SEMICONDUCTOR APPLICATION NOTE

Transcription

1 SEMICONDUCTOR APPLICATION NOTE Order his documen by AN1547/D Prepared by: Y H Chin Power Producs Division One of he main issues for low oupu volage power supplies is he power loss in he power semiconducors. This is especially rue for porable noebook compuers which require a highly efficien power supply o run heir sysems. The laes echnology, HDTMOS is needed in order o mee he noebook power supply requiremens. As digial inegraed circui manufacurers work o implemen more elecrical funcions and circuis ono a single silicon chip, he need for a low hreshhold volage MOSFET arises. They have presenly sandardized he supply volage o a new logic level of 3.3 Vols. This new volage sandard has forced power supply design engineers o explore he possibiliy of using devices oher han juncion diodes for recificaion. The Schoky and fas recovery diodes, however, have limied performance capabiliies. Schoky barrier diodes are limied o a PIV generally below 100 V and fas recovery diodes have excessive reverse recovery imes. The low on resisance, power MOSFETs can break hrough hese barriers and offer a high efficiency swichmode power supply. HDTMOS VERSUS CONVENTIONAL MOSFET The adven of he power MOSFET, wih is ease of drive and fas swiching capabiliy, has led o a subsanial improvemen in swichmode power supplies. High cell densiy TMOS (HDTMOS) is an advancemen in power MOSFET echnology ha reduces power dissipaion. This resuls in lower hermal generaion and a reducion in he componen s oal par coun. HDTMOS provides a subsanial improvemen in curren carrying capabiliy by employing VLSI processing. The use of Edied by: Dave Hollander Power Producs Division VLSI processing and new design echniques allow low volage MOSFETs o be manufacured wih more parallel cells packed ino a given die area. Wih 6 million cells per square inch, he yield is over 5 imes greaer han curren convenional processing can achieve. This new echnology provides a RDS(on) as low as 7 mohms, which is several imes lower han a convenional power MOSFET. The reducion in on resisance achieved by employing HDTMOS echnology makes i possible for power supply designers o increase he power supply oupu raing of a given package. They can even design he power supply for power MOSFETs wihou he need of a heasink. Designers can also ake advanage of is lower on resisance o reduce juncion emperaure and improve sysem efficiency. In addiion o he low on resisance, he reverse recovery characerisic of HDTMOS is anoher meri over convenional diodes. The sof recovery of he inrinsic diode allows i o be used in he oupu recificaion circui, hus eliminaing he need for a parallel freewheeling diode in he oupu circui. A ypical buck converer is shown in Figure 1. Figure 1 shows a converer ha employs one power MOSFET and one Schoky recifier for power conversion. The Schoky recifier acs as a freewheeling diode when he MOSFET is urned off. Alhough he forward volage drop of a Schoky recifier is very low when compared o a convenional diode, hese losses can be furher reduced by using a MOSFET as he freewheeling diode. The resuling improvemen is significan, especially in low oupu volage converers. MAIN SWITCHING MOSFET CONTROL IC DC INPUT BATTERY SCHOTTKY FILTER LOAD Figure 1. Typical Buck Converer HDTMOS and MiniMOS are rademarks of Moorola, Inc. MOTOROLA Moorola, Inc

