MOSFET Gate Drive Calculator — switching and conduction loss

Gate charge, drive current, switching loss and conduction loss for a MOSFET at your switching frequency.

Example: At 100 kHz switching 5 A, this MOSFET dissipates 1 W — mostly conduction loss.

Inputs

Total gate charge Q_g(C)

From the datasheet. Type 20n for 20 nC.

Gate drive voltage(V)

Gate resistor(Ω)

Slows switching to control ringing and EMI, at the cost of switching loss.

Switching frequency(Hz)

Drain current(A)

Drain voltage (off)(V)

R_ds(on)(Ω)

At your gate drive voltage, not the headline figure.

Result

Total loss

1 W

Conduction loss: 500 mW
Switching loss: 480 mW
Gate drive loss: 20 mW
Peak gate current: 1 A
Switching time: 20 ns
Dominant loss: conduction

conduction loss dominates at 500 mW of 1 W total. A lower R_ds(on) part is the fix — gate drive changes will not help.
The driver must source 1 A peak. A driver rated below that will switch more slowly than calculated, raising switching loss above this estimate.
R_ds(on) rises with temperature — typically 40–80% higher at 100 °C. Conduction loss at temperature could be closer to 800 mW, which is what the thermal design must handle.

Loss against switching frequency

Frequency	Conduction	Switching	Gate drive	Total
10 kHz	500 mW	48 mW	2 mW	550 mW
50 kHz	500 mW	240 mW	10 mW	750 mW
100 kHz	500 mW	480 mW	20 mW	1 W
250 kHz	500 mW	1.2 W	50 mW	1.75 W
500 kHz	500 mW	2.4 W	100 mW	3 W
1 MHz	500 mW	4.8 W	200 mW	5.5 W

Formula

P_cond = I² × R_ds(on)    P_sw = ½ × V_ds × I × (t_r + t_f) × f

Worked example

At 100 kHz switching 5 A, this MOSFET dissipates 1 W — mostly conduction loss.

I_gate = V_drive / R_gate
10 V / 10 Ω
1 A
t_switch = Q_g / I_gate
20 nC / 1 A
20 ns
A first-order estimate — the real edge is slower through the Miller plateau.
P_cond = I² × R_ds(on)
(5 A)² × 20 mΩ
500 mW
P_sw = ½ × V_ds × I × (t_r + t_f) × f
0.5 × 48 V × 5 A × 40 ns × 100 kHz
480 mW
P_gate = Q_g × V_drive × f
20 nC × 10 V × 100 kHz
20 mW
Burned in the driver and gate resistor, not in the MOSFET.

Frequently asked questions

How do I choose a gate resistor?

It trades switching speed against ringing and EMI. Smaller means faster edges and lower switching loss, but more overshoot and radiated noise. 10 Ω is a common starting point; tune it while watching the drain waveform on a scope.

Why is my MOSFET hot even though R_ds(on) is tiny?

Switching loss. Conduction loss is I²R and may be negligible, but every transition dissipates energy proportional to V × I × transition time × frequency. At high frequency that dominates, and a lower-R_ds(on) part will not help at all.

What gate voltage does a MOSFET need?

Standard parts want 10 V; logic-level parts are specified at 4.5 V or lower. Driving a standard MOSFET from 5 V logic leaves it partly on with a much higher R_ds(on) than the datasheet headline — always read the R_ds(on) curve at your actual V_GS.

Where does the gate drive power go?

Into the driver and the gate resistor, not the MOSFET. Charging and discharging the gate capacitance dissipates Q_g × V_drive per cycle. It only matters at high frequency or with large parts, but it is what heats the driver IC.

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