The more complicated drive required by BJTs is one reason why MOSFETs have replaced them in many applications. Devices such as TI’s UCC28720 and UCC28722 have addressed this issue by dynamically adjusting the drive current with load. At lower load levels, the reduced base current ensures that there is not a large amount of excess charge in the base region upon turn-off.
These devices also feature a 1W pull resistance on the drive pin to short the base-emitter junction during the off time so that the BJT can sustain the rated collector-to-emitter (VCES) voltage. To sustain the VCES rating, it is important to short the base node to ground through a low-impedance connection during the off time, and that the collector current has stopped flowing before the collector voltage rises above Vceo to avoid second breakdown.
The UCC28720 and UCC28722 simplify the drive for system designers and make power BJTs drop-in solutions for low-power flybacks, reducing component count and system cost.
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