AN609 View Datasheet(PDF) - Vishay Semiconductors
Part Name
Description
Manufacturer
AN609 Datasheet PDF : 9 Pages
| |||
AN609
Vishay Siliconix
T(Junction) RT1
RT2
RT3
RT4 T(Ambient)
CT1
CT2
CT3
CT4
Figure: 2: Tank Circuit Configurations
T(Junction) RF1
RF2
RF3
RF4 T(Ambient)
CF1
CF2
CF3
CF4
Figure: 3: Filter Circuit Configurations
GND
We can observe from the following expression that,
mathematically, the "tank" configuration is very easy to
develop.
RT
(t)
=
R1 (t )
* ⎜⎜⎝⎛1 −
e⎜⎝⎛
−
t
τ1
⎟⎠⎞
⎟⎟⎠⎞
+
R2
(t)
* ⎜⎜⎝⎛1 −
e⎜⎝⎛
−
t
τ2
⎟⎠⎞
⎟⎟⎠⎞
Cx
=
τx
Rx
+
R3 (t)
* ⎜⎜⎛1 −
e⎜⎝⎛ −
t
τ3
⎟⎠⎞
⎟⎞
⎟
+
R4
(t)
* ⎜⎜⎛1 −
e⎜⎝⎛ −
t
τ4
⎟⎠⎞
⎟⎞
⎟
⎝
⎠
⎝
⎠
However, the "filter" configuration requires a complex
approach. That is where a linear square integration rou-
tine is employed in the curve fitting to obtain R-C values
in both configurations. Refer to Appendix A for an exam-
ple of R-C models developed using this approach.
Appendix A is also an illustration of the R-C thermal
models offered under the product information menu on
the Vishay Web site. The model validation can be
observed from close-fitting curves: one curve for the raw
data obtained during part characterization and another
curve that is produced by a curve-fitting routine using
model R-C values. Accordingly, this thermal model rep-
resented by R-C electrical parameters can be used for a
thermal analysis of a power MOSFET as discussed in
the following example.
THERMAL SIMULATION EXAMPLES
(a) Example 1
Aim: Demonstrate the use of a junction to ambient (j-a)
model and verify that the junction temperatures obtained
by each model configuration are comparable.
We shall estimate the junction temperature of Vishay
MOSFET part number Si7390DP with the dissipating
power profile described in Table 2.
Table 2
Time (sec)
0
0.0001
0.00099
0.001
0.002
0.0021
0.005
0.0051
0.1
Power (Watts)
0
100
100
0.1
0.1
20
20
0
0
Repeating after every 500 milliseconds.
Ambient temperature = 25 °C
The power profile described in Table 2 can produce high
temperature excursions during each cycle and a cumu-
lative temperature rise over a period of 500 milliseconds,
which is long enough to produce a temperature rise
beyond the part package. Hence, a junction-to-ambient
model is useful for this analysis.
Here are the steps to set up and run simulations:
Step 1
Obtain R-C thermal model file Si7390DP_RC from the
Vishay Web site; see Appendix A.
Step 2
Start a new project on the OrCAD Capture/P-Spice plat-
form, create a new design folder Tank j-a, and add a
schematic page as shown in Figure 4, Tank Configura-
tion.
Junction Temperature
R1
R2
R3
2.0964
9.0489
8.0177
C1
C2
C3
24.2346m
558.4598m 70.7118m
I2
C:\Si7390DP_RC\100W50mSReverse.txt
Power Profile
I
R4
50.6167
C4
1.4427
V1
25Vdc
Ambient
0
Figure 4: Tank Configuration
Use R-C thermal model values for ambient temperature
from the Table 3 R-C values for the tank configuration
from Appendix A.
Document Number 73554
07-Oct-05
www.vishay.com
3
Share Link: 