R. W. Erickson. Department of Electrical, Computer, and Energy Engineering University of Colorado, Boulder

Transcription

1 R. W. Erickso Departmet of Electrical, Computer, ad Eergy Egieerig Uiversity of Colorado, Boulder

2 The Power MOSFET Gate Source Gate legths approachig oe micro p - p Cosists of may small ehacemetmode parallelcoected MOSFET cells, coverig the surface of the silico wafer Vertical curret flow -chael device is show Drai Fudametals of Power Electroics 46 Chapter 4: Switch realizatio

3 MOSFET: Off state source p- - juctio is reverse-biased p p off-state voltage appears across - regio depletio regio - drai + Fudametals of Power Electroics 47 Chapter 4: Switch realizatio

4 MOSFET: o state source p- - juctio is slightly reversebiased p p positive gate voltage iduces coductig chael chael - drai curret flows through - regio ad coductig chael drai drai curret o resistace = total resistaces of - regio, coductig chael, source ad drai cotacts, etc. Fudametals of Power Electroics 48 Chapter 4: Switch realizatio

5 MOSFET body diode p source Body diode p p- - juctio forms a effective diode, i parallel with the chael egative drai-tosource voltage ca forward-bias the body diode drai - diode ca coduct the full MOSFET rated curret diode switchig speed ot optimized body diode is slow, Q r is large Fudametals of Power Electroics 49 Chapter 4: Switch realizatio

6 Typical MOSFET characteristics I D 10A 5A off state V DS = 200V V DS = 10V o state V DS = 2V V DS = 1V V DS = 0.5V 0A 0V 5V 10V 15V V GS Off state: V GS < V th O state: V GS >> V th MOSFET ca coduct peak currets well i excess of average curret ratig characteristics are uchaged o-resistace has positive temperature coefficiet, hece easy to parallel Fudametals of Power Electroics 50 Chapter 4: Switch realizatio

7 A simple MOSFET equivalet circuit D C gs : large, essetially costat C gd : small, highly oliear G C gd C gs C ds C ds : itermediate i value, highly oliear switchig times determied by rate at which gate driver charges/discharges C gs ad C gd S C ds (v ds )= C 0 1+ v ds V 0 C ds(v ds) C 0 V 0 v ds = C 0 ' v ds Fudametals of Power Electroics 51 Chapter 4: Switch realizatio

8 Switchig loss caused by semicoductor output capacitaces Buck coverter example C ds V g + + C j Eergy lost durig MOSFET tur-o trasitio (assumig liear capacitaces): W C = 1 2 (C ds + C j ) V g 2 Fudametals of Power Electroics 52 Chapter 4: Switch realizatio

9 MOSFET oliear C ds Approximate depedece of icremetal C ds o v ds : V C ds (v ds ) C 0 0 v = C ' 0 ds v ds Eergy stored i C ds at v ds = V DS : V DS W Cds = v ds i C dt = v ds C ds (v ds ) dv ds 0 V DS W Cds = C ' 0 (v ds ) v ds dv ds 0 = 2 3 C 2 ds(v DS) V DS same eergy loss as liear capacitor havig value 4 3 C ds(v DS ) Fudametals of Power Electroics 53 Chapter 4: Switch realizatio

10 MOSFET: coclusios A majority-carrier device: fast switchig speed Typical switchig frequecies: tes ad hudreds of khz O-resistace icreases rapidly with rated blockig voltage Easy to drive The device of choice for blockig voltages less tha 500V 1000V devices are available, but are useful oly at low power levels (100W) Part umber is selected o the basis of o-resistace rather tha curret ratig Fudametals of Power Electroics 55 Chapter 4: Switch realizatio

11 Q 1 D1 Mai switch Q 1 Sychroous rectifier Q 2 V g L + + i L Gate driver circuitry: Source of Q 2 is coected to groud Source of Q 1 is coected to switch ode v s (t) Q 2 D 2