2 MAIN SWITCHING MOSFET CONTROL IC DC INPUT BATTERY SYNCHRONOUS RECTIFIER FILTER LOAD Figure 2. Synchronous Recificaion DC o DC Converer In order o achieve synchronous recificaion during MOSFET swiching, an alernae riggering conrol IC is needed o urn he wo swiching ransisors on and off alernaely. The buck converer includes he main swiching device and he synchronous recifier as shown in Figure 2. When he synchronous recifier MOSFET urns on, he curren will flow hrough he MOSFET insead of he Schoky diode. The low on resisance of he synchronous MOSFET reduces he losses ha would normally occur wih a synchronous diode. Hence, he power dissipaion of a diode is given by: Pdiode = Iou x (1 d) x Vf Where Vf = forward volage drop of he diode Iou = power supply raed oupu curren D = duy cycle. From he above, we know ha he power dissipaion of a diode is direcly proporional o he diode s forward volage drop. The power dissipaion of he bes Schoky diode wih less han a 0.35 V forward volage drop is sill much higher han a synchronous recifier ransisor. The power dissipaion of a synchronous recifier is given by: PMOSFET = [ID(1 D)]2 x RDS(on) Where ID = drain curren of MOSFET PMOSFET = power dissipaion of he MOSFET The RDS(on) of HDTMOS is only a few milliohms. The power loss in a synchronous recifier HDTMOS is herefore much lower han he freewheeling diodes in power converer oupu circuis. The purpose of he Schoky freewheeling diode used in he synchronous recificaion oupu circui is o provide a coninuous curren pah during he swich over period of he wo MOSFETs. Wihou he freewheeling diode, he curren will flow hrough a parasiic diode of he low side MOSFET insead of he MOSFET iself. This will increase he power loss during he swiching ransiion of he devices, whereby he parasiic diode of he MOSFET will perform like a fas recovery diode. THE BUCK CONVERTER The buck converer is one of he mos fundamenal opologies of any swiching power supply configuraion. I is basically a forward mode regulaor and is also he basic building block of all he forward mode opologies. The basic circui of a buck converer is shown in Figure 3. When he swich Q1 is urned on, he inpu volage is applied o inducor L1, and power is hen delivered o he oupu. Inducor curren can also build up according o Faraday s Law as shown below: VL = L (di/d) Iin Q1 V1 Vou Vin CONTROL IC D1 FREEWHEELING DIODE Co FILTER R LOAD Vou (a) Buck Converer I in Q 1 V1 STORING ENERGY CAP. CHARGING V ou I in Q1 V1 RELEASING ENERGY DISCHARGING V ou V in D1 FREEWHEELING DIODE C o FILTER R LOAD V in D1 FREEWHEELING DIODE C o FILTER R LOAD (b) ON Sae Figure 3. The Buck Converer (c) OFF Sae 2 MOTOROLA

3 Vin DIODE D1 VOLTAGE Imax Vou Q1 CURRENT Imin CURRENT Iou VOLTAGE Vin Vo Vo DIODE D1 CURRENT Imax Imin Figure 4. Volage and Curren Waveform of a Buck Converer When he swich is urned off, he volage across he inducor reverses and he free wheeling diode D1 becomes forward biased. This allows he energy sored in he inducor o be delivered o he oupu where he coninuous curren is hen smoohed by he oupu capacior. Typical waveforms for a buck converer are shown in Figure 4. The swiching ransisor Q1 is swiched a high frequency (ypically 20 khz o 300 khz) o produce a chopped oupu volage V. The LC filer has an arranging effec on he applied pulsaing inpu producing a smooh dc oupu volage and curren. The oupu volage flows o he load wih only a small ripple and can be conrolled by varying he Mark/space raio of he V. Neglecing he circui losses, he seady sae average volage across he inducor is zero. The basic dc equaion of he buck converer is given by: Vou/Vin = D Where Where D = ransisor swiching duy cycle, defined as D = he conducion ime divided by he swiching D = period. Usually expressed in he form of: D = on/t T = on + off The buck converer is a sep down ype where oupu volage is always lower han inpu. Oupu volage regulaion is obained by varying he duy cycle of he swich. The buck converer is always operaed in coninuous mode. There are no major problems