Before mounting semiconductor power devices , its important to consider the correct mounting techniques. Observe the following check list:

• Check mounting surface for foreign particles and wipe then off.
• Check surface flatness that must be within plus, minus
• Apply a generous amount of thermal compound evenly on the semiconductor contact surface. The thermal compound must overflow until it reaches the semiconductor edges.
• If you are not using Aegis pre-calibrated dynamometer clamp, verify the correct torque that must be applied to the devices according to the data sheet. Use a torque meter to tighten the device. 

NOTE: 
1- Stud type: the torque must be applied to the device and not to the tightening nuts of the stud type device.
2- Disc type: The Aegis clamp dynamometer has three nuts on it. To tighten the device you must tighten the lateral nuts and DO NOT TOUCH the central nut. This center nut is responsible for the pre-calibration of the dynamometer . There is a metal bellow this nut that is released showing when the tightening pressure is correctly applied to the lateral nuts.

There are three thermal resistances that must be considered for the heat exchange between the semiconductor junction and the environment:

• Thermal resistance between the junction and the device's baseplate, which depends on the device's geometry
• Thermal resistance between the baseplate and heatsink surface
• Thermal resistance of the heatsink, which is responsible for dissipating the heat into the environment

The end-user cannot alter the characteristics of the thermal resistance between the baseplate and the junction, once it only depends on the manufacturer.
The thermal resistance between the heatsink's surface and the device's baseplate depends on the flatness and surface smoothness of both the heatsink and semiconductor baseplate surfaces, the use of an adequate thermal compound and of the correct mounting force applied to the device.
The thermal resistance of the heatsink depends on both the method of cooling and the heatsink geometry. The thermal resistance of the semiconductor device can never be smaller than the heatsink's thermal resistance. 

NOTE: In case of forced air or natural cooling the ambient temperature must be carefully observed: 35oC for forced air and 45oC for natural air cooling.

Protect the power semiconductors from excessive heat:

Periodically check of the ambiental temperature, the functionality of the cooling fans and the quality of the water cooling systems can help to avoid catastrophic device failures due to device overheating. The maintenance procedure of power semiconductor devices shall include this checklist.

Avoid electrical overloading of the equipment:

Equipment electric overloads can lead to semiconductor devices overload, thus resulting in failure of the semiconductor devices. Device overloads can also reduce the power semiconductor's lifetime.

Always protect the devices from transient voltages.

Most of the electric equipments already have transient voltage protection. In industrial plants the most common sources of transient voltages are:

• Equipment on/off switching is usually the most common cause of transient voltages. Every time an inductive load is energized or de-energized, a transient voltage is generated. Electrical transformers and motors are highly inductive loads. Electrical transients may be remotely generated and fed back to the power devices via mains. Power transient protection is a matter of how to attenuate the current surge down to a level compatible with the particular device being affected.
• Transients due to the commutation of the rectifier semiconductor junction in association with its reverse recuperation characteristics. The rectification cycle goes from current conduction to cut off, with fast changes of di/dt. When using fast power devices the circuit inductances must be kept to a minimum.
• Regenerative surges caused by inductive dynamic loads. The transient suppression of these surges depends on protection devices able to absorb very large amounts of energy such as capacitors.
• The semiconductor devices must be protected against electrical current overloads by using adequate electrical protection like fuses or current limiter.
• The semiconductor power device external surface must be kept clean. Dust and/or condensed oil vapor deposited on the external surface may cause an electrical current to flow between the anode and cathode of the device.

RECOMMENDED RECTIFIERS OPERATIONAL PARAMETERS

In order to extend the useful life of semiconductors rectifiers the following operational parameters must be observed al all times:

• Ambient air temperature for natural convection cooling : < 45oC 
• Ambient air temperature for forced air cooling : < 35oC
• Air speed for forced air cooling: > 6m/s
• Water flow for water cooled rectifiers: > 4l/min
• Entrance water temperature for water cooled rectifiers: < 40oC 
• Minimum heatsink temperature for rectifiers stack: 90oC
• Minimum heatsink temperature for thyristors stack: 85oC

Note: The stacks must be used at maximum 80% IDmáx