ATEE Adriana FLORESCU

Transcription

1 SWITCHING POWER SUPPLY WITH MONOLITHIC SWITCHING REGULATOR SUBSYSTEMS AND DC-DC STEP-UP CONERTER PART B: Design Example, Pspice Simulaion, Pracical Consideraions, Experimenal Resuls Adriana FLORESCU Poliehnica Universiy of Buchares Elecronics and Telecommunicaions Faculy, Bd. Armaa Poporului 1-3, Sec. 6, Buchares, Romania adriana.florescu@home.ro Absrac The paper presens swiching power s upplies wih MC34063 and µa78s40 monolihic swiching regulaor subsysems and DC-DC sep-up converer. General descripion of MC34063 and µa78s40 operaion modes, mahemaical design and PSpice under ORCAD simulaion of he whole swiching power supply, a numerical example and he pracical implemenaion are included. Some pracical consideraions are also presened. 1. SWITCHING POWER SUPPLIES WITH MC34063 OR µa78s40 AND DC-DC STEP-UP CONERTER DESIGN EXAMPLE Given are he following: ou 28, I ou 175 ma, f min 30kHz, in(min) 12 25% 12 9, ripple(p-p) 0.5% ou 140 m p-p. Swiching power supplies wih MC34063 or µa78s40 for dc-dc sep-up converers mus be designed. Design example goes as follows: 1. The raio of swich conducion on versus diode conducion off ime is deermined using equaion (4): on ou + F in(min) (13) off in(min) 2. The cycle ime of he LC nework can be deermined: 1 1 T 0 (max) + off 30 khz sa (max) n 33.3µ f min 3. From equaions (13) and (14) he swiching imes on and off are: The raio on /( on + off ) is: T off 33.3us off max µ on T on max off 33,3µ s 9.751µ s µ s (max) 9.751µ s < s T on µ 6 7 s Noe ha he raio on /( on + off ) does no exceed he maximum 6/ This maximum is defined by he 6:1 raio of charge-o-discharge curren of iming capacior C T aken from he MC34063 or µa78s40 daa shee elecrical characerisics able. s (14) (15) (16) (17)

2 4. The MC34063 or µa78s40 iming capacior C T of he oscillaor OSC is charged during on a he value I chg(min) 20µA and he ripple volage of C T is?v CT 0.5. The value for iming capacior C T is : A C 20 µ 5 T on 4 10 on µ s pf 0.5 The sandard value C T 1500pF was used. 5. Equaion (6) gives he peak swich curren: I on pk swich 2 I ou 1 ( ) mA ( ) A off 6. Equaion (8) gives he minimum value of he inducance L in he boundary dc-dc sep-up converer operaion mode: L min µ s µ H (20) 1,195 A The value L170µA>L min was chosen in order o allow dc-dc sep-up converer o work in he correc coninuous mode. 7. A value for he curren limi resisor R sc can be deermined by using he curren level of I pk(swich) when in 12: ' in sa I pk( swich) on(max) 23.58µ s A 161.8µ H L min The value for he limi resisor R sc is: R sc Ω I ' pk(swich) where he volage drop of 330m on R sc was calculaed for cc 5 si I dischg 220µA using daa shee elecrical characerisics able. The sandard value R0.22O was chosen. 8. From equaion (10) filer capacior value is: I ou 175mA C 0 on 23.58µ s µ F (23) 140m ripple( p p) ATEE p p Ideally his would saisfy he design goal, however, even a solid sae capacior of his value will have a ypical ESR (Equivalen Resisance Series) of 0.3 O which will conribue 30m of ripple. In saisfying he example shown, he sandard value for he filer capacior has been seled o C 0 330µF. A analum capacior wih ESR of 1.1 O was chosen, bu a suplemenary LC oupu filer wih L1µH and C100µF was inroduced o keep he oupu volage ripple riple(p-p) o he given value. 9. The given nominal oupu volage ou is programmed by (R 1, R 2 ) resisor divider. The oupu volage is: ou ref [( R2 / R1` ) + 1] 1.25 [ ( R2 / R1` ) + 1] (24) The divider curren can go as low as 100µA wihou affecing sysem performance. In selecing a minimum curren divider, R 1 is equal o: R1 1,25 / 500µ A 2. 2kΩ A sandard value R O was chosen. (18) (19) (21) (22) (25)

3 From equaions (24) and (25) yields he value for resisor R 2 : [( 1.25 ) 1] 2.2kΩ ( ) [ 1] 47. Ω R2 R1 ou 08k The value R 2 47k O is a sandard value, so i was kep. (26) 10. Only for µa78s40 his sep is necessary. In his example wih in 12 he oupu drive ransisor is driven ino sauraion wih a forced gain ß20. The required base drive is: I B I pk( swich) / β 1.195A / mA (27) Fig.5. Swiching power supply wih MC34063 for dc-dc sep-up converer

