Turn Off Characteristics of a Power Transistor
Turn Off Characteristics of a Power Transistor:
- The “Turn OFF” process starts with the base drive voltage going negative to a value -VBB. The base-emitter voltage however does not change from its forward bias value of VBE(sat) immediately, due to the excess, minority carriers stored in the base region
- A negative base current starts removing this excess carrier at a rate determined by the negative base drive voltage and the base drive resistance. After a time “ts” called the storage time of the transistor, the remaining stored charge in the base becomes insufficient to support the transistor in the hard saturation region. The transistor enters quasi saturation region and the collector voltage starts rising with a small slope.
- After a further time interval “trv1” the transistor completes traversing through the quasi saturation region and enters the active region. The stored charge in the base region at this point is insufficient to support the full negative base current. VBE starts falling forward –VBB and the negative base current starts reducing. In the active region, VCE increases rapidly towards VCC and at the end of the time interval “trv2” exceeds it to turn on D. VCE remains clamped at VCC, thereafter by the conducting diode D.
- At the end of trv2 the stored base charge can no longer support the full load current through the collector and the collector current starts falling. At the end of the current fall time tfi the collector current becomes zero and the load current freewheels through the diode D. Turn OFF process of the transistor ends at this point. The total Turn OFF time is given by Ts (OFF) = ts trv1 trv2 tfi
- Considerable power loss takes place during Turn OFF due to simultaneous existence of ic and VCE in the intervals trv1, trv2 and tfi. The total energy last per turn off operation is given by the area under this curve. For safe turn off the average power dissipation during trv1 trv2 tfi should be less than the power dissipation limit set by the FBSOA corresponding to a pulse width greater than trv1 trv2 tfi.
- Turn OFF time intervals of a power transistor are strongly influenced by the operating conditions and the base drive design. Manufacturers usually specify these values as functions of collector current for given positive and negative base current and case temperatures. Variations of these time intervals as function of the ratio of positive to negative base currents for different collector currents are also specified.