Presented by:
David Divins
at
October 2007
1
Using Simulation to Estimate
MOSFET Junction Temperature in
a Circuit Application
Presented by:
David Divins
Senior Staff Field Applications Engineer
International Rectifier
ddivins1@irf.com
October 2007
2
Agenda
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
October 2007
Definition of Electro-Thermal Simulation
Simulation Tools and Methods
Methods of Estimating Die Temperature
Creating Quasi-Dynamic MOSFET Model
Model Generation
Example Application
Conclusion
3
Electro-Thermal Simulation
ƒ Purpose of Electro-Thermal Simulation is to predict
MOSFET junction for a given application.
Heatsink
Package
FR4 Board
October 2007
Die
Solder joint
4
Electro-Thermal Simulation
ƒ Applications
‰
‰
‰
‰
‰
‰
October 2007
Solenoid drivers
Motor drive
Lighting ballast
DC/DC converters
Switch model power supplies
Class D amplifiers
5
Simulation Tools and Methods
ƒ Tools
‰ Simplorer (Ansoft) - Circuit/System simulator with VHDLAMS hardware description language
‰ Saber (Synopsis) - Circuit/System simulator with VHDLAMS and MAST hardware description languages
‰ Spector (Cadence) - Circuit/System simulator with Verilog-A
hardware description language
‰ PSPICE (Cadence) – Defacto standard in circuit simulation.
October 2007
6
Simulation Tools and Methods
ƒ Method
‰ Implementing model in the hardware description language
‰ Implementing model using equations and macro modeling
library ieee; use ieee.std_logic_1164.all;
use ieee.electrical_systems.all;
+
entity comparator is
port ( terminal a : electrical;
signal d : out std_ulogic );
end entity comparator;
----------------------------------------------------------------
A
+
Source
dr
V
irfr9024
10
10
Gate
Rdson
comparator_behavior : process is
begin
if vin > ref_voltage / 2.0 then
d <= '1' after 5 ns;
else
d <= '0' after 5 ns;
end if;
wait on vin'above(ref_voltage / 2.0);
end process comparator_behavior;
end architecture ideal;
October 2007
Drain
Id
Vgs
+
architecture ideal of comparator is
constant ref_voltage : real := 5.0;
quantity vin across a;
begin
V
Vds
Ta
V
Equations
10
10
PWR
EQU
Tj
FCT_ABS1
10
Vdst
117.73m K/W
601.26m K/W
dr
PWR
Probe
RTH1
H
Abs
PWR
RTH2
1.38m Ws/K
Θ
CTH3
211.98u Ws/K
H1
2.01 K/W
RTH3
CTH2
32.33m Ws/K
T1
Ta aC
CTH1
7
Simulation Tools and Methods
ƒ Quasi-Dynamic MOSFET model implementation
‰ Macro modeling with use of linking equations
‰ Using a multi domain simulator that allows for ElectroThermal simulation.
• Volts and Amps
• Heat flow (Watts) and temperature
+
H
Thermometer
October 2007
Heat
V
Voltmeter
A
Ammeter
8
Methods of Estimating Die Temperature
ƒMethods of estimating MOSFET die junction temperature
‰ Equation based + Thermal Impedance curve
P = I ∗V ∗ D
Where:
I = average current during the conduction cycle
V = equivalent voltage across the device during
the conduction cycle
D = duty cycle
October 2007
9
Methods of Estimating Die Temperature
ƒ
ƒ
ƒ
Use Power calculated with P=I*V*D
Use pulse width and duty cycle to determine Zth (thermal impedance)
from device thermal impedance curve
Temperature rise (ΔTjunction)= Zth * P
October 2007
10
Methods of Estimating Die Temperature
ƒ Limitations of the equation based junction temperature
estimate
‰ Only temperature rise from junction to case is taken into account.
Neglects case to ambient temperature rise.
‰ Assumes the power pulse is an ideal square edged pulse train.
‰ It does not allow for transient thermal response.
Temperature
Rise Calculated
Junction
October 2007
Case
Ambient
Case to Ambient Temperature
Rise Not calculated
11
Methods of Estimating Die Temperature
ƒ Simulator based MOSFET junction temperature estimate
‰ Uses circuit simulation to calculate junction temperature in an
application
‰ The circuit can be arbitrary
‰ Transient thermal response is calculated
‰ Component parameters change with temperature
FET Junction Temperature
FET Power
Load Current
150.00
182.47
1.50k
1.00k
100.00
IRF28...
Bus.I [A]
A
500.00
100.00
IRF28...
C
W
Gate Voltage
10.05
Vgs.V ..
5.00
0
-50.00m
0
0
200.00m
2.00m
50.00
4.00m
25.00
0
Vds2 +
D1
+
V
V
V
Vsw
Name
trig
Ipk
Iss
Vbus
R1
R2
Ig
Vds1
30.60
20.00
Vds2....
