Power MOSFET Switching Waveforms: A New Insight Title N75 ower OS- ET witch averms: New utho nter- Device Models orpo- OCI FO

Transcription

1 Power MOSFET Swiching Waveforms: A New Insigh Applicaion Noe Ocober 1999 AN-7502 Tile N75 ) ubc ower OS- ET wich g averms: New si ) uho ) eyords ner- orpo- ion, mincor) reor () OCI FO f- ark ageode seues The examinaion of power MOSFET volage and curren waveforms during swiching ransiions reveals ha he device characerizaion now praciced by indusry is inadequae. In his Noe, device waveforms are explained by considering he ineracion of a verical JFET driven in cascode from a laeral MOSFET in combinaion wih he inerelecrode capaciances. Paricular aenion is given o he drain-volage waveform and is dual-slope naure. The hree erminal capaciances now published by he indusry are shown o be valid only for zero drain curren. For cases where he gae drive is a volage sep generaor wih inernal fixed resisance, he drain volage characerisics are inferred from he gae curren drive behavior and compared o observed waveforms. The naure of he asymmeric swiching imes is explained. A waveform family is proposed as a more descripive and accurae mehod of characerizaion. This new forma is a plo of drain volage and gae volage versus normalized ime. A family of curves is presened for a consan load resisance wih V DS varied. Gae drive during swiching ransiions is a consan curren wih volage compliance limis of 0 and 10 vols. Time is normalized by he value of gae driving curren. The normalizaion shows excellen agreemen wih daa over five orders of magniude, and is bounded on one exreme by gae propagaion effecs and on he oher by ransiion ime self-heaing (ypically ens of nanoseconds o hundreds of microseconds). Device Models The keysone of an undersanding of power MOSFET swiching performance is he realizaion ha he acive device is bimodal and mus be described using a model ha accouns for he dual naure. Buried in oday s power MOS- FET devices is he equivalen of a depleion layer JFET ha conribues significanly o swiching speed. Figure 1 is a cross-secional view of a ypical power MOSFET, wih MOS- FET/JFET symbols superimposed on he srucure. Figure 2 is obained by aking he laeral MOS and verical JFET from his concepion and adding all he possible nodeo-node capaciances. Compued values of he six capaciances for a ypical device srucure sugges ha device behavior may be adequaely modeled using only hree capaciors in he manner of Figure 3. This is he model o be employed for analysis and sudy. FIGURE 1. CROSS-SECTION VIEW OF MOSFET SHOWING EQUIVALENT MOS TRANSISTOR AND JFET FIGURE 2. POLY GATE n+ SOURCE MOS p BODY MOS TRANSISTOR WITH CASCODE-CONNECTED JFET AND ALL CAPACITORS FIGURE 3. SOURCE METAL GLASS GATE OXIDE n+ FIGURE 2 SIMPLIFIED Gae Drive: Consan Volage or Consan Curren Before moving on o he sudy of he equivalen circui saes of he model, a gae-drive forcing funcion which is easy o represen, relaes o realiy, and bes illusraes device behavior mus be chosen. The choice may be immediaely narrowed o wo: (1) An insananeous sep volage wih inernal resisance R, Figure 5. (2) An insananeous sep curren wih infinie inernal resisance, Figure 6. p+ GATE GATE C1 C2 C GS C x n- JFET C6 C4 SOURCE C3 SOURCE 0 10 VOLTS DEPLETION EDGE 40 VOLTS C5 C DS

