MOSFET Power Calculator Formula

Understand the math behind the mosfet power calculator. Each variable explained with a worked example.

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Formulas Used

Conduction Loss

conduction_loss_w = rds_on_ohm * pow(drain_current_a, 2)

Switching Loss

switching_loss_w = 0.5 * drain_voltage * drain_current_a * rise_fall_s * freq_hz

Total Power Loss

total_loss_w = rds_on_ohm * pow(drain_current_a, 2) + 0.5 * drain_voltage * drain_current_a * rise_fall_s * freq_hz

Variables

VariableDescriptionDefault
rds_on_mohmOn-Resistance (Rds_on)(mΩ)50
drain_current_aDrain Current(A)5
switching_freq_khzSwitching Frequency(kHz)100
rise_fall_nsRise + Fall Time(ns)50
drain_voltageDrain Voltage(V)12
rds_on_ohmDerived value= rds_on_mohm / 1000calculated
freq_hzDerived value= switching_freq_khz * 1000calculated
rise_fall_sDerived value= rise_fall_ns / 1000000000calculated

How It Works

MOSFET Power Losses

Conduction Loss

P_cond = Rds(on) x I_D^2

Switching Loss

P_sw = 0.5 x V_DS x I_D x (t_rise + t_fall) x f_sw

Total loss is the sum. Conduction loss dominates at low frequencies; switching loss dominates at high frequencies.

Worked Example

MOSFET with 50 mohm Rds(on), 5 A load, switching 12 V at 100 kHz with 50 ns transitions.

rds_on_mohm = 50drain_current_a = 5switching_freq_khz = 100rise_fall_ns = 50drain_voltage = 12
  1. 01Conduction = 0.05 x 25 = 1.25 W
  2. 02Switching = 0.5 x 12 x 5 x 50e-9 x 100000 = 0.15 W
  3. 03Total = 1.25 + 0.15 = 1.40 W

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