3-5
Thermal Resistance vs Mounting Pad Area
The maximum rated junction temperature, TJM, and the
thermal resistance of the heat dissipating path determines
the maximum allowable device power dissipation, PDM,inan
application.Therefore the application’s ambient temperature,
TA (oC), and thermal resistance RθJA (oC/W) must be
reviewed to ensure that TJM is never exceeded. Equation 1
mathematically represents the relationship and serves as
the basis for establishing the rating of the part.
In using surface mount devices such as the SO-8 package,
the environment in which it is applied will have a significant
influence on the part’s current and maximum power
dissipation ratings. Precise determination of the PDM is
complex and influenced by many factors:
1. Mounting pad area onto which the device is attached and
whether there is copper on one side or both sides of the
board.
2. The number of copper layers and the thickness of the
board.
3. The use of external heat sinks.
4. The use of thermal vias.
5. Air flow and board orientation.
6. For non steady state applications, the pulse width, the
duty cycle and the transient thermal response of the part,
the board and the environment they are in.
Intersil provides thermal information to assist the designer’s
preliminary application evaluation. Figure 13 defines the
RθJA for the device as a function of the top copper
(component side) area. This is for a horizontally positioned
FR-4 board with 2 oz. copper after 1000 seconds of steady
state power with no air flow. This graph provides the
necessary information for calculation of the steady state
junction temperature or power dissipation. Pulse
applications can be evaluated using the Intersil device Spice
thermal model or manually utilizing the normalized maximum
transient thermal impedance curve.
Displayed on the curve are RθJA values listed in the
Electrical Specifications table. These points were chosen to
depict the compromise between the copper board area, the
thermal resistance and ultimately the power dissipation,
PDM. Thermal resistances corresponding to other
component side copper areas can be obtained from Figure
13 or by calculation using Equation 2. The area, in square
inches is the top copper area including the cathode pad
area.
FIGURE 11. AVALANCHE ENERGY TEST CIRCUIT FIGURE 12. AVALANCHE CURRENT AND VOLTAGE
WAVEFORMS
Test Circuits and Waveforms (Continued)
DUT
CURRENT
SENSE +
LR
VDD
L = 20mH
R < 0.1Ω
EAVL = 1/2LI2 [VR(AVL)/(VR(AVL) - VDD)]
Q1= IGBT (BVCES > DUT VR(AVL))
-
VDD
Q1
IV
t0t1t2
IL
VAVL
t
IL
(EQ. 1)
PDM TJM TA
–()
ZθJA
-----------------------------=
RθJA, THERMAL IMPEDANCE
50
100
150
200
CATHODE MOUNTING AREA, TOP COPPER AREA (in2)
0.1 1.0
177oC/W - 0.054in2
217oC/W - 0.0123in2
FIGURE 13. THERMAL RESISTANCE vs MOUNTING PAD
AREA
0.001
350
250
JUNCTION TO AMBIENT (oC/W)
300
0.01
RθJA = 101.6 - 25.82 x ln(AREA)
(EQ. 2)
RθJA 101.6 25.82 Area()ln×–=
RHR1K160