2 VOLTAGE V DD FIGURE 4. V G FIGURE 5. IDEALIZED POWER MOSFET WAVEFORMS STEP-VOLTAGE FORCING FUNCTION Power MOSFET devices are highly capaciive in naure; hence, simple capacior responses o he forcing funcions offer a good vehicle for comparison. The advanageous choice is immediaely obvious: Figure 6. Volage/ime responses dominaed by capaciance are sraigh lines (when consan curren is used). The slope of hese lines is proporional o curren and inversely proporional o capaciance. Analyically, hen, consan curren is mos convenien. I is quie anoher maer, however, o build a bidirecional curren drive ha is accurae across he many decades of boh curren and ime required o esablish experimenal verificaion. Six Saes V GS V G(SAT) To compleely characerize power MOSFET swiching waveforms, he six saes ha a device assumes, Figure 6, mus be addressed: STATE MOS JFET Turn-on 1 Off Off Turn-on 2 Acive Acive Turn-on 3 Acive Sauraed Turn-off 4 Sauraed Sauraed STATES V T i() V DK TURN ON v() V G (1 - e) -/R O C i() V G e -/R O C R O TURN OFF v() V G e -/R O C i() - V G e-/r O C R O R O CONSTANT v() V D(SAT) TIME C i() v() -I PK V G /R O GATE VOLTAGE VOLTAGE -V G I PK V G /R O FIGURE 6. Equivalen Circui STEP CURRENT FORCING FUNCTION The lumped-parameer model of Figure 3, wih he cascodeconneced JFET, can now be reduced o he linear equivalen circui of Figure 7, and he six device saes invesigaed from full off o full on. LEGEND V GS - Gae Volage C DS - Drain Source Capaciance V X - JFET Driving Volage g M - MOSFET Transconducance V D - Drain Volage g MJ - JFET Transconducance C GS - Gae Source - Drain Load Resisance Capaciance C X - MOSFET Feedback Capaciance - Consan Curren Ampliude FIGURE 7. POWER MOSFET EQUIVALENT CIRCUIT Sae 1: MOS Off, JFET Off In a power-mosfet device, no drain curren will flow unil he device s gae hreshold volage, V gs(th), is reached. During his ime, he gae s curren drive is only charging he gae source capaciance. More accuraely, is charging C ISS (C ISS C GS + C GD, C DS shored), he capaciance designaion published by he indusry. The curren generaors, g M V G and g MJ V X are open circuis for zero drain curren, and is presumed o be so low as o represen a shor circui (generally rue for pracical applicaions). This is academic however since C GS is very much larger ha C X. The ime o reach hreshold, hen, is simply: Turn-off 5 Acive Sauraed C ISS Turn-off 6 Acive Acive V gs(th) T 1 The erm sauraed is aken o mean a consan low-volage drain-source I condiion. G - TURN ON v() C i(), 0 < < T TURN OFF -I v() 2V G G C i(), T < < 2T GATE VGS i() C X CGS v() C i() v() g MJ V X V X g M V G SOURCE V D T T T -VG C C DS -

3 Sae 2: MOS AcIve, JFET AcIve This sae graphically illusraes he dramaic influence ha he JFET has on he power MOSFET drain-volage waveform. Insead of having o discharge C x from V DD o ground, he laeral MOSFET need only swing V X o ground, a much smaller volage hanks o he grounded gae JFET. Since he ineracion of wih he device capaciances has a secondorder effec on he drain volage, he equivalen circui of Figure 7 predics a drain volage change of: dv G /d g M l G /[C GS + C X (1 + g M /g MJ )] In all bu he smalles power-mosfet devices, C x is several housand picofarads and g M /g MJ is of he order of 3:1. Power-MOSFET devices exhibi a high dv D /d swiching rae because of he cascode-conneced JFET, no because C RSS (C RSS C GD ) is a small value, as zero-drain-curren daa shee capaciance values migh lead one o believe. If C RSS were, in acualiy, small, long drain volage ails would no exis. The ail response is a direc resul of JFET sauraion. In order o delineae he ransiion from sae 2 o sae 3, a drain volage a which he ransiion occurs mus be defined. V DK is he knee volage a which linear exrapolaions of drain-volage slopes inersec. The ime duraion of sae 2 is: 2 ( 6 ) (V DD - V DK )[C GS + C X (1 + g M /g M J)]/g M Sae 3: MOS Acive, JFET Sauraed When he JFET sauraes, he g MJ V X curren generaor becomes a shor circui and he equivalen circui predics: dv D /d g M /[C GS + C X (1 + g M )] This is he Miller effec so ofen referred o in older exs ha describe he behavior of grounded-cahode vacuum-ube amplifier circuis. Allowing for he fac ha 1 + g M is approximaely equal o g M and C X (1 + g M ) is very much larger han C GS, he expression for drain-volage ail ime is: 3 ( 5 ) (V DK - V D(SAT) )C x /l G Sae 4: MOS Sauraed, JFET Sauraed (Turn-Off) In his sae, in addiion o g MJ V X being shored, he g M V G curren generaor is shored, and is occupied wih charging C X and C GS, in parallel, from he peak value of V G o V G(SAT). The ime required for his is: 4 (V G - V G(SAT) )(C GS + C x )/ Since a value for C GS may be measured independenly of swiching ime, he mehod described is he simples way of deermining C X. On urn-off, he sae ime equaions are equally applicable, bu in reverse order (saes 5 and 6); see he idealized waveform of Figure 4. Experimenal Verificaion The four swiching saes jus analyzed indicae ha for a given device, all four swiching sae imes are inversely proporional o he magniude of he gae drive curren. Figure 8 illusraes he swiching performance of a ypical power MOSFET across hree decades of gae drive curren and ime. In each case he daa slope is almos a perfec -1. A New Device Characerizaion Figure 8 could no be a reasonable device daa shee presenaion because i does no give he designer any informaion on a ypical value for C X, nor does i convey how V DK, g M, g M /g MJ, and V G (sa) vary wih drain curren. Wha would be of enormous value o he designer is a plo of V D (), V G () for seleced values of V DD and I D wihin device raings. A reasonable characerizaion would be as follows: 1. The x axis would be normalized in erms of gae curren drive. 2. The y axis would be normalized in erms of percen maximum raed BV DSS (0 o 100%). 3. BV DSS /I D(max) would define he drain load resisance. 4. Four plos of V D (), V G () a 100%, 75%, 50%, and 25% BV DSS(max) would be shown. 10 V DD 75V I DRO 7.5A Ω V G 10V () - MICROSECONDS FIGURE 8. D(OFF) R F D(ON) CONSTANT GATE CURRENT SWITCHING TIME Figure 9 is such a plo for he power MOSFET. Wih such a plo, a designer can esimae device swiching performance under any resisive gae/drain condiions. DATA THEORY ( ) - MILLIAMPERES

4 % RATES V DSS I T 1mA V G 10 VOLTS V DSS /I D(RMS) 20I T / 40I T / 60I T / 80I T / TIME - microseconds FIGURE 9. NORMALIZED SWITCHING WAVE- FORMS FOR CANSTANT GATE-CURRENT DRIVE. Sep-Volage Gae Drive The majoriy of power MOSFET applicaions employ a sep gae-volage inpu wih a finie source resisance R O. Ofen R O for urn-on is no he same as R O for urn-off. How can swiching imes for hese siuaions be esimaed using he swiching characerizaion curves jus described? The analysis for resisive sep volage inpus, which is complex because he gae curren is no longer consrained o be consan, bu is a funcion of device gae-volage response, is covered in Appendix A. (A second, shorer appendix, B, has been added o illusrae he esimaion of R O for some pracical gae drive circuis.) Table 1 summarizes he common swiching equaions, and indicaes he appropriae 1 G o be used in each sae for relaing sep volage drives o he characerizaion curves. Experimenal Verificaion Since he swiching equaions for sep currens and volages differ only by gae-curren magniudes for he same device ype, one would expec a plo of swiching ime versus 1/R O o be of he same form as hose obained for a sep curren drive. This is exacly he case, as Figure 10 is merely a variaion of Figure 8. Using he relaionships of Table 1, he observed differences beween Figures 7 and 9 can be pinpoined. The wo ses of experimenal curves confirm