Question 1

How do I size a heatsink for a MOSFET or IGBT?

Accepted Answer

Add the junction-path losses Pd = P_cond + P_sw + P_rr, then back-calculate the required sink-to-ambient resistance theta_SA = (Tj_max - Ta)/Pd - theta_JC - theta_CS. For example a MOSFET dissipating Pd = 8.83 W with Tj_max 150 C, ambient 40 C and theta_JC = theta_CS = 0.5 C/W needs theta_SA = (150-40)/8.83 - 1.0 = 11.46 C/W, so a modest extruded heatsink is enough.

Question 2

Why is gate-drive loss excluded from the junction temperature?

Accepted Answer

Gate-drive power P_gate = Q_g * V_GS * f_sw is dissipated in the gate driver and the external gate resistor, not in the transistor's silicon junction. It is real power your supply must provide, so the tool reports it, but it must not be added to the conduction and switching loss when computing the junction temperature or the heatsink size.

Question 3

What does a negative required theta_SA mean?

Accepted Answer

A negative required sink-to-ambient resistance means the design is thermally infeasible: even an ideal infinite heatsink (theta_SA = 0) cannot keep the junction below Tj_max, because the case-to-sink path alone already exceeds the budget. Pd * (theta_JC + theta_CS) is larger than (Tj_max - Ta). You must reduce the loss, lower theta_JC/theta_CS, raise Tj_max with a different part, or lower the ambient.

Question 4

Why correct R_DS(on) for junction temperature?

Accepted Answer

A silicon MOSFET's on-resistance roughly doubles from 25 C to 125 C: R_DS(on)@Tj = R25 * [1 + alpha * (Tj - 25)] with alpha about 0.0045 per K. Using the 25 C datasheet value under-estimates conduction loss by up to a factor of two. This calculator evaluates R_DS(on) at Tj_max for a conservative heatsink size, or fixed-point iterates the coupled R-Tj loop.

Question 5

Energy method or time/overlap method for switching loss?

Accepted Answer

Prefer the datasheet energy method P_sw = (E_on + E_off) * f_sw, scaled to your operating voltage, current and temperature. The time/overlap fallback P_sw = 0.5 * V * I * (t_r + t_f) * f_sw is only for when switching energies are unknown; it ignores the IGBT current tail and the diode reverse-recovery contribution, so it is less accurate.

Power Loss → HeatsinkMOSFET & IGBT Calculator

Power Loss → Heatsink
MOSFET & IGBT Calculator