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What Are the Causes of Blown NPN Transistors?

Unlike PNP (Positive-Negative-Positive) transistors which typically handle switching chores in modern electronic circuits, the most common role of NPN (Negative-Positive-Negative) transistors is that of an amplifier, where they convert a weak signal into a more powerful signal. This requires that the transistor operates in linear mode so instead of being either completely on or off, it varies between the two states. Linear mode is hard on transistors because it reduces efficiency, and the transistor must dissipate the wasted power. Excessive power dissipation can result in blown NPN transistors.
  1. Output Impedance

    • An active load such as a speaker, an antenna, or the input circuit of a larger amplifier has an ever-changing resistance to the signal. This variable resistance is called "impedance." Every amplifier circuit has a specified output impedance, and the load must remain within a range the transistor can handle.

      A loose, open or shorted load connection will cause drastic impedance changes, which can destroy the transistor. Problems can also occur if the user replaces the load with a part that falls outside the design impedance, such as replacement of an 8-ohm speaker with a 4-ohm speaker.

    Supply Voltage

    • A transistorized amplifier circuit is designed to use a specified power supply voltage. The designer specifies resistor values to keep current flow within the transistor's limitations. If a voltage regulator fails or the user applies a higher-than-specified voltage to the circuit, the transistor will consume excessive current. The excess current can break down the transistor junctions immediately, or simply create more heat than the transistor is able to dissipate.

    Input Level

    • Input level is the strength of the signal the transistor must amplify. Similarly to the power supply voltage, the input level must not exceed design specification. An overly-powerful input signal can overcome the base-to-emitter junction. More importantly, it can cause the transistor to draw excessive current in an attempt to "keep up" with the input signal. This excess current is likely to damage the transistor.

    Heat Dissipation

    • Any transistor that operates in linear mode wastes a certain amount of energy, and must be able to dissipate that energy as heat. Higher-powered transistors include a metal tab for connection to a metal heat sink. Usually the tab needs to be electrically isolated from the heat sink, while maintaining a thermal connection. To achieve this end, engineers specify the use of electrically-insulating pads coupled with thermally-conductive paste. If the manufacturer inadvertently leaves the paste out, the transistor may overheat during use. The design may also include a fan to aid in heat removal. If the fan stops working or becomes clogged, the transistors in the circuit can overheat.


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