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In the design of equipment which utilizes solid-state power semiconductors, it is essential that good engineering practices be followed. It is important to remember that prudent judgement dictates operating components within their Absolute-Maximum Ratings. These ratings are defined by an industry standard (JEDEC) statement. The statement says: "Absolute-Maximum Ratings are limiting values of operating and environmental conditions applicable to any electron device of a specified type as defined by its published data, and should not be exceeded under the worst probable conditions." These transistor values are chosen to provide acceptable reliability of the devices. The systems engineer should design the circuit so that no absolute-maximum value is exceeded throughout the equipment's service life.
Power semiconductors are usable in a broad range of applications under many varied operating conditions. Therefore, designers should anticipate the possibility for failure and make certain that the result would not be a safety hazard. Some power semiconductors, such as PowerTech's MT-1267A, can handle 500 Volts and over 1000 Amps; the need for failsafe circuit design is clearly obvious when working with these power levels. The advantage of small-sized solid-state devices is tempered by the relatively small area for thermal dissipation and electrical insulation. As the collectors of some devices are connected to the cases and the collectors may be at some potential other than ground, suitable precautionary measures should be taken. To avoid the risk of hazardous shock, direct contact with the case should be prevented by suitable precautionary measures. For the protection of personnel, as well as the devices, connecting or disconnecting from the circuit should only be done with the power turned off.
Testing and operation of devices should be performed in circuits with reasonable values of current-limiting resistances. In lieu of such current-limiting resistance, some other form of effective current overload protection should be employed. If the current through the device is too excessive, internal heating will result in destruction, possibly even rupture, of the encloser. When soldering devices into the circuit, avoid excessive heating to prevent damage. One technique is to grasp the terminal between the case and the soldering point with a pair of long-nose pliers. The pliers absorb some of the heat from the soldering operation, keeping it from the case. When soldering to the pins close to the pin seals, care must be taken to conduct excessive heat away from the seals or the seals could crack and the device be damaged.
During operation, the mounting flange is at some temperature higher than the ambient temperature. This flange temperature depends on the thermal resistance between the junction and the case, the power dissipated in the transistor, the thermal resistance between the case and the heat sink, and the heat sink. The heat sink must be selected to have enough thermal capacity that the mounting flange temperature is not raised over the rated value for the designed power dissipation of the device. As the case may be the collector of the device, the mounting flange needs to be securely fastened to the chassis or heat sink. However, no attempt should be made to solder the flange directly to the heat sink or chassis. Such an act would probably damage the device due to the excessive heat of the soldering operation on the case. If insulating bushings or spacers are used, they should not exhibit shrinkage or softening under the operating temperatures anticipated. Shrinkage or softening of the insulating bushings or spacers may result in an increase in the thermal resistance between the case and the heat sink. This, of course, would result in a higher case temperature than designed for.
When mounting devices to the heat sink or chassis, excessive torque should be avoided. Excessive torque can result in flange distortion. The distortion of the mounting flange could result in damage: to the transistor. A maximum torque of 8 inch-pounds should not be exceeded. Customer modification of the mounting flange can also result in flange distortion and should not be attempted.
In summary, the design engineer must exercise good judgement when working with power semiconductors. In many of today's low-power circuits, the most severe result of carelessness is a small shock and a damaged device. The engineers designing with power semiconductors must remember that large currents and voltages applied to the human body can be fatal.
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