How do you calculate power loss?

In general, P_V = P_in − P_out = P_in · (1 − η). For a single semiconductor, add conduction and switching losses: for a MOSFET P_V = I_rms² · R_DS(on) + (E_on + E_off) · f_sw, for an IGBT P_V = V_CE0 · I_avg + r_CE · I_rms² + (E_on + E_off) · f_sw.

How do you calculate power loss from efficiency?

Use P_V = P_in · (1 − η) when the input power is known, or P_V = P_out · (1/η − 1) when the output power is known. A power supply with 1,000 W output and 95 % efficiency turns about 53 W into heat.

How do you calculate IGBT power loss?

Conduction losses follow from P_cond = V_CE0 · I_avg + r_CE · I_rms², switching losses from P_sw = (E_on + E_off) · f_sw, scaled to DC-link voltage and current. Add the freewheeling diode losses: V_F0 · I_F,avg + r_F · I_F,rms² + E_rr · f_sw.

How do I calculate MOSFET switching losses?

Most accurately with the datasheet switching energies: P_sw = (E_on + E_off) · f_sw. Without energy data, the linear approximation P_sw ≈ ½ · V_DS · I_D · (t_r + t_f) · f_sw helps. It often underestimates the losses because output capacitance and reverse recovery are missing.

What are conduction losses?

Conduction losses occur as long as a semiconductor is on and carries current. In a MOSFET the channel acts like a resistor (P = I_rms² · R_DS(on)), in an IGBT or diode like a threshold voltage plus a resistor (P = V_0 · I_avg + r · I_rms²). They depend on current and temperature, not on switching frequency.

Why does power loss increase with temperature?

In MOSFETs, R_DS(on) increases with junction temperature, for silicon roughly 1.5 to 2 times between 25 °C and 125 °C. IGBT switching energies also rise with temperature. That is why you calculate with datasheet values at operating temperature and iterate with the junction temperature.

What does Ptot mean in a datasheet?

P_tot is the maximum permissible power dissipation at a case temperature of 25 °C: P_tot = (T_j,max − 25 °C) / R_th,jc. It assumes ideal cooling and is never reached in practice. For the design, what counts is the actual power loss in operation and the total thermal resistance to ambient.

What comes after the power loss calculation?

With P_V you calculate the junction temperature T_j = T_a + P_V · R_th,ja and check that it stays below T_j,max. The other way round, R_th = (T_j,max − T_a) / P_V gives the highest permissible thermal resistance, which determines the heat sink choice.

Power loss calculation for MOSFETs and IGBTs

Power loss is the difference between input and output power: P_V = P_in − P_out = P_in · (1 − η). All of it turns into heat that the heat sink has to remove. In power semiconductors it consists mainly of conduction and switching losses, which this calculator determines from datasheet values.

What is power loss?

No converter is lossless. Whatever electrical power does not reach the output is turned into heat in semiconductors, inductors, capacitors and copper traces. That heat raises the junction temperature and has to pass through the package, the thermal interface material and the heat sink into air or coolant. This is why power loss is the first input of every thermal design. In a power semiconductor it is made up of these parts:

Conduction losses: They occur as long as the device is on and carries current. In a MOSFET they are set by the channel resistance R_DS(on), in an IGBT or diode by the forward voltage.
Switching losses: At every turn-on and turn-off, current and voltage overlap for a short time. The energy per switching event times the switching frequency gives the switching loss.
Diode losses: Bridge circuits add the conduction and reverse-recovery losses of the freewheeling or body diode.
Gate-drive and blocking losses: Gate charge losses are mostly dissipated in the driver and gate resistor, and blocking losses from leakage current are usually negligible. The calculator therefore leaves both out.

Power loss calculator

Known powerOutput power Pout

Efficiency η

Power loss formulas for efficiency, MOSFET, IGBT and diode

All equations give average values over one switching period or one fundamental period. Always use datasheet values at the expected junction temperature.

Power loss from efficiency

P_V = P_in − P_out

Conservation of energy: whatever does not reach the output becomes heat.

P_V = P_in · (1 − η)

When the input power is known, for example from a measurement at the mains input.

P_V = P_out · (1/η − 1)

When the output power is known, for example the rated power of an inverter. At 10 kW output and 97 % efficiency that is about 309 W.

