Difference between Power Dissipation and heat dissipation

[VJKris]

Hi All,

I want to know the difference between heat dissipation and power dissipation?Are they same?

Regards,
VJ

 

[tpetar]

You give answer to yourself with your question :

Heat dissipation and Power dissipation (Heat/Power).

What amount of heat can device dissipate.
What amount of power device can dissipate.

This not the same.

 

[VJKris]

Thanks for the response.
I was looking at the difference w.r.t the dissipation of an IC
What is the SI unit of heat dissipation

 

[godfreyl]

The device converts electrical power into heat, which must be dissipated.

Some other devices work differently. For example a light bulb converts some of the electrical power into light, and the rest into heat. A motor converts some of the electrical power into mechanical power, and the rest into heat. However resistors and ICs only produce heat.

The standard unit for power is watts. It doesn't matter if we are talking about electrical power, mechanical power or heat power.

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However we are often interested in the temperature rise caused by the heat dissipation. In this case we have to take into account thermal resistance, which is given in units of degrees centigrade per watt.

For a small transistor, the thermal resistance from junction to ambient may be given as 200C/W. If that transistor is dissipating 0.5W of heat, then the silicon junction will be 100 degrees hotter than the ambient air temperature. (0.5W * 200 degrees per Watt = 100 degrees)

For power devices the calculation is a bit more complicated.

Let's say we have a power transistor with a thermal resistance of 0.75C/W from junction to case. The transistor is mounted on a heatsink rated at 5C/W using a mica washer and thermal paste which together have a thermal resistance of 1C/W.

The total thermal resistance from the transistor's junction to ambient = 0.75 + 1+ 5 = 6.75 degrees per Watt.

If the transistor is dissipating 10W as heat, then:

• The temperature of the heatsink will be 5 * 10 = 50 degrees hotter than the ambient air temperature.
• The case of the transistor will be 1* 10 = 5 degrees hotter than the heatsink.
• The junction of the transistor will be 0.75 * 10 = 7.5 degrees hotter than it's case.

 

[VJKris]

Thanks for the response.I got the difference