4 Fig.6. Swiching power supply wih µa78s40 for dc-dc sep-up converer Then he driver collecor resisor is equal o: in sa( driver) Rsc Rdriver I + 170Ω B ,11Ω 10 A ( ) / 3 BE( swich) The sandard value of R driver 180O was chosen. The corresponding circuis for he swiching power supplies wih MC34063 and µa78s40 used o conrol dc-dc sep-up converer are presened in fig.5 and fig.6. An inpu capacior filer of 100µF for MC34063 and of 47µF for µa78s40 was inroduced. The wo circuis are idenical as operaion mode because µa78s40 is he improved varian for more sophisicaed applicaions of MC I s inernal block diagram has in addiion an operaional amplifier and a power cach diode (fig.1 in Par A) and he reference regulaor or 1.25 is no inernaly conneced o he comparaor. The conclusion is ha for his example i is sufficien o use MC34063 and furher analysis focus only on i. 6. PSPICE SIMULATION FOR SWITCHING POWER SUPPY WITH MC34063 PSpice under ORCAD was used o sofware verify swiching power supply wih MC34063 for dc-dc sep-up converer in fig.5. PSpice circui model [2] for fig.5 is given in fig.7. Subcircui for MC34063 was included. The mos imporan simulaion resul is oupu volage ou waveform (fig.8) ha proves he sabiliy of wiching power supply wih MC34063 for dc-dc sep-up converer he circui in fig.5. The average value of ou is of 28 as i was given in he example and i is reached in 2ms. The oupu volage ripple ripple(p-p) is arround 1, hree imes bigger han he (28)

5 given design value 0,5% ou 3. This bigger value can be explained observing ha fig.7 is no oaly idenical o fig.5 because i doesn include he opional suplemenary filer ha appears in fig.5. Lp Rs1 100m 170uH MC34063 Rcol 180 SWc DRc vou Rload uF Cou2 IC 27.5 Resr1 150m Lou 1uH Rs2 10m Rupper 47k Rlower 2.2k Dfw DN5819 IC uF Cou1 Resr2 100m 1.5n C SWe Ipk C cc Gnd FB MC34063 U1 Rsense 0.22 inpu 12 0 Converor Boos Fig.7. PSpice circui model for fig.5 7. PRACTICAL CONSIDERATIONS AND EXPERIMENTAL RESULTS The design equaions for L min were based upon he assumpion ha he swiching regulaor is operaing on he onse of coninuous conducions wih a fixed inpu volage, maximum oupu load curren and a minimum charge-curren oscillaor. Typically he oscillaor charge-curren will be greaer ha he specific minimum of 20µA, hus on will be somewha shorer and he acual LC operaing frequency will be greaer han prediced f min. The volage drop developed across he curren-limi resisor R sc was no accouned for in he raio on / off and L min formulas. This volage drop mus be considered when designing high curren converers ha operae wih an inpu volage of less han 5. High frequency circui layou echniques are imperaive wih swiching regulaors. To minimize EMI, all high curren loops should be kep as shor as possible using heavy copper runs. The low curren signal and high curren swich and oupu grounds should reurn on separae pahs back o he inpu filer capacior. The R 1 and R 2 oupu volage divider should be locaed as close o he inegraed circui as possible o eliminae any noise pick-up ino he feedback loop. The circui diagrams were purposely drawn in a manner o depick his. All circuis used permalloy power oroid cores for he magneics where only he inducance value is given. Inpu volage in 14 and oupu volage ou arround 28 waveforms on digial wo-channels oscilloscope PSC64i are shown in fig.9. These waveforms are he mos imporan for a swich power supply. Noe ha in 14 is bigger han he given 12 in he example and he circui in fig.5 keeps he oupu volage ou o i s nominal volage of 28 sill sable. Oupu power is of 4.9W and conversion efficiency is of 87.7%. Some oher waveforms ha confirm he heoreical ones in fig.4 are shown in fig.10 and fig.11.alues for on, off, T, and f can be pracicaly verified.

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7 Fig.8. PSpice simulaion waveform of he oupu volage ou on oscilloscope PSC64i 8. CONCLUSIONS The goal of his paper was o obain simple and complee swiching power supplies wih MC34063 and µa78s40 monolihic swiching regulaor subsysems used o conrol dc-dc sep-up converer. The paper was spli in Par A and Par B. Par A included brief inroducion, general descripion and synheical funcional descripion of swiching regulaor subsysems and mahemaical heory of he dc-dc sep-up converer conrolled by MC34063 and µs78s40. Par B included swiching power supplies wih MC34063 or µa78s40 and dc-dc sep-up converer design example wih i s PSpice under ORCAD modelling and simulaion, pracical consideraions and experimenal resuls. For MC34063 numerical example, oupu volage was of 4.9W and conversion efficiency of 87.7%. Mahemaical heory fis wih he simulaion and experimenal resuls.

8 Fig.9. Inpu volage in and oupu volage ou waveforms on oscilloscope PSC64i Fig.10. olage CE across swich Q1 waveform Fig.11. olage KA across diode D1 waveform REFERENCES [1] Radoi, C., Drogoreanu,., Grigore,., Florescu, A., Lazarescu,., Soichescu, D., Sancescu, S. Elecronica si Informaica Indusriala: Aplicaii pracice. (Indusrial Elecronics and Informaics: Pracical Applicaions), Ediura Tehnica, Bucuresi, [2] Radoi C., Grigore.,Drogoreanu., SPICE: Simularea si analiza circuielor elecronice. (SPICE: Elecronic Circuis Simulaion and Analysis), Amco Press, Bucuresi, [3] Mohan, N., Undeland, T. M., Robbins, W. P., Power Elecronics: Converers, Applicaions and Design, John Wiley & Sons, Inc., USA, 1995 [4] Rashid, M., H., Power Elecronics: Circuis, Devices and Applicaions, Prenice-Hall, 1992 [5]***ON Semiconducor Componens Caalogue, Aprilie 2002