R. W. EricksonDepartment of Electrical, Computer, and Energy Engineering

University of Colorado, Boulder

Fundamentals of Power Electronics Chapter 4: Switch realization46

4.2.2. The Power MOSFET

Drain

n

n-

n np p

Source

Gate

nn

• Gate lengthsapproaching onemicron

• Consists of manysmall enhancement-mode parallel-connected MOSFETcells, covering thesurface of the siliconwafer

• Vertical current flow• n-channel device is

shown

Fundamentals of Power Electronics Chapter 4: Switch realization47

MOSFET: Off state

n

n-

n np p

nn

depletion region

–

+

source

drain

• p-n- junction isreverse-biased

• off-state voltageappears across n-

region

Fundamentals of Power Electronics Chapter 4: Switch realization48

MOSFET: on state

n

n-

n np p

nn

channel

source

drain drain current

• p-n- junction isslightly reverse-biased

• positive gate voltageinduces conductingchannel

• drain current flowsthrough n- regionand conductingchannel

• on resistance = totalresistances of n-

region, conductingchannel, source anddrain contacts, etc.

Fundamentals of Power Electronics Chapter 4: Switch realization49

MOSFET body diode

n

n-

n np pnn

Body diode

source

drain

• p-n- junction formsan effective diode, inparallel with thechannel

• negative drain-to-source voltage canforward-bias thebody diode

• diode can conductthe full MOSFETrated current

• diode switchingspeed not optimized—body diode isslow, Qr is large

Fundamentals of Power Electronics Chapter 4: Switch realization50

Typical MOSFET characteristics

0A

5A

10A

VGS

ID

VDS = 0.5V

VDS = 1V

V DS = 2VV DS

= 10

V

V DS =

200

V

0V 5V 10V 15V

offstate

on state

• Off state: VGS < Vth

• On state: VGS >> Vth

• MOSFET canconduct peakcurrents well inexcess of averagecurrent rating—characteristics areunchanged

• on-resistance haspositive temperaturecoefficient, henceeasy to parallel

Fundamentals of Power Electronics Chapter 4: Switch realization51

A simple MOSFET equivalent circuit

D

S

GCds

Cgs

Cgd

• Cgs : large, essentially constant• Cgd : small, highly nonlinear• Cds : intermediate in value, highly

nonlinear• switching times determined by rate

at which gate drivercharges/discharges Cgs and Cgd

Cds(vds) = C0

1 + vds

V0

Cds(vds) 5 C0V0vds

= C0'

vds

Fundamentals of Power Electronics Chapter 4: Switch realization52

Switching loss caused bysemiconductor output capacitances

+– + –Vg

Cds

Cj

Buck converter example

WC = 12 (Cds + Cj) V g

2

Energy lost during MOSFET turn-on transition(assuming linear capacitances):

Fundamentals of Power Electronics Chapter 4: Switch realization53

MOSFET nonlinear Cds

Cds(vds) 5 C0V0vds

= C0'

vds

Approximate dependence of incremental Cds on vds :

Energy stored in Cds at vds = VDS :

WCds = vds iC dt = vds Cds(vds) dvds0

VDS

WCds = C0' (vds) vds dvds

0

VDS= 2

3 Cds(VDS) V DS2

43 Cds(VDS)— same energy loss as linear capacitor having value

Fundamentals of Power Electronics Chapter 4: Switch realization55

MOSFET: conclusions

● A majority-carrier device: fast switching speed● Typical switching frequencies: tens and hundreds of kHz● On-resistance increases rapidly with rated blocking voltage● Easy to drive● The device of choice for blocking voltages less than 500V● 1000V devices are available, but are useful only at low power levels

(100W)● Part number is selected on the basis of on-resistance rather than

current rating

+–

L

iL+

vs(t)

–

Q1 D1

Q2D2

Vg

Main switch Q1Synchronous rectifier Q2

Gate driver circuitry: • Source of Q2 is connected to ground• Source of Q1 is connected to switch node

+–

L

iL+

vs(t)

–

Q1 D1

Q2D2

+ 12 V

+ 12 V

Logic input

Half-bridge gate driver: • Gate of Q2 is driven by low-side driver• Gate of Q1 is driven by high-side driver• High-side driver is powered by bootstrap

power supply circuit• High voltage integrated circuit

Logic input: • Commands ON/OFF state of MOSFETs• When Q1 is on, Q2 must be off, and vice-

versa• High-side control signal must be level-shifted• Non-overlapping control: insert dead times

+ 12 V

Theveninequivalentmodel

+–

Rth

vth(t)

