- Reduces the number of components on the bill of materials (BOM), allowing a smaller, and more cost-effective solution. Most significantly, you can remove the snubber diode, which is generally a 600V part.
- Reduces the number of components connected to the high-voltage switching node, thus reducing the area of this node. Because of the high instantaneous voltage rate-of-change over time (dv/dt) and the large voltage swing, any parasitic capacitance from this node causes significant unwanted currents. These currents can generate noise signals that can interfere with the controller or other devices on the board, or electromagnetic interference from the power supply, requiring filtering to meet harmonic emission standards.
- Increases efficiency by saving the power dissipated by the steady-state voltage across the snubber resistor (= Vreflected/Rsnubber2 ).
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.