27 April 2017

Thermal resistance

  • The following losses take place during on condition of the SCR.
               On state conductionlosses
               Gate triggering losses
               Forward and reverse blocking leakage current
               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
                       Where θJA = θJC + θCS + θSA
heat-flow-diagram-of-the-scr.png




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