wih he coninuous mode buck. When he ransisor Q1 is urned on, he curren flows hrough he inducor and he load (inducor energy is sored): VL = Vin Vou = L x (dil/d) Assume ha Vin, Vou and L are consan, and ha here is no iniial curren (complee energy ransfer). Hence, he peak o peak curren ripple is: IL = [(Vin Vou)/L] x on When he ransisor is urned off, he inducor curren flows hrough he free wheeling diode and o he load, and he polariy of he volage on L reverses (sored energy is released). During urn off, VL = 0 Vou = L x dil/d Therefore, VL = Vou The curren waveforms are shown in Figure 5. CURRENT IL CHARGING DISCHARGING IL CONTINUOUS MODE DISCONTINUOUS MODE I = Vou /R Figure 5. Inducor Curren Waveform MOTOROLA 3

4 Hence, he peak o peak curren ripple is: IL = on x [(Vin Vou)/L] = D (1 D) [(Vin x T)/L] which reaches maximum when D = 0.5 The maximum curren is: ILmax = Vou/R + IL/2 The minimum curren is: ILmin = Vou/R IL/2 A he boundary condiion beween coninuous and disconinuous modes of operaion: I = Vou/R = IL/2 Hence, IL = [(Vin x T)/L] x D (1 D) Therefore, Inducor L = [D(I D) (Vin x T)]/ IL where IL = 2Vou/R = [D(I D) (Vin x T)R]/2Vou = {D(1 D) [(T/on) Vou x T]R}/2Vou = [T (1 D)R]/2 This is he minimum inducance value required for he inducor. A values lower han his, he converer will operae in disconinuous mode. The oupu volage ripple is obained as follows: The change in charge Q of he oupu filer capacior is represened by he shaded area in he diagram. Q = 1/2 x ( IL/2) x (T/2) = (T x IL)/8 Thus, oupu ripple Vou is: Vou = Q/Cou = [T x IL] x 8Cou = {[Vou x T2] (1 D)D}/8LCou where Cou is he oupu filering capacior. Thus, he minimum values of oupu filering capacior Cou can be deermined by he following: Cou = [(Vou x T2) (1 D)D]/( Vou x 8 x L) = I/(8 f Vou) To deermine he on & off relaionship, firs consider he rae of change of inducor curren: IL = (V/L) x The volage across he inducor is equal o (Vin Vsa Vo), neglecing he volage drop of he effecive resisance of he inducor. When Q1 is urned on, When Q1 is urned off, IL = [(Vou + Vf)/L] x off Vf forward volage of diode Since he rae of change of inducor curren is equal for on and off, [(Vin Vsa Vou)/L] on= [(Vou + Vf)/L]/off Thus, on/off = (Vou + Vf)/(Vin Vsa Vou) Hence, Duy Cycle D = on/(on + off) = (Vou + Vf)/(Vin Vsa + Vf) Again, a minimum value of inducor L can also be deermined by using he above assumpion. Hence, L = (VL/ IL) Lmin = [(Vin Vsa Vou)/ IL]/on The energy sored wihin he inducor during swiching period is: Esore = 1/2 x L x (ipk imin)2 The inpu energy is sored by he flux conained wihin he core of he inducor L. Wih he use of an appropriaely chosen LC filer, he square wave modulaion could be eliminaed and ripple free DC volages equal o he average of he duy cycle modulaed DC inpu would resul. By sensing he DC oupu and conrolling he swiching duy cycle in a negaive feedback loop, he DC oupu could be regulaed agains inpu line and oupu load changes. Synchronous Recifier In order o reduce he free wheeling diode drop as well as increase he converer efficiency, he Schoky diode can be replaced by an N channel HDTMOS device o ac as he main conducing device of he inducor curren during he Q1 urn off period. When considering a converer operaing wih a 50% duy cycle and assuming he forward drop of he Schoky remains unchanged a 0.4 vols, i is obvious ha when using a synchronous recifier wih a RDS(on) of 33 mohms (MTD20N03HD) or less a a curren of 4 amps, he power loss is much lower han when using a free wheeling diode. Power = Volage x Curren or Power = Curren2 x Resisance Effecs of Parallel Schoky Diode To furher minimize he power loss during he dead ime when swiching beween one HDTMOS and he oher, a Schoky diode is added in parallel wih he synchronous recifier. IL = [(Vin Vsa Vou)/L] x on Vsa Q1 urned on sauraion volage 4 MOTOROLA

