MOSFET to uC Guided Learning Q4: – How do I select a MOSFET suitable for a 3.3 VDC microcontroller?

Semiconductor Integrated Circuits (ICs)
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This article is part of a guided learning series to explore real-world applications of MOSFETS and microcontrollers.

:pushpin: Canonical Article: How to Interface a Microcontroller with a Relay Using a MOSFET
:blue_book: Learning Companion (Q&A): Explore All Questions

You are reading: Question 4

How do I select a MOSFET suitable for a 3.3 VDC microcontroller?

This post also answers these closely related questions:

  • What MOSFETs will work with a 3.3 VDC microcontroller?
  • Why won’t my microcontroller turn on my MOSFET?
  • What portion of the MOSFET datasheet should I review to determine maximum load current?

Answer

A 3.3 VDC microcontroller may be used to drive a MOSFET, provided we take precautions to ensure that the MOSFET is fully turned on. Consequently, the load becomes a significant factor in the circuit design. A full description is presented in this article.

What to watch for in datasheets

The steps are summarized as:

  1. Determine the maximum load current.

  2. Select a candidate MOSFET such as the Vishay SIHLU024-GE3.

  3. Using the datasheet-provided curves (drain current as a function of drain to source voltage), ensure that the load current is below the saturation point for the given gate voltage. The chart for the SIHLU024-GE3 is shown in Figure 1 (right). This MOSFET provides good performance with up to 7A drive capability.

Figure 1: Curves for drain current as a function of drain to source voltage for each MOSFET for select values of gate voltage: this includes the IRF520NPbF (left), IRLB8721PbF (middle), and SIHLU24-GE3 (right). The red horizontal line approximates that MOSFET saturation for a 3.3 VDC gate voltage.
Figure 1: Curves for drain current as a function of drain to source voltage for each MOSFET for select values of gate voltage: this includes the IRF520NPbF (left), IRLB8721PbF (middle), and SIHLU24-GE3 (right). The red horizontal line approximates that MOSFET saturation for a 3.3 VDC gate voltage.1035×401 129 KB

Figure 1: Curves for drain current as a function of drain to source voltage for each MOSFET for select values of gate voltage: this includes the IRF520NPbF (left), IRLB8721PbF (middle), and SIHLU24-GE3 (right). The red horizontal line approximates that MOSFET saturation for a 3.3 VDC gate voltage.

Tech Tip: It is difficult to drive a MOSFET directly from a low-voltage (e.g., 3.3 VDC) microcontroller. In almost all cases, the MOSFET is operating near the limit of saturation. This is not a good situation as a small change in an individual MOSFET’s characteristics or a change in operating conductions could cause a MOSFET to fall into the linear operation region. As a consequence, the load may not fully turn on and the MOSFET may overheat.

Precautions for driving a MOSFET from a 3.3 VDC microcontroller

A prudent designer will avoid operating the MOSFET at the theoretical saturation limit. A large safety margin is provided to account for:

  • Individual device variations
  • Temperature
  • Voltage fluctuations on the MOSFET e.g., reduced microcontroller drive voltage especially for battery powered applications

:writing_hand: Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey