27 April 2017

Thermal resistance

The following losses take place during on condition of the SCR.
Thermal Resistance
  • 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 ToC 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 C/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
        Where θJA = θJC + θCS + θSA

heat-flow-diagram-of-the-scr.png




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