Taking into account the amount of time electronic products have been around us, one would think that choosing the correct power supply would be only a trivial task, just an easy off-the-shelf exercise. However, given that a poorly constructed power supply can easily lead to the demise of the entire product (or just parts of it) it is still one of the most important parts on the schematic.

1. Thermal Analysis of a Linear Voltage Regulator http://dev.emcelettronica.com/print/52072
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Thermal Analysis of a Linear Voltage
Regulator
By brumbarchris
Created 02/25/2009 - 07:09
Technology voltage regulator
Taking into account the amount of time electronic products have been around us, one would
think that choosing the correct power supply would be only a trivial task, just an easy
off-the-shelf exercise. However, given that a poorly constructed power supply can easily lead
to the demise of the entire product (or just parts of it) it is still one of the most important parts
on the schematic. It is exactly the large variety of linear and switching regulators that are
available, that makes choosing the correct IC for you application so complicated; and for
products that require a reasonable amount of power to function correctly, the heat
dissipation and thermal management of the power supply is of prime importance. The
purpose of this article is to give an example of how you should determine through
computation whether your linear voltage regulator is safe from a thermal point of view.
Our team has recently been assigned the task of designing a small electronic module for the
automotive industry. Given the harsh environment that a car represents for any electronic
module, the requirements on the power supply were a little more than ordinary. A summary
of these requirements (for the power supply) is given below:
- It should provide an output voltage of 3.3V +/-5%, and a maximum current of 150mA
- It should function (performance) between 8V and 16V, for the automotive temperature
range: -40 Celsius to +85 Celsius.
- It should withstand input voltages of up 24V for 60 seconds without functionality (to
simulate a jump start of the car); once input voltage returns to the normal 8V-16V
range, functionality should be restored automatically
- It should withstand reverse battery voltage of -14V (obviously without functionality);
once input voltage returns to the normal 8V-16V range, functionality should be restored
automatically
- It should withstand start-up pulses ranging from -450V up to +40V (these are rather
short pulses or groups of pulses, sometimes lasting hundreds of microseconds; they
are thoroughly detailed in automotive specifications)
At a first glance, it did not sound like much of a task; to make it even easier, our options were
narrowed to linear voltage regulators only, as the nature of the application would not allow
the use of a switching power supply. Given the situation, one of our junior engineers was
assigned the task, and shortly he came to us with the following schematic (I stripped it down
of any ESD protection components or anything else which is not of any interest for the
thermal management):
1 din 4 25.02.2009 15:31

2. Thermal Analysis of a Linear Voltage Regulator http://dev.emcelettronica.com/print/52072
The components involved are fairly common automotive components. The diode is rated 1A
and 600V repeatable peak reversed voltage while the voltage regulator can withstand input
voltages of over 40V while giving 400mA at 3.3V with a better accuracy than the one
required. These are obvious and clearly stated in the datasheets of the two components
(S1J, TLE4274). Go to Download section -
Datasheet_Thermal_Analysis_Linear_Voltage_Regulator.rar
Moreover, the TLE4274 regulator has a smart way of shutting down at over-temperature,
which would thus render it safe in case an increase of the input voltage would determine an
increase in the dissipated power. So the IC would not be destroyed, but it would safely shut
down until it would cool down enough for it to start working again.
Thus, most of the requirements are fulfilled at a first glance. Since this was mainly a “copy-
paste” schematic from a previous product (with different requirement, though) everybody
assumed it would easily work without any problems. And we all continued to do so until one
of us proceeded to perform the theoretical thermal analysis of this power supply.
The aim of the analysis was to prove that the regulator would be able to provide the 150mA
at a worst case temperature, that of +85 Celsius; and it would have to work under a 16V
battery input voltage. Given the requirements, the circuit should have been able to handle
this situation without getting damaged or without turning off due to over-temperature.
The flow of the calculation would have to be:
1) determine the maximum voltage drop on the regulator
2) determine the maximum dissipated power
3) determine the maximum temperature of the regulator die taking into account an
ambient temperature of +85 Celsius
4) compare the computed die temperature against the maximum allowed (specified in
the regulator datasheet; if the computed value is greater than the one specified, it
means that at some point, the regulator overt-temperature protection would kick in and
turn off the 3.3V output.
The official tool to perform such calculations is Mathcad, but due to lack of licenses at home,
I performed these calculations in MathSuga, a freely available math calculations tool
available under: http://www.gahum.com/
First of all, we need to get from the datasheet all the variables involved in the calculations.
The maximum battery voltage is (taken from the requirements):
The minimum voltage drop on the protection diode is (taken from the diode datasheet):
2 din 4 25.02.2009 15:31

3. Thermal Analysis of a Linear Voltage Regulator http://dev.emcelettronica.com/print/52072
The minimum output voltage of the regulator is (taken from the regulator datasheet):
With these, applying Kirchoff’s second law, it is easy to compute the maximum voltage drop
on the regulator itself:
The maximum current that is to be supplied by the regulator is (taken from requirements):
Knowing these values, it is easy to compute the power dissipated on the voltage
There are two factors which are contributing to the increase of temperature on the regulator
die. One of them is the ambient temperature; the other one is self-heating due to the power
dissipated by the component itself. In order to determine the effect of the dissipated power,
the parameter required is the thermal resistance of the package. This is measured in
degrees/watt and is generally specified in the datasheet of a component; for the given
regulator, taking into account the fact that we use a TO252 package, it is easy to find this
parameter in the datasheet:
This value is valid for the minimum footprint designed for this package. A larger footprint,
which dissipates the heat on a copper plane and on internal copper planes through smart
use of vias would allow us to use a smaller value for this parameter. However, given the
space constraints for this project, it was not possible to ensure anything more than a minimal
footprint for this component.
The effect of the self heating on the die temperature increase (in degrees) may be obtained
directly through the multiplication of the dissipated power by times the thermal resistance of
the package. However, this effect will have to be cumulated to the ambient temperature in
order to determine the real temperature of the die. Thus, taking into account a temperature
of:
We can determine the maximum temperature that would be developed in the silicon:
3 din 4 25.02.2009 15:31

4. Thermal Analysis of a Linear Voltage Regulator http://dev.emcelettronica.com/print/52072
Unfortunately, the result is much higher then the 150 Celsius degrees mentioned in the
datasheet as being the maximum accepted junction temperature. So what would happened
in reality is that once turned on, this regulator would start heating up, and when the die
would reach about 150 Celsius, the over-temperature protection would step in to turn it
down. Given the fact that we proved through calculation a value of more than 200 Celsius, it
became obvious that the proposed solution is was not appropriate. We obviously had to
change the concept.
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Source URL: http://dev.emcelettronica.com/thermal-analysis-linear-voltage-regulator
4 din 4 25.02.2009 15:31