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june 2009 doc id 14904 rev 1 1/25 AN2811 application note 3.5 w non-isolated offline constant-current led driver based on viper17 introduction high brightness leds are becoming a prominent source of lighting. compared to conventional incandescent bulbs, high brightness leds (light emitting diodes) have advantages in higher light efficacy, much longer life and faster reaction time in a smaller profile. since leds cannot sustain high voltage stress directly from an ac source, providing a reliable constant-current source to drive leds becomes fundamental. this solution provides even luminosity, relia bility, the highest efficacy a nd the longest operating life for leds. this application note describes the non-isolated offline constant-current driver based on the viper17hn (high frequency version). this solution operates with an ac line input range from 176 v to 264 vac and provides 500 ma constant current from a 7 vdc source. it can illuminate two leds in series. this device is an offline converter with an 800 v rugged power section, a pwm control, twice the level of overcurrent protection, overvoltage and overload protections, hysteretic thermal protection, soft-start and also safe auto-restart after any fault condition removal. the embedded brownout function protects this switch mode power supply in case the main input voltage falls below the specif ied minimum level for this system. figure 1. steval-ill017v1 demonstration board !-v www.st.com
contents AN2811 2/25 doc id 14904 rev 1 contents 1 safety instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 selected topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 general circuit descripti on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 pcb layout view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4 test results and waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5 connection of ac line an d led lamp to the demo nstration board . . 21 6 conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 8 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
AN2811 list of tables doc id 14904 rev 1 3/25 list of tables table 1. specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 table 2. bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 table 3. basic electrical characteristics of flyback transformer (t1). . . . . . . . . . . . . . . . . . . . . . . . . 11 table 4. bobbin dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 table 5. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
list of figures AN2811 4/25 doc id 14904 rev 1 list of figures figure 1. steval-ill017v1 demonstr ation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. conventional buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. modified buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 4. flyback converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 5. schematic diagram of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 6. top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 7. bottom view with smd parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 8. winding structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 9. bobbin outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 10. efficiency versus input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 11. standby power versus input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 12. vin and iin at 176 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 13. vin and iin at 176 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 14. vin and iin at 264 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 15. vin and iin at 264 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 16. inrush current at line in, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 17. inrush current at line in, two leds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 18. vds and id at 176 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 19. vds and id at 176 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 20. vds and id at 264 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 21. vds and id at 264 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 22. vo and io at 176 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 23. vo and io at 176 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 24. vo and io at 264 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 25. vo and io at 264 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 26. startup of vo and io at 176 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 27. startup of vo and io at 176 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 28. startup of vo and io at 264 vac, one led . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 29. startup of vo and io at 264 vac, two leds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 30. vdd and vds at 264 vac, output in short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 31. io at 264 vac, output in short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 figure 32. vdd and vds at 264 vac, output in open-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 33. startup of vdd and vds at 264 vac, output in open-circuit . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 34. completed demonstration board connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 35. connection of ac line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 36. connection of led lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
AN2811 safety instructions doc id 14904 rev 1 5/25 1 safety instructions warning: the demonstration board must be used in a suitable laboratory by only qualified personnel who are familiar with the installation, use, and maintenance of electrical systems. intended use the demonstration board is a component designed for demonstration purposes only, and shall be used neither for domest ic installation nor for industri al installation. the technical data as well as the information concerning the power supply and working conditions shall be taken from the documentation included with the demonstration board and strictly observed. installation the installation of the demonstration board shall be taken from the present document and strictly observed. the components must be protected against excessive strain. in particular, no components are to be bent, or isolating distances altered during the transportation, handling or usage. the demonstration board contains electro-statically sensitive components that are prone to damage through improper use. electrical components must not be mechanically damaged or destroyed (to avoid potential risks and health injury). electrical connection applicable national accident prevention rules must be followed when working on the mains power supply. the electrical installation shall be completed in accordance with the appropriate requirements (e.g. cross-sectional areas of conductors, fusing, and pe connections). board operation a system architecture which supplies power to the demonstration board shall be equipped with additional control and protective devices in accordance with the applicable safety requirements (e.g. compliance with technical equipment and accident prevention rules).
