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april 2002 1/11 g. augustoni - f. salanitri AN1484 application note a 3.6 watts travel adaptor using viper12a ? abstract the viper12a is an integrated pwm and mosfet circuit for low power application in the 5w range, typically in cellular phone adapters. it is housed in surface mount so-8 and dip8 packages. with the availability of viper12a in so-8 package and the limited number of external components for a real pwm operation, building a compact and performant power supply becomes simple. the travel adaptor design presented here, has been made with the aim of minimising overall cost for a secondary voltage and current regulated adapter, topology widely used in cellular phone adapters. the viper12a presents itself as the answer for lower consumption in standby, like in home appliances where it will be required to limit non negligible ooff the useo consumptions, as recommended by the aeuropean commission of energyo. today regulations demand less than 1 watt. thanks to viper12a low power consumption, it is possible to achieve 100mw standby power in a wide range of operations. 1.0 principle of operation the circuit is a standard flyback converter with secondary current and voltage regulation driving the viper12a feedback pin through an optocoupler. table 1: system performances 1.1 the viper12a 1.1.1 start-up phase viper12a, as any member of the viper family, has an integrated high voltage current source to charge c3 vdd capacitor until it reaches its startup level (15v). when vdd gets to 15v, the viper switches, supplied by the energy stored in c3, until it is supplied through the auxiliary winding. 1.1.2 auxiliary supply viper12a has a wide operating voltage range from 9v to 40v, respectively maximum and minimum values for undervoltage and overvoltage protections. this wide voltage supply range simplifies the design of the viper12a supply but, to ensure proper operation of the application in any case, it is advised: - in normal operation, to supply viper12a within its operating range; - in standby mode, to minimize auxiliary consumption to achieve very low standby power; - in short circuit, to limit output power by going into an hiccup mode; - in constant current mode, to ensure current regulation below 2v before going into hiccup mode. parameters 100vdc 380vdc standby power 90mw 119mw efficiency at 3.6w 62% 66% short circuit power 1w 1.3w load regulation 3% 2%
AN1484 - application note 2/11 1.1.3 burst mode the viper12a integrates a current mode pwm with a power mosfet and includes the leading edge blanking function. the burst mode is a feature which allows viper12a to reduce its average switching frequency when the energy drained by the output load goes below e=(tb*vin) 2 *fsw/2lp (tb blanking time, vin dc input voltage, fsw switching frequency, lp primary inductance). this is obtained with a small ripple current around shut down current of feedback pin and maintaining the vdd voltage above 9v. if vdd goes below 9v there is the abad burst modeo in which viper12a repeats the restart cycle continuously, with a worst standby consumption and a higher secondary ripple voltage. 1.1.4 compensation and duty cycle control the internal structure of viper12a feedback and compensation pin (fb pin 3) is shown in figure 1. the current injected on the fb pin is added to the one coming from the sensefet in r2 and then compared to an internal 0.23v vref. when fb voltage is closed to ground, the viper12a delivers its full power. on the other side, when fb voltage is above 0.23*(r1+r2)/r2, the viper12a stops switching. the fb pin is directly driven by the emitter of the optocoupler, behaving as a current source. this current is filtered by a small 47nf capacitor c5 to guarantee cycle to cycle stability. important : it is necessary to keep c5 very close to the viper12a feedback pin to avoid high frequency instability on the compensation loop. 1.1.5 primary drive in a flyback power supply, the transformer is used as an energy tank fuelled during the on time of the mosfet. when the mosfet turns off, its drain voltage rises from low value to the input voltage + reflected voltage when the secondary diode conducts, regaining on the secondary the magnetic energy stored in the transformer. as primary and secondary