![]() ![]() There are some key features that previously were not achievable: However, innovative digital-control technology has enabled development of new BJT-control-IC-based solutions with very high efficiency. Also, as a current-driven device, BJT driver design is more complicated. It is generally considered to have high switching losses because of its slow switching speed. ĭigital Control Technology Enables High-Efficiency BJT DesignĪdmittedly, there are drawbacks to the BJT. In this case, the source of CM noise is reduced by about 25% in the BJT-based design.įigure 1: CM noise source comparison: the measured CM voltage between the transformer primary and second windings. ![]() The BJT-based design has about 2.7 peak-to-peak CM voltage, while the MOSFET-based design (with the same 5V/1A rating) has about 3.6V peak-to-peak CM voltage. BJT-based solutions directly reduce the sources of EMI noise, such as the switching di/dt and, in particular, the dv/dt which is the major source of CM noise.įigure 1 compares the source of CM noise between BJT and MOSFET drives, which are the measured CM voltages between the transformer primary and secondary windings in 5W/5V/1A flyback converters. Comparing a power MOSFET to a BJT, the MOSFET has much faster switching speed and much shorter turn-on time than the BJT, and therefore generates higher dv/dt and more CM noise. The switching turn-on transitions greatly affect CM noise. ![]() To try to meet this requirement, MOSFET-based solutions will often require very complicated transformers, which increase complexity and cost, and still don’t guarantee EMI results. Consequently, modern cell phone charger manufacturers forbid the use of Y capacitors, greatly challenging EMI design. Furthermore, Y capacitors hinder cell phone communications, as they provide the path for the EMI noises generated in the transformer primary side to flow to the secondary side, where the phone is connected. Y capacitors can lead to leakage current from the input lines to the loads at the output sides, so eliminating them makes it easier to pass leakage current safety tests. CM chokes are bulky and expensive, and therefore unacceptable in cost- and size-sensitive applications such as cell phone chargers. EMI design necessitates filters, normally in both differential mode (DM) and common mode (CM), at the input or output interface of the system.ĬM filters consist of so-called “CM choke” and Y capacitors. The challenge is to pass EMI regulations as efficiently as possible in terms of size and cost. Switching power converters are tremendous EMI sources. 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. 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.įor example, a 3-watt, BJT-based, complete universal-AC input power supply design can use only 21 components. The power BJT technology matured in the mid-1960s, while the MOSFET did not become practical until the late 1970s. 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? 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. 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. 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. ![]()
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