design considerations AN2811 6/25 doc id 14904 rev 1 2 design considerations 2.1 selected topology this is a 500 ma constant-current source conversion from 176 vac ~ 264 vac line input. the specifications shown in ta bl e 1 are for refrigerator lighting usage. according to the specifications the maximum operating power is 3.5 watts. no power factor correction circuit is required. therefore, both buck and flyback topologies are suitable for this application. figure 2 shows the conventional buck converter while figure 4 illustrates the flyback converter. to convert high voltage to low voltage, a conventional buck converter just requires a few components. output current ripple is small due to vout obtained from inherent filter l1 and c1, thus the voltage and current stresses on these power components are small. in order to properly drive the mosfet (q1), a controller and an additional transformer are required. additional winding with l1 to bias q1 as well as a feedback current to manage output in constant-current mode are needed. figure 2. conventional buck converter table 1. specifications parameter value ac input 220 vac 20% output current 500 ma output voltage 7 v max dimensions 30 mm x 30 mm isolation not required topology constant-current source !-v
AN2811 design considerations doc id 14904 rev 1 7/25 figure 3. modified buck converter for ease in driving q1 using a conventional buck converter, a modified buck converter has been introduced as shown in figure 3 . such topology is widely used to drive leds. with this modified solution, the mosfet is no longer floating. in this case the output (vout) is not connected to ground, and it becomes quite difficult to sense the output current in the output stage directly. compared to a buck converter, the flyback converter may be the better choice. figure 4 shows the typical circuit of a flyback converter. figure 4. flyback converter the auxiliary winding can be added to the transformer (t1) to provide bias for q1. unlike the buck converter, t1 provides isolation between vin and vout. since such isolation is not required for this application, a current sense resistor can be placed across the primary ground and negative polarity of vout. thus, vout shares the same primary ground. in this topology, the mosfet is not floating. thanks to viper17 the board is built with a high- performance low-voltage controller chip with an 800 v avalanche rugged power mosfet. designed with viper17, only a few external components are required which allows a smaller profile in the design. !-v !-v
general circuit description AN2811 8/25 doc id 14904 rev 1 3 general circuit description 3.1 schematic diagram figure 5 shows the complete schematic diagram of the demonstration board. it consists of an input full-bridge rectifier with filtering circuit, flyback converter and output stage. figure 5. schematic diagram of demonstration board am01057v1
AN2811 general circuit description doc id 14904 rev 1 9/25 referring to the schematic diagram in figure 5 , fuse1 is the input fuse to prevent hazards if the system current exceeds the fu se rating. d1 is the bridge rectifier to convert ac to dc. the filter is formed by c11, l2 and c3 that are used to attenuate the high frequency harmonic interference. t1 is the flyback transformer and u1 is formed by the pwm controller and output mosfet. the auxiliary winding (pin 5 and 6) and di ode d5 provide bias supply for each control circuit. the output stage includes d6, c9 and c13. r9 and r12 are the output current sense resistors providing current sense signal. these are connected in parallel to share the power dissipation. the constant-current control circuit consists of u2, q1, u1 and some passive components. the output current sense signal feeds to op amp in u2. r7 and c7 consist of the compensation network for the output signal of u2 in order to properly drive q1. the 0.3 v reference voltage on pin 6 of u2 is obtained by voltage divider r11 and r10. the collector junction of q1 is connected to the feedback pin of u1 and completes the feedback loop. the output voltage is indirect ly monitored by the auxiliary win ding (pin 5 and 6 of t1) and feedback to pin 3 of u1 through r1 and r4. once the voltage at pin 3 of u1 exceeds 3 v, u1 shuts down, then enters the auto-restart mode. thanks to u1, which includes an overload protection function, if the led is absent in the application (no load), this solution provides a safeguard. the led and the application board are both fully protected. 3.2 bill of material table 2. bill of material name value rated type c1 1 nf 25 v ceramic cap [0603] c2, c8 100 nf 25 v ceramic cap [0603] c3, c11 2.2 f 400 v al elcap cappr3.5-8x12 c4 56 nf 25 v ceramic cap [0603] c5, c12 10 f 25 v al elcap cappr2-5x11 c6 2.2 nf 25 v ceramic cap [0603] c7 12 nf 25 v ceramic cap [0603] c9 220 f 16 v al elcap cappr3.5 - 8x11.5 c10 470 pf 25 v ceramic cap [0603] c13 1 f 25 v ceramic cap [0805] d1 mb6s pkg30 e3 1 a 600 v bridge rectifier vishay d3 bat46jfilm small signal schottky diode stmicroelectronics [sod323] d5 stth1r06a 1 a 600 v ultrafast rectifier stmicroelectronics [sma] d6 stps2h100a 2 a 100 v schottky rectifier stmicroelectronics [sma] fuse1 500 ma 250 v fuse_5_8.5*8_bel l2 lps3314-105ml 1 mh, 0.1 a inductor, coilcraft l_lp3314 q1 bc817-40 npn general-purpose transistor [sot-23] r1 24 0 k 1% resistor [0603]
