MOSFET power loss calculator

How it works

The calculator adds the standard first-order loss terms for a MOSFET that switches an inductive load hard, with a flat-topped current I_D while it is on:

P_cond = I_rms² · R_DS(on)(T_j),   I_rms = I_D · √D
P_sw = ½ · V_DS · I_D · (t_r + t_f) · f_sw
P_oss = ½ · C_oss · V_DS² · f_sw
P_diode = V_F · I_D · 2t_dead · f_sw
P_gate = Q_g · V_GS · f_sw

• Conduction uses R_DS(on) at the junction temperature: either R₂₅·(1 + α(T_j − 25)) or the exponential form R₂₅·(1 + α)^{T_j − 25} of the Infineon note below, with α per kelvin.
• Switching treats each transition as a linear overlap of voltage and current, which the Infineon note describes as sufficient for a loss balance and as the worst case.
• Output capacitance: the energy in C_oss is lost in the channel at each hard turn-on. C_oss falls with voltage, so use the energy-related value C_o(er) if the datasheet gives one.
• Body diode, if selected, conducts I_D through both dead times of each period, as in a synchronous converter.
• Gate drive power is dissipated in the gate drive path (the driver, the gate resistor and the MOSFET’s internal gate resistance), mostly in the driver when the gate resistance is low. It is shown but not added to the MOSFET’s loss.

Junction temperature

With a thermal resistance, the calculator solves T_j = T_a + R_th · P(T_j), where R_th is R_th(j-a) or R_th(j-c) + R_th(c-s) + R_th(s-a). Because R_DS(on) rises with temperature, the loss and the temperature are found together. If the loss grows faster with temperature than the thermal path can remove it, there is no steady state, and the calculator reports thermal runaway.

Sources

• D. Graovac, M. Pürschel and A. Kiep, MOSFET Power Losses Calculation Using the Data-Sheet Parameters, Infineon application note, 2006: conduction losses, R_DS(on) temperature dependence, linear switching transitions.
• L. Balogh, Fundamentals of MOSFET and IGBT Gate Driver Circuits, Texas Instruments application report SLUA618A: gate charge loss and where it is dissipated.
• The output capacitance and dead-time diode terms are the usual first-order estimates.

Limits

• Hard switching only. Soft-switched (ZVS or ZCS) designs lose much less in switching than this predicts.
• Ringing, the diode’s reverse recovery, common-source inductance and current ripple are not modeled. Reverse recovery can add substantially to turn-on loss in hard-switched bridges.
• Datasheet rise and fall times are measured at the datasheet’s test conditions (gate resistance, current, voltage). Real transitions depend on your gate drive and layout.
• One temperature for the whole die, and steady state: no thermal transients.

Worked examples

1. 48 V, 10 A, 25% duty, 200 kHz, with T_j set to 100 °C. R_DS(on) = 5 mΩ at 25 °C, α = 0.6 %/K linear, t_r = 20 ns, t_f = 15 ns, C_oss = 300 pF, Q_g = 30 nC at 10 V. R_DS(on) at 100 °C is 5 · (1 + 0.006 · 75) = 7.25 mΩ, and I_rms = 10 · √0.25 = 5 A, so conduction is 0.181 W. Switching is ½ · 48 · 10 · 35 ns · 200 kHz = 1.68 W and C_oss adds 0.0691 W: 1.93 W in the MOSFET. The driver dissipates 0.0600 W.

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2. The same MOSFET on a 20 K/W heat sink at 40 °C ambient, with R_th(j-c) = 1 K/W and an interface of 0.5 K/W. The junction settles at 81.2 °C, where R_DS(on) is 6.69 mΩ and the MOSFET dissipates 1.92 W.

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3. 12 V, 20 A, 60% duty, 500 kHz, no heat sink. R_DS(on) = 2 mΩ at 25 °C with the exponential model at 0.5 %/K, t_r = t_f = 10 ns, C_oss = 1 nF, Q_g = 40 nC at 5 V and R_th(j-a) = 40 K/W at 50 °C ambient. The junction reaches 132 °C with 2.06 W in the MOSFET, 18 °C below a 150 °C limit.

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