Thermal resistance

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

• On state conduction losses
• 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.

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 T_A  ^oC and T_B  ^oC due to power loss P, the thermal resistance is

       P = ( T_A – T_B )  / 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 ( θ_JC )

• It is ratio of difference between junction temperature and case temperature to the power loss

       θ_JC = ( T_J – T_C ) / P  ^oC/Watt

• The value of thermal resistance θ_JC is given in the SCR datasheet.

Thermal resistance – Junction to case ( θ_CS )

• It is ratio of difference between case temperature and surface of heat sink to the power loss

       θ_CS = ( T_C – T_S ) / P  ^oC/Watt

The thermal resistance θ_CS 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 θ_CS 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 ( θ_SA )

• It is ratio of difference between heat sink temperature and atmosphere temperature to the power loss.

       θ_SA = ( T_S – T_A ) / P  ^oC/Watt

The thermal resistance θ_SA 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

        θ_JA = ( T_J – T_A ) / P  ^oC/Watt

           P = ( T_J – T_A ) / θ_JA Watt

        Where θ_JA = θ_JC + θ_CS + θ_SA

HEAT FLOW DIAGRAM OF AN SCR.PDF

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