general circuit description AN2811 10/25 doc id 14904 rev 1 name value rated type r2 6.8 k 1% resistor [0603] r3 100 k 1% resistor [0603] r4 56 k 1% resistor [0603] r5 10 k 1% resistor [0603] r6 10 1% resistor [0603] r7 82 k 1% resistor [0603] r8 3 k 1% resistor [0603] r9, r12 1.2 1% resistor [1206] r10 3.3 k 1% resistor [0603] r11 24 k 1% resistor [0603] r13 2.7 k 1% resistor [0603] r14 1 m 1% resistor [0603] t1 (1) t_ee10/11_tdk 1 mh tdk flyback transformer u1 viper17hn offline high voltage converter stmicroelectronics [dip-7] u2 tsm103w dual op and voltage reference stmicroelectronics [so-8] 1. t1, the transformer design, is shown in section 3.4 on page 11 . table 3 gives the basic electrical characteristics, figure 8 shows the winding structure, and figure 9 illustrates the bobbin outline. table 2. bill of material (continued)
AN2811 general circuit description doc id 14904 rev 1 11/25 3.3 pcb layout view the pcb views are shown in figure 6 and figure 7 . 3.4 transformer design figure 8. winding structure figure 6. top view figure 7. bottom view with smd parts !-v !-v table 3. basic electrical characteristics of flyback transformer (t1) name value core type ee10/11-pc40 bobbin type be10-118cpsfr primary inductance 1 mh +/- 10% leakage inductance 10 h typical !-v
general circuit description AN2811 12/25 doc id 14904 rev 1 figure 9. bobbin outline table 4. bobbin dimensions dimension value a 7.2 mm b 3.5 mm c 6.6 mm e 3.85 mm x 10.2 mm y 10.2 mm z9 mm p 0.5 mm !-v
AN2811 test results and waveforms doc id 14904 rev 1 13/25 4 test results and waveforms figure 10 shows the overall efficiency versus a range of ac line voltage loads with one led and two leds. under both load conditions, we can observe that the efficiency drops when input voltage increases. the maximum efficiency occurs at minimum ac line input (176 vac). comparing a load condition of one led with a load condition of two leds in series, the efficiency increa ses by 7%. the efficiency with 1 led is close to 75%. figure 10. efficiency versus input voltage figure 11 shows us the standby power which is measured when the led is disconnected. standby does not mean burst mode under light load. in standby, the overvoltage protection works. under various ac line inputs, the maximum standby power is 0.18 w at 264 v input. figure 11. standby power versus input voltage with the aid of the filter formed by c11, l2 and c3, no high-frequency interference can be observed at the input current which definitely helps in meeting the conducted emi standard. in figure 12 and figure 13 the waveform is captured at 176 vac. in figure 14 and figure 15 the waveform is captured at 264 vac. to choose the proper rating of the fuse, we always refer to the inrush current. there are two inrush current plots at the ac line input 220 v: figure 16 with one led and figure 17 with two leds. !-v !-v
test results and waveforms AN2811 14/25 doc id 14904 rev 1 figure 12. vin and iin at 176 vac, one led f igure 13. vin and iin at 176 vac, two leds top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 ms/div top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 ms/div figure 14. vin and iin at 264 vac, one led figure 15. vin and iin at 264 vac, two leds top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 ms/div top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 ms/div !-v !-v !-v !-v
AN2811 test results and waveforms doc id 14904 rev 1 15/25 figure 16. inrush current at line in, one led fi gure 17. inrush current at line in, two leds iin: 5 a/div, 40 us/div max. value: 14.2 a iin: 5 a/div, 40 us/div max. value: 20.28 a !-v !-v
test results and waveforms AN2811 16/25 doc id 14904 rev 1 the viper17 integrates one 800 v mosfet and the drain current is limited at 0.6 a. the drain-source voltage and drain current waveforms are shown in figure 18 through 21 . in figure 18 and figure 19 the waveform is captured at 176 vac. in figure 20 and figure 21 the waveform is captured at 264 vac. the peak drain voltage, 496 v, is obtained at 264 v load with two leds (see figure 21 ). under the same condition, the peak drain current is 384 ma. figure 18. vds and id at 176 vac, one led figure 19. vds and id at 176 vac, two leds top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 us/div top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 us/div figure 20. vds and id at 264 vac, one led figure 21. vds and id at 264 vac, two leds top trace: vds (200 v/div) bottom trace: id (200 ma/div) time: 4 us/div top trace: vin (200 v/div) bottom trace: iin (200 ma/div) time: 4 us/div !-v !-v !-v !-v
