How to Calculate Conduction Losses in MOSFET: Simple Formula & Example

To calculate conduction losses in a MOSFET, multiply the square of the current flowing through it by its on-resistance (R_{DS(on)}). The formula is P = I^2 × R_{DS(on)}, where P is the conduction loss in watts and I is the current in amperes.

Syntax

The basic formula to calculate conduction losses in a MOSFET is:

P = I^2 × R_{DS(on)}

• P: Conduction loss in watts (W)
• I: Current flowing through the MOSFET in amperes (A)
• R_{DS(on)}: MOSFET's on-state resistance in ohms (Ω)

This formula assumes the MOSFET is fully on and the current is steady.

python

P = I**2 * R_DS_on

Example

This example calculates conduction losses for a MOSFET with an on-resistance of 0.05 Ω carrying 10 A of current.

python

def conduction_loss(I, R_DS_on):
    return I**2 * R_DS_on

current = 10  # Amperes
R_DS_on = 0.05  # Ohms
loss = conduction_loss(current, R_DS_on)
print(f"Conduction Loss: {loss} W")

Output

Conduction Loss: 5.0 W

Common Pitfalls

• Using the wrong resistance value: Always use the R_{DS(on)} at the operating temperature and gate voltage.
• Ignoring current waveform: For non-constant current, calculate losses using RMS current, not peak current.
• Neglecting switching losses: Conduction loss is only part of total losses; switching losses also matter.

python

def wrong_loss(I, R_DS_on):
    # Using peak current instead of RMS
    return I**2 * R_DS_on  # Incorrect formula

# Correct way

def correct_loss(I_rms, R_DS_on):
    return I_rms**2 * R_DS_on

Quick Reference

Remember these key points when calculating conduction losses:

• Use RMS current for varying currents.
• Check R_{DS(on)} from the MOSFET datasheet at your operating conditions.
• Conduction loss increases with the square of current.
• Combine conduction and switching losses for total power loss.

Key Takeaways

Conduction loss in a MOSFET is calculated as P = I² × R_DS(on).

Always use RMS current and correct R_DS(on) values from the datasheet.

Conduction losses grow with the square of the current, so small current increases cause bigger losses.

Conduction loss is only part of total MOSFET losses; consider switching losses too.

Verify operating conditions to select the right R_DS(on) for accurate loss calculation.