5 Q1 ON OFF CYCLE Q1 ON OFF Q1 ON OFF Q1 ON Q2 ON OFF CYCLE Q2 ON DEAD TIME OFF Q2 ON OFF Q2 ON SCHOTTKY CURRENT MOSFET SWITCHED OVER Figure 6. Dead Time and Schoky Diode Waveforms Q1 MAIN SWITCHING MOSFET L1 DC INPUT BATTERY CONTROL IC Q2 SYNCHRONOUS RECTIFIER D1 FREEWHEELING DIODE FILTER LOAD Figure 7. Synchronous Recifier wih Parallel Schoky Diode When MOSFET Q1 is swiched off before Q2 urns on, he free wheeling diode D1 will provide a coninuous pah for eiher he curren or he energy sored in inducor L1 o coninue flowing hrough he diode. This waveform is shown in Figure 6. Wih he parallel Schoky diode D1 in place, he overall efficiency of he power supply is slighly improved. This circui is shown in Figure 7. The comparison of efficiency beween a synchronous recifier wih a parallel Schoky diode and ha of a Schoky diode alone is shown in Figure % 90% SYNCHRONOUS RECTIFIER + SCHOTTKY SYNCHRONOUS RECTIFIER 80% SCHOTTKY DIODE CONVENTIONAL DIODE Figure 8. The Improvemen in Efficiency by Employing Synchronous Recificaion MOTOROLA 5

6 Furher Improvemens The buck converer is one of he simples and mos fundamenal configuraions in swichmode power supply design. In order o furher reduce he losses in he converer, eiher [1] increase he urn on and urn off swiching speed of he MOSFETs riggering gae pulses o reduce he swiching loss during ransiion, [2] reduce he swiching frequency of he converer, or [3] use low effecive (inernal) resisance componens, i.e., inducors, inpu and oupu capaciors, low forward drop Schokys, and low RDS(on) on swiching MOSFETs. The swiching loss of a MOSFET is shown in Figure 9. DC TO DC BUCK CONVERTER DESIGN The buck converer is designed for he presen marke needs of noebook compuers and oher porable equipmen where efficiency & size are imporan. The efficiency of he converer is an imporan consideraion for all baery powered equipmen since high efficiency prolongs he baery s operaing life. Anoher imporan facor is he size of he power supply, as noebook compuers and oher porable producs are space limied. Therefore, he ouline of he power supply mus be designed o be as small as possible. The design is shown in Figure 10. The Conrol IC The choice of he conrol IC is exremely imporan. Selec he one ha provides he bes performance in synchronous recificaion. The Maxim 797 is chosen for he synchronous recifier PWM conrol device. Is feaures include: 1. Swiching frequency can go as high as 300 khz (150 khz/300 khz fixed frequency PWM operaion for Maxim 797) 2. Very low quiescen curren. Typically 375 µa 3. High efficiency; can achieve as high as 96% 4. Adoped boh high side and low side N channel MOSFET for synchronous recificaion operaion. This will reduce losses by using P channel for high side configuraion. 5. High sink/source riggering curren. Typically 1A 6. Small ouline and low profile surface moun package. Ideal for noebook compuers. (Refer o he Maxim 797 daa shee for deailed informaion) Swiching Frequency of Buck Regulaor The oupu volage of he buck regulaor Vou = Vin (on / T) is independen of he value of period T. The quesion arises as o wheher or no here is an opimum period and on wha basis he period is seleced. A firs hough i migh seem bes o minimize he size of he filer componens L & Cou by going o a frequency ha is as high as possible. While increasing he swiching frequency will lead o an increase of AC losses in he circui, he swiching speed of he MOSFET remains consan a differen operaing frequencies. Therefore, he AC losses of he converer are inversely proporional o he swiching