Undersized heatsinks are a major reason motor drivers fail in the field. Once the junction temperature gets too high, the driver will derate or shut down—sometimes, it fails outright. This Motor Driver Heatsink Calculator helps you determine total device heat loss and how much heat a heatsink actually needs to pull away, based on real operating parameters like motor current, Rds(on), switching frequency, supply voltage, and the key thermal resistance numbers. This is a real concern in robotics, automation, and linear actuator control, where thermal headroom is tight and duty cycles can be aggressive. You'll find the main thermal equations, a step-by-step example, technical background on conduction and switching losses, and a focused FAQ below.
What is motor driver heat dissipation?
Motor driver heat dissipation is simply about moving the excess heat from inside the power devices to the outside world, so the junctions don’t overheat. The most direct way is to use a heatsink. The idea is to increase the area from which the device can lose heat into the air, making it easier to keep the device in a working temperature range.
Simple Explanation
Picture a motor driver as a valve in a high-pressure pipe. Each time it opens or closes, it wastes a bit of energy as heat. Higher currents and faster switching mean more wasted heat. A heatsink is just a chunk of metal designed to absorb that heat and spread it out, letting the surrounding air cool things down before the chip gets too hot.
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Motor Driver Thermal System Diagram
Motor Driver Heatsink Calculator
How to Use This Calculator
This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.
- Fill in motor current (A), driver Rds(on) (Ω), switching frequency (kHz), and supply voltage (V).
- Enter ambient and max junction temperatures (both in °C) — these come from your application and the part's datasheet.
- Add the junction-to-case (Rth(j-c)) and case-to-sink (Rth(c-s)) resistances, which are in the datasheet or thermal interface material specs.
- Hit Calculate.
📹 Video Walkthrough — How to Use This Calculator
Motor Driver Heat Dissipation Interactive Visualizer
You can see directly how motor current, switching frequency, and thermal resistances change both heat output and required heatsink performance. The visual gives a basic thermal resistance network, tracing the heat path from silicon to air.
CONDUCTION LOSS
5.0 W
SWITCHING LOSS
0.24 W
REQ HEATSINK RTH
21.9°C/W
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Thermal Design Equations
Power Dissipation Calculations
Conduction Losses:
Calculate conduction losses as follows — this is the bulk of the heat in typical H-bridge drivers:
Pcond = I²Rds(on)
Switching Losses:
Switching losses occur each time a MOSFET turns on or off. Estimate them with this formula:
Psw = ½ × Vsupply × I × (trise + tfall) × fsw
Total Power Dissipation:
Add conduction and switching losses for your total device heat load:
Ptotal = Pcond + Psw
Thermal Resistance Network
Required Heatsink Thermal Resistance:
Heatsink selection comes straight from temperature margin, device power loss, and the thermal path from junction to air:
Rth(heatsink) = (Tj(max) - Tambient) / Ptotal - Rth(j-c) - Rth(c-s)
Junction Temperature:
This formula gives your estimated chip temperature from power, total thermal resistance stack-up, and ambient conditions:
Tj = Tambient + Ptotal × (Rth(j-c) + Rth(c-s) + Rth(s-a))
Simple Example
For illustration: 10 A motor current, 0.05 Ω Rds(on), 20 kHz switching, 24 V supply, 25°C ambient, 150°C max junction, 1.5°C/W (j-c), 0.5°C/W (c-s).
- Conduction loss: 10² × 0.05 = 5.000 W
- Switching loss: 0.5 × 24 × 10 × (50ns + 50ns) × 20,000 = 0.240 W
- Total power: 5.000 + 0.240 = 5.240 W
- Required heatsink Rth: (150 − 25) / 5.240 − 1.5 − 0.5 = 21.85°C/W → Any small natural convection heatsink usually meets this.
Understanding Motor Driver Heat Dissipation
If you work on automation or actuators, motor drivers are unavoidable. Heat comes from two effects: conduction loss as current passes through the FET, and switching loss every time it changes state. You have to deal with both if you want reliable operation and long life—you’re always fighting thermals.
