Switching Trajectory and Switching Losses in a Power Transistor
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 and plotted.
The areas under these curves indicate the energy loss during each switching operation (Turn ON and Turn OFF). Indicating these areas as EON and EOFF during Turn ON and Turn OFF operations respectively one can write.
If the switching frequency of the transistor is fSW, then the average switching power loss is given by:
The value of the maximum switching frequency is given by:
For safe switching operation, however it is not sufficient to merely restrict the switching power loss. It will be necessary to restrict the switching trajectory (an instantaneous plot of ic vs VCE during switching with time as a parameter) within the FBSOA /RBSOA region corresponding to a pulse width greater than TSW (ON) or TSW (OFF).
The conduction power loss is given bywhere TON is the time period for which the base drive voltage remains positive. Usually ts –TSW(ON) << TON
Switching aid circuits or “snubbers” (as they are popularly known) are used to enhance the switching performance of a power transistor.
Shape the switching trajectory such that the voltage and current rating of a transistor can be fully utilized.
Reduce the switching power loss inside the device.