mosfet sizing

Electronics #2 – MOSFET sizing challenges and considerations

MOSFET sizing dilema

I had to deal with an issue revolving around MOSFET sizing; hence today’s video. The fact of the matter is that sizing a transistor should be one of the fundamentals an electrical engineer should know, but many struggle with the concept. Remember to ALWAYS read your datasheets and breadboard the circuit before soldering it onto your PCB.

Everyday Transistors

A transistor is a semiconductor based device. It generally (there are exceptions to this generalization; keep that in mind) has three pins which can be used to drive a load through the means of a signal. There are hundreds of transistor circuits in which they are used as amplifiers, signal generators, oscillators and other useful devices. However, the most fundamental application is to drive a load with an ON/OFF signal coming from a controller. For this purpose, one must satisfy several conditions:

  • Correct Load Voltage/Current – The device must be able to drive the given load; its electrical characteristics must meet or exceed the ones of the load.
  • Correct Toggle Voltage/Current – The transistor must be able to transition between ON & OFF states based on the supplied current or voltage.

MOSFET driven loads

Driving your loads such as motors, heaters, relays and speakers can be done with a MOSFET. This type of a transistor can be toggled by a voltage level at the gate pin and is very often used in DIY projects; especially with Arduino. There are several “watch-outs” when it comes to using these devices correctly and achieving the expected outcome. As mentioned above, you need to make sure that the MOSFET you have selected, will be able to trigger with the voltage level transition of the micro controller (0v-5v in case of an Arduino) and be able to drive your load.

N-Channel MOSFET

The N-Channel MOSFET will have the Source pin connected to ground which facilitates your task. You will need to make sure that the threshold voltage for Vgs is between 1v and 5v. This means that it will turn ON at that voltage level allowing you to drive the load.
mosfet sizing

P-Channel MOSFET Challenges

The P-Channel MOSFET can be seen as a mirror image of the N-Channel counterpart. The high voltage is now connected to the Source pin. This induces the opposite calculation for the threshold. You will have to take into account the voltage of your load minus the driver signal in order to calculate if the device will in fact trigger.

Conclusion

ALWAYS read the datasheet and make sure to pay attention to the threshold voltage. ALWAYS create a diagram on paper to make sure you are using the right voltage levels and will be able to toggle your load.