EET426 Power
Electronics II
Thermal Management
Prepared by : Mohd Azrik Roslan
EET 426 – Power Electronis II
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What you should know after this lecture
Thermal basic
Dissipated power vs junction temperature
Thermal management
Mosfet
Schottky
EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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Thermal Basics
EET 426 – Power Electronis II
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ELECTRICAL
voltage
potential difference
current
power
conductivity
resistivity
EET 426 – Power Electronis II

V
DV
I
P


ANALOGY

T
DT
PD
Q
 th
th
THERMAL
temperature
temperature difference
power dissipated
heat
thermal conductivity
thermal resistivity
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IDEAL WORLD
PD,max
PD
REAL WORLD
TJ,max
Tambient
PD = dissipated power
slope 
1
RTH ( J  A )
T
  J A
RTH = Thermal Resistance
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PD,max
slope  RTH , J  A   J  A  PD 
PD
slope  RTH , J  A   J  A  PD 
Tambient
EET 426 – Power Electronis II
TJ,max
T
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PD,max
PD
f n ( RTH , J  A)
Tambient
EET 426 – Power Electronis II
TJ,max
T
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f n (external _ system)
f n (device _ package)
PD,max
PD
f n ( RTH , J  A)
Tambient
EET 426 – Power Electronis II
TJ,max
T
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DEVICE PD / T
PD
Tambient
T
NOMINAL OPERATING POINT
SYSTEM HEAT REMOVAL = DEVICE DISSIPATION
dPrem oval dPD

dT
dT
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RTH ( J  A)
LOW
 TH ( J  A)
HIGH
dPremoval dPD

dT
dT
SYSTEM HEAT REMOVAL
SYSTEM HEAT REMOVAL > PDEVICE
PD
DEVICE PD / T
TJN,op
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T
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RTH ( J  A)
HIGH
 TH ( J  A)
LOW
dPrem oval dPD

dT
dT
DEVICE PD / T
SYSTEM HEAT REMOVAL < PDEVICE
PD
SYSTEM HEAT REMOVAL
TJN,op
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T
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Non-linear characteristic
dPrem oval dPD

dT
dT
PD
dPremoval dPD

dT
dT
Tambient
EET 426 – Power Electronis II
DEVICE PD / T
T
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dPrem oval dPD

dT
dT
DEVICE PD / T
PD
Tambient
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T
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Thermal instability
stable
boundary
unstable
DEVICE PD / T
PD
T
Tambient
TJN,op1
dPrem oval dPD

dT
dT
EET 426 – Power Electronis II
TJN,op2
dPrem oval dPD

dT
dT
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non-linear power dissipation
vs
device junction temperature
curve has a boundary
operational junction temperature
above which
power dissipation > the removal capability resulting in thermal instability
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Schottky off-state and mosfet on-state
exhibit non-linear power dissipation curves
versus device junction temperature
and
have a boundary operational junction temperature
above which
power dissipation exceeds the removal capability
resulting in thermal instability
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Thermal Management
Mosfets
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Tj
Rth,j-c
Tc
Rth, j c
DT TJ  Tc


P
P
D
Rth,c-s(case)
Ts
Rth,s-a(heatsink)
Ta
ELECTRICAL
voltage
potential difference
current
power
conductivity
resistivity
EET 426 – Power Electronis II

V
DV
I
P


ANALOGY

T
DT
PD
Q
 th
th
THERMAL
temperature
temperature difference
power dissipated
heat
thermal conductivity
thermal resistivity
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HIGHER VOLTAGE RATED MOSFETS

LARGER RDSON

LARGER Pconduction loss
HIGHER VGS DRIVE

LOWER RDSON
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RDSON

TENDS to be INDEPENDENT of IDS
if HIGH VGS DRIVE
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RDSON value
RDSON INCREASES at HIGHER TJN op