The Physics of Heat Generation
MOSFET-based drivers heat up from both conduction and switching. Conduction losses follow P = I²R and are straightforward: higher current, higher Rds(on), more heat. Switching losses happen only during transitions, but add up with frequency. If you use faster edge speeds or higher supply voltage, switching loss climbs quickly, even for the same load current.
All switching devices show some combination of these two. Conduction loss often dominates at high current, switching loss can matter more at high frequency or low current. Choose your FET and gate drive accordingly to balance these.
Thermal Resistance Networks
Heat flows much like current in a resistor ladder: temperature is analogous to voltage, and thermal resistance adds up in series. For a typical driver, there are three thermal bottlenecks:
- Junction-to-Case (Rth(j-c)): Heat leaves the silicon, reaching the case or tab.
- Case-to-Sink (Rth(c-s)): Heat moves through the interface (thermal pad, paste, etc.).
- Sink-to-Ambient (Rth(s-a)): Finally, heat is dumped into the surrounding air.
Total thermal resistance tells you how many degrees above ambient the chip will actually run at for a given wattage. You can't cheat physics here—with a high-enough total Rth, even a tiny power loss will overheat the junction, no matter what datasheet limit you picked. This calculator works with the full resistance chain, no shortcuts.
Practical Design Considerations
Natural convection heatsinks are reliable—no moving parts, no extra wiring—but you’ll need a much bigger fin surface to make up for the lack of airflow. Adding a fan drops the required heatsink size fast, but now you have to worry about dust, mechanical failure, and noise.
Aluminum is the default for most jobs—lighter, cheaper, and works well enough for most driver applications. Copper spreads heat slightly better, but comes with weight and cost penalties. Unless you have a really high-power job or extreme space constraint, aluminum usually does the job at lower cost.
Worked Example: Servo Motor Driver
Example from actual actuator work (numbers are practical, not theoretical):
- 5 A continuous motor current
- 0.01 Ω per FET, two in parallel → effective 0.005 Ω
- 20 kHz switching
- 24 V supply
- 40°C typical ambient
- 125°C allowable junction
Plugging in:
Conduction losses: 5² × 0.005 = 0.125 W
Switching losses: (using 50 ns per edge) 0.5 × 24 × 5 × (50×10⁻⁹ + 50×10⁻⁹) × 20,000 = 0.12 W
Total power: 0.125 + 0.12 = 0.245 W
If your device has Rth(j-c) = 2°C/W, and the pad adds 0.5°C/W:
Required heatsink Rth: (125 - 40) / 0.245 - 2 - 0.5 = 344°C/W
With such low power, almost any piece of extruded aluminum will do the job. No need to oversize the sink for most servo or light actuator applications with these numbers.
Advanced Thermal Management Techniques
Big power drivers sometimes need more than just a finned block. Liquid cooling gets you low Rth (<0.1°C/W), but with pumps, piping, and a lot of system overhead. PCM (phase change) pads can help with short heat bursts, but don't help with steady loads as much.
Thermal interface choices matter. Standard silicone grease is enough for most builds, but if you’re assembling at volume, pads are less messy and repeatable—just keep in mind they run hotter. Only high-end, high-density designs justify exotic materials such as liquid metal to squeeze Rth(c-s) as low as possible.
Integration with Linear Actuator Systems
Linear actuators may see much higher peak current on startup than during steady running. Thermal sizing needs to take account of these transients—not just the continuous draw. Use the calculator to check both regular operation and startup pulses if you want to avoid nuisance cutouts or early failures.
With more functions getting packed into modern actuator controllers, heat density rises and every degree matters. Underestimating thermal load or the effect of real-world installation (enclosures, cable routing, vertical/horizontal sinks) usually catches up quickly in actuator, automation, or robotics work.
Frequently Asked Questions
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About the Author
Robbie Dickson
Chief Engineer & Founder, FIRGELLI Automations
Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.
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