High Current PCB Design for Motor Drivers and Power Electronics

Routing low-power microcontrollers requires adhering to standard design rules, but routing high-current motor drivers and DC-DC power electronics demands a deep understanding of thermal dissipation, copper trace resistance, and stray parasitic inductance. When switching 10A to 30A+ through brushless DC (BLDC) motor drivers or H-bridges, inadequate trace widths generate intense Joule heating (I²R), delaminating PCB tracks and destroying semiconductor switches. Here are practical engineering rules for high-current circuit design.

1. Calculating Trace Widths with IPC-2152

A standard 10 mil (0.25mm) trace cannot safely carry more than 1A without significant thermal rise. According to IPC-2152 standards for 1 oz copper (35µm) on an outer layer with a 10°C allowable temperature rise, carrying 5A requires a minimum trace width of approximately 3.8mm (150 mil), while carrying 15A requires over 14mm. When spatial constraints prevent 14mm wide traces, engineers employ three techniques:

• Upgrade to 2 oz Copper (70µm): Halves the required trace width for the same current carrying capacity.
• Solid Polygon Pours: Replace standard traces with wide, contiguous copper pours across both Top and Bottom layers, stitched together with matrices of vias.
• Solder Mask Openings: Expose copper along high-current tracks on the outer layer, allowing technicians to flow thick tin-lead solder or solder copper bus wire directly on top of the trace.

2. Thermal Vias and Heat Spreading

Surface-mount MOSFETs (such as DFN5x6 or TO-263 packages) dissipate thermal energy through their exposed bottom drain pads directly into the PCB substrate. To channel heat away from the silicon die, place a dense grid of 0.3mm thermal vias directly inside and surrounding the drain pad, connecting to internal ground or bottom-layer copper heat spreading planes. Space thermal vias approximately 1.0mm to 1.2mm apart.

3. Kelvin Connections for Current Sensing

Measuring motor current requires low-ohmic shunt resistors (e.g. 1mΩ to 5mΩ). Because the shunt resistance is so small, the parasitic resistance of the copper trace connecting the shunt can introduce severe measurement errors. Always use a Kelvin (4-wire) connection: route dedicated differential voltage-sensing traces directly from the inner pads of the shunt resistor back to the current-sense amplifier, ensuring no high motor load currents flow through the sense traces.