Transistors – Part 1: How to choose the right transistor for your project?

We have many different options in order to switch power from our embedded devices. Depending on the load we want to control, the key parameters to take into account to make a sound decision are voltage, current, and control frequency.

The following figure summarizes very well the different domains from the different available technologies:

Figure extracted from:

For most of our projects, where the actuators require voltages bellow 1000 V and currents bellow 1000 A, we will have to choose between MOSFETs or BJTs.

As we can see in the graph, BJT are suited for low frequencies up to 10 kHz and can handle more current that MOSFETs. Therefore, they are perfectly suited for static loads that require high currents, such as for example a motor.

MOSFETs on the other are capable of handling higher frequencies, which can make them a better choice if we want to drive our load with a PWM over 10kHz.

BJT’s waste some current whenever they’re switched on, regardless of whether the load is drawing anything. In a battery-powered device, using a BJT to power something whose load is highly variable but is often low will end up wasting a lot of energy. If a BJT is used to power something with a predictable current draw, though (like a LED), this problem isn’t as bad; one can simply set the base-emitter current to be a small fraction of the LED current.

Finally we need to take the price into account. For low power and low frequency control switches both BJT and MOSFETs are a good solution. If we are not concerned about the extra power consumed by the BJT during operation, their cheaper price can make them a better choice.

In the next post we will explore how to include these transistors to our circuits.

Bibliography

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