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march 2007 rev 4 1/30 AN2042 application note vipower: dimmable driver for high brightness leds with viper22a-e introduction this application note introduces an innovative solution to drive high brightness 1w leds (light emitting diode), using viper22a-e in flyback configuration with output current control. the power supply is able to drive an array of 1 to 8 leds in european range, i.e. 185-265 vac with no modifications. by means of an input voltage doubler, it is possible to use the same viper device also in u.s. input voltage range, guaranteeing the specs. a new control technique is used to adjust the duty cycle of the output current, in order to dim the luminosity of the leds down to 10% of the maximum value (patent pending by stmicroelectronics). the proposed driver can be suitably used in app lications such as landscape lighting, street lighting, car parks, bollards, garden lighting, large area displays and so on. also domestic applications such as room lighting, decorative fixtures and architectural lighting can benefit from the advantage of this dimmable light source. 10w dimmable leds driver board layout www.st.com
contents AN2042 2/30 contents 1 light sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 light emitting diode and colour vi sion . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 commercial leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 new dimming technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.1 dimming control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.2 transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.3 dali interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6 experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 8 emi measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 9 non dimmable version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 10 input section arrangement for u.s. market . . . . . . . . . . . . . . . . . . . . . . 26 11 conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 12 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
AN2042 list of figures 3/30 list of figures figure 1. light emitting diode structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 2. the electromagnetic spectrum and visible region of light . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. human relative vision curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 4. c.i.e. chromaticity diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 5. forward current vs. forward voltage in a typical commercial leds . . . . . . . . . . . . . . . . . . . 8 figure 6. pwm technique for dimming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 7. brightness variation versus duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 8. dimming technique using series switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 9. dimming technique using the new methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 10. new dimming technique: typical waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 11. converter schematic for european input voltage range . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 12. transformer features: (a) schematic, (b) me chanical characteristics and (c) pinout . . . . . 17 figure 13. v ds and i d at 230 v ac : 1 led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 14. v ds and i d at 230 v ac : 8 leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 15. typical waveforms: drain voltage and output current ripple at 230 v ac . . . . . . . . . . . . . . . 19 figure 16. typical waveforms: startup at 265 v ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 17. drain voltage v ds and output current i out : 1 led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 18. drain voltage v ds and output current i out : 8 leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 19. drain voltage v ds and output current i out at 50% dimming: 1 led . . . . . . . . . . . . . . . . . 20 figure 20. drain voltage v ds and output current i out at 50% dimming: 8 leds . . . . . . . . . . . . . . . . 20 figure 21. drain voltage v ds and output current i out at 10% dimming: 1 led . . . . . . . . . . . . . . . . . 20 figure 22. drain voltage v ds and output current i out at 10% dimming: 8 leds . . . . . . . . . . . . . . . . 20 figure 23. control signals at 230 v ac : 1 led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 24. control signals at 230 v ac : 8 leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 25. control stage at 230 v ac : 1 led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 26. control stage at 230 v ac : 8 leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 27. open load condition at 230 v ac : no dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 28. open load condition at 230 v ac : minimum dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 29. efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 30. pcb layout (not in scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 31. conducted emissions at full load: line 1 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 32. conducted emissions at full load: line 2 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 33. non dimmable solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 34. application circuit for u.s. input voltage range: changes on the input section . . . . . . . . . . 26 figure 35. steval-ill001v1 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
