MOSFET to uC Guided Learning Q3: – How are MOSFET specifications misleading?

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 3

How are MOSFET specifications misleading?

This post also answers these closely related questions:

  • Why is my MOSFET not fully turning on?
  • Why is my MOSFET overheating?
  • What happens when a MOSFET falls out of saturation?

Clarification:

The DMN67D8L-7 MOSFET featured in the canonical article is rated for 230 mA. Why is this metric inappropriate for the featured application? Stated another way, could our Figure 3 relay with its 17 mA coil be replaced with a 200 mA relay?

Answer

The MOSFET is rated for ideal conditions that may or may not be applicable to the user’s circuit. In this example, the MOSFET is rated for an ID max of 230 mA when driven with a gate voltage of 10 V. In our application, the gate voltage is limited to 5 VDC. This results in a cascade of problems when we attempt to drive a 200 mA load:

  • With low gate voltage, the channel resistance increases.

  • The MOSFET is no longer saturated.

  • A non-saturated MOSFET operates in the linear range.

  • The load (relay) may not fully turn on.

  • The MOSFET may overheat leading to self destruction.

Tech Tip: Misinterpretation of MOSFET parameters is a difficult lesson. For good reasons, most datasheets present the ideal MOSFET characteristic. The engineer is responsible for the application details. Mistakes can damage your reputation as a circuit designer.

Next steps to learn more about MOSFETs

  • This relay drive application is further explored in this associated article. The article explores the limits of MOSFET application by driving larger loads with a 3.3 VDC microcontroller.

  • There are benefits to using a Darlington transistor as described in this article. Note that the transistor does not suffer from the high channel resistance when driven by a 3.3 VDC source. With a high DC current gain, the 3.3 VDC drive signal is sufficient to fully drive the transistor into saturation. The featured experiment is shown in Figure 1 where a 3.3 VDC Arduino microcontroller drives a Darlington pair transistor which is used to control a large 24 VDC contactor.

Figure 1: Prototype of the Arduino Nano 33 IoT microcontroller driving a Schneider LC1D09BD contactor via a BDX33C Darlington transistor
Figure 1: Prototype of the Arduino Nano 33 IoT microcontroller driving a Schneider LC1D09BD contactor via a BDX33C Darlington transistor975×731 110 KB

Figure 1: Prototype of the Arduino Nano 33 IoT microcontroller driving a Schneider LC1D09BD contactor via a BDX33C Darlington transistor.

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