MOSFET power loss calculation: conduction and switching losses

P_cond = I_rms² · R_DS(on)(T_j)

Conduction losses from RMS current and on-state resistance at operating temperature. For silicon MOSFETs, R_DS(on) rises roughly 1.5 to 2 times between 25 °C and 125 °C. The exact factor comes from the R_DS(on) versus T_j curve in the datasheet.

P_sw = (E_on + E_off) · f_sw

Switching losses from the datasheet switching energies. They come from a double-pulse test and include effects such as output capacitance and reverse recovery of the opposite diode. They only apply to the voltage, current, gate resistance and temperature of that test.

P_sw ≈ ½ · V_DS · I_D · (t_r + t_f) · f_sw

Linear approximation for hard switching when no switching energies are given. It often underestimates the losses because output capacitance and reverse recovery are missing, and the datasheet switching times only hold for the gate resistance stated there.

P_V = P_cond + P_sw

Total MOSFET losses. The gate-drive power Q_G · V_GS · f_sw is mostly dissipated in the driver and gate resistor and barely heats the chip.

IGBT power loss and IGBT switching loss

P_cond = V_CE0 · I_avg + r_CE · I_rms²

The on-state characteristic is approximated by a straight line made of threshold voltage V_CE0 and slope resistance r_CE. That is why average and RMS current enter separately.

P_sw = (E_on + E_off) · f_sw · (V_DC / V_ref) · (I / I_ref)

Switching losses from the datasheet energies, scaled linearly to your operating point. This is an approximation: the energies do not depend exactly linearly on voltage and current, and more detailed models use exponents.

P_sw = (1/π) · (E_on + E_off) · f_sw · (V_DC / V_ref) · (Î / I_ref)

In an inverter with sinusoidal current, each IGBT switches only during one half-wave and at a changing current. Using the peak value Î and the factor 1/π gives the average over one fundamental period.

I_avg = Î · (1/(2π) + m · cos φ / 8), I_rms² = Î² · (1/8 + m · cos φ / (3π))

Average and RMS IGBT current in a sinusoidally modulated inverter, depending on modulation index m and power factor cos φ. For the diode the same expressions apply with a minus sign before the m term.

Freewheeling diode losses

P_D = V_F0 · I_F,avg + r_F · I_F,rms² + E_rr · f_sw

Diode conduction losses plus reverse-recovery losses at commutation. Scale E_rr to the operating point in the same way as the transistor energies.

P_rr ≈ Q_rr · V_DS · f_sw

Estimate for MOSFET half bridges when only the reverse-recovery charge Q_rr of the body diode is given. A large part of this loss occurs in the opposite switch at turn-on.

Worked example: SiC MOSFET in a 20 kHz converter

A 650 V SiC MOSFET in a TO-247 package runs in a hard-switched DC/DC converter. It switches 20 A at 400 V with f_sw = 20 kHz, and the RMS current through the transistor is 15 A. For an assumed junction temperature of 100 °C, the datasheet gives R_DS(on) = 40 mΩ and, at 400 V and 20 A, a switching energy E_on + E_off = 250 µJ. The thermal path consists of R_th,jc = 0.6 K/W, an insulating thermal pad with 0.4 K/W and a heat sink with 2.5 K/W at T_a = 40 °C. The values are generic but typical for this device class.

P_cond = I_rms² · R_DS(on) = (15 A)² · 0.040 Ω = 9.0 W
P_sw = (E_on + E_off) · f_sw = 250 µJ · 20 kHz = 5.0 W
P_V = P_cond + P_sw = 9.0 W + 5.0 W = 14.0 W
For comparison, linear approximation with t_r + t_f = 35 ns: ½ · 400 V · 20 A · 35 ns · 20 kHz = 2.8 W
T_j = T_a + P_V · R_th,ja = 40 °C + 14.0 W · (0.6 + 0.4 + 2.5) K/W = 89 °C

At 89 °C the junction temperature is below the assumed 100 °C, so the R_DS(on) used is slightly conservative and no further iteration is needed. The linear approximation would have clearly underestimated the switching losses at 2.8 W instead of 5.0 W.

After the power loss: junction temperature and heat sink

Power loss alone does not tell you whether a device runs too hot. Only together with the thermal resistance does it give you a temperature.

Check the junction temperature

T_j = T_a + P_V · R_th,ja

Power loss and the total thermal resistance from junction to ambient give you T_j. If it is well below T_j,max, the cooling is sufficient.

