Thermal Design

What are the basic concepts about thermal design of using semiconductor devices?

The power consumed by semiconductor devices is converted into heat.
This generated heat causes a rise in temperature of semiconductor devices.

Semiconductor devices will operate normally as long as the temperature does not exceed an upper limit (specified as the ambient temperature and the temperature of the junctions inside the semiconductor).
When this upper limit is exceeded, the semiconductor stops operating normally and becomes damaged.
Therefore, it is necessary to successfully dissipate the generated heat so as to keep the temperature within specified level.
Thermal design is indispensable when using the semiconductor for high power application or using it under the high operating temperature.
The concept of thermal resistance is used when considering heat dissipation.
This approach is based on the similarity of the conduction of heat and the conduction of electricity.
While the fine details are omitted here, by substituting heat (=power) for current, temperature for voltage, and thermal resistance for resistance, the calculation of electricity can be applied.
In other words, we have:

Potential difference   = Resistance             x current
Temperature difference = Thermal resistance     x heat (power)
Thermal resistance     = Temperature difference / heat (power)

Hence, the unit of thermal resistance is °C/W.
Moreover, θ is used as a symbol. The thermal resistance between 2 points is indicated using subscript (θj-a: Thermal resistance between a junction and ambient air).

Caution
In the case of heat, the difference between a good conductor and an isolator (insulator) is not so large as for electricity. Therefore, accurate calculations using thermal resistance are not very meaningful.

What type of modeling is thermal design based on?

The part that consume electric power (= generates heat) in a semiconductor device is the junction.
(In the case of a MOS, there are no junctions, but for the sake of simplicity, the same expression is used.)

The heat that is generated at the junction propagates through the internal structure of the device and reaches its surface (case).
The heat is released into the ambient air through case.
In other words, there are two stages (thermal resistances) in series from the junction that generates the heat until the ambient air that ultimately absorbs the heat.

Junction: Junction temperature (Tj)

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   >  Thermal resistance between a junction and case (θj-c) (°C/W)
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Case surface: Case temperature (Tc)

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   >  Thermal resistance between case and ambient air (θc-a) (°C/W)
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Ambient air: Ambient temperature (Ta)

Where does thermal resistance occur?

The thermal resistance (resistance to the conductivity of heat) between the junction and the case is determined by the structure of the device.
Therefore, the thermal resistance of this part is treated as a fixed value.

The thermal resistance between the case and the ambient air varies greatly according to the mounting conditions.
The attachment of a radiator represents the addition of a heat dissipation route from the case surface via the radiator.
The concept here is the introduction of thermal resistance via a radiator in parallel with the device's original thermal resistance between the case and the ambient air.
Since the resistance is connected in parallel, it is possible to reduce the thermal resistance between the case and the ambient air by the amount corresponding to the radiator.

What is the relationship between the junction temperature and the thermal resistance?

When using a power device, the relationship between the junction temperature and the thermal resistance, which indicates the resistance to the conductivity of heat, is important.
The junction temperature (Tj) can be expressed as follows:

Tj = Ta + θj-a x P
  Ta: Ambient temperature
  θj-a: Thermal resistance between a junction and ambient air 
    (= between junction and case + between case and ambient air) 
  P: Power consumption 


Based on this fact, the following can be stated 

In the case of the same power consumption, the smaller thermal resistance makes smaller rise in temperature at the junction.
Therefore, the device can be used in a higher ambient temperature.
Conversely, if using the device at the same ambient temperature, the smaller thermal resistance allowes greater power consumption.

Could you show some cases of thermal design?

There are no special documents with collections of examples.
The following three can be cited as points of thermal designing.

• Suppress the total power dissipation by derating.
• Decrease the thermal resistance by radiating heat (by using a heat sink, cooling fan, or water cooling).
• Lower the temperature range in the operating environment.

For how to calculate the thermal resistance and junction temperature of transistors, see "Discrete > Transistor > Thermal Resistance".

Here, how to calculate the thermal resistance of the following semiconductor device when the total power dissipation increases is explained.



The thermal resistance θj-a of this device is 35°C/W (= (150°C - 80°C)/2 W) between the junction and atmosphere.
If the power consumption exceeds 2 W, the thermal resistance must be lower than that. For example, shall we consider a case where this device consumes 3 W at an ambient temperature of up to 80°C?

To transfer a heat generation of 3 W at a temperature difference of 70°C (= 150°C - 80°C), the value of θj-a must be as follows or less.


Condition (3) states that this device has θj-c = 5°C/W. Therefore, thermal resistance θc-a between the case and the atmosphere must be as follows or less.


Incidentally, where θc-a = 0°C/W, a temperature difference of 70°C is generated in θj-c. The maximum power consumption is therefore 14 W (= 70°C/5(°C/W)) when it is assumed that heat radiated from the case is 100% without temperature rise.



In Japan, if you have an ASIC that needs a stricter thermal design, please contact the NEC Electronics-related distributor where you requested development.

(2008/02)

What size heat sink is necessary?

The amount of heat dissipation that can be achieved with a flat plate of aluminum is indicated below for reference purposes.

Assuming the same conditions as in the above case, a surface of approximately 50 square centimeters is needed when using an aluminum plate with a thickness of 1.5 mm.
Actually, heat dissipation characteristics greatly differ depending on factors such as the material of the heat sink and air convection.

Caution
When attaching the heat sink, it is necessary to optimize heat propagation between the device and the heat sink by using silicon grease, etc.

Why is the thermal resistance of the package not prescribed for some ICs?

In the case of microcontrollers and peripheral ICs, the power consumption is not restricted by ratings, unlike the case for power semiconductor devices, and the power consumption is determined as a result, such that maximum operating speed, voltage, ambient temperature and the package can be prescribed. Therefore, the thermal resistance of the package is not prescribed. If the input/output current of the signal lines grows large, so does the amount of heat that is emitted, but the limit voltage between the high level and the low level is prescribed with the current as a condition, and when that current value is exceeded, degradation and/or destruction risk occurring. Therefore, thermal design is not necessary, and if the operating ambient temperature exceeds the rated range, the implementation of cooling is sufficient.
In the case of discrete and power supply ICs, ASICs, etc., for which the usage method cannot be known in advance, the thermal resistance of the package is shown so as to allow thermal design.

(2007/11)