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Motor

Motor Driver Power Dissipation

Calculate motor driver IC or discrete MOSFET power dissipation including conduction loss and switching loss at a given PWM frequency.

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Formula

P_cond = I² × R_DS × D, P_sw = f × Qg × V

R_DS— On-state resistance (Ω)
Qg— Gate charge (nC)

How It Works

Power dissipation in a motor driver IC consists of conduction losses (I² × R_DS(on) × duty cycle) and switching losses (proportional to V × I × switching frequency). At low PWM frequencies, conduction losses dominate; at high frequencies, switching losses dominate. The junction temperature must stay below the IC's maximum rating: T_j = T_ambient + P_dissipated × R_θJA (thermal resistance junction-to-ambient). Exceeding T_j_max triggers thermal shutdown and causes intermittent motor dropouts.

Worked Example

A DRV8876 motor driver IC (R_DS(on) = 565 mΩ total high+low side, R_θJA = 35 °C/W) drives a 12 V motor at 3 A continuous and 80% duty cycle. Ambient temperature is 40 °C. Step 1 — Conduction loss: P_cond = I² × R_DS(on) × D = 3² × 0.565 × 0.80 = 4.07 W Step 2 — Switching loss (assume f_PWM = 20 kHz, t_sw = 100 ns): P_sw ≈ V × I × t_sw × f = 12 × 3 × 100e-9 × 20000 = 0.072 W (negligible here) Step 3 — Total dissipation: P_total = 4.07 + 0.07 = 4.14 W Step 4 — Junction temperature: T_j = T_amb + P × R_θJA = 40 + 4.14 × 35 = 40 + 144.9 = 184.9 °C Step 5 — Maximum T_j for DRV8876 = 150 °C → EXCEEDED Solution: Reduce motor current to 2 A or add thermal pad to PCB copper pour. At 2 A: P_cond = 2² × 0.565 × 0.80 = 1.81 W → T_j = 40 + 1.81 × 35 = 103.4 °C ✓ Result: At 3 A, this driver overheats in free air at 40 °C. Derate to 2 A, or use a copper pour or external heat sink to reduce R_θJA.

Practical Tips

  • ✓Expose the thermal pad on the bottom of QFN/DFN motor driver packages and solder it to a copper pour with at least 4 thermal vias to the ground plane on the opposite layer
  • ✓Measure driver IC temperature with an IR thermometer during initial power-on testing — surface temperature above 80 °C indicates insufficient cooling and requires PCB layout improvement
  • ✓For high-duty-cycle or continuous operation, select a motor driver with synchronous rectification (low-side MOSFET freewheeling instead of body diode) to halve freewheeling conduction losses

Common Mistakes

  • ✗Using the bare R_θJA (junction-to-ambient) value from the datasheet without considering PCB copper area — a large copper pour can reduce effective R_θJA by 30–50%
  • ✗Ignoring duty cycle in conduction loss calculations — conduction loss scales with duty cycle, so a motor idling at 20% duty cycle dissipates only 1/4 the power of 80% duty cycle at the same current
  • ✗Calculating power dissipation at rated current without accounting for actual operating current — motors rarely draw rated current continuously; use the RMS current for accurate loss estimation

Frequently Asked Questions

R_θJA is junction-to-ambient resistance — the total thermal path from the die to surrounding air. R_θJC is junction-to-case resistance — only the path from die to package surface. Use R_θJA for free-air calculations without a heat sink. Use R_θJC plus a separate heat-sink resistance for externally cooled packages. For ICs with exposed thermal pads, the PCB copper area strongly determines the effective R_θJA.
Lower R_DS(on): choose a driver with lower on-resistance or use discrete MOSFETs. Reduce current: use a gearbox to allow lower motor current at the same output torque. Reduce duty cycle: operate at a lower average speed. Improve cooling: increase PCB copper pour area, add vias to inner layers, or attach a heatsink.
At low motor loads, switching losses can dominate if the PWM frequency is very high. Quiescent current and bootstrap capacitor charging losses are also fixed overheads. Additionally, if the motor is stalled (high current at zero speed), all power goes to heat with no mechanical output. Check whether the motor is actually running or stalled during the hot condition.

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