light sources AN2042 4/30 1 light sources incandescent lights are basically electric space heaters that give off light as a by-product. they are very inefficient, wasting most of the power they consume as heat. an innovative light source is represented by led technology, with very low power consumption and virtually no heating effect, making leds ideal for several domestic and commercial applications. the long lifetime characteristic of leds means savings on maintenance costs. unlike traditional light sources, leds are not subject to sudden failure or burnout. since led based light sources last at least 10 times longer than a normal light source (up to 10 years or 100.000 hours for the higher quality products), it is possible to reduce or eliminate the maintenance ongoing costs. this can be useful in many critical applications where the location makes replacement difficult (radio tower, aircraft warning lights, bridge and tunnel lights?) or in applications where a failure of the light source is not acceptable (emergency exit lights, back up lighting, security lighting?). led lighting technology features many advantages compared to conventional lighting: higher energy efficiency, in terms of lumens per watt; direct light beam for increasing system performance; dynamic color control technology; full dimmable without color variation; no mercury and no uv or heat in light beam; low voltage operation, suitable for safety purpose in selv systems. the most important limitation for using high brightness leds is the manufacturing cost, which is still relatively high. in ta b l e 1 a comparison between traditional light sources and a typical commercial led is shown. table 1. performance of typical light sources compared with white luxeon leds lighting source luminous efficiency (lm/w) lifetime (hours) theoretical optical power (min and max) incandescent bulbs 18 25 1000 ? 2000 15 ? 1000 w halogen lamps 15 ? 25 2000 ? 5000 5 ? 2000 w fluorescent lamps 60 ? 110 14000 ? 20000 4 ? 60 w mercury lamps 15 ? 60 12000 ? 24000 50 ? 1000 w leds (white luxeon) 25 100000 0.7 ? 5 w
AN2042 light emitting diode and colour vision 5/30 2 light emitting diode and colour vision light-emitting diodes (leds) used for illumination are solid-state device s that produce light by passing electric current across layers of semiconductor chips that are housed in a reflector, which in turn is encased in an epo xy lens. the semiconductor material determines the wavelength and subsequent color of the light. the lens converts the led into a multidirectional or unidirectional light source based on specification. the first generation of le d was based on gallium arseni de (gaas), gallium arsenide phosphide (gaasp), gallium phosphide (gap) tec hnology, but thanks to the growth of solid state technology, new structur es have been introduced based on aluminum indium gallium phosphide (alingap), indium gallium nitride (ingan) or gallium aluminum arsenide (algaas), mainly for the high brightness leds branch. in figure 1 the basic led structure and the energy bands are shown. figure 1. light emitting diode structure the junction in an led is forward biased and when electrons cross the junction from the n to the p type material, the electron-hole recombination results in a process called electroluminescence: when the applied voltage drives the electrons and holes into the active region between the n-type and p-type material, the energy can be converted into infrared or visible photons. this implies that the electron-hole pair drops into a stabler bound state, releasing energy on the order of electron volts by emission of a photon of energy, according to ( equation 1 ). equation 1 the human eye is excited in response to electromagnetic radiations with wavelengths in a tight range of the electromagnetic spectrum, as shown in figure 2 , from 400 nm to 700 nm which corresponds to extreme red and violet respectively. e g h c h c ----- = ? =
light emitting diode and colour vision AN2042 6/30 figure 2. the electromagnetic sp ectrum and visible region of light the red extreme of the visible spectrum, 700 nm, requires an energy release of 1.77 ev to provide the quantum energy of the photon. at the other extreme, 400 nm in the violet, 3.1 ev is required. the human vision efficacy is not constant in the entire visible region, but decreases near the edges, as shown in figure 3 featuring a peak value for a wavelength of 555 nm (green- yellow). figure 3. human relative vision curve wavelength can be defined in terms of dominant wavelength and x-y chromaticity coordinates, which define the color as perceived by the human eye. the dominant wavelength is derived from the c.i.e. (commission internationale de l'eclairage - international commission on illumination) chromaticity diagram, as shown in figure 4 this is an international standard for primary colors established in 1931. based on the fact th at the human eye is able to separately sense three different portions of the spectrum (we i dentify these peak sensitivities as red, green and blue), the eyes response is best described in terms of such primary colors. all the other colors are defined as weighted sum of them.
