When to use a BJT power switch

Today, it is almost a given that switching power supplies will use MOSFETs as power switches. However, there are instances where bipolar junction transistors (BJTs) may still have some advantages over MOSFETs. Especially in offline power supplies, cost and high voltage (greater than 1kV) are two reasons to use BJTs instead of MOSFETs.

In low power (3W and below) flyback supplies, it is hard to beat the BJT on cost. A 13003 NPN transistor can be purchased for as little as $0.03 when purchased in large quantities. Not only can this device handle 700V VCE, but it can drive several hundred milliamps without excessive base current. With a BJT, gain and power dissipation may limit practical use to low power applications. At these low power standards, the efficiency difference between MOSFETs and BJTs is very subtle. Figure 1 below compares the efficiency of two similar 5V/1W designs. The first design is  the PMP8968  using MOSFETs, while the other design is  the PMP9059  using BJTs. This is not a completely fair comparison because the two supplies are designed to operate from different input voltages, but it illustrates how similar their efficiency is.

Figure  1:  Efficiency comparison of the PMP8968  MOSFET design and  the PMP9059 BJT design

Some new controllers are actually designed to drive BJTs in order to provide the lowest cost solution. In most cases, a controller with an external BJT is less expensive than one that includes an integrated MOSFET. When designing with a BJT controller, care must be taken to ensure that the base drive and gain of the BJT is sufficient to provide the necessary peak current in the transformer.

At slightly higher power levels, the efficiency difference between FETs and BJTs becomes more pronounced due to the BJT’s poorer switching characteristics and voltage drop. However, for applications where the input voltage is higher than the typical household and commercial voltage range of 100-240VAC, the BJT may still have an advantage. Industrial applications and power meters are two examples of this, which may require higher input voltages. Reasonably priced MOSFETs are only available for less than 1kV. In some power meter applications, the line voltage may exceed 480VACrms. After the rectifier, voltages of more than 680Vdc can be achieved. For three-phase input, this number can be even higher. The power switch needs to be able to withstand this voltage as well as the reflected output voltage and leakage peaks. In these applications, MOSFETs may not be an option at all, so BJTs become the simplest and lowest cost solution (see PMP9044 , link provided below).

We have discussed before that switching losses in BJTs can become a problem as power levels increase above 3W. Using a cascade connection to drive the BJT can alleviate this problem. Figure 2 below (from the PMP7040) shows the operation of a cascade connection. The base of the BJT (Q1) is connected to the VCC rail, while the emitter is pulled low to turn on the switch. Inside  the UCC28610  , a low voltage MOSFET pulls the DRV pin low, and an internal current sense is used to schedule the peak switch current. Fast turn-off is achieved by the internal MOSFET because it is in series with the external high voltage BJT.

Figure  2: PMP7040 schematic showing the operation of the cascade connection

In summary, there are still reasons why BJTs may be important in your power supply. In applications below 3W, they may offer a low cost advantage without sacrificing performance. At higher voltages, they offer more options where MOSFET selection may be limited. In addition, we have seen how cascade connections can be used to improve BJT switching performance. Here are some  links to PowerLab  designs that highlight these aspects...

Low Power, Low Cost  BJT Flyback Solution:

• PMP9059  — 120VAC input, 5V/200mA
• PMP9074  — 85VAC-265VAC Input, 12V/3W

High Input Voltage  BJT Flyback Solution:

• PMP6741  — 85Vdc-576Vdc Input, 24V/12W
• PMP9044  — 3-phase AC input, 3.3V/0.5A

Cascade Drive  BJT Flyback Solution:

• PMP6710  — 85VAC-265VAC Input, 12V/1A
• PMP7040.1  — 147-400VAC input, 20V/0.25A