AN609 View Datasheet(PDF) - Vishay Semiconductors
Part Name
Description
Manufacturer
AN609 Datasheet PDF : 9 Pages
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AN609
Vishay Siliconix
Thermal Simulation of Power MOSFETs on the P-Spice Platform
Author: Kandarp Pandya
INTRODUCTION
R-C thermal model parameters for Vishay power
MOSFETs available under the product information
menu offer a simple means to evaluate thermal behav-
ior of the MOSFET under a defined transient operating
condition.
Steady state values of thermal impedance, Rth(j-a) and
Rth(j-c) / Rth(j-f), along with normalized thermal tran-
sient impedance characteristics published in a power
MOSFET datasheet, are adequate to analyze the ther-
mal behavior of a part under a regular wave-shaped,
single pulse or the periodic power dissipation of known
duty cycle.
However, thermal analysis for transient or irregular
wave-shaped power profiles requires extending the
thermal characterization offered in the datasheet.
What is really needed is a thermal model emulating the
thermal transient behavior of the power MOSFET on a
suitable software platform. The thermal transient
impedance characteristics published in a datasheet
are a net effect of the thermal resistance and thermal
capacitance of the physical structure of a device.
Hence the latter can be used for developing a thermal
model for the part, but it is necessary to parameterize
the thermal characteristics. Incidentally, there exists a
direct behavioral analogy between thermal compo-
nents / parameters and electrical components / param-
eters; see Table 1.
Table 1
Description
Ohm’s law analogy
Resistance
Potential
Energy flow
Capacitance
Electrical
R=V/I
R - Electrical
Resistance in Ohms
V - Electrical
Potential
Difference in Volts
I - Electrical
Current in Ampere
C - Electrical
Capacitance
Thermal
Rth = °C / W
Rth - Thermal
Resistance in °C / W
°C - Temperature
Difference in Celsius
W - Power
Dissipation in Watts
Cth - Thermal
Capacitance
Using the analogy described above, we can use elec-
trical simulation software like P-Spice to analyze ther-
mal behavior by using the corresponding parameters.
This requires a means to obtain the electrical parame-
ters equivalent to the corresponding thermal parame-
ters. Typical thermal characteristics as represented in
a datasheet of a power MOSFET are shown in Figure 1.
Curve-fitting techniques can be applied to such time -
varying thermal characteristics while using a combina-
tion of electrical resistances (R) and capacitances (C)
in pairs as variable parameters. The nature of transient
thermal characteristics for power MOSFETs necessi-
tates at least four R-C pairs to obtain the best curve fit.
These R-C pairs in turn represent thermal model
parameters.[1] These pairs can be used on the same
OrCAD Capture and P-Spice platform to run electrical
simulations and carry out thermal analyses by cor-
rectly employing the analogy discussed earlier.
Document Number 73554
07-Oct-05
www.vishay.com
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