AN2042 commercial leds 7/30 figure 4. c.i.e. chromaticity diagram 3 commercial leds in the last years, light emitting diodes can be chosen from a wide variety of products designed to meet specific needs to provide more efficient, longer life time alternatives to traditional incandescent lamps. they are manufactured of gan and related compounds of algan and ingan due to the wide bandgap, which allows emission of light ranging from the red to the ultraviolet (uv) wavelength. blue and green leds are of special interest and are being used in a wide range of applications from outdoor video displays to automotive and cell phone backlights. leds for solid-state white lighting offer high efficiency, long lifetime and a high degree of design flexibility for a variety of lighting applications. thanks to new solid state technology, it now de livers from 25 to more then 120 lm/w in white and comparable light output in other colors. in ta bl e 2 are listed the main specifications for typical commercial high efficiency leds are listed, while figure 5 shows a typical v-i characteristic for a high efficiency led. table 2. typical characteristic for commercial leds (from luxeon) color operating voltage (v) operative forward current (ma) dominant wavelength/ color temperature typical luminous flux (lm) white 3.42 350 5500 k 18 blue 3.42 350 470 nm 5 cyan 3.42 350 505 nm 30 green 3.42 350 530 nm 25 amber 2.85 350 590 nm 20 red 2.85 350 625 nm 25
new dimming technique AN2042 8/30 figure 5. forward current vs. forward voltage in a typical commercial leds 4 new dimming technique nowadays, thanks to the growth of process, packaging and thermal transfer technologies, light output continues to evolve. this invo lves especially the ingan technology, which produces light output across bl ue, cyan, green and white, with high reliability and efficiency. the wavelength of the light emitted is strongly dependent on the forward current driven through the device and in order to avoid shifts in color the dimming strategies have to be chosen carefully. the most common method of dimming a led is by varying either forward current or voltage across it. unfortunately, due to the characteristics of ingan, varying current or voltage will shift the wavelength. this effect is proportional to the wavelength, with the longer wavelengths undergoing the strongest shift variation versus current. in many applications this effect cannot be accepted and, employing a pwm technique, it is possible to dim a led in the right manner, without wavelength shift. the led is switched on and off at constant forward current (i f ) by varying the duty cycle, as shown in figure 6 . if the pwm frequency is higher than 100 hz, the human eyes cannot perceive the single pulses, but they integrate and interpret those pulses as brightness, which can be changed linearly by varying the duty cycle linearly, with no wavelength shift. figure 7 shows the brightness variation versus duty cycle.
AN2042 new dimming technique 9/30 figure 6. pwm technique for dimming figure 7. brightness variation versus duty cycle as shown in figure 8 , the most common method to dim leds consists in a series connection of a power switch which is controlled by pwm. due to the relatively high operative forward current, the switch has to be selected carefully in order to handle the conduction losses.
new dimming technique AN2042 10/30 figure 8. dimming technique using series switch to overcome this problem, a patented solution has been implemented, which allows to eliminate the series switch, with a considerable improvement in terms of efficiency. the new technique consists in a double control loop: a current and a voltage control loops. the first one drives the leds with constant current when the maximum luminosity is required. during the dimming operation, the current control loop will still limit the maximum output current, while the volt age loop will maintain the outpu t voltage below the threshold voltage of the leds array. also disconnecting the leds, the maximum output voltage will be limited by the voltage loop. in figure 9 and figure 10 the block diagram of the new dimming technique and the temporal diagrams are respectively shown. thanks to the absence of the power switch, it is possible to have a more efficient and cheaper solution. figure 9. dimming technique using the new methodology
