Skip to content
RFrftools.io
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.

Loading calculator...

Formula

Pcond=I2×RDS×D,Psw=f×Qg×VP_cond = I² × R_DS × D, P_sw = f × Qg × V
R_DS— On-state resistance (Ω)
Qg— Gate charge (nC)

How It Works

This calculator determines power dissipation and junction temperature in motor driver ICs from on-resistance, switching frequency, and thermal resistance parameters. PCB designers, embedded systems engineers, and thermal analysts use it to verify that driver ICs remain within safe operating temperature. Exceeding maximum junction temperature triggers thermal shutdown (typically at 150-175°C) and causes intermittent motor dropouts.

Per semiconductor physics, driver dissipation consists of conduction losses and switching losses: P_total = P_cond + P_sw. Conduction loss follows: P_cond = I² × R_DS(on) × D, where D is duty cycle. Switching loss approximates: P_sw ≈ 0.5 × V × I × (t_rise + t_fall) × f_sw. For typical 3A motor drivers at 20 kHz, conduction losses dominate (2-5W vs. 0.1-0.3W switching).

Junction temperature calculation per JEDEC JESD51: T_j = T_ambient + P_total × R_θJA. Manufacturer-specified R_θJA assumes minimal PCB copper (1 in² per JEDEC standard). With optimized thermal design—4-layer PCB, thermal vias, large copper pour—effective R_θJA reduces by 30-50%. The Texas Instruments DRV8876 (R_DS(on) = 565 mΩ, R_θJA = 35°C/W) at 3A continuous dissipates 5.1W and reaches 178°C junction in free air—exceeding its 150°C maximum. Proper PCB thermal design reduces R_θJA to 20-25°C/W, achieving safe 127-152°C operation.

Worked Example

Verify thermal performance for a DRV8833 dual H-bridge driving two 12V/1.5A motors. IC specifications: R_DS(on) = 320 mΩ (per H-bridge), R_θJA = 42°C/W (TSSOP-16), T_j max = 150°C. PWM frequency is 25 kHz, duty cycle 75%.

Step 1 — Calculate conduction losses per channel: P_cond = I² × R_DS(on) × D = 1.5² × 0.320 × 0.75 = 0.54W per channel Total for dual H-bridge: 0.54 × 2 = 1.08W

Step 2 — Estimate switching losses: Assuming t_sw = 50 ns rise + 50 ns fall: P_sw = 0.5 × 12 × 1.5 × (100e-9) × 25000 × 2 channels = 0.045W (negligible)

Step 3 — Calculate total dissipation: P_total = 1.08 + 0.045 + 0.02 (quiescent) = 1.15W

Step 4 — Determine junction temperature at 40°C ambient: T_j = T_amb + P × R_θJA = 40 + 1.15 × 42 = 88.3°C Margin to limit: 150 - 88.3 = 61.7°C — acceptable

Step 5 — Calculate maximum allowable current: P_max for 150°C at 40°C ambient: (150-40)/42 = 2.62W I_max = √(P_max / (R_DS(on) × D × 2)) = √(2.62/(0.32×0.75×2)) = 2.34A per channel

Result: At 1.5A per motor and 75% duty cycle, junction reaches 88°C—well within limits. The driver can handle up to 2.3A per channel before thermal shutdown at 40°C ambient. Adding thermal vias (reduces R_θJA to 30°C/W) allows 2.7A operation.

Practical Tips

  • ✓Per Texas Instruments layout guidelines, expose thermal pads on QFN/DFN packages and connect with at least 9 thermal vias (0.3mm diameter) to inner ground plane—reduces R_θJA by 30-40%
  • ✓Measure IC surface temperature with IR thermometer during initial testing: surface >85°C indicates junction near limits; >100°C surface requires immediate PCB redesign per reliability guidelines
  • ✓For currents >5A, consider discrete MOSFETs (R_DS(on) < 10 mΩ available in TO-220) instead of integrated drivers (50-500 mΩ typical)—external FETs dissipate 10-50× less power at same current

Common Mistakes

  • ✗Using bare R_θJA from datasheet without considering PCB: Per JEDEC JESD51, R_θJA is measured on minimal copper; a 4-layer PCB with thermal vias and ground plane reduces effective R_θJA by 30-50%, enabling 1.5× higher current
  • ✗Ignoring duty cycle in power calculations: At 50% duty cycle, conduction loss is half that of 100% duty cycle—a driver hot at 90% duty may be cool at 50%, allowing higher peak current for PWM-controlled speed
  • ✗Calculating peak power instead of average: Per thermal dynamics, junction temperature responds to average power (thermal time constant is 10-100 ms); use RMS current for accurate thermal analysis in PWM applications

Frequently Asked Questions

Per JEDEC JESD51: R_θJA (junction-to-ambient) is the complete thermal path from die to surrounding air—used for natural convection calculations. R_θJC (junction-to-case) covers only die to package surface—used with external heatsinks where T_case is controlled. For exposed-pad packages, R_θJC is typically 2-10°C/W while R_θJA is 30-80°C/W. With a good heatsink, effective junction-to-ambient approaches R_θJC + R_θheatsink, typically 5-15°C/W.
Four strategies per motor driver application notes: (1) Select lower R_DS(on) driver or discrete MOSFETs—reducing R_DS(on) from 500 mΩ to 50 mΩ cuts conduction loss 10×; (2) Use lower PWM frequency (5-10 kHz) to reduce switching losses by 50-75%; (3) Reduce motor current via gearing—a 10:1 gearbox allows 1/10th motor current at same output torque; (4) Optimize duty cycle—run motor at voltage producing desired speed rather than PWM modulating full voltage.
Three common causes per troubleshooting guides: (1) Motor is stalled—even at low command, stall current = V/R_winding which can be 5-10× running current; (2) High PWM frequency with low motor inductance increases current ripple and RMS current above average; (3) Shoot-through during dead-time transitions—inadequate dead-time causes brief supply shorts each PWM cycle, adding fixed losses independent of motor load. Check actual motor current with a current probe, not just command signal.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

NEMA 17 Stepper Motor

NEMA 17 bipolar stepper motors for precision motion control

Amazon →

Stepper Motor Driver (A4988)

A4988 stepper driver modules for microstepping control

Amazon →

DC Motor with Encoder

12 V DC motors with encoders for closed-loop drive applications

Amazon →

Related Calculators

Motor

H-Bridge

Calculate H-bridge MOSFET requirements including peak current, conduction losses, and minimum current rating for DC motor drivers.

Power

MOSFET Power Loss

Calculate MOSFET conduction loss, switching loss, total power dissipation, junction temperature, and efficiency for power electronics design

Motor

Motor Efficiency

Calculate motor efficiency, power losses, and heat dissipation from electrical input and mechanical output measurements.

Motor

DC Motor

Calculate DC motor speed, torque, power, and efficiency from electrical parameters