12 + 12 V Q 1 D1 L V v s (t) D Q 2 2 Logic iput i L Half-bridge gate driver: Gate of Q 2 is drive by low-side driver Gate of Q 1 is drive by high-side driver High-side driver is powered by bootstrap power supply circuit High voltage itegrated circuit Logic iput: Commads ON/OFF state of MOSFETs Whe Q 1 is o, Q 2 must be off, ad viceversa High-side cotrol sigal must be level-shifted No-overlappig cotrol: isert dead times

13 + 12 V R th A simple Thevei-equivalet circuit: Thevei equivalet model v th (t) + v th (t) is ope-circuit voltage produced by gate driver R th is effective output resistace of driver, approximately give by o-resistace of output-stage trasistors withi the driver D For a driver rated at 12 V ad 1 A, R th = (12 V)/(1 A) = 12 Ω MOSFET, with capacitaces ad body diode explicitly show G C gd C gs C ds S

14 + 12 V Q 1 D 1 v th (t) L + + i L v gs (t) v th (t) R th C gd + + C gs v gs (t) Gate driver model MOSFET model v s (t) v s (t) t

15 Specific o-resistace R o as a fuctio of breakdow voltage V B Majority-carrier device: AARR #$ = kk μμ $ εε ) EE +, VV. / A device area V B E c device breakdow voltage critical electric field for avalache breakdow µ electro mobility e s semicoductor permittivity Uiversity of Colorado Boulder 4

16 Compariso of Power Semicoductor Materials Material Badgap [ev] Electro mobility µ [cm 2 /Vs] Critical field E c [V/cm] Thermal coductivity [W/m o K] Si x SiC x GaN (AlGaN/GaN 2DEG) Wide-badgap device advatages x Much larger E c, hece much lower specific R o at high breakdow voltages Majority carrier devices: o curret tail, o reverse recovery Capability of operatio at icreased juctio temperature Uiversity of Colorado Boulder 5

17 Compariso of Power Semicoductor Materials Material Badgap [ev] Electro mobility µ [cm 2 /Vs] Critical field E c [V/cm] Thermal coductivity [W/m o K] Si x SiC x GaN (AlGaN/GaN 2DEG) x But: SiC is iferior to Si at sub-600v voltages because of lower electro mobility GaN devices are lateral (ot vertical), more difficult to scale to higher voltages ad currets GaN substrate issues: GaN-o-Si Uiversity of Colorado Boulder 6

18 A Historical Perspective: Trasitio from BJTs to MOSFETs Si BJT Si MOSFET Power MOSFET techology advaced rapidly ad replaced power BJTs relatively quickly (i spite of cost disadvatages) Majority carrier device, much faster, much lower switchig losses Sufficietly low o-resistace ca be achieved ecoomically (up to V) Easier to drive ad cotrol Power MOSFETs led to sigificat efficiecy ad power desity improvemets Uiversity of Colorado Boulder 8

19 Trasitio from Si MOSFET to GaN i sub-600v applicatios? Si MOSFET? GaN HEMT Similar trasitios: from Si to SiC at 600+ V Si IGBT SiC MOSFET Si Diode SiC Schottky Uiversity of Colorado Boulder 9

20 State of the Art Device Compariso Example Si MOSFET GaN Voltage ratig 600 V 650 V R o at 25 o C-150 o C mw mw Q g at V DS = 400V 123 C (10V) 12 C (6V) C oss (eergy eq.) 184 pf 177 pf C oss (time eq.) 1900 pf 284 pf V SD 0.8 V 4 V Q rr 8.7 uc - t rr 440 s - Uiversity of Colorado Boulder 10

21 State of the Art Device Compariso Example Si MOSFET GaN Voltage ratig 600 V 650 V R o at 25 o C-150 o C mw mw Q g at V DS = 400V 123 C (10V) 12 C (6V) C oss (eergy eq.) 184 pf 177 pf C oss (time eq.) 1900 pf 284 pf V SD 0.8 V 4 V Q rr 8.7 µc - t rr 440 s - Si MOSFET body diode reverse recovery Q rr V DS f s = 350 W at 400V, 100 khz Uiversity of Colorado Boulder 11