Power Darlington Transistor
Fig: Monolithic Darlington connected power transistor. (a) circuit diagram, (b) schematic cross section.
- 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.
- The effective current gain of a Darlington pair is given by
So that even when individual β’s are small, effective β can still be quite large.
- In the Darlington configuration the base drive current for the main transistor is derived from the collector biasing power supply through a drive transistor.
- This drive transistor should have the same voltage rating as the main transistor but lower current rating.
- In a monolithic design both are fabricated from the same crystal.
- The silicon protrusion through the p layer (the base region for both transistors) isolates the two bases from each other.
- A discrete diode D is added to speed up the turn off time of the main transistor.
- The major quantitative difference in the operating characteristics of a Power Darlington is due to the fact that the main transistor cannot go into hard saturation.
- The ON state voltage drop of the drive transistor prevents forward biasing of the C-B junction of the main transistor.
- The ON state power dissipation of the main transistor will be larger than that of an otherwise comparable single BJT. The switching times will also be somewhat larger for the Darlington transistor.