current: figure 17. power loss vs. dimmed rms line voltage (120 v line) figure 18. output current vs. dimmed rms line note that the dimming curve for the schottky diode unit is much smoother and slightly lower at the low end. this allows the unit to meet the requirements of nema ssl 6-2010, as shown in figure 25 . ! - v 0 o w e r , o s s 7 a t t s $ i m m e d 2 - 3 l i n e v o l t a g e . o $ i o d e " ! 4 ! - v , % $ c u r r e n t $ i m m e d 2 - 3 l i n e v o l t a g e . o d i o d e " ! 4 power converter performance AN4130 14/41 doc id 023315 rev 1 3 power converter performance 3.1 led current vs. line voltage and load voltage nominal (18 leds) voltage is 54-56 v. performance is excellent over a very wide range of load conditions, even with the ac injection. 3.2 efficiency and power dissipation as expected, efficiency drops off at low voltage. the rather high r ds(on) of the hvled815pf and the input filter series resistances increase i 2 r loss due to higher required input current. figure 19. led current vs. line voltage with 18 series led load figure 20. led current vs. led voltage at 120 v in ! - v , % $ # u r r e n t ! m p s , i n e v o l t a g e ! - v , % $ # u r r e n t , % $ v o l t a g e figure 21. efficiency vs. line voltage figure 22. power loss vs. line voltage ! - v % f f i c i e n c y , i n e v o l t a g e ! - v 0 o w e r , o s s 7 a t t s , i n e v o l t a g e AN4130 power converter performance doc id 023315 rev 1 15/41 3.3 power factor and total harmonic distortion 3.4 dimming figure 25. relative dimmed output figure 23. power factor vs. line voltage figure 24. thd vs. line voltage ! - v , i n e v o l t a g e ! - v , i n e v o l t a g e ! - v 2 e l a t i v e l i g h t o u t p u t 4 r i a c $ i m m e d 2 - 3 , i n e 6 o l t a g e . e m a 3 3 , l i m i t s ) n c a n d e s c e n t r e l a t i v e o u t p u t . o r m a l i z e d , % $ c u r r e n t power converter performance AN4130 16/41 doc id 023315 rev 1 3.5 conducted emi the conducted emissions plot for the two input lines are virtually identical. the plots are the maximum (peak hold) of 10 scans for peak power. 3.6 startup figure 28. unit startup where: yellow = (not shown) line voltage, triggers scope magenta = line current blue = led voltage green = led current. the unit produces usable light in about 0.15 seconds, and nearly full output in about 0.5 seconds. figure 26. conducted emi, line 1 figure 27. conducted emi, line 2 AN4130 power converter performance doc id 023315 rev 1 17/41 3.7 component stress 3.7.1 thermal the unit was mounted above the bench in free air with the narrow end (ac input) down. temperatures were recorded after 45 minutes of operation. the dimmed temperatures were taken with a triac dimmer feeding the unit. the dimmer was adjusted to the point where the power analyzer reported greatest loss. undimmed input voltage was 120 vrms. dimmed input voltage was 108 vrms, and conduction angle about 150 degrees. table 1. component thermal stress thermal stress undimmed dimmed efficiency 87.6% 85.7% power loss 1.41 w 1.56 w ambient 23.9c 24.9c r1 36.7c 50.3c r2 50.8c 73.2c br1 46.6c 50.6c l2 42.1c 46.6c u1 66.5c 78.1c t1 56.3c 60.2c d3 56.2c 58.7c c11 41.9c 43.8c power converter performance AN4130 18/41 doc id 023315 rev 1 3.7.2 electrical the plot below was taken near the peak of line voltage, where both voltage and current stresses are greatest. figure 29. component electrical stress where: yellow = drain voltage magenta = diode voltage blue = drain current. note the very small overshoot in the drain voltage and diode voltage waveforms. this is due to the transformer's low leakage inductance, about 0.6 microhenries. the current ringing is due to the snubber capacitor on the secondary side resonating with the leakage inductance at fet turn-on and turn-off. the circulating current loop on the pc board is very small and is only weakly coupled to the ac line, so it does not cause an emi problem. 3.8 summary performance is excellent for an led driver of this size and simplicity. the added bonuses of dimmability and power factor correction compel consideration of this design. AN4130 power converter performance doc id 023315 rev 1 19/41 figure 30. schematic ! - v 3 2 # ) , % $ $ - ' # / - 0 . ! $ r a i n $ r a i n # 3 6 c c ' . $ $ r a i n $ r a i n . ! . # . ! . ! 5 ( 6 , % $ 0 & 2 + 2 + 2 2 , % $ , % $ 7 ( 4 , % $ , % $ 7 ( 4 ! a v e r a g e ! p p x 6 ! # 6 ! # 6 " 2 " 2 ) $ ' % 3 - 4 # n & 3 e e n o t e 2 / h m & |