HEATING, COOLING AND MOUNTING OF THYRISTOR or SCR

• proper heating, cooling, and mounting of thyristors are critical for their reliable operation and long life. Careful consideration should be given to the selection of the heat sink, cooling system, and mounting method to ensure that the thyristor operates within its safe operating range.
• Some power loss occurs in a thyristor during its working. The various components of this power loss in the junction region of a thyristor are as under :(a)Forward conduction loss,(b)Loss due to leakage current during (c)forward and reverse blocking,(d)Switching losses at turn-on and turn-off,(e)Gate triggering loss
• At industrial power frequencies between zero and 400 Hz, the forward conduction loss, or on-state conduction loss, is usually the major component. But switching losses become dominant at high operating frequencies. 
• These electrical losses produce thermal heat which must be removed from the junction region.The thermal losses and hence the temperature rise of the device increase with the thyristor rating.The cooling of thyristors, therefore, becomes more difficult as the SCR rating increases.
• The heat produced in a thyristor by electrical loss is dissipated to ambient fluid (air or water) by mounting the device on a heat sink. 
• When heat due to losses is equal to that dissipated by the heat sink, steady junction temperature is reached.
• Thyristor heating and hence its junction temperature rise is dependent primarily on current handled by the device during its working.As such, current rating of thyristors is often based on thermal considerations.

Thermal Resistance

• Thermal energy, or heat, flows from a region of higher temperature to a region of lower temperature.This is similar to the flow of current from higher to lower potential in an electric circuit.
• Thermal Resistance is thus an analogy to electrical resistance, is the resistance offered to thermal power flow. Thermal resistance is denoted by θ. If power loss,P_av in watts, causes the temperature of two points to be at T₁°C and T₂°C where T₁ > T₂, 
• Then thermal resistance is given by θ₁₂ = (T₁-T₂)/P_av  °C/W
• The heat generated in a thyristor due to internal losses is taken to be developed at a junction within the semiconductor material. The heat flow in a thyristor is then as under: (i) from the junction to thyristor case, (ii) from the thyristor case to heat sink and (iii) from the heat sink to the surrounding ambient fluid (air or water).

Thermal Equivalent Circuit for an SCR

• There is thus thermal resistance θ_jc between junction temperature T_j and case temperature T_c.Similarly, there is thermal resistance θ_cs between T_c and sink temperature T_s and θ_sa between T_s and ambient temperature T_A. Using the electrical analogy, a thermal equivalent circuit depicting the flow of heat from junction to ambient fluid can be drawn as shown in fig..
• Here    P_av = (T_j-T_c)/θ_jc = (T_c-T_s)/θ_cs = (T_{j -} T_A)/θ_jA
• where θ_jA = θ_jc + θ_cs + θ_sA is the total thermal resistance between junction and ambient.
• The difference in temperature between junction and ambient can be written as T_j -T_A = P_av(θ_jc + θ_cs + θ_sA)

Heat Sink Specifications

• The thyristor datasheet specifies maximum junction temperature T_j thermal resistances θ_jc and θ_cs The manufacturers of heat sinks provide catalogue in which sufficient data on heat sink is available.
• Heat sinks are made from metal with high thermal conductivity. Aluminium is the most commonly used metal. Copper, being a costly metal, is seldom used as a heat sink material.
• Heat dissipation from heat sink takes place primarily by convection. As such, thyristor cooling by convection can be made more effective by enlarging the cooling surface area by providing the heat sink with peripheral fins. Heat dissipation also takes place by radiation.Heat sinks are usually provided with black anodized finish to enhance the heat dissipation by radiation.
• Sometimes the size of naturally-cooled finned heat sink may become large. In such a case,size of the heat sink can be reduced by using forced air cooling which involves a fan blowing air over the fins. With forced air cooling, heat-removing capability of the finned heat sink increases by a factor of two to three. For dissipating large losses in high power thyristors,water-cooling is usually employed to get a compact size of the heat sink.

Heating

• Thyristors have a maximum junction temperature rating, which should not be exceeded to prevent damage to the device. The maximum temperature rating is typically around 125°C for most thyristors.
• To prevent overheating, it is important to ensure that the thermal resistance between the thyristor and the heatsink is low.
• The heatsink should be chosen to have a thermal resistance that is suitable for the specific application and the power dissipation of the thyristor.

Cooling

• The heatsink should be properly designed to ensure adequate cooling of the thyristor.
• The heatsink should have sufficient surface area and good thermal conductivity to dissipate the heat generated by the thyristor.
• Forced air cooling may be required for high power applications.

Thyristor Mounting Techniques

• The thyristor should be mounted securely to the heatsink using a suitable thermal interface material such as thermal grease or thermal pad.
• The mounting torque should be applied as per the manufacturer's recommendations.
• The mounting surface should be clean and flat to ensure good thermal contact between the thyristor and the heatsink.
• When the current passes through SCR is greater than the rated value, the thermal stress produced in it which generates mechanical force. 
• If this mechanical force does not control, the SCR may damaged. 
• The protection of the SCR in such a condition is done by proper mounting of it. The mounting method depends upon the rating of the SCR.
• Internal power losses in a thyristor cause high thermal stresses which further give rise to mechanical forces. A thyristor must be braced to withstand such mechanical forces.
• SCR mounting must be so designed as to facilitate heat flow from junction to the case. 
• Depending upon the low or high power ratings of thyristors, there are five major mounting techniques for SCRS as described below:

Lead-mounting

• This method is used when load current is of small value. 
• The SCR does not require cooling device or heat sink in this method because most of the heat is dissipated by radiation and convection.

Stud-mounting

• There are two molybdenum plates kept on both sides of SCR. 
• The anode is soldered with aluminium resulting one stud is created. The SCR is joined to heat sink by this stud. 
• If there is not necessary electrical isolation, mica or fibre type washers are used. 
• The conduction of heat is done easily though mica or fibre type washers. It will also works as electrical insulator.

Bolt-down mounting

• This is also called flat-pack mounting. This type of device mounting has tabs with one or more holes.
• The heat sink and SCR are joined by the bolt. 
• The mica or fibre insulation is kept in between the heat sink and SCR. This type of mounting is used in the small and medium rating SCR.

Press-fit mounting

• Press-fit (or pressure-fit) package is designed for insertion into an appropriate sized hole in the heat sink.
• The insertion may be done by using a vice and pressing the device into the hole using wooden block etc. 
• For large sizes, the insertion is carried out by means of a hydraulic ram. This type of mounting is used for large rated thyristors.

Press-pak mounting

• This type of mounting is also called "disc" or "hockey-puck" mounting because of its shape. 
• The SCR is clamped between two heat sinks and external pressure is applied evenly so that there is no deformation of any part. 
• The heat sinks may be air, water or oil cooled. Such type of mounting is used for thyristors of very high current ratings.