MOSFET has been the absolute main power switch device field. However, in some instances, compared with the MOSFET, a bipolar junction transistor (BJT) may still have certain advantages. Especially in the off-line power supply, cost and high voltage (greater than 1kV) are two major reasons for using BJT instead of MOSFET.

Low power consumption (3W and below) flyback power supply, it is difficult to beat BJT in terms of cost. When purchased in bulk, the price of a 13003 NPN transistor as low as $ 0.03. The device not only can handle 700V VCE, and without too much base flow can drive a few hundred mA. Use BJT, gain and power dissipation might actually restrict the use of low-power applications. In these low-power standards, the efficiency difference between the MOSFET and BJT very subtle. Figure 1 compares two similar 5V / 1W design efficiency. The first design is the "230VAC input, 5.5V / 250mA non-isolated flyback converter" Using MOSFET, while the other is designed to "120VAC input, 5V / 200mA flyback converter" Using BJT. This is not entirely fair comparison because the two different input power supply voltage operation in the design, but it illustrates how similar their efficiency.

Some new controller is actually designed to drive the BJT, the purpose is to provide the lowest cost solution. In most cases, the controller has an external BJT cheaper than the controller includes integrated MOSFET. When using BJT controller design must be taken to ensure that BJT base drive and gain sufficient to provide the necessary peak current in the transformer.

At a slightly higher level of power, efficiency and BJT FET difference will become more obvious because BJT poor switching characteristics and pressure drop. However, the input voltage is higher than 100-240VAC typical household and commercial applications for the voltage range, BJT may still have the advantage. Industrial applications and power meter just two examples of this, they may require a higher input voltage. Affordable MOSFET can only be used below 1kV. In some applications, power meter, line voltage may exceed 480VACrms. After the rectifier voltage will reach more than 680Vdc. For a three-phase input, this figure may be even higher. Power switch needs to be able to withstand such voltage and the reflected output voltage and the drain peak. In these applications, MOSFET may simply not be available as an option, so the BJT has become the easiest, lowest cost solution.

We discussed earlier, when the power level increased to more than 3W, BJT switching loss may become a big problem. Using a cascade connection to drive BJT can alleviate this problem. Figure 2 (taken from PMP7040) is the work of a cascade connection. BJT (Q1) of the base connected to VCC power rail, while the emitter is pulled down to turn on the switch. In UCC28610 inside, a low-voltage MOSFET will DRV pin is pulled low by an internal current sensing to arrange the peak switch current. Fast Shutdown by the internal MOSFET to achieve, because it is in series with the external high voltage BJT.

In short, BJT may be important in your power, and still there are some reasons. In less than 3W applications, they may be under less compromising performance, low cost advantages. At higher voltages, they may have limitations situation may choose to provide more choices in the MOSFET under.