AN2042 application description 11/30 figure 10. new dimming technique: typical waveforms 5 application description the proposed converter is based on viper22a-e, a smart power with a current mode pwm controller, startup circuit and protections integrated in the same monolithic chip, using stmicroelectronics vipower m0 technology. the power stage consists in a vertical power mosfet with 730 v breakdown voltage and 0.7 a typical peak drain current. the application consists in an isolated constant current power supply, intended to supply an array of eight high efficiency leds, as shown in figure 11 . the board has been designed referenced to the specifications listed in ta bl e 3 it is important to highlight that the converter works in single range, but both u.s. and european range can be selected, with only a few modifications in the input section. table 3. smps specifications parameters value selectable input voltage range 85v ac 135 v ac or 185 v ac 265 v ac nominal output voltage range 3.5 v28 v maximum output voltage at open load 32 v output current 350 ma dimming range 0%90% emi standard en55015:2000
application description AN2042 12/30 in the input stage, an emi filter is implemented (c 1 , cm, c 2 ) for both differential and common mode noise, in order to fit the en55015:2000 standard (limits for electrical lighting and similar equipment). the input resistor r 1 , limits the inrush current of the capacitors at plug-in and a standard fuse is also introduced to prevent catastrophic failure. the clamping network (r 2 -c 4 -d 5 ), limits the peak of the leakage inductance voltage spike, assuring reliable operation of the viper22a-e. the auxiliary winding on the primary side, is connected in forward mode, since the output voltage ranges from 3.5 v to 28 v and the voltage on vdd pin varies from 17 v to 24 v. a brown-out circuit (r 3 , r 4 , r 5 , q 1 , q 2 and c 7 ) is implemented in order to avoid the flickering of the leds during switch off. the values of r 3 , r 4 and r 5 are chosen in order to get the given thresholds, while c 7 stabilizes the voltage on the base of q 1 . the output filter selection is a very critical point to consider during the design. since leds are switched on and off during the dimming phase the value of the output capacitor has to be as low as possible. therefore, in order to avoid exceeding the maximum output current ripple, care must be paid to design the right lc post filter. 5.1 dimming control circuit the current loop is controlled by the second operational amplifier of tsm104w and the sense resistor r 10 . the voltage threshold is generated by means of a resistor bridge (r 12 , r 13 and r 14 ) connected to the 2.5 v internal voltage reference v ref . the resistors of the bridge should be 1% precision in order to ge t the best precision on the regulation. the current control equations are given by ( equation 2 ) and ( equation 3 ). equation 2 equation 3 the sense resistor r 10 , is chosen taking into account the maximum dissipation during full load. the voltage loop is controlled by the third operational amplifier and the voltage divider r 8 and r 9 directly connected to the output. the va lues are chosen according the equations ( equation 4 ) and ( equation 5 ). equation 4 equation 5 where v out(max) is the maximum acceptable output voltage, when the leds array is disconnected. the transistor q 3 , connected to the dimming control section, is on during normal operation. v iout () v ref r 14 ? r 12 r 13 r 14 ++ ----------------------------------------- - = i out v iout () r 10 ? = v oref v ref r 13 r 14 + () ? r 12 r 13 r 14 ++ -------------------------------------------------- - = v oref v out max () r 8 r 9 + ----------------------------- =
AN2042 application description 13/30 the feedback to the primary side is achieved thanks to the diodes d 9 and d 10 , which decouple the two loops and drive the optocoupler opt. the legs r 23 -c 11 and r 24 -c 12 are connected for feedback stabilization. the zener diode d z2 is connected at the non-inverting input of the voltage control operational amplifier in order to clamp the maximum voltage on the pin in any operative condition. the pwm control is realized using the first operational amplifier to generate a sawtooth waveforms at 270 hz (given by the leg r 19 -c 13 ), which is compared with a variable voltage (set by the potentiometer r 21 ): the generated signal will drive the npn transistor q 3 . when the transistor is "on", the smps works in "current control" mode limiting the max output current while, when the transistor is "off", it works in "voltage control" mode, regulating the output voltage below the leds threshold and consequently switching them off. during the dimming operation, the transistor q 3 is switched off and the voltage on pin 11 of ic2 is pulled up and limited to v dz1 . consequently, the viper stops switching and the output current falls to zero, while the output voltage decrease down to v out = n v f(off) , where n is the number of leds and v f(off) is the threshold voltage. further decrease of the output voltage is not possible because of the high output impedance. doing so, the output voltage never falls to zero, resulting in a big improvement in the dynamic behavior of the dimming function, with a slight impact on the efficiency p diss = (v out -v dz2 )/r 8 . in open load condition, the maximum voltage is regulated by r 8 , r 9 and d z2 according to the reference voltage given by ( equation 5 ).
