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A stepper driver is thermally acceptable only at a defined motor current, operating sequence, board construction and enclosure condition. The current printed on a driver summary is not a continuous-current promise for every PCB. A useful review separates heat generated inside the driver from heat produced in the windings, sense resistors, connectors and nearby regulators, then checks how the assembled product removes that heat.

This guide focuses on choosing and verifying the thermal arrangement of a bipolar integrated stepper-driver board. For pulse timing and current-control architecture, use the stepper motor driver circuit guide. Here the deliverable is a comparison of copper area, airflow and operating states that another engineer can reproduce. The calculations below are illustrative assumptions, not measured ³Ô¹Ïtv product results.

Build the loss budget around real phase current

Write down the operating states before selecting copper

Start with a small state table: enabled but stationary, acceleration, constant-speed travel, deceleration, reduced-current hold and disabled. Record each state’s duration, phase-current command, measured current waveform, supply voltage and enclosure temperature. Add abnormal but credible states such as a jammed mechanism or a fan that stops. A motor can generate substantial driver heat while the shaft is not moving because holding torque still requires phase current.

Keep three different time quantities separate. Mechanical duty describes how long the mechanism moves. The driver chops winding current at its own electrical rate. The microstep sequence changes the two phase-current references. Multiplying a driver’s full-current loss by the fraction of time the shaft moves misses stationary holding loss and can greatly understate the thermal problem. Firmware current reduction belongs in the electrical specification, with its delay and restoration behavior identified.

List exactly what a current number means: instantaneous peak, winding RMS over a stated interval, or a device-specific current-limit setting. Include the sense-resistor tolerance and current-regulation behavior. An instrument that reports only average supply current cannot directly reveal winding RMS current. Energy storage and recirculation mean supply current and phase current have different waveforms. Where necessary, capture the winding current with instrumentation appropriate to its common-mode voltage and bandwidth.

Calculate a transparent conduction example

For an educational first pass, let Rpath represent the combined on-resistance of the two conducting FETs in one bridge at the relevant junction temperature. If the two bridges have comparable paths, estimate conduction loss as Pcond = Rpath × (IA,rms² + IB,rms²). For ideal sinusoidal microstepping over a complete electrical cycle, with phase peaks Ipk, the sum of the squared phase RMS currents is Ipk². This simplification does not describe every full-step current convention, decay interval or distorted low-speed waveform.

Assume Ipk = 1.8 A and a hot Rpath of 0.40 ohm. The resulting illustrative conduction loss is 1.296 W. Reducing both phase-current references to 60 percent during hold reduces this conduction term to 0.4666 W, because 0.6 squared is 0.36. It does not reduce every driver loss by 64 percent. For a repeated sequence containing 40 percent run and 60 percent reduced hold, the weighted conduction term is 0.7983 W. These numbers describe the chosen assumptions, not a particular IC rating.

If instead both windings continuously carry 1.8 A, the squared-current sum is twice the ideal sinusoidal-cycle value. The corresponding conduction term becomes 2.592 W with the same assumed resistance. That is why a current setting must be tied to the actual drive mode. Use measured or appropriately simulated RMS values when the waveform departs from the assumed model. Do not silently carry the sinusoidal simplification into a stationary test with two fixed phase currents.

Then add the relevant switching, control and recirculation losses for the exact device and decay strategy. Texas Instruments’ explains why on-resistance and switching behavior both belong in the budget. Use hot resistance data from the selected part, rather than a generic multiplier. External sense-resistor dissipation is a separate component loss, although it can raise the local board temperature seen by the driver.

Use thermal resistance as a comparison, not a universal property

A quick steady-state screening calculation is Tj ≈ Ta + Pdevice × Rtheta,system, provided the effective thermal resistance represents the actual board and cooling arrangement. If an assumed 1.8 W total driver loss and 45 degrees C local ambient are combined with an assumed 35 degrees C/W effective resistance, the estimate is 108 degrees C. At 50 degrees C/W it becomes 135 degrees C. The contrast illustrates sensitivity; neither resistance is a measured value for your construction.

