Guide to Selecting and Controlling a MOSFET for 3.3 VDC Logic Applications

[APDahlen]

Guide to Selecting and Controlling a MOSFET for 3.3 VDC Logic Applications
It is common knowledge that MOSFETs provide an easy-to-use solution for controlling digital loads. In theory, a microcontroller can turn on a MOSFET by directly applying voltage to the MOSFET’s gate terminal. In practice, things are not so simple, especially if we use 3.3 VDC logic.

The MOSFET is a voltage-controlled device where the gate voltage determines the conductivity. Classic MOSFETs such as the IRF520 require a gate voltage of approximately 10 VDC to reach the datasheet specified low on resistance. This presents a challenge when driving a MOSFET from a 3.3 VDC microcontroller such as the Arduino Nano 33 IoT pictured in Figure 1.
This engineering brief explores techniques and identifies MOSFETs compatible with the 3.3 VDC gate voltage. This article is part of a series that includes:

• How to Interface a Microcontroller with a Relay Using a MOSFET

• Driving a Large Relay From a 3.3 VDC Microcontroller Using an NPN Darlington Transistor

• Circuit Myths: MOSFETs Require No Gate Current

• Breadboard an Arduino Controlled BLDC Motor Driver

The first article shows how to drive moderately sized loads using a 5.0 VDC Arduino Nano Every. The second article presents a Darlington pair driven by the 3.3 VDC Arduino Nano 33 IoT. The next two present dynamic considerations when a MOSFET is used in a high speed PWM circuit.

Figure 1: Image of the SIHLU24-GE3 MOSFET with the Arduino Nano 33 IoT microcontroller in the background. The 1 kΩ gate current limiting resistor and the 100 kΩ discharge resistor are in the foreground.

Tech Tip: A digital semiconductor switch such as the MOSFET featured in this article may be described in terms of static (steady on) or dynamic (rapidly switching). This article explores the steady state static conditions. This article explores driver requirements for dynamic switching, such as when a MOSFET is used as part of a Pulse Width Modulation (PWM) circuit.

MOSFET specifications

Let’s take a few moments to review the MOSFET datasheet before we describe the microcontroller and the MOSFET interface. For our purposes, it is sufficient to review the channel resistance and design maximums such as drain to source voltage and current.

For this discussion, we limit the MOSFET to simple on-off control of a load. Pulsed or Pulse Width Modulation (PWM) control would require a deeper exploration of the datasheet.

MOSFET channel resistance

Recall that a MOSFET is constructed as a semiconductor channel, drain to source. The conductivity of this channel is a function of the applied gate to source voltage. The channel is closed (no conduction) when the gate voltage of the enhanced MOSFET is zero. The channel conductivity increases as the gate voltage increases. While this is a complex process with temperature dependency, we can obtain a first approximation by reviewing the curves that describe the drain current as a function of the source voltage.

Figure 2 presents the curves for several representative MOSFETs including the IRF520NPbF, IRLB8721PbF, and SIHLU024-GE3. When viewed from left to right, each curve has a linear increasing region followed by a horizontal section. Recall that the rising section describes the linear Ohmic response of the MOSFET while the flat portion indicates the saturation region where current is relatively constant for increasing drain to source voltages for a given gate voltage.

These graphs can provide go, no-go information for a given gate voltage such as when a 3.3 VDC logic device is used. For example, consider the 3.0 VDC curve for the IRLB8721PbF (Figure 2 middle), the regions under the curve are acceptable while regions above are no-go zones. For example, with a 3.0 VDC gate drive, the IRLB8721PbF MOSFET will not support a 5 A current.

A red horizontal line representing the MOSFET saturation for a gate voltage of 3.3 VDC has been added to each chart. Based on these charts, we see:

• the IRF520 (left) is inappropriate for use with 3.3 VDC logic as the curve for the 3.3 VDC gate voltage does not even appear on the chart. The 3.3 VDC gate voltage appears to be below the MOSFET, cut-off region.

• the IRLB8721 (middle) is capable of providing 10 A

• the SIHLU024-GE3 (right) is capable of providing about 7 A

Figure 2: 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: The MOSFET drain to source specification is often misunderstood. A common mistake is to assume the MOSFET has the ideal on-resistance that is prominently displayed on the first page of the datasheet. The resulting designs will have higher than anticipated power dissipation in the MOSFET.