windings are not perfectly magnetically coupled, there is a serial leakage inductance that behaves like an open inductor charged at ipeak that makes the mosfet drain voltage reach higher values. if the peak voltage is higher than the vdss of the viper12a mosfet, the device will be destroyed. so the drain voltage must be kept below its avalanche voltage of 730v. commonly a clamper based on a rcd network or a diode with a transil to clamp the rise of the drain voltage is used. the presence of the clamper is an extra consumption in standby mode, especially with rcd clamper respect to the transil clamper. because the power consumption is manageable with transil clamper, this solution has been chosen here. figure 1: viper12a internal structure 1.2 secondary regulation 1.2.1 voltage regulation the voltage regulation is achieved with a zener diode d6 directly driving the optocoupler. the resistor r3 limits the current in both the zener and the opto in case of overvoltage. the viper12a feedback pin is current controlled and its requirement goes from few ua at full load to 1ma in standby. the same current change is experimented by the regulating zener on the secondary side of the converter leading to around 5% load regulation. it is possible to improve the load regulation, by connecting a resistor between the zener and the vout. of course, this will degrade the standby power consumption. 1.2.2 current regulation the current regulation uses the drop voltage across a shunt resistor r6/r7/r8 to bias the t1 transistor base-emitter junction. the t1 collector drives the optocoupler limiting the output power. 60khz oscillator pwm latch s q r 0.23v id drain source fb r1 r2 c +vdd secondary feedback i fb is 1k w 230 w
AN1484 - application note 3/11 figure 2: application schematic +vout 6v/600ma 5.1v fb vdd drain source control ic2 viper12a -+ d1 s1zb6d 0.6a, 600v r1 10 c1 4.7uf 400v c2 4.7uf 400v inducto r c4 47nf tr1 transformer r2 22 d2 bav103 c3 10uf 63v ic1 sfh517 d5 smbyw01-200 r6 2.7 r5 1k t1 bc847b d6 ac in ac in gnd 470uf c6 25v r3 56 100nf c8 r8 2.7 r7 2.7 47uf c7 16v c5 1.5nf 2kv i1 1mh d3 smaj188a d4 bgy20g tp3 tp2 tp1 tp4 tp5 tp6 tp7
AN1484 - application note 4/11 the accuracy of this circuit is limited especially in temperature but is unrivalled in terms of cost. the addition of r5 base resistor is necessary in short circuit to avoid destroying t1 base-emitter junction. r3 is also necessary in short circuit otherwise t1 collector current directly flows through d6 and the optocoupler is not driven anymore, leading to an increase of short circuit power consumption. c8 limits the gain in frequency of t1, stabilising the loop. 2.0 the transformer an important part of a smps design lies in the transformer. its performances are a key to the system performances. the requirements for this application are: small size and limited voltage on the drain. table 2: transformer target specification 2.1 primary inductance a simple calculation gives the range of values of primary inductance suitable for this application. viper12a has a drain current limitation of 360ma min. the energy transferred is e=1/2lpip 2 *fsw in discontinuous mode. emin=5w, ip=360ma and fsw=50khz giving lp>1.54mh. the transition mode is when ton*vin=toff*vr (ton*vin=lp*ip). the expression of lp is: lp=1/2*(ton*vin) 2 *fsw/e with vin=150v, vr=50v, ton=5us, e=5w give lp=2.8mh. vin was chosen to reach the continuous mode at low input voltage level. vr is low to limit the drain peak voltage. the transformer optimisation has led to a final value of 2.5mh partly to reduce the primary turns and their power dissipation with an e12.5 bobbin. 2.2 transformer structure a standard transformer structure (so called with windings order primary/auxiliary/secondary) gives the following results on viper12a supply: table 3: viper12a vdd with a standard transformer two concerns can be seen from this table: - the viper12a is not going in hiccup mode in short circuit - the supply voltage is too low in standby with the risk of a obad burst modeo with higher standby consumption and poor regulation (viper12a undervoltage is at 9v max with 8v typical). the solution, implemented in the demoboard, is the optimized structure (so called with windings order: primary/secondary/auxiliary) shown on figure 3. figure 3: transformer structure parameters value power 5w saturation current > 400ma primary inductance 2.5mh reflected voltage 50v leakage inductance < 100 m h or <3% primary capacitance > 20pf conditions 100vdc 380vdc stand-by 10v 8v load 6v/100ma 15v 15v load 6v/600ma 25v 26v short circuit 9v 10v core bobin pri m ary seco ndary auxiliary co re bob in prim ary secondary auxi l i ary opt i m isation