Calculate junction temperature →

Size the heat sink

R_th,sa ≤ (T_j,max − T_a) / P_V − R_th,jc − R_th,cs

The other way round, P_V gives the highest permissible thermal resistance of the heat sink. Plan in a margin to T_j,max.

Calculate thermal resistance →

In practice: making the results reliable

The formulas are quick to apply. Whether the result holds up depends on the input values.

Values at operating temperature: Datasheets usually give R_DS(on), V_CE0, r_CE and the switching energies at 25 °C and at 125, 150 or 175 °C. For the design, the value at the expected junction temperature is what counts. For SiC MOSFETs, the rise of R_DS(on) depends strongly on manufacturer and technology.
Iterate: Losses depend on T_j and T_j depends on the losses. Estimate T_j, calculate P_V, use it to recalculate T_j and repeat with the matching datasheet values. Two or three passes are usually enough.
Maximum instead of typical values: For a safe design, take the maximum value of R_DS(on) or V_CE(sat) and scale it with the normalised temperature curve. Typical values underestimate the losses once part-to-part spread is considered.
Switching energies apply to the test circuit: Gate resistance, driver voltage, stray inductance and the opposite diode have a strong effect on E_on and E_off. If your setup differs a lot from the datasheet test, your own double-pulse measurement gives the most reliable values.
Inverters with sinusoidal current: Each switch conducts only during one half-wave, and the switched current changes all the time. Use the formulas with modulation index and cos φ for I_avg and I_rms, and the peak value Î with the factor 1/π for switching losses.
Soft switching: With zero-voltage switching (ZVS), turn-on losses largely disappear. In that case you essentially only count the turn-off energy.
Several devices on one heat sink: The losses of all switches and diodes on the same heat sink add up. The heat sink sees the sum, while each device has its own junction temperature.

The datasheet values you need

A reliable loss calculation only needs a few parameters. What matters is reading them in the right place and at the right temperature.

Parameter	Where in the datasheet	Note
R_DS(on)	Static characteristics, R_DS(on) versus T_j diagram	Value at the expected T_j, maximum value for the worst case.
E_on, E_off	Switching characteristics, E versus I_D or I_C and versus R_G diagrams	Valid for the test voltage, current, gate resistance and temperature.
t_r, t_f	Switching times	Only for the linear approximation, strongly dependent on driver and R_G.
V_CE0, r_CE	Output characteristic I_C versus V_CE at 125 or 150 °C	Fit a straight line through the operating range: the intercept with the voltage axis is V_CE0, r_CE = ΔV_CE / ΔI_C.
V_F0, r_F	Diode forward characteristic I_F versus V_F	Linearise as for the IGBT.
E_rr, Q_rr	Diode reverse-recovery characteristics	Usually given as E_rr for IGBT modules, often only Q_rr of the body diode for MOSFETs.
R_th,jc	Thermal characteristics	For the next step, calculating the junction temperature.

How the calculator rates the result

The rating refers to the power loss that one device or one heat sink has to remove. The limits are guide values: in the end, what decides is the permissible thermal resistance and the heat flux density, i.e. watts per square centimetre of chip or base plate area. The thermal resistance column assumes a permissible heat sink temperature rise of about 40 K above ambient.

Power loss per device	Typical cooling	Required R_th,sa at 40 K
below 20 W	Air cooling with an extruded heat sink, often by natural convection	above 2 K/W
20 to 150 W	Bonded-fin or pin-fin heat sink with fan, liquid cooling where space is tight	about 2 to 0.3 K/W
above 150 W	Mostly liquid cooling with a cold plate, large air-cooled heat sinks for modules with a large base plate	below 0.3 K/W

Sources and further reading

Graovac, D.; Pürschel, M.; Kiep, A.: MOSFET Power Losses Calculation Using the Data-Sheet Parameters. Infineon Technologies, Application Note, 2006.
Graovac, D.; Pürschel, M.: IGBT Power Losses Calculation Using the Data-Sheet Parameters. Infineon Technologies, Application Note, 2009.
Mohan, N.; Undeland, T. M.; Robbins, W. P.: Power Electronics: Converters, Applications, and Design. 3rd edition, Wiley, 2003.
Wintrich, A.; Nicolai, U.; Tursky, W.; Reimann, T.: Application Manual Power Semiconductors. 2nd edition, SEMIKRON International, 2015.