Output I-V characteristics of a Power Transistor
Output ( ic – vCE ) characteristics of an n – p – n type Power Transistor:
Fig: Output ( ic – vCE ) characteristics of an n – p – n type Power Transistor
- In the cut off region (iB ≤ 0) the collector current is almost zero. The maximum voltage between collector and emitter under this condition is termed “Maximum forward blocking voltage with base terminal open (iB = 0)” and is denoted by VCEO.
- The blocking voltage can however be increased to a value VCBO by keeping the emitter terminal open. In this case IB < 0.
- VCBO is the breakdown voltage of the collector base junction. However, since the open base configuration is more common the value of VCEO is used by the manufacturers as the maximum voltage rating of a power transistor.
- Power transistors have poor reverse voltage withstanding capability due to low break down voltage of the base-emitter junction. Therefore, reverse voltage (C negative with respect to E) should not appear across a power transistor.
- In the active region the ratio of collector current to base current (DC current Gain (β)) remains fairly constant upto certain value of the collector current after which it falls off rapidly. Manufacturers usually provide a graph showing the variation of β as a function of the collector current for different junction temperatures and collector emitter voltages which is used for designing the base drive of a Power transistor. The value of the dc current gain of a Power transistor is much smaller compared to their signal level counterpart.
- The maximum collector-emitter voltage that a power transistor can withstand in active region is determined by the Base collector avalanche break down voltage. This voltage, denoted by VSUS in Fig, is usually smaller than VCEO. The voltage VSUS can be attained only for relatively lower values of collector current. At higher collector current the limit on the “total power dissipation” defines the boundary of the allowable active region.
- At still higher levels of collector currents the allowable active region is further restricted by a potential failure mode called “the Second Breakdown”. It appears on the output characteristics of the BJT as a precipitous drop in the collector-emitter voltage at large collector currents. The collector voltage drop is often accompanied by significant rise in the collector current and a substantial increase in the power dissipation.
- This dissipation is not uniformly spread over the entire volume of the device but is concentrated in highly localized regions. This localized heating is a combined effect of the intrinsic non uniformity of the collector current density distribution across the cross section of the device and the negative temperature coefficient of resistively of minority carrier devices which leads to the formation of “current filaments” (localized areas of very high current density) by a positive feed-back mechanism. Once current filaments are formed localized “thermal runaway” quickly takes the junction temperature beyond the safe limit and the device is destroyed.
- It is in the saturation region that the output characteristic of a Power transistor differs significantly from its signal level counterpart. In fact the saturation region of a Power transistor can be further subdivided into a quasi-saturation region and a hard saturation region.