To better understand the resistance specification, consider Figure 2. A green dot has been added indicating the position from which the datasheet provided resistance measurements were computed. Observe that the dot is placed high on the family of curves where it is associated with the higher gate voltages. For example, the IRLB8721PbF (middle), resistance is calculated with a gate voltage of 10 VDC and a 31 A current. The corresponding drain to source voltage in approximately 0.2 VDC. This yields a drain to source resistance of approximately 6.5 mΩ.

Again, this resistance is valid for a single operation point. A more realistic value for our 3.3 VDC logic is seen as the blue dot placed on the graph for the IRLB8721PbF (middle). This dot is positioned at the junction of 10 A and 0.4 VDC drain to source. The calculated resistance is 40 mΩ which is far from the 8.7 mΩ specified in the datasheet.

MOSFET drain to source voltage

For a given power supply, we must select a MOSFET that can withstand the maximum anticipated voltage. We should also include a safety margin to protect against extreme operating conditions. For the MOSFETs featured in Figure 3, we see the design maximum voltages as:

• 100 VDC for the IRF520NPbF

• 30 VDC for the IRLB8721PBF

• 60 VDC for the SIHLU024-GE3

Providing a 75% safety margin, these components are generally acceptable for nominal 75, 24, and 45 VDC supplies respectively.

Tech Tip: Flyback voltage must be carefully considered when selecting a MOSFET. Recall that an inductive load such as a relay or motor will produce a high voltage spike upon turn off. Diodes such as the venerable 1N4004G or other types of snubbers are used to capture this high voltage spike. The flyback design has important voltage and time components as described in this article. You will see that accommodating the high voltage spike is desirable as the relay / contactor will turn off faster. However, this is constrained by the natural limitations of the drive MOSFET’s maximum V_{DS} or the drive transistors maximum V_{CE}.

Note that loss of the protection diode or snubber will often result in the destruction or significantly shorten the life of the drive semiconductor.

MOSFET maximum drain current

The MOSFET’s design maximum is another prominent specification typically found at the top of the datasheet. The typical device has a continuous and a pulsed specification. For example, the featured IRLB8721PbF has an absolute rating of 44 A continuous and 250 A pulsed with junction temperature as a limitation.

MOSFET maximum power dissipation

Junction temperature is a limiting factor for all semiconductors. Since we are using the MOSFET as a switch, we shouldn’t need to worry about junction temperature. That is provided we select a MOSFET with a low on resistance relative to the chosen load. The resulting low V_{DS} will ensure the MOSFET runs cool without a heatsink.

As a review, recall that static power dissipation is calculated as

P_{static} = I_{D} x V_{DS}

As an example, the blue dot on the IRLB8721PBF curve (Figure 2 middle) represents a power dissipation of:

P_{static} = 10 x 0.4 = 4 W

This is a large amount of power necessitating a proper heat sink as the TO-220 packaged device is unable to dissipate the power without a significant increase in junction temperature.

As we conclude this section, we once again look at the blue dot and consider the impact of the gate voltage. As we increase the gate voltage while holding the current constant, the dot slides to the right. The result is a lower V_{DS} and a reduction in power dissipation. This is desirable but has a natural limit as there is a design maximum for the gate voltage.

Demonstration circuit

The Arduino microcontroller to MOSFET demonstration is shown in Figure 3 with a schematic in Figure 4. The Arduino and MOSFET can be seen in the foreground, The DIN rail contains a Phoenix contact control relay and a Schneider Electric 3-phase contactor. Both devices feature 24 VDC coils and both devices have integral flyback protection diodes.

Figure 3: Image of the MOSFET based relay driver. An interposing relay is used between the MOSFET and the large three phase contactor.

Figure 4: Schematic of the 3.3 VDC Arduino to MOSFET circuit. An interposing relay is used to accommodate the higher flyback voltage of the larger contactor’s TVS diode.

Tech Tip: The 1 KΩ series gate resistor is adequate for this simple design on off design. You may want to include an appropriate gate driver as described in this article.

Flyback voltage requires an interposing relay

Close inspection of the Figure 4 schematic show that the control relay features a conventional flyback diode while the larger contactor features a Transient Voltage Suppression (TVS) diode. As described in this article the TVS diode is an essential component that allows the contactor to quickly open. While this is highly desirable to prevent contact welding and arc suppression it makes the contactor difficult to drive. Assuming a 24 VDC rail, the TVS diode clamps at approximately 70 VDC relative to ground as opposed to a conventional diode that camps at approximately 24.7 VDC.