application description AN2042 14/30 figure 11. converter schematic for european input voltage range
AN2042 application description 15/30 table 4. component list reference description note fs 1 a-250 v fuse r1 10., 1/2 w metallic oxide resistor ? no flammable r2 1m., 1/2 w r3 560 k., 1/4 w r4 12 k., 1/4 w r5 24 k., 1/4 w r6 1 k., 1/4 w r7 150., 1/2 w r8 5.6 k., 1/4 w r9 220., 1/4 w r10 0.47., 1/4 w sense resistor r11 2.7 k., 1/4 w r12 12 k., 1/4 w r13 10 k., 1/4 w r14 1.5 k., 1/4 w r15 4.7 k., 1/4 w r16, r18, r22 22 k., 1/4 w r17 100., 1/4 w r19 33 k., 1/4 w r20 15 k., 1/4 w r21 20 k., 1/4 w potentiometer r23, r24 220 k., 1/4 w r25 1.2 k., 1/4 w r26 6.8 k., 1/4 w c1 100 nf, 275 v x2 capacitor c2 10 f, 400 v electrolytic capacitor c4 100 pf, 630 v polypropylene capacitor c5 33 f, 25 v electrolytic capacitor c6, c13 220 nf polyester capacitor c7 47 nf polyester capacitor c8 33 f, 16 v electrolytic capacitor c9 1 f, 50 v electrolytic capacitor c10 3.3 f, 50 v electrolytic capacitor
application description AN2042 16/30 5.2 transformer specifications the transformer has four winding s, included two auxiliaries. one is used to supply the viper and the other one to supply the tsm104 and the dimming control circuit on the secondary side. since the output voltage is variable between 3.5 v (with 1 led) and 28 v (with 8 leds), the two auxiliary windings are coupled in forward mode to the primary winding. in order to limit the reflected voltage to a maximum value (100 v), the primary-to-secondary turn's ratio has been set according to the maximum count of leds. the transformer characteristics are listed in ta bl e 5 and the winding arrangement as well as the mechanical specifications are shown in figure 12 5.3 dali interface in order to control the board in remote fashion a connector has been introduced to interface it with the dali reference design (st7dali-eval). referring to the schematic in appendix a , it is possible to move from analog control by the trimmer r 21 to the digital one by dali, removing the jumper j1 and j2. then, connect the 1..10 v output of the dali interface on connector j2 of the st7dali-eval demo board to cn1 connector of the steval-ill001v1, providing the correct voltage range, i.e. from 0 to 2.5 v. c11, c12 2.2 nf polyester capacitor c14 2.2 nf, 250 v y1 capacitor d1, d2, d3, d4 1n4007 d5 stmicroelectronics stth1r06 d6, d8, d9, d10, d11 1n4148 d7 stmicroelectronics stth102 dz1, dz2 zener diode 5.1 v, 1/4 w q1, q3 stmicroelectronics bc337 npn transistor q2 stmicroelectronics bc327 pnp transistor l1 47 h radial tf tdk srw16es-exxh003 cm coilcraft bu9-103r25b 2x10 mh common mode choke opt sfh610a ic1 stmicroelectronics viper22adip-e ic2 stmicroelectronics tsm104 table 4. component list (continued) reference description note
AN2042 application description 17/30 figure 12. transformer features: (a) schematic, (b) mechanical characteristics and (c) pinout table 5. transformer specifications parameters value ferrite pc40ef16 core geometry e16 primary inductance 2.0 mh12% leakage inductance 60 h max np 135 turns ? awg 35 naux1 9 turns ? awg 35 naux2 5 turns ? awg 29 nsec 36 turns ? awg 29 a b c
experimental results AN2042 18/30 6 experimental results in this section typical waveforms are gi ven under several load conditions. in figure 13 and figure 14 the drain-source voltage and the drain current at minimum load (1 leds) and full load (8 leds), at nominal input voltage (230 v ac ) are shown, respectively. in figure 15 the output current ripple is shown, which is fixed to about 20% iout, in order to keep the output filter small and improve the output dynamic behavior. in figure 17 to figure 22 the output current and drain-source voltage are shown during dimming operations. it is important to point out that the driver is able to dim the leds array down to 10% of its maximum luminosity. in figure 23 and figure 24 typical waveforms of the dimming control section, as introduced in section 5.1 , are shown: the sawtooth waveform, vsaw, defines