The DXTN07 and DXTP07 bipolar junction transistor families can be used for high current switching in industrial, consumer and automotive applications.
MOSFETs rule the world of transistors these days, and I understand the practical reasons for this. However, we do not want to marginalize the BJTs. I wonder if bipolar junction transistors are sometimes overlooked, not because they are actually the bottom of a certain application, but simply because we have become used to using MOSFETs.
I guess that’s why a recent advertisement for a Diodes Incorporated product caught my eye. They have released two new BJT families. The DXTN07 series consists of four NPN devices, and the DXTP07 series consists of four PNP devices. These transistors are designed for power and load switching circuits. I find it interesting because it seems to me that, for these types of applications, development efforts are focused on MOSFETs.
BJT vs MOSFET?
I have discussed this problem before, as did Lonne Mays when discussing which power semiconductor to choose for power stage design. Today, however, I am taking a new approach: I will not make any attempt to compare these two types of transistors or to make recommendations as to which one is more appropriate for a given application. I’m just not familiar with the latest techniques used in discrete transistors design and manufacturing, and I’m concerned that the traditional approach to comparing field effect devices and bipolar devices may not be as relevant as it used to be.
The only advice I’ll offer is this: don’t use field effect transistors simply because FETs are more common or because the older version of the design used a FET. If you really want to optimize performance, I think a design based on a recently released BJT is often worth considering and even evaluating.
The DXTN07 and DXTP07 devices
I would put these transistors in the medium-high power category. They tolerate collector-emitter voltages ranging from 25 V to 100 V, and have a continuous collector current rating of 2 A or 3 A. Note, however, that many switching applications involve a pulsed load current at instead of a continuous load. stream. Therefore, you should take a closer look at the specifications to determine if these parts are a good fit for your system.
The graph below provides an example of how the maximum collector current varies with the duration of the pulse.
Plot taken from data sheet DXTN07025BFG.
One spec that I noticed is the collector-emitter saturation voltage, particularly for part number DXTN07045DFG. This device has a typical ECV (SAT) of 140 mV with a collector current of 1 A. This seems pretty low, and a low saturation voltage is good because it means less power dissipation for a given load current.
As shown in the following graph, the saturation voltage is affected by the collector current and the ratio of collector current to base current; It can be significantly less than 140 mV.
Plot taken from data sheet DXTN07045DFG.
We can put this number into perspective by comparing the power dissipation of this transistor to that of a MOSFET under similar operating conditions. With a collector current of 1 A and a collector-emitter voltage of 140 mV, we have 140 mW of power dissipation. To achieve the same power dissipation with a drain current of 1 A, a FET should have a resistance of 140 mΩ in the state.
I rarely miss an opportunity to emphasize the importance of incorporating thermal analysis into voltage regulators, motor controllers, and other circuits that involve a non-trivial amount of power dissipation. Looking at the maximum current specifications is not enough; current leads to power dissipation, and power dissipation leads to heat, and heat leads to temperatures that can damage or even compromise the functionality of a circuit.
The DXTN07 and DXTP07 devices are housed in an interesting package (shown below). Diodes Incorporated calls it the PowerDI3333. They describe it as more thermally efficient than a SOT223, and it requires 70% less space on the board. This is an important consideration, as package size and thermal performance are generally in conflict – a smaller package makes it difficult for heat to pass from the chip to the PCB and into the surrounding environment.
Image courtesy of Diodes Inc.
Do you have any thoughts on the topic MOSFET vs. BJT? Do you think a newly released BJT could replace a MOSFET in one of your next designs? Let us know in the comment section.