## 27 April 2017

### Thermal resistance

• The following losses take place during on condition of the SCR.

Switching losses

• The heat generates in the SCR dissipates in the surrounding medium by heat sink.
•  If the heat generated by the SCR is same at that of heat dissipated by the heat sink, the steady state temperature is achieved.
• The generation of heat in the SCR depends upon the current passes through it and current rating of the SCR depends upon its thermal rating.
• The thermal resistance is defined as the resistance offered to power flow in the SCR.
• The thermal resistance of the SCR is a ratio of temperature difference across it and power losses due to this temperature difference.
• If there is temperature difference between two points TA  oC and ToC due to power loss P, the thermal resistance is
P = ( TA – TB )  / P  oC/Watt
• The unit of thermal resistance is degree centigrade per watt. The heat flow diagram of the SCR to its surrounding medium is shown in the figure A.

### Thermal resistance – Junction to case ( θJC )

• It is ratio of difference between junction temperature and case temperature to the power loss
θJC = ( TJ – TC ) / P  oC/Watt
• The value of thermal resistance θJC is given in the SCR datasheet.

### Thermal resistance – Junction to case ( θCS )

• It is ratio of difference between case temperature and surface of heat sink to the power loss
θCS = ( TC – TS ) / P  oC/Watt
• The thermal resistance θCS depends upon following factors.
Size of device
Smooth case surface of the semiconductor
Contact pressure with heat sink
Use of grease between semiconductor and heat sink
• The value of thermal resistance θCS lies between 0.05 oC/Watt and 0.5 oC/Watt. It also depends upon lubrication medium between two surface and installation process.

### Thermal resistance – Heat sink to surrounding medium ( θSA )

• It is ratio of difference between heat sink temperature and atmosphere temperature to the power loss
θSA = ( TS – TA ) / P  oC/Watt
The thermal resistance θSA depends upon following factors.
• The material, size and shape of heat sink
• Cooling medium ( water or air )
The thermal resistance between junction of SCR and surrounding atmosphere is
θJA = ( TJ – TA ) / P  oC/Watt
P = ( TJ – TA ) / θJA Watt