Motor Driver Heat Calculator

This tool calculates total MOSFET power dissipation—including conduction loss (I²R_DS(on)) and switching losses (due to rise/fall times and PWM frequency)—then estimates junction temperature using ambient temperature and user-defined thermal resistance. It helps determine if a heatsink is needed and what thermal performance is required.

Use it during component selection and thermal validation—especially when driving brushed DC or stepper motors at medium-to-high currents (>1 A) or high PWM frequencies (>5 kHz). It’s essential before PCB layout, heatsink procurement, or reliability testing to avoid MOSFET thermal runaway.

R_DS(on) is the on-state drain-to-source resistance of the MOSFET—critical for conduction loss calculation. It’s specified in mΩ on datasheets (e.g., 4.5 mΩ, 12 mΩ) because typical values are small; entering it directly in mΩ avoids decimal errors and aligns with standard semiconductor documentation.

Switching losses occur each time the MOSFET transitions between on/off states. Shorter t_r/t_f reduce energy lost per transition, but higher PWM frequency increases the number of transitions per second—so total switching loss scales linearly with f_PWM and quadratically with t_r+t_f.

For discrete MOSFETs with gate drivers, t_r/t_f typically range from 10–100 ns; integrated motor drivers (e.g., DRV8871) may be 50–200 ns. PWM frequencies commonly fall between 10–50 kHz—higher frequencies reduce audible noise and current ripple but increase switching losses significantly.

First verify input values (especially worst-case motor current and R_DS(on) at elevated temperature). Then consider lowering PWM frequency, selecting MOSFETs with lower R_DS(on) and faster switching, improving PCB copper area, adding forced airflow, or upgrading to a larger heatsink with lower θ_JA or θ_SA.

No—it assumes ideal complementary switching and neglects body diode reverse recovery and shoot-through (cross-conduction). These effects can add 10–30% extra loss in H-bridge designs, especially at low duty cycles or high dV/dt. For precision analysis, perform SPICE simulation or consult your driver IC’s thermal application notes.

Accuracy depends heavily on the thermal resistance value (θ_JA or θ_SA) you assume. The calculator uses the standard formula T_J = T_A + P_TOT × θ_JA. For best results, use θ_JA from your actual PCB layout (not datasheet “typical” values) or measure θ_SA for your heatsink + interface material combination.

Yes—but enter the *peak phase current*, not RMS or average. Microstepping increases effective switching activity and may elevate average power slightly due to higher effective PWM duty cycle complexity. For conservative sizing, increase the calculated total loss by ~15% or validate with oscilloscope current/voltage waveforms.

This calculator models a single MOSFET channel (e.g., one high-side or low-side switch), so for a full H-bridge, multiply the result by 2 for worst-case simultaneous conduction—though in practice only two MOSFETs conduct at once per PWM cycle. Always verify per-phase loss in multi-channel ICs using their specific thermal derating curves.