© Ned Mohan, 2005

2- 1

Chapter 2 Design of Switching Power-Pole

2-1 Power Transistors and Power Diodes 2-2 Selection of Power Transistors 2-3 Selection of Power Diodes

2-4 Switching Characteristics and Power Losses in Power-Poles

2-5 Justifying Switches and Diodes as Ideal

2-6 Design Considerations 2-7 The PWM Controller IC

References Problems

Appendix 2A Diode Reverse-Recovery and Power Losses

© Ned Mohan, 2005

2- 2

POWER TRANSISTORS AND POWER DIODES

• Voltage Rating

• Current Rating

• Switching Speeds

• On-State Voltage

© Ned Mohan, 2005

2- 3

SELECTION OF POWER TRANSISTORS

• MOSFETs

• IGBTs

• IGCTs

• GTOs

• Niche devices: BJTs, SITs, MCTs

Figure 15-1 Power semiconductor devices.

Thyristor IGBT MOSFETIGCT(a)

101 102 103 104

102

104

106

108

Thyr

isto

r

IGBT

MOSFET

Pow

er (V

A)

Switching Frequency (Hz)

IGCT

(b)

Thyristor IGBT MOSFETIGCT(a)

Thyristor IGBT MOSFETIGCT(a)

101 102 103 104

102

104

106

108

Thyr

isto

r

IGBT

MOSFET

Pow

er (V

A)

Switching Frequency (Hz)

IGCT

(b)

101 102 103 104

102

104

106

108

Thyr

isto

r

IGBT

MOSFET

Pow

er (V

A)

Switching Frequency (Hz)

IGCT

101 102 103 104

102

104

106

108

Thyr

isto

r

IGBT

MOSFET

Pow

er (V

A)

Switching Frequency (Hz)

IGCT

(b)

© Ned Mohan, 2005

2- 4

MOSFETs

2.5 2.7( )

toDS on DSSR Vα

Figure 2-1 MOSFET: (a) symbol, (b) i-v characteristics, (c) transfer characteristic.

G

D

SGSV+

−

DSV

Di

+

−

(a) (c) (b)

GSV 11V=

9V7V5V

DSV

Di

0( )GS GS thV V≤

( ) 1/slopeDS onR =

GSV

Di

0

oI

( )GS thV ( )oGS IV

© Ned Mohan, 2005

2- 5

IGBTs

Figure 2-2 IGBT: (a) symbol, (b) i-v characteristics.

−

+

C

E

G CEV

GEV−

+

Ci

GEV

CEV

Ci

(a) (b)

−

+

C

E

G CEV

GEV−

+

Ci

−

+

C

E

G CEV

GEV−

+

Ci

GEV

CEV

Ci

GEV

CEV

Ci

(a) (b)

© Ned Mohan, 2005

2- 6

Power Semiconductor Price Trends

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1990 1995 2000 2005

USD/A

Pricing (USD/A) 1200 V IGBTs

© Ned Mohan, 2005

2- 7

SELECTION OF POWER DIODES

• Line-frequency diodes

• Fast-recovery diodes

• Schottky diodes

• SiC Schottky diodes

Figure 2-3 Diode: (a) symbol, (b) i-v characteristic.

(a) (b)

A K

AKi

AKv0

(a) (b)

A K

AKi

AKv0

© Ned Mohan, 2005

2- 8

SWITCHING CHARACTERISTICS AND POWER LOSSES IN POWER-POLES

Figure 2-4 MOSFET in a switching power-pole.

(a) (b)

inV+

−

Di

oI

Di

0DSv

off

onoIGGR

GGV DSv−

+

inV(a) (b)

inV+

−

Di

oI

Di

0DSv

off

onoIGGR

GGV DSv−

+

inV

© Ned Mohan, 2005

2- 9

Turn-on Characteristic

Figure 2-5 MOSFET turn-on.

