The power MOSFET has been the major semiconductor switching device in the modern power-electronics and power-management industry, and is widely studied and used by power-system designers. Conversely, the power bipolar junction transistor (BJT) is generally considered an outdated device that has poor switching performance and is unfamiliar to today’s power system engineers.
Recently, however, electronics market leaders in low-power AC/DC off-line power-supply designs have adopted BJT control ICs in applications such as high-volume cellular-phone chargers. This paper explores the questions: Why consider a Power BJT rather than a MOSFET? What are its advantages? Can BJT-based solutions achieve high efficiency?
Low cost is key
The power BJT technology matured in the mid-1960s, while the MOSFET did not become practical until the late 1970s. Fundamentally, BJTs cost less than power MOSFETs because their fabrication involves fewer layers and simpler processes than the power MOSFET, in particular for high voltage (>700V) and low power (below 5 watts) applications.
BJTs normally have less switching di/dt and dv/dt, easing electromagnetic interference (EMI) design with no-Y capacitor, no common-mode choke, and simpler transformer construction for easy, low-cost manufacturing. Also, due to slow di/dt at turn off, some energy in the transformer leakage inductance can be dissipated at the BJT turn-off transitions.
Therefore, without causing voltage stress to exceed the safe operation region, the snubber circuits can possibly be removed in flyback power supply designs of less than 3 W. Removing the snubbers helps reduce component count and bill-of-material (BOM) costs and eliminates power loss associated with the snubber circuits.
For example, a 3-watt, BJT-based, complete universal-AC input power supply design can use only 21 components. Furthermore, very-high-voltage (900 V and above) BJTs are economically available, making BJT-based designs attractive in offline power supplies for the industrial market (white goods, motor control and smart meters), and in regions with widely-varied utility voltages.
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