LARGER Pconduction loss
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RDSON
NORMALISED
value
RDSon  RDS,normalised  RDS,25o C
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PD  ON  STATE LOSS   SWITCHING LOSS   INTERNAL DIODE LOSS 
+ gate charge loss
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PD  ON  STATE LOSS 
Irms 2 RDS on
fn (T)
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Thermal Runaway
Thermal runaway refers to a situation where
an increase in temperature changes the
conditions in a way that causes a further
increase in temperature, often leading to a
destructive result.
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TJN
fn (TJN)
fn (PD)
fn (RDSon)
TJN
RDSon
PD
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transcendental equations
resulting from the non-linear characteristics
requires graphical or computer solutions
to avoid laborious iteration
Mosfet on-state loss Irms2 RDS,on
is the problem due to the non-linear
RDS,on versus temperature characteristic
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Solving Transcendentals
Iterative Process
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ASSUME TJN op < TJN max
RDSon @ TJN op
conservative
re SELECT
HIGHER TJN op
data sheet
calculate PD @ TJN op
SELECT Tambient
re SELECT
LOWER TJN op
Y
RTH J-A data sheet
TJN << TJN op
calculate D TJ-A
calculate TJN
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Y
TJN > TJN op
N
OK
N
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Mosfet conduction loss
THERMAL INSTABILITY
THERMAL STABILITY
DETERMINE
THERMAL STABILITY
BOUNDARY
T (oC)
Tamb
TJN,op < TJN,boundary
EET 426 – Power Electronis II
TJN,boundary
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Mosfet conduction loss
slope 
1
RTH ( J  A )
  J A
Pop
T (oC)
Tamb
TJN,op
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LINE PLOTTING : JUNCTION-CASE
Tjunction
Rth,j-c
Tcase
DP
Rth,c-s(contact)
Tsink
DT
Rth,s-a(heatsink)
Tamb
T (oC)
DT  5 oC
Rth  1 oC / W
DP 
Tcase
TJN,op
slope 
1
RTH ( jn  case )
DT
 5W
Rth
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LINE PLOTTING: CASE-AMBIENT
Tjunction
Rth,j-c
Tcase
DP
Rth,c-s(contact)
Tsink
PD,op
DT
Rth,s-a(heatsink)
Tamb
T (oC)
DT  20 oC
Rth  5 oC / W
DP 
TJN,op
Tamb
DT
 4W
Rth
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slope 
1
RTH (case amb )
Tcase
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Thermal Management
Schottky Rectifiers
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Schottky Rectifiers
VF  forward voltage drop
TJN  TJ  junction temperature
TJN ,OP  junction temperature operating point
TJN , MAX  maximum junction temperature
I F  forward current
VR  reverse voltage
I R  reverse leakage current
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SCHOTTKY RECTIFIER
IF
2
Vf ( Tj)  a Tj  b Tj  c
VF
T
EET 426 – Power Electronis II
TJmax
VF
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SCHOTTKY RECTIFIER
VF
IF
T
TJmax
VF
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SCHOTTKY RECTIFIER
VF
T
TJmax
Pcon, schottky  VF ,on  I ave  VF ,on  Don, schottky  I out
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SCHOTTKY RECTIFIER
IR mA
IR
T
TJmax
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VR
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SCHOTTKY RECTIFIER
IR
T
TJmax
Prev, schottky  Vrev  I rev ,ave  Vrev  (1  Don, schottky)  I out
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SCHOTTKY RECTIFIER
ON-STATE
predominant
EET 426 – Power Electronis II
OFF-STATE
predominant
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TJN
fn (TJN)
fn (Prev)
fn (Irev)
TJN
IREV
Prev
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transcendental equations
resulting from the non-linear characteristics
requires graphical or computer solutions
to avoid laborious iteration
Schottky reverse leakage current
versus temperature
has an exponentially rising characteristic
that creates the problem
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EET 426 – Power Electronis II
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SCHOTTKY TOTAL LOSS
THERMAL INSTABILITY
DETERMINE
THERMAL STABILITY
BOUNDARY
THERMAL STABILITY
T (oC)
Tamb
TJN,op < TJN,boundary
EET 426 – Power Electronis II
TJN,boundary
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20 oC margin is a rule of thumb
“Thermal stability requires a heat removal capability
that is greater than the heat dissipation”
SCHOTTKY operational junction temperature
should be lower than the temperature
at the tangent from Tambient to the PSCH1 curve
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D1
L
NP
NR
NS
D2
C
R
Vout
Ein
DR
SCH2 conduction loss
P
SCH1conduction loss
Schottky rectifier
forward loss curves
reduce with
junction temperature increase
due to the reduction in
forward voltage drop
Psc ,on (T )  Dsch  I out  Vak (T )
T
Dsch,1  Dsw
EET 426 – Power Electronis II
Dsch,2  (1  Dsw )
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D1
L
NP
NR
NS
D2
C
R
Vout
Ein
DR
SCH1 reverse loss
Schottky rectifier
reverse loss curves
increase exponentially
with junction temperature
P
SCH2 reverse loss
T
EET 426 – Power Electronis II
Psc,off (T )  (1  Dsch ) Vrev  I rev (T )
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17.5 (W)
P (W)
SCH2
SCH1 Power dissipation
starts the upward rise at lower temperature
and has a much greater increase with temperature
hence this curve determines
the designers operational boundary.
SCH1
3.5 (W)
T (oC)
Tamb
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Tbdy
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below Teff,msax the Schottky losses are
predominantly on-state losses and
the combined ‘on’ and ‘off’ losses
exhibit a dropping loss curve as
temperature increases due to the
reduction in Schottky forward voltage
drop
with
rise
in
junction
temperature
efficiency
Ptotal
PSCH2
PSCH1
Pmos
Teff,max
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combined forward and reverse losses
usually exhibit a dropping loss curve
at lower junction temperatures
where the conduction losses are
predominant. As the junction
temperature rises and reverse loss
starts to increase faster than the
conduction loss falls the combined
curve then starts an upward path
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P1tot + P2tot
17.5 (W)
SCH2
19.3 W
P2tot
15.8 W
P (W)
SCH1
P1tot
3.5 W
3.5 (W)
T (oC)
Tamb
EET 426 – Power Electronis II
Tjn1,op
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THERMAL IMPEDANCE
THERMAL RESISTANCE
POWER PULSE DURATION
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Determine a single heat sink thermal management design for
the mosfet and Schottky rectifiers operating at 50oC ambient
temperature and with a minimum 20oC Schottky junction
temperature boundary margin.
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(W)
Ptot,SCH2
Pop , SCH 2  9W
Ptot,SCH1
Pmosfet
T jn , SCH 2  140C
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Tjunction
Finding Tsink
Tsink,SCH2  T jn , SCH 2   Pop , SCH 2  Rth , j  s , sch 
 140   9  2 
 122C
Rth,j-c
Tcase
Rth,c-s(contact)
Tsink
Rth,s-a(heatsink)
Tamb
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(W)
Ptot,SCH2
Pop , SCH 2  9W
Ptot,SCH1
Pop , SCH 1  4W
Pmosfet
Tsink, SCH 2  122C
T jn , SCH 2  140C
T jn , SCH 1  130C
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Tjunction
Finding junction temperature for mosfet
o
Based on Rth,ju-sink =0.42 C/W
0.42C
1W
Difficult to draw line  too small
What if we try to find how much power change if
temperature increase by 5oC
Rth,j-c
Tcase
Rth,c-s(contact)
Tsink
Rth,s-a(heatsink)
Tamb
0.42C 5C