AN1484 - application note 5/11 the position of the auxiliary winding on top gives the following benefits: - better coupling of primary and secondary windings thus lower leakage inductance and energy stored in the ringing circuit - no coupling between primary / secondary leakage inductance and auxiliary windings - less capacitive coupling between primary and auxiliary windings figure 4a and figure 4b show the drain voltage of the viper12 (trace 1) and vdd voltage before r2 (trace 2) at full load and in short circuit with the auxiliary in sandwich (1) and on top (2) of the windings. figure 4a 1 : at full load at 100v (sandwich) figure 4a 2 : at full load at 100v (on top) figure 4b 1 : in short circuit at 100v (sandwich) figure 4b 2 : in short circuit at 100v (on top) the viper12a auxiliary supply shows the following voltage on vdd pin: table 4: viper12a vdd with an optimized transformer conditions 100v 380v standby 12v 11v load 6v/100ma 18v 18v load 6v/600ma 20v 19.5v short circuit hiccup hiccup
AN1484 - application note 6/11 in these conditions, the viper12a is properly operating. it draws less than 100mw in standby and the hiccup mode is safe in short circuit (figure 5a and figure 5b). figure 5a: hiccup mode at 100v figure 5b: hiccup mode at 380v 2.3 peak drain voltage this transformer allows the reduction of drain peak voltage in any condition. the gain of this structure is 70v. with the lower reflected voltage (100v to 50v), the gain is up to 120v. there is a good voltage margin at full load under 380vdc. this means that a standard 200v transil clamper will not take any energy in normal operation. the clamper is still necessary during start-up and short circuit, the drain voltage goes above the 730v viper12a avalanche voltage. table 5: mosfet peak voltage at 380vdc 2.4 emc compatibility most of the emc performances are due to the ofloatingo voltage of the secondary winding or to the voltage across c7 emc capacitor. this ofloatingo amplitude is linked to all the parasitic capacitances along the wire between primary and secondary windings. with the optimized transformer, the emc performances are degraded. but it is possible to turn this problem into an advantage. the secondary winding is placed between the primary and the auxiliary ones. the auxiliary winding is used to compensate the induction from primary to the secondary. figure 6 shows this compensation. figure 6: emc compensation technique - the cold point is wound close to the secondary winding, limiting the voltage swing of the closest one. conditions standard transfo optimized transfo full load vpeak 750v 630v reflected voltage 100v 50v leakage inductance 105 m h25 m h primary capacitance 22pf 26pf primary inductance 3mh 2.6mh full load ipeak 260ma 275ma auxiliary winding primary winding r2 vin secondary
AN1484 - application note 7/11 - the voltage variation of the primary and the auxiliary side of the converter must be opposite. in this design, the d3 diode has been placed on the ground so the voltage swing is opposite on the transition. 2.5 transformer specification lp = 2.5mh @ 50khz ll =30 m h @ 50khz cp = 35pf @ 1mhz voltages: 55v-pri / 7.2v-sec. / 20.0v-aux isat > 400ma pout = 5w geometry: e12.5 winding order: primary / secondary / auxiliary primary winding: 180 turns awg 38 auxiliary winding: 66 turns awg 38 secondary winding: 25 triple isolation 0.20mm 3.0 system performances 3.1 efficiency the power losses are distributed at 6v / 600ma output power as follows: - 400mw in the output diode - 700mw in the viper12a - 300mw in the transformer - 380mw in the shunt resistor overall efficiency is 3.6w/ (3.6w+1.78w)=67%. if the losses in the shunt resistor are considered as available power, the converter efficiency becomes 3.98w/(3.98w+1.4w)=74%. this is possible using secondary controller like stm's tsm101. figure 7a: efficiency at 100v figure 7b: efficiency at 380v 3.2 regulation figure 8: v out vs. i out 3.3 standby consumption the demoboard consumes less than 100mw at 100vdc and 120mw at 380vdc. this power level is far below today regulation's requirement. the charts in figure 9a/9b shows the details of the charger standby consumption at minimum and maximum input voltage. the major contribution to the standby consumption is the viper12a own consumption of just 35mw and is independent from input voltage. 