period T. The free wheeling diode D1 can also conribue some losses during he reverse recovery period. This period is he ime i akes for he diode o cease drawing reverse leakage curren. The period is measured from he insan ha he diode has been subjeced o reverse volage. The free wheeling diode should herefore be specified as an ulra fas recovery ype or a Schoky, boh of which have recovery imes as low as 30 ns. Even a fas recovery ime diode can sill dissipae significan power during he urn off period and is also proporional o he swiching frequency. While increasing he swiching frequency does reduce he size of he filer elemens L & Cou, i also adds o he oal losses and conribues o he requiremen for a larger hea sink for he swiching devices (if HDTMOS devices are no used). To compromise, he swiching frequency of he DC DC buck converer discussed in his applicaion noe was seleced a 300 khz o minimize he inducor and capacior size for noebook requiremens. I & V V I OFF ON VDS(on) ENERGY IV PRODUCT ENERGY LOSSES DURING SWITCHING Figure 9. Swiching Loss of MOSFET 6 MOTOROLA

7 Oupu inducor The curren waveform of he oupu inducor is shown in Figure 5. Is predominan characerisic is a ramping up and down waveshape during he charging and discharging cycle. I can also be noed ha he curren a he cener of he ramp is equal o he DC oupu curren Io. As he DC oupu curren changes, he slope of he ramp remains consan as he volage across L remains consan. Where VL = Vin Vou & L = di/d x VL bu he curren a he cener of he ramp (Io) decreases. The value of he inducor is no fixed and can be adjused freely in order o make radeoffs among size, cos, and efficiency. Alhough lower inducor values will minimize size and cos, hey will also reduce efficiency due o higher peak currens. To permi he use of he physically smalles inducor in he circui, lower he inducance value unil he circui is operaing a he border beween coninuous and disconinuous modes. Reducing he inducor values even furher, below his crossover poin, resuls in disconinuous conducion operaion even a full load. This helps reduce oupu filer capaciance requiremens bu causes he core energy sorage requiremens o increase again. On he oher hand, higher inducor values will increase efficiency, bu a some poin he resisive losses due o he exra urns of wire will exceed he benefi gained from lower AC curren levels. Also, high inducor values can affec he load ransien response of he converer. Inducance L, Lmin = [(Vin Vsa Vou)/ IL] x on Consider maximum inpu volage, Vin = 30 V Vsa = 1 V (for MTD20N03HD) Vou = 3.3 V IL = 2A (assume 0.5 ime of Iou) on = 0.66 µs (assuming D = 0.2, on = f = 300 khz) Thus, Lmin = 8.5 µh This is he minimum inducance requiremen, below his value he converer will operae in disconinuous mode. Hence, a 10 µh ferrie core inducor was chosen for he sample circui. The inducor s DC resisance is a key parameer for efficien performance and mus be ruhlessly minimized, preferably o less han 25 mohms a Iou = 3A. If a sandard off he shelf inducor is no available, choose a core wih an LI2 raing greaer han L x Ipk2 o preven sauraion and wind i wih he larges diameer wire ha can fi he winding area. In 300 khz applicaions, ferrie core maerial is srongly preferred; for 150 khz applicaions, Kool mu (Aluminum alloy) and even iron powder is accepable. For high curren applicaions, shielded core geomeries (such as oroidal or po core) help keep noise and EMI o a minimum. Curren Sense Resisor The curren sense resisor values are calculaed according o he wors case, curren limi hreshold volage (beween CSH & CSL pins of he Maxim conrol IC) and he peak inducor curren. The coninuous mode peak inducor curren calculaions ha follow are