1W
xW
5 1
x
 11.9 W
0.42
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(W)
Ptot,SCH2
Pop , SCH 2  9W
Pop ,mosfet  4.4 W
Ptot,SCH1
Pop , SCH 1  4W
Pmosfet
Tsink, SCH 2  122C
T jn , SCH 2  140C
T jn , SCH 1  130C T jn ,mosfet  124C
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Find Ptotal
Ptotal  Pop, sch1  Pop, sch 2  Pop ,mosfet
 4  9  4.4
 17.4W
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Rth , s  amb
Tsink  Tamb

Ptotal
122  50

17.4
 4.14 C W
EET 426 – Power Electronis II
Tjunction
Rth,j-c
Tcase
Rth,c-s(contact)
Tsink
Rth,s-a(heatsink)
Tamb
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(W)
Ptotal  17.4W
Ptot,SCH2
Pop , SCH 2  9W
Pop ,mosfet  4.4 W
Ptot,SCH1
Pop , SCH 1  4W
Pmosfet
Tamb  50C
Tsink, SCH 2  122C
T jn , SCH 2  140C
T jn , SCH 1  130C T jn ,mosfet  124C
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Q2) Thermal Management
Rth,junction-case
1.5 oC / W
Rth,case-sink
0.5 oC / W
Determine the common heat sink design requirement for thermal
management of the Schottky rectifiers at an ambient temperature
of 50 oC if the operating junction temperature of SCH1 is 125 oC.
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Pop , SCH 1  4W
T jn , SCH 1  125C
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Finding Tsink
Tsink,SCH1  T jn , SCH 1   Pop , SCH 1  Rth , j  s 
Rth,junction-case
1.5 oC / W
Rth,case-sink
0.5 oC / W
 T jn , SCH 1   Pop , SCH 1   Rth , j c  Rth ,c  s  
 125   4  1.5  0.5  
 125  8  117C
Tjunction
Rth,j-c
Tcase
Rth,c-s(contact)
Tsink
We want to make sure that
Tsink,SCH1  Tsink,SCH2  117C
EET 426 – Power Electronis II
Rth,s-a(heatsink)
Tamb
67
Pop , SCH 2  9W
Pop , SCH 1  4W
Tsink  117C
T jn , SCH 1  125C T jn , SCH 2  135C
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Tjunction
Verify
Tsink,SCH2  T jn , SCH 2   Pop , SCH 1  Rth , j  s 
 T jn , SCH 1   Pop , SCH 1   Rth , j c  Rth ,c  s  
 135  9  1.5  0.5  
 135  18
 117C
EET 426 – Power Electronis II
Rth,j-c
Tcase
Rth,c-s(contact)
Tsink
Rth,s-a(heatsink)
Tamb
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Find Ptotal
Ptotal  Pop, sch1  Pop, sch 2
 49
 13W
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Rth , s  amb
Tsink  Tamb

Ptotal
117  50

13
 5.15 C W
EET 426 – Power Electronis II
Tjunction
Rth,j-c
Tcase
Rth,c-s(contact)
Tsink
Rth,s-a(heatsink)
Tamb
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Ptotal  13W
Pop , SCH 2  9W
Pop , SCH 1  4W
Tamb  50C
Tsink  117C
T jn , SCH 1  125C T jn , SCH 2  135C
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