0 10 20 30 40 50 60 70 80 0 0.5 1 1.5 2 2.5 3 3.5 4 pout (w) efficiency cv efficiency cc 0 10 20 30 40 50 60 70 80 0 0.5 1 1.5 2 2.5 3 3.5 4 pout (w) efficiency cv efficiency cc 0 1 2 3 4 5 6 7 0 100 200 300 400 500 600 700 800 iout (ma) vout (v) 100v 380v
AN1484 - application note 8/11 figure 9a: standby consumption at 100v figure 9b: standby consumption at 380v the only change is due to the internal startup current source of 22ua which consumption goes from 2mw at 100v up to 8mw at 380v. another factor due to the viper12a is the current used on the feedback pin, regulated around 1ma in standby that leads to a 10mw consumption in the primary and 6mw in the secondary (with an opto gain of 1). note that it is necessary to keep a certain level of current in the regulating zener to improve the load regulation. as load increases, the current in the opto and the zener decreases lowering the output regulated voltage. overall, viper12a needs 50mw to operate in standby. it is possible to spare some mw in the auxiliary supply, especially the 22 w serial resistor which is necessary to regulate the transformer ringing voltage peak. in the demoboard the transformer voltage has a narrow dynamic so r2 becomes useless. so the standby consumption is decreased if the resistor is removed and the transformer is tuned to set 10v or less on viper12a vdd. the standby consumption is less than 60mw at 100v and 80mw at 380v: fairly good considering the 50mw required by viper12a. 4.0 design material 4.1 pcb solder side figure 10a: bottom view of charger's board. figure 10b: pcb art work 4.2 silk screen solder side figure 11: smd components clamper 0mw 22 w 20mw viper12a 37mw opto 11mw zener 5mw opto 1mw rectifier diode 1mw rectifier diode 5mw switching losses 2mw 22ua 2mw 0mw transfo. 4mw v i p e r 1 2 a vin 100v 12v vout 6v primary : 81mw secondary : 7m clamper 0mw 22 w 31mw viper12a 34mw opto 10mw zener 5mw opto 1mw rectifier diode 1mw rectifier diode 7mw switching losses 12mw 22ua 8mw 10mw transfo. 4mw v ip er 12a vin 380v 11v vout 6v primary : 112mw secondary : 7m
AN1484 - application note 9/11 4.3 silk screen component side figure 11a: top view of the charger's board figure 11b: through hole components
AN1484 - application note 10/11 4.4 component list ref. part list description supplier r1 wirewound res. 10ohm 5% 2w vitrohm tyohm r2 chip res. 22ohm 5% 0.125w s0805 r3 chip res. 56ohm 5% 0.125w s0805 r5 chip res. 1kohm 1% 0.125w s0805 r6-r7-r8 chip res. 2.7ohm 5% 0.25w s1206 c8 chip cap. 100nf 25v x7r s0805 c4 chip cap. 47nf 50v x7r s0805 c5 chip cap. 1.5nf kx cd wkp murata tdk roederstein c1-c2 elect. cap. 4.7uf 400v 10x12.5 sd vz kmg p/n 2222 151 90021 nhg samwha nichicon/sanyo nippon chemi-con philips panasonic c3 elect. cap. 10uf 63v 5x11 lxz yxg fc pw wd/wl nippon chemi-con rubicon panasonic nichicon samwha c6 elect. cap. 470uf 25v 10x16 c7 elect. cap. 47uf 16v 6.3x7 d1 phase bridge rectifier s1zb60 mb6s shindengen g.i. d2 diode bav103 minimelf d5 diode smby01-200 sma stmicroelectronics d6 diode zener 5.1v 2% minimelf i1 inductor 1mh series sps tdk ic1 optocoupler sfh617-a3 ps2561l-1d pc123fy/2 tcet1106g siemens nec sharp temic ic2 i.c. viper12a stmicroelectronics t1 bc847b sot23 tr1 transformer pf0037 pulse jp1 jumper tinned copper wire 0.7
AN1484 - application note 11/11 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties 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 devices or systems without express written approval of stmicroelectronics. the st logo is a trademark of stmicroelectronics ? 2002 stmicroelectronics - printed in italy- all rights reserved. stmicroelectronics group of companies australia - brazil - canada - china - finland - france - germany - hong kong - india - israel - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - u.s.a. http://www.st.com

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