the dimming frequency while varying the reference voltage, v ref , by means of the potentiometer r 21 , it is possible to change the pwm duty-cycle and consequently the leds luminosity. it is important to point out that the output voltage never goes to zero, but is always above a minimum value depending on the number of leds in the array. in figure 25 and figure 26 the output during dimming is shows. finally, figure 27 and figure 28 shows the drain voltage and output voltage in open load condition with 1 or 8 leds connected respectively. under this condition the output voltage is limited to about 33 v both in steady state and dimming operation. figure 13. v ds and i d at 230 v ac : 1 led figure 14. v ds and i d at 230 v ac : 8 leds ch1 freq - 58.18 khz (black) ch2 max - 196 ma (green) ch1 freq - 58.18 khz (black) ch2 max - 196 ma (green)
AN2042 experimental results 19/30 figure 15. typical waveforms: drain voltage and output current ripple at 230 v ac figure 16. typical waveforms: startup at 265 v ac ch1 freq - 548 v (black) ch2 max - 348 ma (red) ch3pk-pk - 68 ma (red) ch1 max - 610 v (black) figure 17. drain voltage v ds and output current i out : 1 led figure 18. drain voltage v ds and output current i out : 8 leds ch1 max - 418 v (black) ch2 max - 348 ma (green) ch1 max - 542 v (black) ch2 mean - 352.6 ma (green)
experimental results AN2042 20/30 figure 19. drain voltage v ds and output current i out at 50% dimming: 1 led figure 20. drain voltage v ds and output current i out at 50% dimming: 8 leds ch2 mean - 170.6 ma (green) ch2 duty - 51.63% (green) ch2 freq - 246 hz (green) ch2 mean - 171.1 ma (green) ch2 duty - 50.52% (green) ch2 freq - 245 hz (green) figure 21. drain voltage v ds and output current i out at 10% dimming: 1 led figure 22. drain voltage v ds and output current i out at 10% dimming: 8 leds ch2 mean - 33.9 ma (green) ch2 duty - 10.18% (green) ch2 freq - 252 hz (green) ch2 mean - 31.5 ma (green) ch2 duty - 8.8% (green) ch2 freq - 249 hz (green)
AN2042 experimental results 21/30 figure 23. control signals at 230 v ac : 1 led figure 24. control signals at 230 v ac : 8 leds ch2 mean - 33.9 ma (green) ch2 duty - 10.18% (green) ch2 freq - 252 hz (green) ch2 mean - 31.5 ma (green) ch2 duty - 8.8% (green) ch2 freq - 249 hz (green) figure 25. control stage at 230 v ac : 1 led figure 26. control stage at 230 v ac : 8 leds ch1 mean - 200 ma (black) ch2 max - 3.48 v (green) ch2 min - 2.44 v (green) ch2 mean - 197.4 ma (black) ch2 max - 26.6 v (green) ch2 min - 20.2 v (green)
experimental results AN2042 22/30 the efficiency of the system, one of the key parameters of the application, has been measured in the whole input voltage range varying the number of leds from 1 to 8, and the experimental results are shown in figure 29 . figure 29. efficiency figure 27. open load condition at 230 v ac : no dimming figure 28. open load condition at 230 v ac : minimum dimming ch1 freq - 613 hz (black) ch2 max - 32.8 v (green) ch1 freq - 250 hz (black) ch2 max - 33.6 v (green)
AN2042 layout considerations 23/30 7 layout considerations as any switched mode power supply, for proper operations, basic rules have to be taken into account in order to optimize the current path, especially in the routing of high current path. in fact, since emi issues are also related to layout, the current loop area has to be minimized. in addition to this, in order to avoid any noise interference between the control section and the power section, the control ground paths have to be kept separated from each other. all the high current traces have to be as short and wide as possible, in order to minimize the resistive and inductive effect. a particular care has to be taken regarding the optimal routing of the input emi filter path and the correct placement of any single component. a final consideration regards the thermal management: a copper area has to be provided on the viper drain, in order to reduce the thermal resistance r th and consequently keep the device temperature reasonably low. all the aforementioned considerations have been taken into account in the lab prototype, as shown in figure 30 . figure 30. pcb layout (not in scale)