A simple Thevenin-equivalent circuit: • vth(t) is open-circuit voltage produced by

gate driver •  Rth is effective output resistance of driver,

approximately given by on-resistance ofoutput-stage transistors within the driver

• For a driver rated at 12 V and 1 A,Rth = (12 V)/(1 A) = 12 Ω

Fundamentals of Power Electronics Chapter 4: Switch realization51

A simple MOSFET equivalent circuit

D

S

GCds

Cgs

Cgd

• Cgs : large, essentially constant• Cgd : small, highly nonlinear• Cds : intermediate in value, highly

nonlinear• switching times determined by rate

at which gate drivercharges/discharges Cgs and Cgd

Cds(vds) =C0

1 + vdsV0

Cds(vds) � C0V0vds = C0

'

vds

MOSFET, with capacitances and body diode explicitly shown

+–

L

iL+

vs(t)

–

Q1 D1+ 12 V

+–

Rth

vth(t)

Cgd

Cgs+

vgs(t)–

Gate drivermodel

MOSFETmodel

vth(t)

vgs(t)

vs(t)

t

4University of Colorado Boulder

Specific on-resistance Ron as a function of breakdown voltage VB

A device areaVB device breakdown voltageEc critical electric field for avalanche breakdownµn electron mobilityes semiconductor permittivity

𝐴𝐴𝑅𝑅#$ =𝑘𝑘

𝜇𝜇$𝜀𝜀)𝐸𝐸+,𝑉𝑉./Majority-carrier device:

5University of Colorado Boulder

Comparison of Power Semiconductor MaterialsMaterial Bandgap [eV] Electron

mobility µµn[cm2/Vs]

Critical field Ec [V/cm]

Thermal conductivity

[W/moK]Si 1.12 1400 3 x 105 130

SiC 2.36-3.25 300-900 1.3-3.2 x 106 700GaN 3.44 1500-2000

(AlGaN/GaN2DEG)

3.0-3.5 x 106 110

Wide-bandgap device advantages• Much larger Ec, hence much lower specific Ron at high breakdown voltages• Majority carrier devices: no current tail, no reverse recovery• Capability of operation at increased junction temperature

6University of Colorado Boulder

Comparison of Power Semiconductor MaterialsMaterial Bandgap [eV] Electron

mobility µµn[cm2/Vs]

Critical field Ec [V/cm]

Thermal conductivity

[W/moK]Si 1.12 1400 3 x 105 130

SiC 2.36-3.25 300-900 1.3-3.2 x 106 700GaN 3.44 1500-2000

(AlGaN/GaN2DEG)

3.0-3.5 x 106 110

But:• SiC is inferior to Si at sub-600V voltages because of lower electron mobility• GaN devices are lateral (not vertical), more difficult to scale to higher

voltages and currents• GaN substrate issues: GaN-on-Si

8University of Colorado Boulder

A Historical Perspective: Transition from BJTs to MOSFETs

Power MOSFET technology advanced rapidly and replaced power BJTs relatively quickly (in spite of cost disadvantages)

• Majority carrier device, much faster, much lower switching losses

• Sufficiently low on-resistance can be achieved economically (up to 600-900V)

• Easier to drive and control

• Power MOSFETs led to significant efficiency and power density improvements

Si BJT Si MOSFET

9University of Colorado Boulder

Transition from Si MOSFET to GaN in sub-600V applications?

?GaN HEMTSi MOSFET

Si IGBT SiC MOSFET

Similar transitions: from Si to SiC at 600+ V

Si Diode SiC Schottky

10University of Colorado Boulder

State of the Art Device Comparison Example

Si MOSFET GaNVoltage rating 600 V 650 VRon at 25oC-150oC 24-60 mW 25-65 mWQg at VDS = 400V 123 nC (10V) 12 nC (6V)Coss (energy eq.) 184 pF 177 pFCoss (time eq.) 1900 pF 284 pFVSD 0.8 V 4 VQrr 8.7 uC -trr 440 ns -

11University of Colorado Boulder

State of the Art Device Comparison Example

Si MOSFET GaNVoltage rating 600 V 650 VRon at 25oC-150oC 24-60 mW 25-65 mWQg at VDS = 400V 123 nC (10V) 12 nC (6V)Coss (energy eq.) 184 pF 177 pFCoss (time eq.) 1900 pF 284 pFVSD 0.8 V 4 VQrr 8.7 µµC -trr 440 ns -

Qrr VDS fs = 350 W at 400V, 100 kHz

Si MOSFET body diode reverse recovery