The datasheet’s junction-to-ambient metric describes specified test conditions and should not be treated as a board-independent material constant. Likewise, package-top temperature is not junction temperature. The TI distinguishes thermal resistances from characterization parameters such as psi-JT. Match the measurement location and intended use of the metric. Do not add junction-to-case and junction-to-ambient values together as if they were a simple series chain.

Short bursts require additional care. The average-power method may approximate a slowly varying, thermally settled system but does not bound the hottest point during a long acceleration burst. Record both the transient maximum and the repeating steady cycle. Use an appropriate transient thermal model where available, then validate the installed design. Thermal shutdown is a protective response, not an acceptable normal temperature-control strategy or a production qualification result.

Compare constructions and release a repeatable thermal test

Make copper useful by preserving the heat path

For a package with an exposed thermal pad, follow its electrical connection and assembly instructions. A large copper island does little if heat must cross a narrow neck before reaching it. Inspect the pad connection, local spreading region, thermal vias, internal copper continuity and paths to the bottom surface. Nearby plane clearances and routed channels may interrupt spreading even when the total copper percentage looks generous in a CAD summary.

TI’s explains the importance of the soldered exposed pad for that package family. Use the exact land pattern and process recommendations for the selected device. Discuss solder wicking, paste segmentation and via treatment with the assembler; a generic via count is not a substitute for a buildable pattern. The site’s via-in-pad manufacturing guidance helps frame that conversation without assuming that every thermal via requires the same filling process.

Extra copper thickness can improve current distribution and spreading, but its value depends on geometry and the bottleneck. Compare the whole stackup before paying for a heavy-copper PCB construction. A thicker outer layer cannot correct a poorly connected pad or absent enclosure airflow. It may also affect fine-feature constraints, etching allowances and assembly heat demand. Ask for the actual finished-copper specification and dielectric arrangement used in the proposed comparison.

Consider where heat goes after reaching the board. An internal plane spreads heat laterally but still needs a route toward cooler surfaces. Mounting posts, enclosure walls and neighboring hot components influence that route. A heatsink bonded to an electrically active surface also introduces insulation and mechanical requirements. Document these interfaces explicitly rather than treating a bench fan or a temporary metal clamp as part of an unspecified production design.

Run a comparison that isolates the design decision

Do not compare an open-board baseline with an enclosed modified board and attribute the entire temperature difference to copper. Repeat the baseline after the test sequence if ambient or mounting may have drifted. Define sensor locations before starting, including a local air reference that is not heated directly by the package. For infrared measurements, emissivity and reflections can make bare metal pads misleading. Preserve the measurement method as carefully as the temperature values.

Changing hold current is often attractive, but it creates a mechanical verification task. Check holding torque, disturbance recovery, vertical-axis behavior and re-acceleration using the machine’s accepted method. The robot-control PCB overview covers the surrounding system interfaces; it does not establish safe machine behavior from a current reduction alone. The firmware revision and configuration must remain linked to the thermal evidence.

Carry the result into the fabrication and assembly package

A prototype report should state the PCB revision, copper weights by layer, thermal-via geometry, fitted driver and sense components, solder-paste choice, mounting hardware, cooling arrangement and tested current sequence. Record pass criteria before interpreting results. Include maximum allowed component and junction temperatures from the applicable documentation, measurement uncertainty, operating ambient range and the design team’s chosen margin. Do not invent one universal margin for all products.

For PCB fabrication review, mark the copper regions and stackup features that the thermal argument depends on. For assembly and inspection planning, identify the exposed-pad joint and agree which inspection evidence is needed. A visible joint or an X-ray image alone does not measure the installed thermal resistance; inspection and operating validation answer different questions.

The handoff should end with one controlled configuration and a list of unresolved assumptions. If copper area, via treatment or a component alternative is still open, send the specific alternatives with the design files and quantities through a stepper-driver board review request. Attach the thermal comparison and current trace manually. This lets manufacturing evaluate the construction while keeping torque, temperature and system qualification with their appropriate engineering tests.