ha, on he basis of he shor-circui drive curren V G /R O equalling he consan, D(on), R, D(off), and F will all be longer, as prediced by he raios of he gae drive currens of Table 1. Noice also ha R, F swiching symmery is disruped by he use of a sep volage wih source resisance R O. For saes 2 and 6 he ime raio is: TABLE 1. COMMON SWITCHING EQUATIONS CONSTANT CURRENT STATE 1: MOS OFF, JFET OFF CONSTANT VOLTAGE T U R N O N C ISS V GS(TH) [1] R O C ISS In [1 - V GS(TH) /V G ] I T STATE 2: ACTIVE, ACTIVE (V G - V GS(TH) )/R O [V DD - V DK ] [C GS + C x (1 + g M /g MJ )] g M I T STATE 3: ACTIVE, SATURATED (V G - V G(SAT) )/R O (V DK - V D(SAT) )C X I T STATE 4: SATURATED, SATURATED -V G /R O T U R N O F F (C GS + C X )(V G - V G(SAT) ) R O (C GS + C X ) In (V G /V G(SAT) ) I T STATE 5: ACTIVE, SATURATED (V G - V G(SAT) )/R O (V DK - V D(SAT) )C X I T STATE 6: ACTIVE, ACTIVE (V G - V G(SAT) )/R O [V DD - V DK ] [C GS + C X (1 + g M /g MJ )] g M

5 Experimenal Verificaion Since he swiching equaions for sep currens and volages differ only by gae-curren magniudes for he same device ype, one would expec a plo of swiching ime versus 1/R O o be of he same form as hose obained for a sep curren drive. This is exacly he case, as Figure 10 is merely a variaion of Figure 8. Using he relaionships of Table 1, he observed differences beween Figures 7 and 9 can be pinpoined. The wo ses of experimenal curves confirm ha, on he basis of he shor-circui drive curren V G /R O equalling he consan, D (on), R, D (off), and F will all be longer, as prediced by he raios of he gae drive currens of Table 1. Noice also ha R, F swiching symmery is disruped by he use of a sep volage wih source resisance R O. For saes 2 and 6 he ime raio is: For saes 3 and 5 he ime raio is: Uilizaion of available maximum gae drive volage and curren can be opimized for fases power MOSFET swiching speed hrough he use of consan-curren gae drive a he expense of increased gae-drive circui complexiy. () - MICROSECONDS FIGURE 10. TURN-ON TURN-OFF TURN-ON TURN-OFF V G(SAT) V G - V GS(TH) V G(SAT) V G - V G(SAT) CONSTANT GATE VOLTAGE SWITCHING TIME Using he Characerizaion Curve, Figure 9 DATA THEORY D(OFF) R F D(ON) To esimae he swiching imes for an power MOSFET under he condiions V G 10V, V DD 75V, R O 100 ohms, and 10 ohms, precedes as follows: /R O V DD 75V I DVG 7.5A 10V Sae 1: MOS Off, JFET Off This ime can be esimaed wihou recourse o he curves Sae 2 & 6: MOS Acive, JFET Acive Sae 3: MOS Acive, JFET Sauraed Sae 4: MOS Sauraed, JFET Sauraed Sae 5: MOS Acive, JFET Sauraed Figure 11 shows waveforms using he condiions specified in he example. FIGURE (1200 x ) ln [1/(1-4/10)] 61 ns (10-4)/100 60mA (curve divisions) x I T µs (10-7)/100 30mA (curve divisions) x I T µs ns 467ns C GS + C x (gae volage slope)(es curren) (1.5 x 10-6 s/5 vols)(10ma) 3000pF 100(3000 x ) ln [10/6.6] 125ns VOLTAGE - VOLTS /100 66mA (curve divisions) x I T µs V D V GS 121ns TIME - MICROSECONDS STEP GATE VOLTAGE INPUT TO AN CALCULATED TIME V DD 75 VOLTS 10 OHMS V G 10 VOLTS R O 100 OHMS MEASURED TIME STATE RATIO ( C, ns) ( M, ns) ( C / M )

6 For peak gae volages oher han 10 vols, and load resisances oher han BV DSS /I D(MAX), he equaions of Table 1 may be used in conjuncion wih slope esimaes from he characerizaion curves for C X and C GS + C X (1 + g M /g MJ ) a he appropriae drain-curren level. Characerizaion-Curve Limis The swiching-ime range over which he characerizaion can be applied is very impressive. For gae currens of he order of microamperes, device dissipaion is he limiing facor. For gae currens of he order of amperes, he device response will be slowed by gae propagaion