Relationship between power dissipation and thermal resistance

Understanding Semiconductor Thermal Resistance Data | Diodes Incorporated

relationship between power dissipation and thermal resistance

I want to know the difference between heat dissipation and power Let's say we have a power transistor with a thermal resistance of C/W. Thermal resistance is a heat property and a measurement of a temperature difference by which Main articles: analogical models and Onsager reciprocal relations . is the absolute thermal resistance of the bond between the transistor's case the maximum power that the transistor can be allowed to dissipate, so he can. Thermal resistance and power dissipation each have several key specifications that are . interface layer between the device case and the system heat sink. Column tool, Figure 2, the thermodynamic relationships can be probed for a better.

relationship between power dissipation and thermal resistance

In practice, obtaining accurate temperatures at the required measurement points is difficult, even using non-contact infrared instruments. Then with an accurate measurement of temperature at that point Tx the true thermal resistance can be calculated as: This figure depends only on the physical properties of the heat flow path and is independent of the amount of power dissipated or the size of board the device is mounted on.

The junction to lead thermal resistance values provided by Diodes Incorporated in its datasheets are measured using Method 1. The value is independent of board size and so helps compare the thermal performance of the lead frames of various packages.

Thermal resistance

It is calculated using an equation similar to that used in Method 1: Determining the junction temperature Tj: Tj Rth JA …As shown in Chart 3 the variation of Rth JA with junction temperature is minimal but the impact due to different heatsinks is more significant.

Consequently when using datasheet Rth JA values care must be taken to make sure that device mounting conditions in actual applications are close to those stated in the datasheet. The differences in heatsink arrangements volume and conductivity of the heatsink connected to the lead frame tab of the device can cause significant errors when estimating the junction temperature using Rth JA. In order to measure the datasheet Rth JL value using this method, it is necessary to attach a massive heatsink to the lead frame tab to ensure that most of the heat from the junction flows out of the lead frame tab into the heatsink.

Understanding Semiconductor Thermal Resistance Data

In a practice this is rarely the case as there will be other parallel heat flow paths that decrease the accuracy of Rth JL. Chart 2 shows the dependency of Rth JL on heat sinking when practical-sized heatsinks are used.

relationship between power dissipation and thermal resistance

For this reason, this value should not be treated as a true thermal resistance but only a thermal parameter and therefore should only be used for comparison between various packages. Chart 1 show that this value not only depends on the size of the heatsink but also the operating junction temperature.

The value decreases with increasing junction temperature due to the convection of the air around the device. Even though the measurement is done under still air conditions, the hot surface of the device will still cause air to circulate resulting in a convection effect. Consequently this value should not be used unconditionally in trying to determine the junction temperature in real applications.

This approach eliminates the effect of different heatsinks from the equation. However care must be taken while measuring the case temperature so 1 a non-contact thermal measurement instrument is recommended and 2 the point of measurement on the case should be as close as possible to the center of its surface.

relationship between power dissipation and thermal resistance

Instead, Chart 4 shows a much closer correlation between junction temperature and case temperature that is far less dependent on the size or effectiveness of any heatsink.

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relationship between power dissipation and thermal resistance

The heat causing the rise in temperature flows into the environment and an equilibrium is reached. The amount of heat that flows depends on the temperature of the environment or ambient temperature and the thermal resistance between the material and the environment. In fact is is very similar to electricity with temperature being equivalent to voltage, current being equivalent to heat and thermal resistance being equivalent to resistance.

Why should I care? You care because you can calculate if something will get too hot and burn out in advance of it happening. If one end of you thermal resistance is anchored at the ambient temperature the other end gets hotter, the temperature it get to is dependent on the power dissipated and the thermal resistance.

Thermal resistance - Wikipedia

You can reduce the latter by applying a heat sink but no amount of heat sink is going to affect the former. In fact manufacturers of devices often use the concept of an infinite heat sink in getting their headline figures. That means a heat sink so big that the case temperature and the ambient temperature are the same thing. As a rule of thumb: Hence the current move away from 5V devices to 3V3 or even 1V8 and lower.

relationship between power dissipation and thermal resistance

Modern micro controller systems often have a 3V3 interface with a 1V2 core, requiring two supply voltages but dissipating less power. Switching That is also why FETs are much preferred nowadays over transistors for switching large currents. Where as a transistor can have a saturation voltage of between 0.

Where as a transistor with 0. One mistake beginners often make is to think if they are switching say a 50W load they are going to dissipate 50W in the switch, this is not the case. You only dissipate the power in the switch given by the current down it and either the series resistance or the saturation voltage across it.