(c)(b)

Di

DSv

oIon

off

A

0

0

0

GSv

t

t

oIDiinV DSv

( )d ont rit fvt

GGv

(a)

oI

inV

−

+G

S

DDi

0GGv B

DSv

inV

( )oGS Iv( )GS thv

(c)(b)

Di

DSv

oIon

off

A

0

0

0

GSv

t

t

oIDiinV DSv

( )d ont rit fvt

GGv

(a)

oI

inV

−

+G

S

DDi

0GGv B

DSv

inV

( )oGS Iv( )GS thv

© Ned Mohan, 2005

2- 10

Example 2-1 In the converter of Fig. 2-4a, the transistor is a MOSFET which carriesa current of 5 A when it is fully on. If the current through thetransistor is to be limited to 40 A during a malfunction in which casethe entire input voltage of 50 V appears across the transistor, whatshould be the maximum on-state gate voltage that the gate-drive circuitshould provide? Assume the junction temperature jT of the MOSFET

to be 0175 C .

Solution The transfer characteristic of this MOSFET is shown in Fig. 2-6. It shows that

if 7.5GSV V= is used, the current through the MOSFET will be limited to 40 A.

Figure 2-6 MOSFET transfer characteristic.

40 A

10 A

1A

0.1A

100 A

4.0 5.0 6.0 7.0 8.0 9.010.07.5V

DI

GSV

40 A

10 A

1A

0.1A

100 A

4.0 5.0 6.0 7.0 8.0 9.010.07.5V

DI

GSV

© Ned Mohan, 2005

2- 11

Turn-off Characteristic

Figure 2-7 MOSFET turn-off. (a)

oI

inV−

+

GS

DDi

0GGv

(b)

Di

DSv

oIon

off

D

0

C

(c)

0

0

GSv

t

t

oIDi

inV

DSv

( )d offt rvt fit

GGv( )oGS Iv( )GS thv

DSv

inV

(a)

oI

inV−

+

GS

DDi

0GGv

(b)

Di

DSv

oIon

off

D

0

C

(c)

0

0

GSv

t

t

oIDi

inV

DSv

( )d offt rvt fit

GGv( )oGS Iv( )GS thv

DSv

inV

© Ned Mohan, 2005

2- 12PSpice Modeling: C:\FirstCourse_PE_Book03\Power_pole_PSpice_Diode.sch

© Ned Mohan, 2005

2- 13

Time

0s 0.2us 0.4us 0.6us 0.8us 1.0us 1.2us 1.4us 1.6usV(M2:d,M2:s) -I(V2)

-10

0

10

20

30

40

50

DSv

Di

Simulation Results: MOSFET Voltage and Current

© Ned Mohan, 2005

2- 14

Calculating Power Losses Within the MOSFET (assuming an ideal diode)

Conduction Loss: 2( )cond DS on oP d R I=

Switching Losses: , ,1 ( )2sw in o c on c off sP V I t t f= +

,c on ri fvt t t= +

,c off rv fit t t= +

Figure 2-8 MOSFET switching losses.

0t

oI DSv

,c ont

rit fvt

0t

Di

inV

,c offtrvt fit

DSvinV

Di

,c ont

,c offt

swp in oV I in oV Iswp

0t

oI DSv

,c ont

rit fvt

0t

Di

inV

,c offtrvt fit

DSvinV

Di

,c ont

,c offt

swp in oV I in oV Iswp

© Ned Mohan, 2005

2- 15

Gate Driver Integrated Circuits (ICs) with Built-in Fault Protection

Figure 2-9 Gate-driver IC functional diagram.