0
+
AM1
0
V
IRF2804S_7P_Therm
IRF2804S_7P_Therm1
A
200.00m
Vds1
STATE2
TRANS1
R2
(C)
150.00
-39.20m
Tj.MAX
0
0
Load_current
0
TR
Probe
C1
27.80
(V)
A
-2.44
192.00
200.00m
A
0
100.00
-20.00n
0
100.00m
200.00m
Measured Values
200.00
Name
Bus.I [A]
Load_current.MAX
Temp_Rise
100.00
Load_...
167.65
200.00m
8.00u
500.00u
A
1.00m
Experiment Table
Tj
368.00
300.00
180.00
Vds1....
0
C
191.80
Vds2
1.40
0
N0009.V
10.00
1.50u
200.00m
Peak Load Current vs Gate Drive Current
EQU
Load Current
100.00m
200.00m
Equations
Driver
Load Voltage
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
Vgs.V ..
5.00
Peak Tj for IRF2804_7P
Tj
TR
Probe
R1
4.35m
Gate Voltage vs. Ig
10.05
Parameters
ICA:
STATE1
Value
1.00m
200.00
40.00
28.00
175.00m
700.00m
100.00u
Value
42.76
186.82
148.25
RunNo
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
Ig
10.00u
16.68u
27.83u
46.42u
77.43u
129.16u
215.44u
359.38u
599.48u
1.00m
Load_current.MAX
167.68
174.47
179.20
182.85
185.65
187.68
189.41
190.50
191.25
191.71
Tj.MAX
366.02
314.35
271.71
207.12
156.68
116.89
83.11
63.17
60.75
60.25
FET Junction Temp Rise vs. Gate Current
366.25
300.00
C
Rdrv
2.00m
Parameters
Vbus V
(V)
I1
-20.00n
50.00m72.28m
Bus
IRF2804S_7P_Therm
Vgs +
25.00m
0
IRF2804S_7P_Therm
200.00
Tj.MAX
60.00
10.00u 30.00u
100.00u200.00u
A
1.00m
25.00
0
October 2007
100.00m
200.00m
12
Methods of Estimating Die Temperature
ƒ Assumptions made for junction temperature estimates
using simulation
‰ No other source of heat considered (Temperature rise due
to self heating only)
‰ Only MOSFET RDS(on) and threshold voltage changes with
temperature
‰ Since simulation solves Ordinary Differential the junction is
assumed to be a point source of heat.
October 2007
13
Creating Quasi-Dynamic Thermal MOSFET Model
ƒ Gathering information:
‰ 25C Spice Model of MOSFET
‰ Datasheet information
• RDS(on) vs. Temperature curve
• Thermal Impedance Curve
with thermal RC ladder
network
.SUBCKT irf1404 1 2 3 * SPICE3 MODEL WITH
THERMAL RC NETWORK
************************************** * Model Generated
by MODPEX * *Copyright(c) Symmetry Design
Systems* * All Rights Reserved * * UNPUBLISHED
LICENSED SOFTWARE * * Contains Proprietary
Information * * Which is The Property of * *
SYMMETRY OR ITS LICENSORS * *Commercial Use
or Resale Restricted * * by Symmetry License
Agreement * ************************************** * Model
generated on April 2, 01 * MODEL FORMAT: SPICE3
* Symmetry POWER MOS Model (Version 1.0) *
External Node Designations * Node 1 -> Drain * Node
2 -> Gate * Node 3 -> Source M1 9 7 8 8 MM L=100u
W=100u .MODEL MM NMOS LEVEL=1 IS=1e-32
+VTO=3.74133 LAMBDA=0.00250986 KP=514.947
+CGSO=7.17952e-05 CGDO=1.60578e-08 RS 8 3
0.00282867 D1 3 1 MD .MODEL MD D IS=1.89845e10 RS=0.00218742 N=1.20398 BV=40 +IBV=0.00025
EG=1.2 XTI=1.85712 TT=2.00014e-05
+CJO=5.42237e-09 VJ=2.67939 M=0.566441 FC=0.1
RDS 3 1 1e+06 RD 9 1 0.000681391 RG 2 7 3.16781
D2 4 5 MD1 * Default values used in MD1: * RS=0
EG=1.11 XTI=3.0 TT=0 * BV=infinite IBV=1mA
.MODEL MD1 D IS=1e-32 N=50 +CJO=3.13813e-09
VJ=0.970446 M=0.823421 FC=1e-08 D3 0 5 MD2 *
Default values used in MD2: * EG=1.11 XTI=3.0 TT=0
CJO=0 * BV=infinite IBV=1mA .MODEL MD2 D IS=1e10 N=0.4 RS=3e-06 RL 5 10 1 FI2 7 9 VFI2 -1 VFI2 4
0 0 EV16 10 0 9 7 1 CAP 11 10 7.84089e-09 FI1 7 9
VFI1 -1 VFI1 11 6 0 RCAP 6 10 1 D4 0 6 MD3 *
Default values used in MD3: * EG=1.11 XTI=3.0 TT=0
CJO=0 * RS=0 BV=infinite IBV=1mA .MODEL MD3 D
IS=1e-10 N=0.4 .ENDS irf1404
October 2007
14
Creating Quasi-Dynamic Thermal MOSFET Model
ƒ 25C Spice Model
‰ Characterized to the datasheet
‰ Does not change performance characteristics as power is
calculated
‰ Used as base model for Quasi-Dynamic MOSFET model
October 2007
15
Creating Quasi-Dynamic Thermal MOSFET Model
MOSFET
Model in
Simulation
Power
Calculation
Temperature (Tj)
Thermal
Network
Rdson=F(Tj)
Vth=F(Tj)
October 2007
16
Model Generation
ƒ Ladder Network
‰ A thermal RC network used to model the dynamic thermal behavior
of the package + mounting system.