also useful for sizing he swiches and specifying he inducor curren sauraion raings. Where Ipeak = (Vou/R) + ( I/2) = Iload + [(Vin Vsa Vou) on]/2l Thus, Ipeak = 4 + [( ) 0.66 µ]/(2 x 10 µ) = 4.9A In order o simplify he calculaion, he sense resisor Rsense can be obained as follows: Rsense = 100 mv/ipeak Thus, = 100 mv/4.9 = 20.4 mohms Hence, 15 mohms was used in he converer circui o increase he curren limi o 5.8A. When selecing he sense resisor, low inducance resisors, such as surface moun meal film resisors, are preferred. Noe: The curren limi for he CSH & CSL is ypically 100 mv. Inpu Capacior Capaciance Place a small ceramic capacior, C8 (0.1 µf) beween V+ and GND close o he device in parallel wih he bulk capaciors C1, C11, C22 and C23. This is o accommodae he high frequency componens of he ripple curren. Also, connec a low ESR bulk capacior direcly o he drain of he high side MOSFET. Selec he bulk inpu filer capacior based on how much ripple he supply can olerae on he dc inpu line. The less ripple expeced, he larger he capacior and he higher he surge curren during he power up period can be. In addiion, he designer should consider replacing he single inpu bulk capacior wih wo parallel unis, each wih half he capaciance value of he buck capacior. This is o reduce he capacior ESR o one half ha of he single uni. The value of he inpu bulk capacior can be calculaed by he following formula: Cin = (I x )/ V Where Cin = inpu capaciance, µf I = inpu curren, A = 3A (Assume 90% efficiency I = & 4A load) = ime he capacior spen supplying curren, = ms = on 1 ms = µs (Assume duy cycle a 0.8) V = allowable peak o peak ripple, V = 0.2 V DV = (Assume 0.2 V ripple) Therefore, Cin = (3 x 2.66 µ)/0.2 Cin = 40 µf However, four 22 µf capaciors were conneced in parallel o reduce ESR and increase is capaciance for lesser inpu ripple. MOTOROLA 7

8 Oupu Filer Capacior The choice of he oupu filer capacior depends upon he ype of converer being used as well as he maximum operaing curren and swiching frequency. Mos of oday s applicaions call for an elecrolyic capacior, preferably a low ESR ype. The ESR of he filer capacior has a direc effec on he oupu ripple and also he life of he capacior iself. Since he ESR is a dissipaive elemen, he power loss in he capacior generaes hea which, in urn, shorens he capacior s life. To ensure sabiliy, he capacior mus mee boh minimum capaciance and allowable maximum ESR values as given by: Vou = (1/Cou) i d Thus, Vou = ( IL x T) (4 Cou x 2) = I/8 f Cou The sample board used hree 220 µf capaciors conneced in parallel o furher reduce ESR and also o minimize oupu ripple. In order o ensure minimum oupu ripple, he ESR of he capacior may be calculaed by he following relaionship: ESRmax = Vou/ IL I is imporan o noe ha proper selecion of he LC filer is essenial since i will influence wo imporan parameers in he performance of he swiching power supply. Firs, he LC filer combinaion has a very srong influence on he overall sabiliy of he swiching sysem. Second, a small L and large C will resul in a low surge impedance of he oupu filer. This means ha he power supply will have a good ransien response due o load sep changes. Rearranging he erms, he minimum oupu capaciance is: Cou = I/8 f Vou = 2/(8 x 300K x 0.03) Assume oupu ripple = is 30 mv = 27 µf Vin = o 30 V J1 ON/OFF C1 22 µf 35 V C11 22 µf 35 V C22 22 µf 35 V C23 22 µf 35 V R8 1M R6 1M R7 100K C8 0.1 µf V+ VL SHDN PWM CONTROL IC MAX797 BST DH LX SYNC DL PGND CSH SKIP CSL FB SS GND REF SCHOTTKY D1 MBR0530 C3 0.1 µf Q1 MTD20N03HDL L1 10 µh Q2 MTD20N03HDL SCHOTTKY D2 MBRS140T3 J2 R1 25 mohm +5 V a %ma C2 +Vou 220 µf 10 V C µf 10 V C µf 10 V 3.3 V or 5.0 V TO V L 3.3 V (SHORTED THIS PIN TO FB FOR 5 V OUTPUT) +Vou C µf C µf 5.0 V (SHORTED THIS PIN TO FB FOR 3 V OUTPUT) The power supply is 3.3 V or 5 V selecable. Figure 10. A 5 VDC o 3.3 VDC, 4 Amp Synchronous Sep Down Power Supply Conroller Using N Channel MOSFETs and Schoky Diodes (Componen Lis is in Table) 8 MOTOROLA