delay. This delay, of course, degrades he linear swiching relaionship o gae curren. However, as Figure 12 graphically shows, he characerizaion is valid across five decades of gae curren and swiching ime, allowing all bu a very few swiching applicaions o be described by he characerizaion curves of Figure 9. TIME() - MICROSECONDS D (OFF) R F D (ON) GATE CURRENT ( ) - MICROAMPERES FIGURE 12. FIVE DECADES OF LINEAR RESPONSE saes mus be examined using he same device equivalen circui as was used for he consan-gae-curren case, bu wih he forcing funcion replaced wih a sep volage wih inernal resisance R O, Figure A-1. V G GATE R O VGS C X LEGEND V GS - Gae Volage C DS - Drain Source Capaciance V X - JFET Driving Volage g M - MOSFET Transconducance V D - Drain Volage g MJ - JFET Transconducance C GS - Gae Source - Drain Load Resisance Capaciance C X - MOSFET Feedback Capaciance - Consan Curren Ampliude FIGURE A-1. CGS g MJ V X V X g M V G SOURCE POWER MOSFET EQUIVALENT CIRCUIT Sae 1: Mos Off, JFET Off As before, boh curren generaors are open circuis, reducing he equivalen circui o simply charging C ISS hrough R O. R O C ISS In(1/(1 - V GS(TH) /V G )] C DS Sae 2: Mos Acive, JFET Acive Before proceeding, i is wise o examine an acual device response and make use of available simplificaions. Figure A-2 shows i G () and i D () for a ypical power MOSFET driven by a sep gae volage. For ruly resisive swiching, realize ha hese waveforms are only mirror images of heir volage counerpars v G () and v D (). Using Figure A-2, applicable gae currens for each of he device saes may be lised. V D Conclusions The viabiliy of he proposed characerizaion curves using consan curren has been demonsraed and he limis of applicaion defined. The exisence of a verical JFET in a power MOSFET makes daa-shee capaciances of lile use for esimaing swiching imes. The classical mehod of defining swiching ime by 10% and 90% is a poor represenaion for power MOSFETs because of he dual-slope naure of he drain waveforms. Swiching influences are masked because he 10% level is conrolled by one mechanism and he 90% level by anoher. Device comparisons based on he classical swiching definiion can be very misleading. CURRENT I PK1 I PK2 I PK3 i G () TIME I PK4 i D () I PK6 IPK5 Appendix A - Analysis for Resisive Sep Volage Inpus FIGURE A-2. i G () AND i D () FOR A TYPICAL POWER MOSFET DRIVEN BY A STEP GATE VOLTAGE Sep Volage Gae Drive To obain he necessary relaionships, six device swiching

7 Turn-On Turn-Off Sae 1: MOS Off, JFET Off I PK1 V G /R O Sae 2: MOS Acive, JFET Acive I PK2 (V G - V GS(TH) )/R O Sae 3: MOS Acive, JFET Sauraed I PK3 (V G - V G(SAT) )/R O Sae 4: MOS Sauraed, JFET Sauraed I PK4 V G /R O Sae 5: MOS Acive, JFET Sauraed I PK5 V G(SAT) /R O Sae 6: MOS Acive, JFET Acive I PK6 V G(SAT) /R O The equivalen circui of Figure A-1 predics ha: dv D /d (-g M (V G - V GS(TH) )e -/T1 ) /T1 where T1 R O C GS + (1 + g M /g MJ )R O C X Noe ha g M (V G - V GS(TH) ) is usually an order of magniude greaer han V DD, indicaing ha he drain volage is discharging oward a very large negaive value. The device operaion, hen, is on he early, almos linear, porion of he exponenial, where e -/T1 approximaes uniy. The drain curren of Figure A- 2, and hence he drain volage, does indeed exhibi a linear decrease wih ime. Thus, for sae 2: where I PK2 (V G - V GS(TH) )/R O [V DD - V DK ][C GS + C X (1 + g M /g MJ )] g M I PK2 Sae 3: Mos Acive, JFET Sauraed Because of he Miller effec, he gae volage and, hence, he gae curren, is almos consan during he ail ime. The equivalen circui hen predics: Sae 5: Mos Acive, JFET Sauraed The JFET curren generaor V x g