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Transient protection for bipolar switching transistors

Often, the terms snubber, damper, and clamp are used interchangeably. Although dampers and clamps limit load-line excursions, slumbers exert more modification of the entire load line. Protective circuit called a damper is much used in the horizontal output stage of TV sets.

All of these protective circuits share the common feature that, when properly deployed, destruction by secondary breakdown can be avoided. Such destruction otherwise tends to occur in switching applications when the energy stored in load or circuit inductance penetrates the RBSOA boundary of the transistor when it is turned off. Protection is accomplished via absorption of much of this excess energy.

Snubbing networks are nearly always used with inductive loads such as motors and solenoids. The inclusion of series resistance R, can provide additional dissipation of excess energy when the transistor is turned off. Over a wide range, the amount of inductance in the switching circuit is not the governing factor of RBSOA vulnerability to destruction; rather, it is the time constant of the inductive circuit, given by L/Rs,. Note that the insertion of R, reduces this time constant. R, can also be viewed as a Q spoiler of the resonant circuit set up between L and circuit stray capacitance. High Q resonant circuits are reservoirs of high values of circulating energy; it is of course, preferable to dissipate LC (inductive-capacitive) energy in a resistance than in the collector junction of the transistor. In many prac­tical circuits, there is often sufficient resistance in the inductive load itself to serve this function.

In circuit C, an appropriately selected zener diode protects the transistor by clamping the collector-emitter voltage at, or below Vcex(sus). Although all zener diodes are fast acting, it is preferable to use zener diodes specially made for absorption of transient energy, such as Motorola's line of Mosorbs. Circuit G is commonly encountered in automotive ignition systems. Here, FBSOA is worsened because RBSOA energy regeneratively extends the on time of the transistor. That is considered a worthwhile trade off because transistors tend to be more electrically rugged in their FBSOA modes than when forced to absorb RBSOA energy. Some of the excess RBSOA energy is dissipated as heat in resistance R. (1951)

ТЕКСТ 9

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