Finding the maximum watt and temperature of regulator

[Murugesh_89]

Hi,
I want to know how to calculate the maximum wattage and there by the temperature without heatsink, an regulator iC say 7808 can withstand?

Say the input voltage - 24V, output voltage - 8V, current = 350mA.

View attachment 7808_St.pdf

I attached the datasheet here for your reference.

Even if these terms are already mentioned in the datasheet kindly tell me it.


Regards,
Murugesh

 

[betwixt]

There are two steps:

1. ignoring the very tiny current through the ground pin which is insignificant, the power dissipated in the device is the voltage across it's input pin and output pin multiplied by the current flowing through it. This gives a figure in Watts.

2. Look in the data sheet for the 'Rthj-amb' parameter, it varies according to the package type you are using. The figure indicates the temperature rise per Watt in ambient (no heatsink) conditions. Multiply this by the number of Watts in calculation 1. to find the temperature rise.

The maximum temperature it is allowed to reach is 150C but that is very high and you should aim for a much lower temperature for reliability and safety of surrounding components.

Brian.

 

[Gorgon]

All 78xx regulators has an internal thermal shutdown if the temperature gets too high. This is an emergency shutdown, and acts as a safety measure.
The thermal resistance from junction to air is around 60 C/W, so it means that you can have less than 1.5W disipation from a chip with no cooling.

For an output of 8V from a 24V supply you can only draw 1.5/16= 93 mA before the regulator is too hot, and may shut down.

These figures differ a bit between manufacturers, but you should be conservative.