9 Tes Resuls Figures 11a and 11b show he measured waveform of he gae riggering volage of he high side swiching ransisor. IL 3.1A ( I) 11a) A Vin = 8 V 11a) A 11a) A Iou = 4A Vou = 5 V 3.36 µs 11b) A Vin = 30 V 11b) A 11b) A Iou = 4A Vou = 5 V 0.88 µs µs 3.36 µs DUTY CYCLE = 73.8% Figure 11. DUTY CYCLE = 19% Figure 12 shows he inducor curren waveform a full load (4A) wih maximum Vin and 5 V oupu. Figure 12. Inducor Curren Waveform Where Io = 4A, I = 3.1A The oupu volage ripple measured was 30 mvp p a full load. The ransien response subjec o a load change ha swiched from 1A o 4A a he rae of 500 Hz, was 42 mv. The circui was firs esed on a breadboard using wires for all connecions. The resuls are raher poor in all aspecs. The sysem was unsable, noisy and also unable o supply full load. The curren limi was low due o high noise presen a he Rsense resisor caused by he connecing wires. The following daa was obained by using a double sided PCB wih all of he appropriae componens in place. This synchronous recifier yields resuls ha reach 92% efficiency, whereby he Schoky diode converer only aains 89% efficiency. Efficiency daa was also aken under hree differen line volage condiions (minimum, nominal, and maximum) using synchronous recificaion. The resuls show ha here is lile variaion in he efficiency a low line condiions. A high line volage, however, here is a significan dip in efficiency due o high losses in he inducor. The resuls, which were bes a low line, are lised in Table 1 and also ploed in graphic form in Figure 13. 4A Vin = 4.75 V Vin = 6 V Vin = 30 V Io (A) Iin (A) Vin (V) Vo (V) Eff Iin (A) Vin (V) Vo (V) Eff Iin (A) Vin (V) Vo (V) Eff Table 1. Efficiency Daa Under Various Inpu Volage Condiions MOTOROLA 9

10 Efficiency 100% Vin = 4.5 V 90% Vin = 6 V 80% Vin = 30 V Iou (A) Figure 13. Comparison of Efficiency a Differen Line Inpu Volage The efficiency comparisons beween a synchronous recifier, a Schoky diode and an ulra fas diode each used as he free wheeling device are shown in Tables 2a and 2b. The overcurren proecion esed was shudown a 5.8A. I can be varied by simply changing he value of Rsense. The major losses of he converer were mainly due o he following componens: 1. I2R losses (hese include inducor DC losses, RDS(on) of MOSFET, Rsense and copper loss of PCB). 2. Gae charge losses. 3. Diode conducion losses. 4. Swiching losses or ransiion losses of power MOSFET. 5. Capacior ESR losses. 6. PWM Conrol IC losses. Synchronous Recifier + Schoky Synchronous Recifier Io (A) Iin (A) Vin (V) Vo (V) Efficiency Iin (A) Vin (V) Vo (V) Efficiency Table 2a. Efficiency Comparison Schoky Diode Io (A) Iin (A) Vin (V) Vo (V) Efficiency Iin (A) Vin (V) Vo (V) Efficiency Table 2b. Efficiency Comparison 10 MOTOROLA

11 Designaion Qy Descripion C1, C11, C22, C23 C2, C21, C µf, 35 V low ESR capacior AVX 22 µf 35 V analum capacior Sprague 22 µf 35 V analum capacior µf, 10 V low ESR capacior AVX 220 µf 10 V analum capacior Sprague 220 µf 10 V analum capacior C3, C µf ceramic capacior C µf, 16 V analum capacior AVX 4.7 µf 16 V analum capacior Sprague 4.7 µf 16 V analum capacior C µf ceramic capacior C µf ceramic capacior D1 1 Schoky diode Moorola MBR0530 D2 1 Schoky diode Moorola MBRS140T3 Moorola MBRS340T3 J1, J2 2 2 pin header L µh, 2A inducor Coilcraf DO Q1, Q2 2 N channel MOSFET Moorola MTD20N03HDL R Sense resisor Dale WSL 2010 R025 F R6, R8 2 1 M, 5% resisor