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 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

3 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

4 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

5 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

6 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

7 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

8 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

9 Power GaN HEMT High Electro Mobility Trasistor (HEMT) A heterojuctio field effect trasistor Source Gate Drai Lateral device No oxide layers AlGaN -type GaN itrisic AlGaN: low badgap GaN: high badgap Substrate Fudametals of Power Electroics 1

10 The Two-Dimesioal Electro Gas (2DEG) Source Gate Drai 2DEG AlGaN -type Substrate GaN itrisic The eergy bad diagram takes a step at the heterojuctio. Uder the correct coditios, a 2DEG forms at the surface of the GaN layer. These electroics exhibit very low resistivity (high mobility), ad ca coduct curret betwee source ad drai. A majority carrier device havig: High breakdow field Low o resistace Fudametals of Power Electroics 2

11 The HEMT is a JFET The basic device is a depletio-mode juctio field-effect trasistor: Normally o To tur off, reverse-bias gate Gate-chael juctio is a diode that ca coduct curret whe forward-biased Additioal semicoductor desig ca shift threshold voltage: Ehacemet-mode JFETs are available The device is off whe v gs = 0 G D S Fudametals of Power Electroics 3

12 Electrical Cosideratios O state: v gs > V th with V th ~ 3.5 V But do t apply v gs that is too large: gate-source diode will become forward-biased ad coduct large curret. Off state: v gs 0 Reverse coductio: No body diode Chael ca coduct curret i either directio With v gs = 0, a egative v ds (< V th ) ca tur device o Behavior is similar to havig a body diode, except Large forward drop ~ V th No reverse recovery G D S Fudametals of Power Electroics 4

13 Bipolar Juctio Trasistor (BJT) Base Emitter Iterdigitated base ad emitter cotacts Vertical curret flow p p device is show miority carrier device - o-state: base-emitter ad collector-base juctios are both forward-biased Collector o-state: substatial miority charge i p ad - regios, coductivity modulatio Fudametals of Power Electroics 56 Chapter 4: Switch realizatio

14 BJT switchig times v s (t) V s2 V s1 V CC v BE (t) 0.7V R L v s (t) + i B (t) R B i C (t) + v BE (t) + v CE (t) V s1 i B (t) v CE (t) 0 I B1 I B2 V CC I Co R o i C (t) I Co 0 (1) (2) (3) (4) (5) (6) (7) (8) (9) t Fudametals of Power Electroics 57 Chapter 4: Switch realizatio

15 Ideal base curret waveform i B (t) I B1 I Bo 0 t I B2 Fudametals of Power Electroics 58 Chapter 4: Switch realizatio

16 Coclusios: BJT BJT has bee replaced by MOSFET i low-voltage (<500V) applicatios BJT has bee replaced by IGBT i applicatios at voltages above 500V A miority-carrier device: compared with MOSFET, the BJT exhibits slower switchig, but lower o-resistace at high voltages Fudametals of Power Electroics 63 Chapter 4: Switch realizatio

17 The Isulated Gate Bipolar Trasistor (IGBT) Emitter A four-layer device Gate Similar i costructio to MOSFET, except extra p regio p - p Collector p miority carrier ijectio O-state: miority carriers are ijected ito - regio, leadig to coductivity modulatio compared with MOSFET: slower switchig times, lower o-resistace, useful at higher voltages (up to 6500 V) Fudametals of Power Electroics 64 Chapter 4: Switch realizatio

18 The IGBT Symbol collector gate Locatio of equivalet devices emitter Equivalet circuit C p i 2 i 1 p G - i 1 i 2 p Fudametals of Power Electroics E 65 Chapter 4: Switch realizatio

19 Curret tailig i IGBTs C IGBT waveforms i L v A (t) i A (t) curret tail V g diode waveforms } 0 0 i L t G 0 i B (t) 0 v B (t) t i 1 i 2 V g E p A (t) = v A i A V g i L area W off t 0 t 1 t 2 t 3 t Fudametals of Power Electroics 66 Chapter 4: Switch realizatio

20 Switchig loss due to curret-tailig i IGBT V g + i A physical IGBT v A + DT s + T s v B gate + driver i L (t) ideal diode Example: buck coverter with IGBT i B L trasistor tur-off trasitio IGBT waveforms diode waveforms i L v A (t) i A (t) curret tail } V g i B (t) 0 v B (t) V g i L t t P sw = 1 T s switchig trasitios p A (t) dt =(W o + W off ) f s p A (t) = v A i A V g i L area W off Fudametals of Power Electroics 67 t 0 t 1 t 2 Chapter 4: Switch realizatio t 3 t

21 Coclusios: IGBT Has become the device of choice i 500 to 1700V+ applicatios, at power levels of kW Positive temperature coefficiet at high curret easy to parallel ad costruct modules Forward voltage drop: diode i series with o-resistace. 2-4V typical Easy to drive similar to MOSFET Slower tha MOSFET, but faster tha Darligto, GTO, SCR Typical switchig frequecies: 3-30kHz IGBT techology is rapidly advacig: 6500 V devices: HVIGBTs 150 khz switchig frequecies i 600 V devices Fudametals of Power Electroics 69 Chapter 4: Switch realizatio

22 Efficiecy vs. switchig frequecy Add up all of the eergies lost durig the switchig trasitios of oe switchig period: W tot = W o + W off + W D + W C + W L +... Average switchig power loss is P sw = W tot f sw Total coverter loss ca be expressed as P loss = P cod + P fixed + W tot f sw where P fixed = fixed losses (idepedet of load ad f sw ) P cod = coductio losses Fudametals of Power Electroics 89 Chapter 4: Switch realizatio

23 Efficiecy vs. switchig frequecy P loss 100% 90% 80% = P cod + P fixed + W tot f sw dc asymptote f crit Switchig losses are equal to the other coverter losses at the critical frequecy f crit = P cod + P fixed W tot 70% 60% 50% 10kHz 100kHz 1MHz This ca be take as a rough upper limit o the switchig frequecy of a practical coverter. For f sw > f crit, the efficiecy decreases rapidly with frequecy. Fudametals of Power Electroics f sw 90 Chapter 4: Switch realizatio