The MOSFETs featured in this article can easily handle the 24.6 VDC turn off event. However, they will not withstand the 70 VDC turn off spike. Consequently, the smaller interposing relay is necessary.

MOSFET protection

In addition to the flyback diode, the MOSFET is protected by two resistors. The 1 kΩ resistor limits the current as the MOSFET’s gate capacitance is charged and then discharged by the microcontroller. The 100 kΩ resistor provides protection for periods when the Microcontroller’s output is in a high impedance condition such as when the microcontroller is initially powered, programmed, and to protect against programming errors. This is a simple circuit that will discharge the MOSFET’s gate charge thereby turning the MOSFET off.

Tech Tip: The placement of the two protection resistors is critical when 3.3 VDC logic is used. With the circuit presented in Figure 4, the full 3.3 VDC is applied to the MOSFET gate thereby providing the lower on resistance possible. Were the 100 kΩ resistor moved to the gate side of the MOSFET, it would form an undesirable voltage divider with the 1 kΩ resistor.

Parting thoughts

MOSFET selection was an important part of this project. Research suggests that there is a design trade-off for MOSFETs operating at low gate voltage. Low voltage operation appears to come at the expense of V_{DS}. The featured SIHLU024-GE3 was one of the best general purpose devices as it allows moderate current (approximately 7 A with V_{GS} = 3.3 VDC) with the ability to work in a 60 VDC system. Unfortunately, the V_{DS} limitation were insufficient to drive the large contactor with its integral TVS diode. Consequently, the solution presented in this article is not as elegant as the Darlington solution.

As a challenge, see if you can use the DigiKey tools to locate a single MOSFET with the capability of directly driving the contactor with its 70 VDC clamped voltage. Leave a comment below if you are successful. Also, be sure to test your knowledge by answering the questions and critical thinking questions at the end of this note.

Best wishes,

APDahlen

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About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided an additional 500 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls. A collection of Dahlen’s Arduino educational articles can be found at Arduino education content.

Questions:

The following questions will help reinforce the content of the article.

1. State the conditions required to turn on a MOSFET.

2. Define the terms static and dynamic as they relate to the on and of conditions for a semiconductor switch such as a MOSFET.

3. What challenges are encountered when driving a MOSFET with a 3.3 VDC logic signal.

4. Describe the relationship between a MOSFET’s gate voltage and the channel resistance.

5. Why was it necessary to use an interposing relay to drive the large 3-phase contactor?

6. Why is the venerable IRF520 unsuited for direct 3.3 VDC logic drive?

7. Use the curves in Figure 2 to determine the power dissipation of the IRLB8721PbF (middle) when sinking 3 A when $V_{GS} = 3.0 VDC.

8. With respect to the previous question, use the datasheet to estimate the junction temperature assuming no heatsink and ambient temperature. Should a heat sink be used? Hint: Did you consider operation at elevated temperatures?

9. The SIHLU024-GE3 MOSFET offers high performance when we consider the on resistance, current, and voltage. Search DigiKey and see if you can locate an improved N-channel MOSFET that can operate with a 3.3 VDC gate voltage, provide at least 10 A, while operating on a 100 VDC source.

10. With regards to the Figure 4 schematic, why is the 100 kΩ resistor placed on the Arduino side of the 1 kΩ resistor? What would happen if it were move to the other side of the 100 kΩ resistor.

Critical thinking questions

These critical thinking questions expand the article’s content allowing you to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.

11. MOSFETs are commonly found in motor control circuits such as three-phase H-bridge for a brushless DC (BLDC) motor. Research this application and then identify the specific components that allow the MOSFET to be efficiently driven by low voltage logic.

12. Lowering the voltage for logic circuits is a positive trend that lowers the power consumption. We have seen 5 VDC logic replaced by 3.3 VDC with 1.8 VDC circuit seen in select applications. Describe the suitability of a MOSFET with 1.8 VDC logic.

13. With respect to the previous question, what technology is best suited for direct to microcontroller interface. Hint: Contrast and compare a MOSFET and a Darlington BJT.

14. This article is focused on static application of the MOSFET. There is an implicit suggestion that the Darlington BJT may be better suited for this application. Does this situation change when a dynamic PWM drive is used?

15. Research MOSFET specifications and develop a strategy for determining the datasheet specified channel resistance. Hint: What is the desired marketing target and what things are considered in the optimization process.