# Texas Instruments | Method of Graphing Safe Operating Area (SOA) Curves in DC-DC Converter | Application notes | Texas Instruments Method of Graphing Safe Operating Area (SOA) Curves in DC-DC Converter Application notes

```Application Report
SLVA766 – March 2016
Method of Graphing Safe Operating Area (SOA) Curves in
DC-DC Converter
Anousone Sibounheuang, Na Kong, and Kit Nguyen
ABSTRACT
This document describes how to graph the SOA curves with airflow in the DC-DC power supply converter.
To reduce the overall cost of a system, the converter solution reduces the printed-circuit-board (PCB) area
while maintaining the highest efficiency possible. These requirements limit the electrical and material
stresses of a component to operating safety over the wide range of temperature. As a result, the designer
must know the safe operating temperature range versus the maximum output loading in the switching
power supply converter.
1
Introduction
Any component in the power supply converter, such as a controller or a power conversion MOSFET, has
its own safety operating temperature range. The converter will fail if any component operates outside its
temperature range. As result, the SOA curves of a converter are serving as a design guideline
recommendation at a given operating temperature range.
This document shows how to generate the SOA curves with natural convection and air velocity linear feet
per minute (LFM) in the system. Figure 1 shows an example of typical SOA curves of the TPS546C23.
110
Ambient Temperature (qC)
100
90
80
70
Natural Convection
100 LFM
200 LFM
400 LFM
60
50
0
5
10
15
20
Output Current (A)
25
30
35
D001
Figure 1. TPS546C23 Typical SOA Curves
PMBus is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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1
Plotting SOA Curve Procedures
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2
Plotting SOA Curve Procedures
2.1
Natural Convection SOA Curve
2.1.1
Oven Temperature Unit Setup
A labview program interface is used to control both the evaluation board and the oven temperature. Figure
2.1 shows the front interface of the oven temperature (TESTEQUITY Model 115) and evaluation board
inside the oven. Figure 2.2 shows the upper chamber inside the oven which is used to provide the
programmed ambient temperature to the lower chamber. Figure 2.3 shows the thermal wire used to
regulate the programmed temperature. Figure 2.4 shows the evaluation board set up at the lower chamber
inside the oven and limits the hot air flowing directly on top of the board.
Upper Chamber has Fan,
Heater Element, and
Control Unit
Oven Test Unit Control,
EVM Inside
Block Air Flow
1
2
4
3
Hot Air Flows to the DUT
Chamber on This Side
Contains Thermal Wire
to Monitor the
Temperature
Figure 2. Evaluation Board Inside Oven Temperature Set Up
2.1.2
Junction Temperature Measurement:
An integrated circuit (IC) controller in the DC-DC converter is considered to be the heart of the switching
power supply. The junction temperature of this IC controller is normally designed to operate safety in the
–40°C to 125°C range. Therefore, it is necessary to have the SOA curves at the maximum junction
temperature at 125°C. Depending on the complexity of the controller, one method to measure the junction
temperature is through the body diode of the MOSFET at the power good pin. This method gives the
reference diode voltage at the junction, 125°C. Table 1 shows an example of the baseline of the diode
voltage versus the junction temperature. This junction temperature measures at the condition of no power
applied to the evaluation board and 10 minutes settling time for each temperature.
2
Method of Graphing Safe Operating Area (SOA) Curves in DC-DC Converter
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Plotting SOA Curve Procedures
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Table 1. Reference Junction Temperature
2.1.3
Junction Temperature (0°C)
Measured Diode (V)
25
0.57347
50
0.55329
85
0.51591
100
0.49423
125
0.45831
150
0.42045
Data Collection Setup
A labview program interface is used to collect the data as shown in Table 2. The procedures to collect the
data follow:
1. Set the ambient temperature (Ta): eq. at 25°C
2. Wait for 10 minutes for the temperature inside of the oven to reach the programmed temperature in
step 1.
3. Change the load current (IOUT): eq. start at 0 A
4. Wait for 10 minutes for thermal equilibrium, then record the data
5. Change the load current to another value: eq. at 5 A
6. Wait for 10 minutes for thermal equilibrium then record the data.
7. Repeat step 3 to 6 until at full load.
8. Repeat step 1 to 7 at each temperature.
Table 2. Natural Convection Data Collection Example
VIN (V)
VOUT (V)
IOUT (A)
FS (kHz)
Ta (°C)
12
1.0026253
20
500
12
1.0026383
20
500
12
1.0024828
20
12
1.0023922
20
12
1.0022756
12
12
Thermal
Couple (°C)
VDiode (V)
TPMBus (°C)
25
0.55469582
46.9
50
0.52804621
71.9
500
70
0.50639045
91.9
500
80
0.49322489
101
20
500
85
0.48643681
106.6
1.0020165
20
500
100
0.46480054
122
1.001887
20
500
105
0.45720211
127.1
12
1.0017575
20
500
110
0.44938101
133.1
12
1.0015373
20
500
120
0.43356414
143.1
12
1.0014207
20
500
125
0.42541352
148.9
VIN and VOUT are the input and output voltage to the evaluation board, respectively. IOUT is the output
loading of the converter. FS is the switching frequency for the converter. Ta is the ambient temperature
inside the oven. TPMBus is the junction temperature which measures using the digital method through
PMBus™ data lines. In the case of a controller without the PMBus capability, the junction temperature can
be measured through the thermal coupling wire placed on the top surface of controller device.