117.73m K/W
PWR
Probe
RTH1
H
Abs
RTH2
1.38m Ws/K
Θ
CTH3
211.98u Ws/K
H1
RTH3
CTH2
32.33m Ws/K
T1
Ta aC
CTH1
Package + Mounting
Equivalent RC Ladder Network
The ladder network can be synthesized from the thermal impedance curve
or is given by the MOSFET manufacturer
October 2007
17
Model Generation
ƒ Tying the thermal model to the 25C Spice model
‰ Create the equation that represents RDS(on) vs. temperature
dRdson(Tj )
= Rdson(25C ) * (2 * a * Tj + b)
dTj
dRdson = Rdson ( 25C ) * ( 2 * a * Tj + b) * (Tj − 25)
This expression gets
implemented in the model
Note: a, b and c are calculated via a curve fitting routine.
The Rdson vs Temperature curve is assumed to be quadratic.
October 2007
18
Model Generation
ƒ Create the voltage source that represents the
temperature dependence of Vth (threshold voltage)
Vth (Tj ) = −0.007 * (Tj − 25)
Expression used in model.
The voltage source is in series with the MOSFETs gate.
October 2007
19
Model Generation
ƒCalculating the power in the MOSFET for use in the
thermal network.
P = Id * Vds
This calculated power is the source for the thermal
network.
October 2007
20
Model Generation
ƒ Putting it together
Drain
dr
+ Vds
irf2804s7p
Vth
irf2804s7p1
V
+
Vdst
Tj 10
V
Ta 10
Gate
PWR 10
Source
Tj
PWR
Probe
PWR
Abs
H
305.073m K/W
0.195105 K/W
RTH1
RTH2
RTH3
Θ
H1
PWR_abs
3.808m Ws/K
CTH1
26.941m Ws/K
T1
Ta aC
CTH2
Equations
EQU
dt:=Tj.T-Ta
Tj:=Tj.T-273.15
if(Vds.V<0.1) {Rdson25:=abs(Vds.V/dr.I)} else {dr:=1m}
if(Vds.V<0.1) {dr:=(7.41u*Tj+3.519m)*dt*Rdson25} else {dr:=1u}
PWR:=PWR_abs.VAL
Vth:=-7m*(Tj-25)
October 2007
21
Model Generation
ƒ Final model
‰ 25C Spice model
‰ Added voltage source Vth in gate implements Vth(Tj)
‰ dr implements
‰ Vds and the current in dr are used to calculate RDS(on) 25C
‰ Vdst and the current in dr is used to calculate the total power
‰ PWR_abs is used to insure that the thermal network is driven with
positive power.
October 2007
22
Example Application
ƒ High side switch
ƒ MOSFET being driven by a opto isolated drive
‰ Very low drive current capability
ƒ Load is capacitive
ƒ Issue: How does driving this load effect the junction
temperature of the MOSFET
October 2007
23
Example Application
ƒ Simulation Schematic
Vbus
108 V
W
R1
1k Ohm
Vcc
24 V
C1
A2
C2
STATE2
S1
TRANS1
PVI1050N
IRFP4232
A1
STATE1
IRFP4232_Therm
Ig
Vop1
Case
A
Von1
N0052
Vop2
Heatsink
Von2
PVI1050N
Vgs
+
V
C1
400u F
October 2007
+
PWR_FET
R2
10 Ohm
24
Example Application
ƒ Assumptions
‰
‰
‰
‰
October 2007
Tambient=25C
Heak sink is modeled as just a thermal resistor
C1 & R2 represent a load system i.e. power supply
Ig, Vgs, PWR_FET, States 1 & 2, Trans1 and S1 are
measurements, input stimulus and ideal switch
25
Example Application
Gate Voltage, Junction Temperature, Power and Current
ƒ Results
15
MOSFET Current
(A)
10
5
0
1k
MOSFET Power
(W)]
500
0
15
Gate Voltage
(V)
10
5
0
150
Junction Temperature
(C)
100
50
0
0
October 2007
20.00m
40.00m
60.00m
80.00m
100.00m
120.00m
157.50m t [s]
26
Conclusion
ƒ Electro-Thermal simulation allows for analysis in both
electrical and thermal domains
ƒ Quasi-Dynamic Thermal MOSFET model allows for
self-heating to alter RDS(on) and Vth during simulation
as a function of temperature
ƒ Quasi-Dynamic Thermal MOSFET Model generation
is a data gathering task
ƒ The example shows why it is difficult to switch a
capacitive load with an opto-driver and a MOSFET
due to the excessive junction temperature spike
during turn-on.
October 2007
27