MOSFET as a power switch in Power Electronics - Step-by-Step Execution

Concept Flow - MOSFET as a power switch

Apply Gate Voltage

↓

MOSFET Channel Conducts?

↓

Current Flows

↓

Load Powered

The flow shows how applying voltage to the MOSFET gate controls whether current flows through it, turning the load on or off.

Execution Sample

Power Electronics

V_gate = 5V
if V_gate > Threshold:
  MOSFET = ON
  Current flows
else:
  MOSFET = OFF
  No current

This code checks gate voltage to decide if the MOSFET switches on or off, controlling current flow.

Analysis Table

Step	Gate Voltage (V_gate)	Condition (V_gate > Threshold?)	MOSFET State	Current Flow	Load State
1	0	0 > 2? No	OFF	No	Off
2	1	1 > 2? No	OFF	No	Off
3	2	2 > 2? No	OFF	No	Off
4	3	3 > 2? Yes	ON	Yes	On
5	5	5 > 2? Yes	ON	Yes	On
6	0	0 > 2? No	OFF	No	Off

💡 Execution stops when gate voltage is below threshold, MOSFET turns OFF, stopping current.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
V_gate	0	0	1	2	3	5	0
MOSFET State	OFF	OFF	OFF	OFF	ON	ON	OFF
Current Flow	No	No	No	No	Yes	Yes	No
Load State	Off	Off	Off	Off	On	On	Off

Key Insights - 2 Insights

Why does the MOSFET not conduct when gate voltage equals the threshold voltage?

Because the MOSFET typically requires gate voltage to be greater than the threshold to fully turn ON, as shown in steps 3 and 4 where 2V (threshold) is not enough but 3V is.

What happens to current flow when gate voltage drops back to zero?

Current stops flowing immediately because the MOSFET switches OFF, as seen in step 6 where gate voltage is 0V and current flow is No.

Visual Quiz - 3 Questions

Look at the execution table at step 4. What is the MOSFET state and why?

A ON, because gate voltage is above threshold

B OFF, because gate voltage equals threshold

C ON, because current is flowing

D OFF, because load is off

💡 Hint

Check the 'Condition' and 'MOSFET State' columns at step 4 in the execution table.

At which step does the current start flowing through the MOSFET?

A Step 2

B Step 3

C Step 4

D Step 6

💡 Hint

Look at the 'Current Flow' column in the execution table to find when it changes from No to Yes.

If the threshold voltage was 1V instead of 2V, at which step would the MOSFET first turn ON?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Compare gate voltages with the new threshold and check the 'MOSFET State' in variable_tracker.

Concept Snapshot

MOSFET as a power switch:
- Apply voltage to gate terminal
- If gate voltage > threshold, MOSFET turns ON
- ON state allows current flow to power load
- If gate voltage ≤ threshold, MOSFET is OFF
- OFF state stops current, load is off

Full Transcript

This visual execution shows how a MOSFET works as a power switch. When the gate voltage is below the threshold, the MOSFET stays OFF and no current flows, so the load is off. When the gate voltage rises above the threshold, the MOSFET turns ON, allowing current to flow and powering the load. The execution table traces gate voltage changes step-by-step, showing MOSFET state and current flow. Key moments clarify why the MOSFET only conducts above threshold voltage and what happens when voltage drops. The quiz tests understanding of these steps and how threshold changes affect switching.

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