emi measurements AN2042 24/30 8 emi measurements conducted emi measurements have been performed according to en55015:2000, the specific european standard on electrical lig hting and similar equipment, using a 50 lisn and a spectrum analyzer with peak detector. the results are shown in figure 31 and figure 32 , for line 1 and line 2 respectively, under full load condition at nomina l input voltage, i.e. 230 v ac . the emissions level are well below the quasi peak limit although the measurements have been performed using the peak detector, conforming the conducted emi compliance of the system. figure 31. conducted emissions at full load: line 1 emissions figure 32. conducted emissions at full load: line 2 emissions
AN2042 non dimmable version 25/30 9 non dimmable version a lower cost solution is introduced as shown in figure 33 , if the dimming function is not required. in this case the tsm104 used for the dimming control is replaced by the simpler tsm1011 and the brown-out circuit is not necessary anymore during the switch off of the circuit. no other changes need to be introduced neither the transformer specifications nor the voltage and current thresholds have to be changed. the dimming control section is eliminated and the tsm104 is replaced by the simplest tsm1011. moreover, the brownout circuit is not necessary during the switch off. the same rules to design to define the transformer specifications and voltage and current thresholds are still valid. figure 33. non dimmable solution
input section arrangement for u.s. market AN2042 26/30 10 input section arrangement for u.s. market the proposed system has been designed for the european voltage range, i.e. 187-264 v ac , but by means of a voltage doubler, consisting of d 1 -d 2 and c 2 -c 3 , it can also be used with the u.s. voltage range, i.e. 88-132 v ac . the only modification needed is related to the input capacitor c 2 which has to be replaced by two capacitors c 2 and c 3 with half the value of the european voltage range, connected as shown in figure 34 . figure 34. application circuit for u.s. input voltage range: changes on the input section
AN2042 conclusions 27/30 11 conclusions in this document an innovative solution for driving high efficiency leds has been introduced. the power converter is based on a flyback topology with the smart power viper22a-e. it is able to drive with no circuital modifications 1 to 8 leds array and to perform an optimal dimming function by means of a patented pwm technique. a simplified version of the system has also been introduced in order to address the low end applications which do not require the dimming function. a lab prototype has been developed and fully tested under several conditions, confirming the suitability of the prop osed approach to such an emerging application. the reference board will be av ailable at stock through th e order code: steval-ill001v1.
steval-ill001v1 schematic AN2042 28/30 appendix a steval-ill001v1 schematic figure 35. steval-ill001v1 schematic +vcc +vcc +vcc vref vref +vcc fs + c5 r20 r21 q1 d8 c4 r9 d9 r17 r19 c13 + c8 l1 + c10 r23 r16 d11 r6 r2 opt r12 q3 dz1 c6 d3 10 - - + - + - + - + 11 12 5 6 7 4 9 8 13 2 3 1 15 14 16 2 3 4 1 ic2 + c2 r8 d10 c7 r10 ld1 q2 r3 r14 c1 ld2 d4 r13 d5 r15 r22 d6 cm r7 c14 r11 dz2 + c9 d2 ld8 c12 control fb drain source vdd ic1 c11 d1 d7 r24 r5 r4 r26 r25 r18 in in j1 cn1 j2 out + c3 j3
AN2042 revision history 29/30 12 revision history table 6. revision history date revision changes oct-2004 1 first issue feb-2005 2 d5 & q2 values change in component list table feb-2005 3 ? figure in cover page changed ? bil of material modified 21-mar-2007 4 ? the document has been reformatted ? figure in cover page changed ? pcb layout changed ? steval-ill001v1 schematic insertion
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