mj, is operaive. I PK5 V G(SAT) /R O Sae 6: Mos Acive, JFET Acive The Miller effec is now reduced by he acivaion of V G g MJ, and he equivalen circui predics: I PAK6 V G(SAT) /R O Appendix B - Esimaing R O for Some Typical Gae-Drive Circuis Case 1: Typical Pulse-Generaor Drive, Figure B-1 FIGURE B-1. [V DK - V D[SAT] )C X I PK5 [V DD - V DK ][C GS + C X (1 + g M /g MJ )] g M I PAK6 V GEN R GEN V G TYPICAL PULSE-GENERATOR DRIVE CIRCUIT Turn-On and Turn-Off R O R GEN R GS /(R GEN + R GS ) For he ypical case where R GEN 50Ω, and a coaxial-cable erminaion of 50 ohms, R O 25Ω and V G V GEN /2. Case 2: Volage-Follower Gae Drive, Figure B-2 + R GS V DD V DD dv D g M l G l G d C GS + (1 + g M )C X C X l G I PK3 (V G - V G(SAT) )/R O (V DK - V D[SAT] )C x and I PK3 Sae 4: Mos Sauraed, JFET Sauraed (Turn-off) Boh equivalen-circui generaors are shor circuis, and he gae drive is discharging C X in parallel wih C GS hrough R O. R O (C GS + C X ) ln[v G /V G(SAT) ] I PK4 V G /R O FIGURE B-2. VOLTAGE-FOLLOWER GATE-DRIVE CIRCUIT Turn-On R O is approximaely equal o 1/g M for R S very much greaer han 1/g M. Turn Off R S gm ransconducance of driving MOSFET ransisor. R O R S

8 Case 3 :Common-Source Gae Drive, Figure B-3 + R D V DD 10V 0V FIGURE B-3. COMMON-SOURCE GATE-DRIVE CIRCUIT Turn-On R O R D (drain-o-ground capaciance of driving device adds o C GS of driven MOSFET.) Turn Off R O r DS(ON) of driving MOSFET when R D is very much greaer han R DS(ON)

9 TRADEMARKS The following are regisered and unregisered rademarks Fairchild Semiconducor owns or is auhorized o use and is no inended o be an exhausive lis of all such rademarks ACEx Boomless CoolFET CROSSVOLT DenseTrench DOME EcoSPARK E 2 CMOS TM EnSigna TM FACT FACT Quie Series STAR*POWER is used under license DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS LIFE SUPPORT POLICY FAIRCHILD S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF FAIRCHILD SEMICONDUCTOR CORPORATION As used herein: 1 Life suppor devices or sysems are devices or sysems which, (a) are inended for surgical implan ino he body, or (b) suppor or susain life, or (c) whose failure o perform when properly used in accordance wih insrucions for use provided in he labeling, can be reasonably expeced o resul in significan injury o he user PRODUCT STATUS DEFINITIONS Definiion of Terms FAST â FASTr FRFET GlobalOpoisolaor GTO HiSeC I 2 C ISOPLANAR LileFET MicroFET MicroPak MICROWIRE OPTOLOGIC â OPTOPLANAR PACMAN POP Power247 PowerTrench QFET QS 2 A criical componen is any componen of a life suppor device or sysem whose failure o perform can be reasonably expeced o cause he failure of he life suppor device or sysem, or o affec is safey or effeciveness Daashee Idenificaion Produc Saus Definiion â QT Opoelecronics Quie Series SILENT SWITCHER â SMART START SPM STAR*POWER Sealh SuperSOT -3 SuperSOT -6 SuperSOT -8 SyncFET TinyLogic TruTranslaion UHC UlraFET â VCX Advance Informaion Preliminary No Idenificaion Needed Formaive or In Design Firs Producion Full Producion This daashee conains he design specificaions for produc developmen Specificaions may change in any manner wihou noice This daashee conains preliminary daa, and supplemenary daa will be published a a laer dae Fairchild Semiconducor reserves he righ o make changes a any ime wihou noice in order o improve design This daashee conains final specificaions Fairchild Semiconducor reserves he righ o make changes a any ime wihou noice in order o improve design Obsolee No In Producion This daashee conains specificaions on a produc ha has been disconinued by Fairchild semiconducor The daashee is prined for reference informaion only Rev H5