12extV V=CCV

cv

S

12extV V=CCV

cv

S

© Ned Mohan, 2005

2- 16

JUSTIFYING SWITCHES AND DIODES AS IDEAL

Very High Converter Efficiencies

• Low on-state voltage drops across devices

• Low switching losses

© Ned Mohan, 2005

2- 17

• Switching Frequency

• Selection of Transistors and Diodes

• Magnetic components

• Capacitor Selection

DESIGN CONSIDERATIONS

max max

ˆrms

pw

LIIAk J B

= ,

max max

conv y y rmsp

w s

k V IA

k B J f= ∑

Figure 2-10 Capacitor ESR and ESL.

C ESL ESRC ESL ESR

© Ned Mohan, 2005

2- 18PSpice Modeling: C:\FirstCourse_PE_Book03\Capacitor_Characteristics.sch

© Ned Mohan, 2005

2- 19

Frequency

1.0KHz 3.0KHz 10KHz 30KHz 100KHz 300KHz 1.0MHzI(L2) I(L1) -I(V3)

0A

10A

20A

30A

40A

50A

Simulation Results: Individual and Total Admittances

© Ned Mohan, 2005

2- 20

Thermal Design

( )j a jc cs sa dissT T R R R Pθ θ θ= + + +

Figure 2-11 Thermal design: (a) semiconductor on a heat sink, (b) electrical analog.

jcRθ

dissP

(b)

csRθ sa

Rθ

aT

aTsTcTjT

(a)

chip

heat sinkisolation pad

jTcT

sT aT

ambient

casejc

Rθ

dissP

(b)

csRθ sa

Rθ

aT

aTsTcTjT

(a)

chip

heat sinkisolation pad

jTcT

sT aT

ambient

case

© Ned Mohan, 2005

2- 21

Design Tradeoffs

Magnetics andcapacitors

size

Heatsink

Sf

Magnetics andcapacitors

size

Heatsink

Sf

Figure 2-12 Size of magnetic components and heat sink as a function of frequency.

© Ned Mohan, 2005

2- 22

PWM CONTROLLER IC

( )( ) ˆc

r

v td tV

=

Figure 2-13 PWM IC waveforms.

0sd T

sTt

r̂V( )cv t

t

rv

( )q t0

1

0sd T

sTt

r̂V( )cv t

t

rv

( )q t0

1

© Ned Mohan, 2005

2- 23

APPENDIX 2A: Diode Reverse Recovery and Power Losses

Diode Forward Loss: , (1 )diode F FM oP d V I= − ⋅

Diode Reverse Recovery Characteristic:

Diode Switching Losses: , ,1( )2diode sw RRM b d neg sP I t V f= ⋅ ⋅

Figure 2A-1 Diode reverse recovery characteristic.

0

rrtat bt

RRMI

t

t0

FMV

,d negV

rrQ

0

rrtat bt

RRMI

t

t0

FMV

,d negV

rrQ

© Ned Mohan, 2005

2- 24PSpice Modeling: C:\FirstCourse_PE_Book03\ Power_pole_MUR2020.sch

© Ned Mohan, 2005

2- 25

Time

0s 0.2us 0.4us 0.6us 0.8us 1.0us 1.2us 1.4us 1.6usV(M2:d,M2:s) -I(V2)

-10

0

10

20

30

40

50

Simulation Results: MOSFET Voltage and Current

© Ned Mohan, 2005

2- 26

Example 2A-1 In the switching power-pole of Fig. 2-4a, 40inV V= and the output current is 5oI A= . The switching frequency 200sf kHz= . The MOSFET switching times are 15 and 15ri fvt ns t ns= = . The diode snaps-off at reverse recovery such that

20rr at t ns= = (such that 0bt = ) and the peak reverse-recovery current 2RRMI A= . Calculate the additional power loss in the MOSFET due to the diode reverse recovery.

Figure 2A-2 Waveforms with diode reverse-recovery current.

DSv

0

0

0 t

t

t

rit

inV

fvt

a rrt t=

oI

Di

RRMI

swp

fvt

DSv

0

0

0 t

t

t

rit

inV

fvt

a rrt t=

oI

Di

RRMI

swp

fvt