7.Heat Compensation in Analogue electronics.pptx

The presentation is about heat compensation in Analog electronics

1. Heat compensation

2. Heat Sinks The power dissipated in a transistor increases its internal temperature above the ambient temperature. If the device or junction temperature Tj becomes too high, the transistor may suffer permanent damage. Special precautions must be taken in packaging power transistors and in providing heat sinks so that heat can be conducted from the transistor.

3. Additionally, heat sinks can be used to remove the heat developed in the transistor:

4. Electrical equivalent circuit of thermal-conduction process Temperature of transistor junction Ambient temperature Temperature difference  Voltage difference Thermal resistance between the junction and ambient  Electrical resistance Thermal power through the element  Electric current.

5. Manufacturers’ data sheet for power devices generally give: maximum operating junction (device) temperature, T J max ; thermal resistance from the junction to the case,  JC ; The temperature conduction process may be represented as follows:

6. The following equation can be used to describe the temperature conduction process: If the heat sink is not used, then;

7. A MOSFET has the following parameters; Determine the maximum power dissipation in the transistor and determine the temperature of the transistor case and heat sink. EXAMPLE

8. Solution Maximum power (without heat sink) Maximum power (with heat sink)

9. Heat sink temperature

10. Case temperature Note : The use of heat sink allows more power to be dissipated in the device.

11. Power derating curve Manufacturer usually specifies: the maximum temperature T J max ; the maximum power dissipation P D max , at a particular ambient temperature T A (usually 25 C); and the thermal resistance  JA . In addition, a graph – power derating curve is provided.

12. Power derating curve For operation below T A , the device can safely dissipate the rated value of P D watts. If the device is to be operated at higher ambient temperature, the maximum allowable power dissipation must be derated according to the straight line.  

13. Exercise Determine what maximum dissipation will be allowed for an 80-W silicon transistor (rated at 25°C) if derating is required above 25°C by a derating factor of 0.5 W/°C at a case temperature of 125°C. Sol