R K, 5% resisor U1 1 Maxim MAX797 Table 3. Componen Lis for DC DC Converer CONCLUSIONS Synchronous Recificaion is possible wih all commonly used converer opologies. I is achieved by simply replacing he free wheeling diode wih a MOSFET uilizing an addiional gae drive circui. As a resul of his replacemen, he efficiency will improve significanly. Efficiency of 92% and higher can be achieved by using very low on resisance MOSFETs a a lower frequency if size is no a consrain o he design. A a higher swiching frequency, a fas swiching gae drive and low gae charge MOSFET is required o reduce losses. To furher increase he efficiency, use a low ESR capacior for inpu and oupu filering. In addiion, reduce I2R losses by using a low dc losses inducor and increasing he PCB copper rack widh on he high curren pah. LIST OF REFERENCES 1. High Frequency Swiching Power Supply, George C. Chryssis, McGRAW HILL inernaional ediion. 2. Swiching Power Supply Design, Abraham I. Pressman, McGRAW HILL inernaional ediion. 3. Achieving 90% Efficiency Power Conversion, Bijian Mohandes, Siliconix Ld. Newbury, Unied Kingdom. 4. A Simple and Efficiency Synchronous Recifier for Forward DC DC Converers, N. Murakami, H. Namiki, Inerdisciplinary Research Laboraories. 5. Pracical Swiching Power Supply Design, Mary Brown, Moorola, Academic Press, Inc. 6. Power Supply Cookbook, Mary Brown, Moorola, EDN. 7. Maxim796/Maxim797/Maxim799 Daa Shee, Sep Down Conrollers wih Synchronous Recifier CPU Power. 8. HDTMOS Power MOSFETs Excel in Synchronous Recifier Applicaion, Applicaion Noe AN1520, Sco Deuy, Moorola. 9. High Cell Densiy MOSFETs, Engineering Bullein EB201, Kim Gauen and Wayne Chavez, Moorola. MOTOROLA 11

12 Moorola reserves he righ o make changes wihou furher noice o any producs herein. Moorola makes no warrany, represenaion or guaranee regarding he suiabiliy of is producs for any paricular purpose, nor does Moorola assume any liabiliy arising ou of he applicaion or use of any produc or circui, and specifically disclaims any and all liabiliy, including wihou limiaion consequenial or incidenal damages. Typical parameers can and do vary in differen applicaions. All operaing parameers, including Typicals mus be validaed for each cusomer applicaion by cusomer s echnical expers. Moorola does no convey any license under is paen righs nor he righs of ohers. Moorola producs are no designed, inended, or auhorized for use as componens in sysems inended for surgical implan ino he body, or oher applicaions inended o suppor or susain life, or for any oher applicaion in which he failure of he Moorola produc could creae a siuaion where personal injury or deah may occur. Should Buyer purchase or use Moorola producs for any such uninended or unauhorized applicaion, Buyer shall indemnify and hold Moorola and is officers, employees, subsidiaries, affiliaes, and disribuors harmless agains all claims, coss, damages, and expenses, and reasonable aorney fees arising ou of, direcly or indirecly, any claim of personal injury or deah associaed wih such uninended or unauhorized use, even if such claim alleges ha Moorola was negligen regarding he design or manufacure of he par. Moorola and are regisered rademarks of Moorola, Inc. Moorola, Inc. is an Equal Opporuniy/Affirmaive Acion Employer. How o reach us: USA / EUROPE: Moorola Lieraure Disribuion; JAPAN: Nippon Moorola Ld.; Tasumi SPD JLDC, Toshikasu Osuki, P.O. Box 20912; Phoenix, Arizona F Seibu Busuryu Cener, Tasumi Koo Ku, Tokyo 135, Japan MFAX: RMFAX0@ .sps.mo.com TOUCHTONE (602) HONG KONG: Moorola Semiconducors H.K. Ld.; 8B Tai Ping Indusrial Park, INTERNET: hp://design NET.com 51 Ting Kok Road, Tai Po, N.T., Hong Kong MOTOROLA AN1547/D