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Plotting SOA Curve Procedures
2.1.4
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SOA Curve Graphing Method
The first step must graph the relationship between VDiode and TPMBus columns as shown in Figure 3. With
the trend line of this graph, solve the VDiode at 125°C junction temperature. This calculated diode voltage
should be close to the measured diode voltage in Table 1 . The next step must graph the relationship
between VDiode and Ta columns as shown in Figure 4. Again, with the trend line of this graph, solve the
ambient temperature at 125°C junction temperature through the VDiode in the previous step. These two
graphing methods must repeat for all the output load ranges. At the end, one can graph the natural
convection SOA curve as shown in Figure 2. This SOA curve is considered as the worst case of the SOA
graph.
0.6
0.5
y = 0.0013x + 0.6202
VDiode (V)
0.4
0.3
0.2
0.1
20 A
Linear (20 A)
0
20
40
60
80
100
PMBus TJ (qC)
120
140
160
180
D002
Figure 3. Junction Temperature vs Diode Voltage
0.6
0.5
y = 0.0013x + 0.5934
VDiode (V)
0.4
0.3
0.2
0.1
20 A
Linear (20 A)
0
0
20
40
60
80
100
Ambient Temperature (qC)
120
140
160
180
D003
Figure 4. Ambient Temperature vs Diode Voltage
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2.2
2.2.1
Airflow SOA Curves
Airflow Unit Setup
Figure 5 shows the airflow tunnel setup. Figure 5.1 shows the setup which includes the fan controller,
thermal image window (if needed to get access to the evaluation board), airflow meter, airflow sensor, and
output airflow direction. Figure 5.2 shows how the evaluation board is mounted for the airflow directional.
This airflow unit regulates the DC fan to produce the air velocity in terms of LFM.
Controllable Fan
Airflow Meter
and Sensor
Honeycomb Straws
Help Straighten
the Airflow
Thermal
Camera
Window
EVM Mounting
Bracket
1
2
Figure 5. Airflow Tunnel Setup
2.2.2
Data Collection Steps
A labview program interface is used to collect the data as shown in Table 3. The procedures to collect the
data follow:
1. Set airflow at highest LFM first: eq. 400 LFM
2. Change the load current: eq. at 0A
3. Wait for 10 minutes for thermal equilibrium, then record the data
4. Change the load current to another value: eq. 5A
5. Wait for 10 minutes for thermal equilibrium then record the data.
6. Repeat step 2 to 5 until at full load.
7. Repeat step 1 to 6 at the next lower LFM.
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Conclusion
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Table 3. Airflow Data Collection Example
Nat Conv
2.2.3
100 LFM
IOUT (A)
TPMBus (°C)
VDIODE
IOUT (A)
TPMBus (°C)
VDIODE
0
29.3
0.572431
0
28.9
0.57259
5
32.1
0.570947
5
31.4
0.57141
10
35.8
0.567997
10
34.6
0.56871
15
40.1
0.56353
15
38.8
0.5647
20
44.8
0.556845
20
43.5
0.5587
25
54.6
0.547753
25
50.5
0.55067
30
67.6
0.535576
30
62.7
0.53976
35
80
0.516387
35
72.2
0.52617
LFM SOA Curves Graphing Method
The airflow unit is too big to be inside the oven and has difficulty controlling the uniform air velocity flow
across the evaluation board if the unit is in the oven. At result, the airflow unit is located in the typical room
temperature and is not in the regulated ambient temperature environment as in the natural convection
case. To calculate the ambient temperature of the SOA curve with airflow, it is necessary to calculate the
relative difference of diode voltage in Table 3 to the line equations in Figure 3 and Figure 4. The
procedures follow:
1. Calculate the VDiode difference between natural convection and 100 LFM in Table 3.
2. Subtract the result, in step 1, from the calculated VDiode of Figure 3 at 125°C junction temperature.
3. Use the result of VDiode, in step 2, to calculate the ambient temperature by using the trend line equation
of graph 4.
4. Repeat steps 1 to 3 for each load and each LFM curve.
Finally, graph all of the SOA curves as shown in Figure 2. For the device without the ability to read the
junction temperature through the PMBus data lines, the calculation procedures are the same by using the
thermal coupling wire data column. For more accurate calculations with this type device, there is a
temperature difference between the junction temperature and the top surface of device. At result, the
calculation VDiode of Figure 3 should have included this difference.
3
Conclusion
The SOA curves of any DC-DC power supply converter are generated by following the procedures
described in Section 2. The purpose of SOA curves is to serve as a design guideline of the thermal
performance to a system designer. In the actual implementation of a real system, the SOA curves will vary
depending on many other factors such as (PCB) area, heat source, inconsistent airflow velocity, and so
forth. As a result, the natural convection SOA curve serves as the worst-case guideline for the system
designer.
6
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