MOSFET as a power switch in Power Electronics - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does MOSFET stand for?

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. It is a type of transistor used to control electrical power.

beginner

How does a MOSFET act as a power switch?

A MOSFET controls the flow of electricity by turning on or off when voltage is applied to its gate terminal, allowing it to switch power to a load efficiently.

beginner

What is the role of the gate in a MOSFET used as a power switch?

The gate controls the MOSFET by receiving voltage signals that open or close the channel between the source and drain, turning the switch on or off.

intermediate

Why are MOSFETs preferred for switching power in electronics?

MOSFETs have low resistance when on, fast switching speeds, and require very little current to control, making them efficient and reliable power switches.

beginner

What happens if the gate voltage is below the threshold voltage in a MOSFET?

If the gate voltage is below the threshold, the MOSFET remains off, and no current flows between the source and drain, acting like an open switch.

What terminal of a MOSFET controls its switching action?

A Drain

B Source

C Gate

D Body

The gate terminal receives voltage signals that control the MOSFET's switching between on and off states.

When a MOSFET is fully on, it behaves like:

A A closed switch with low resistance

B A resistor with high resistance

C An open switch

D A capacitor

A fully on MOSFET acts like a closed switch with very low resistance, allowing current to flow easily.

Which of these is a key advantage of using MOSFETs as power switches?

A Low gate current requirement

B High power loss

C Slow switching speed

D Requires mechanical movement

MOSFETs require very little current at the gate to switch, making them efficient for power control.

What happens if the gate voltage is less than the threshold voltage?

A MOSFET turns on

B MOSFET remains off

C MOSFET explodes

D MOSFET acts as a capacitor

Below the threshold voltage, the MOSFET stays off, blocking current flow.

In a MOSFET used as a power switch, the source and drain terminals are connected to:

A Control signals

B The gate

C Ground only

D Power supply and load

The source and drain connect the power supply and the load, allowing current to flow when the MOSFET is on.

Explain how a MOSFET works as a power switch in simple terms.

Think about how a light switch controls electricity to a lamp.

Describe the advantages of using a MOSFET instead of a mechanical switch for controlling power.

Consider why electronic devices use MOSFETs instead of regular switches.

Practice

1. What happens when a voltage is applied to the Gate of a MOSFET used as a power switch?

easy

A. The MOSFET heats up but does not conduct

B. The MOSFET turns OFF and blocks current

C. The MOSFET turns ON and allows current to flow

D. The MOSFET permanently damages

Solution

1. Step 1: Understand MOSFET Gate control

   Applying voltage to the Gate creates an electric field that allows current flow between Drain and Source.

2. Step 2: Effect on current flow

   When ON, the MOSFET acts like a closed switch, letting current pass through.

3. Final Answer:

   The MOSFET turns ON and allows current to flow -> Option C

4. Quick Check:

   Gate voltage ON = MOSFET ON [OK]

Hint: Gate voltage ON means MOSFET conducts current [OK]

Common Mistakes:

• Confusing ON and OFF states
• Thinking MOSFET heats up to conduct
• Assuming Gate voltage damages MOSFET

2. Which of the following is the correct symbol for an N-channel MOSFET used as a power switch?

easy

A. Arrow on the source leg pointing inward

B. A circle with a diagonal line through it

C. A rectangle with two terminals only

D. Arrow on the source leg pointing outward

Solution

1. Step 1: Identify N-channel MOSFET symbol

   The N-channel MOSFET symbol has an arrow on the source leg pointing inward.

2. Step 2: Compare options

   Arrow on the source leg pointing inward matches this description; arrow on the source leg pointing outward describes a P-channel MOSFET.

3. Final Answer:

   Arrow on the source leg pointing inward -> Option A

4. Quick Check:

   N-channel MOSFET arrow points inward [OK]

Hint: N-channel MOSFET arrow points inward on source [OK]

Common Mistakes:

• Mixing N-channel and P-channel symbols
• Confusing arrow direction
• Choosing symbols with missing terminals

3. Consider a MOSFET used as a power switch with a Gate voltage of 10V applied. What is the expected behavior of the MOSFET?

easy

A. It remains OFF because 10V is too high

B. It permanently damages the MOSFET

C. It oscillates between ON and OFF

D. It turns ON and allows current to flow

Solution

1. Step 1: Check Gate voltage level

   10V is a typical Gate voltage sufficient to fully turn ON a power MOSFET.

2. Step 2: Determine MOSFET state

   With Gate voltage applied, the MOSFET channel forms and current flows freely.

3. Final Answer:

   It turns ON and allows current to flow -> Option D

4. Quick Check:

   Gate voltage 10V = MOSFET ON [OK]

Hint: High Gate voltage turns MOSFET ON [OK]

Common Mistakes:

• Assuming high voltage damages MOSFET instantly
• Thinking MOSFET stays OFF at high voltage
• Confusing oscillation with switching

4. A MOSFET switch circuit is not turning ON even when 5V is applied to the Gate. What could be the likely cause?

medium

A. The MOSFET is a logic-level type

B. The Gate voltage is below the MOSFET's threshold voltage

C. The MOSFET is connected correctly

D. The Source and Drain are shorted

Solution

1. Step 1: Understand threshold voltage concept

   MOSFETs require a minimum Gate voltage (threshold voltage) to turn ON.

2. Step 2: Analyze 5V Gate voltage

   If the MOSFET is not logic-level type, 5V may be insufficient to reach threshold voltage, so it stays OFF.

3. Final Answer:

   The Gate voltage is below the MOSFET's threshold voltage -> Option B

4. Quick Check:

   Gate voltage < threshold = MOSFET OFF [OK]

Hint: Check if Gate voltage exceeds threshold voltage [OK]

Common Mistakes:

• Assuming 5V always turns MOSFET ON
• Ignoring MOSFET type (logic-level or not)
• Overlooking wiring errors

5. You want to use a MOSFET as a power switch to control a 12V motor that draws 5A. Which MOSFET characteristic is most important to ensure efficient switching?

hard

A. Low Drain-Source ON resistance (Rds(on))

B. High Gate threshold voltage

C. High Gate capacitance

D. Low maximum Drain current rating

Solution

1. Step 1: Identify key MOSFET parameter for power loss

   Low Drain-Source ON resistance (Rds(on)) reduces power loss and heat when conducting high current.

2. Step 2: Evaluate options

   High threshold voltage or gate capacitance reduce efficiency; low current rating risks damage.

3. Final Answer:

   Low Drain-Source ON resistance (Rds(on)) -> Option A

4. Quick Check:

   Low Rds(on) = efficient power switching [OK]

Hint: Choose MOSFET with low Rds(on) for power efficiency [OK]

Common Mistakes:

• Selecting MOSFET with high threshold voltage
• Ignoring current rating
• Overlooking gate capacitance effects