AN2811 test results and waveforms doc id 14904 rev 1 17/25 the current sense circuit (r9 and r12 in figure 5 ) is one portion of output voltage. the additional voltage drop is 300 mv. the following figures show the output voltage and current waveforms for the load (one led vs. two leds). in figure 22 and figure 23 the waveform is captured at 176 vac. in figure 24 and figure 25 the waveform is captured at 264 vac. we can observe that the output ripple current always less than 30 ma. independent of the load condition, the output current is regulated at precisely 500 ma. figure 22. vo and io at 176 vac, one led figure 23. vo and io at 176 vac, two leds top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 2 ms/div top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 2 ms/div figure 24. vo and io at 264 vac, one led figure 25. vo and io at 264 vac, two leds top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 2 ms/div top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 2 ms/div !-v !-v !-v !-v
test results and waveforms AN2811 18/25 doc id 14904 rev 1 during the startup phase the output voltage response is optimized. no output voltage overshoot nor voltage spike has occurred thanks to the soft-start function and optimum regulation performance provided by the viper17. in figure 26 and figure 27 the waveform is captured at 176 vac. in figure 28 and figure 29 the waveform is captured at 264 vac. figure 26. startup of vo and io at 176 vac, one led figure 27. startup of vo and io at 176 vac, two leds top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 10 ms/div top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 10 ms/div figure 28. startup of vo and io at 264 vac, one led figure 29. startup of vo and io at 264 vac, two leds top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 10 ms/div top trace: io (200 ma/div) bottom trace: vo (2 v/div) time: 10 ms/div !-v !-v !-v !-v
AN2811 test results and waveforms doc id 14904 rev 1 19/25 the load can be open-circuit (led absent or wrong polarity at installation) or short-circuit due to the system undergoing installation or an operating anomaly. the led lamp can be damaged due to overtemperature, for example. the system should be able to withstand damage until removal of the anomaly, thanks to the viper17 which provides full protection against output short-circuit as well as output open-circuit. in figure 30 and figure 31 the waveform from the short-circuit load condition is captured at the highest ac line input 264 v, which is the most hazardous condition to the system board. in figure 32 and figure 33 the waveforms are captured at the highest ac line input 264 v with the output load in open- circuit condition. figure 30. vdd and vds at 264 vac, output in short-circuit figure 31. io at 264 vac, output in short- circuit top trace: vdd (10 v/div) bottom trace: vds (100 v/div) time: 100 ms/div io: 200 ma/div time: 100 ms/div !-v !-v
test results and waveforms AN2811 20/25 doc id 14904 rev 1 figure 32. vdd and vds at 264 vac, output in open-circuit figure 33. startup of vdd and vds at 264 vac, output in open-circuit top trace: vdd (10 v/div) bottom trace: vds (100 v/div) time: 200 ms/div top trace: vdd (10 v/div) bottom trace: vds (100 v/div) time: 10 ms/div !-v !-v
AN2811 connection of ac line and led lamp to the demonstration board doc id 14904 rev 1 21/25 5 connection of ac line and led lamp to the demonstration board figure 34. completed demonstration board connection figure 35. connection of ac line figure 36. connection of led lamp !-v !-v !-v
conclusion AN2811 22/25 doc id 14904 rev 1 6 conclusion this document introduces a non-isolated offlin e constant-current led driver based on the viper17. the input range is 220 vac +/- 20% and the device is capable of driving two 500 ma white light leds. the led current is sensed and regulated through the tsm103w and attains a constant output current. by using resistors with 1% precision, the output current achieves a maximum tolerance less than 5%. the input fuse and input filter are built on a 30 mm x 30 mm pcb. overtemperature protection, led open-circuit and led short- circuit protection are all inte grated functions which enhance the reliability of the device.
AN2811 references doc id 14904 rev 1 23/25 7 references viper17, off-line high voltage converter (datasheet) tsm103w, dual operational amplifier and voltage reference (datasheet) stps2h100a, power schottky diode (datasheet) stth1r06a, turbo 2 ultrafast high voltage rectifier (datasheet) bat46jfilm, small signal schottky diode (datasheet)
revision history AN2811 24/25 doc id 14904 rev 1 8 revision history table 5. document revision history date revision changes 16-jun-2009 1 initial release
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