? AN1074/0998 1/6 application note auxiliary power supply using viper20 e. chou stmicroelectronics 1 1 introduction the pres ent board prototype is a 120 to 375 vdc input off-line single switch fly back, w orking at 100 khz. it is based on a new off-line smart switcher : the viper20. the viper20 is a current mode pwm with a 620v/16 w power switch, able to withstand an avalanche current during normal o peration. the start up of the circuit is done with an int ernal high voltage current source, which is switched off during steady state. it also includes a regulation function designed to minimise the pin count, while maintaining design flexibility. such a device can be easily used in any off line flyback smps, with a 20w minimum power capability for a single input voltage range or 10w with a wide input range. this stand-alone off-line sm art swit cher concept provides a cost effective solution for smps of many applications. the benefits for the customer are a simpler design phase and a reduced ov erall components count due to the optimisation of the product configuration. 2 electrical definition the complete s chematics of the power supply is given in page 2. figure 1 is table of key electrical parameters. component list component reference value unit resistors r9 10 w r8 1.2 k w r7 120 w r6 4.7 k w r5 4.7 k w r4 47 w r3 8.2 k w r2 5.1 k w r1 5.1 k w figure 1 : table of electrical parameters pa ra meter speci fi ca tion input voltage range 120 ~ 375 vdc output voltage 5v +/- 5% output power (continuous) 6w load regulation (0.1 ~ 1a) 250 mv p-p max. efficiency >70% output ripple voltage 100mv max. vout in ovp 6.9v max.
2/6 application note 13v osc co mp source dr a in vdd - + 1 23 54 ic1 viper20 r1 5.1k c1 3.9nf c2 0.47uf r2 5.1k d2 1n4148 r9 10 c3 47uf 16v r3 8.2k t1 zd p6ke200 d3 stps140 l1 6.8uh c4 1000uf 10v c5 2200uf 6.3v r7 120 r4 47 ohms r8 1.2k r5 4.7k c6 470nf pc pc817a r6 4.7k ic2 tl431 23 1 d1 byt11 -600 zd1 zener:4.3v c7 22nf c8 100nf/400v dc input gnd input +5v gnd * these devices are opti onal. it depends on the voltage spike on drain pin of viper. if tra nsformer has small leakage inductance in small power appli cation, such snubber circuit is unnecessary. 3 schematic component list (continued) component reference value unit capaci tors c8 100 (400v) nf c7 22 nf c6 470 nf c5 2200 m f c4 1000 m f c3 47 m f c2 0.47 m f c1 3.9 nf diodes zd p6ke200* zd1 4.3v zener d1 byt11-600* d2 1n4148 d3 stps140 ic ic1 viper20 ic2 tl-431 optocoupler pc pc817a transformer t1 1400 m h inductance l1 6.8 m h figure 2 : electrical schematic
3/6 application note 4 electrical specification of transformer primary turns n1--2x 62t (two primary split windings) secondary winding n2-- 7t (for 5v o\p) auxiliary supply n3-- 15t. primary inductance required is 1.4mh for 6w 0\p power at 100khz. primary leakage inductance required is 15uh max. at 100khz. core# - pc30 ee16-z (tdk) bobbin# - be16-118cph (tdk) 5 layout recommendation some simple rules insure a correct running of switching power supplies. they may be classified into two categories: - to minimise power loops: the way the switched power current must be carefully analysed and the corresponding paths must present the smallest inner loop area as possible. this avoids radiated emc noises, conducted emc noises by magnetic coupling, and provides a better efficiency by eliminating parasitic inductances, especially on secondary side. - to use different tracks for low level signals and power ones. the interferences due to a mixing of si gnal and power may result in instabilities and/or anom alous behaviour of the device in case of violent power surge (input overvoltages, output short circuits...). in the case of viper, these rules apply as shown in figure 2. the loops dc input capacit ance-t1-ic1, c3-d2-t1, c4-d3-t1 must be minimised. the signal components c1, pc-2 and r2 are using a dedicated track to be connected directly to the source of the device. * * * 10 8 7 6 1 5 n1 n2 n3 4* #0.2mm wire #0.2mm wire 0.2mm wire n1 (62 turns) n3 (15 turns) n2 (7 turns) n1 (62 turns) tape figure 4 :transformer cons truction figure 3 : pin allocation of transformer
4/6 application note 6 pcb board and component layout (66 * 43 mm)
5/6 application note appendix a: simulation result with viperxxx software ver. 1.01 project input value unit min. ac input voltage 85 v max. ac input voltage 265 v line frequency 50 hz switching f requency 100 khz output voltage 5v nominal output power 6w min. output power 0.1 w efficiency 0.8 input ripple 10 v first cell output ripple 0.5 v second cell output ripple 0.1 v ro*co 65 ohm* m f soft startup 5msec optocoupler gain 0.7 primary inductance 1400 m h turnsratio 17.715 np/ns primary leak inductance 42 m h secondary turns 7 auxiliary turns 17 regulation type secondary regulation device type viper50-sec..reg.-zener project output value unit operation mode discontinous min. dc input voltage 111.6 v max. dc input voltage 375 v computed inut ripple 8.58 v max. duty circle 0.41 primary peak curren :0.33 a primary rms current 0.12 a secon dary peak current 4.07 a secon dary rms current 1.8 a max. drain voltage 585 v standard input capacitor 82 m f oscillator r2 value 5.03 kohm oscillator c4 value 3.9 nf standard c7 capacitor 1000 m f standard c8 capacitor 1000 m f reflected output voltage 106.29 v zener clamp voltage 210 v
6/6 application note information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsability for the co nsequences of use of such information nor for any inf ringement of patents or other rights of third pa rties which may results from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support de vices or systems without express written approval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics ? 1998 stmicroelectronics - printed in italy- all rights reserved. stmicroelectronics gr oup of companies australia - brazil - canada - china - france - germany - italy - japan - korea - malaysia - malta - mexico - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a.
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