Switching characteristics of a Power Transistor
Switching characteristics of a Power Transistor:
In a power electronic circuit the power transistor is usually employed as a switch i.e. it operates in either “cut off” (switch OFF) or saturation (switch ON) regions. However, the operating characteristics of a power transistor differ significantly from an ideal controlled switch in the following respects.
- It can conduct only finite amount of current in one direction when “ON”
- It can block only a finite voltage in one direction.
- It has a voltage drop during “ON” condition
- It carries a small leakage current during OFF condition
- Switching operation is not instantaneous
- It requires non zero control power for switching
- Of these the exact nature and implication of the first two has been discussed in some depth in the previous section. The third and fourth non idealities give rise to power loss termed the conduction power loss.
The basic switching characteristics of a power transistor are
- Turn On characteristics of a Power Transistor
- Turn Off Characteristics of a Power Transistor
- Switching Trajectory and Switching Losses in a Power Transistor
- Base Drive Design and Power Darlington
Turn On characteristics of a Power Transistor:
Minority carriers must be moved across different regions of a power transistor in order to make it switch between cut off and saturation regions of operation. The time delay in the switching operation of a power transistor is due to the time taken by the minority carriers to reach appropriate density levels in different regions.
Turn off Characteristics of a Power Transistor:
During Turn OFF a power transistor makes transition from saturation to cut off region of operation. Just as in the case of Turn ON, substantial redistribution of minority charge carriers are involved in the Turn OFF process.
Switching Trajectory and Switching Losses in a Power Transistor:
Energy loss takes place in a power transistor during each switching operation. Instantaneous power loss during switching can be calculated through plotted graphs. The areas under these curves indicate the energy loss during each switching operation (Turn ON and Turn OFF).
Base Drive Design and Power Darlington:
Power transistors have low values of dc current gain (β) compared to their signal level counterpart. Particularly, if a low value of VCE (sat) is desired at full load current, β can be as low as 5. With such low gain large current switching becomes difficult since the base drive circuit is required to handle about 20% of the full load current, Monolithic, Darlington connected transistors can solve this problem.