Power Tips: when to use a BJT power switch

Other Parts Discussed in Post: PMP8968, PMP9059, PMP9044, UCC28610

Today, it is almost a fore-gone conclusion that a switching power supply will use a MOSFET as the power switch.  However in some instances, a bipolar junction transistor (BJT) may still offer an advantage over a MOSFET.  Specifically, in off-line power supplies, cost and high voltage (greater than 1kV) are two reasons to use a BJT instead of a MOSFET.

In low power (3W and less) flyback supplies, it is hard to beat the cost a BJT.  In high volume, a 13003 NPN transistor can be as cheap as $0.03.  This device can handle 700V VCE and can drive a few 100mA without requiring excessive base current.  With BJTs, the gain and power dissipation tend to limit the practical usage to low power applications.   At these low power levels, there is very little difference in efficiency between a MOSFET and a BJT.  Figure 1 below compares the efficiency of two similar 5V/1W designs.  One design, PMP8968 uses a MOSFET and the other, PMP9059, uses a BJT.  This is not a completely fair comparison, because these two supplies were designed to run from different input voltages, but it illustrates how similar the efficiencies can be.

PowerLab Notes: When to choose a BJT power switch efficiency curve comparing MOSFET and BJT

Figure 1: Efficiency of the PMP8968 MOSFET design compared to the PMP9059 BJT design 

Some new controllers are actually designed to drive BJTs, with the intent of providing the lowest cost solution.  In most cases, a controller with an external BJT will be less expensive than a controller that contains an integrated MOSFET.  When designing with a BJT contoller, care must be taken to make sure that 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 the FET and BJT becomes more significant due to the poor switching characteristics and voltage drop of the BJT.  However, a BJT might still offer an advantage in applications that run from input voltages that are higher than the typical residential and commercial range of 100-240VAC.   Industrial applications and power meters are a couple of examples of situations that may require higher input voltages.  Reasonably priced MOSFETs tend to be limited to less than 1kV.  In some power meter applications, the line voltage can be over 480VACrms.  After the rectifier, this results in a voltage of over 680Vdc.  With a three phase input, it can be even higher.  The power switch needs to withstand this voltage, plus the reflected output voltage and leakage spike.  In these applications, a MOSFET might not even be an option at all, leaving the BJT as the most simple and lowest cost solution (see PMP9044, link provided below).

Switching losses in the BJT can start to become a big problem as the power level is increased above the 3W level that we discussed earlier.  Using a cascode connection to drive the BJT can alleviate this problem.  Figure 2 below, from PMP7040, shows how the cascode connection works.   The base of the BJT (Q1) is tied to the VCC rail, and the emitter is yanked low to turn on the switch.  Inside the UCC28610, a low-voltage MOSFET pulls the DRV pin low and an internal current sense programs the peak switch current.  A fast turn-off is obtained by the internal MOSFET, since it is connected in series with the external high-voltage BJT. 

PowerLab Notes: when to use a BJT power switch, figure 2: PMP7040 schematic

Figure 2: PMP7040 schematic showing how cascode connection works

In summary, there are still a few reasons why a BJT might make sense in your power supply.  In applications below 3W, they can offer a cost advantage without sacrificing too much performance.  At higher voltages, they can offer more selection where the choice of MOSFETs might be limited.  We also saw how a cascode connection can be used to improve the switching performance of a BJT.  Here are links to a few designs in PowerLab highlighting some of these aspects…

Low Power, Low Cost BJT Flybacks:

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

High Input Voltage, BJT Flybacks:

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

Cascode-driven BJT Flybacks:

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