USB-C Power Delivery (PD) Negotiation & Trace Width Sizing in Handhelds

The Scale of USB-C Power Delivery

The USB Type-C Power Delivery (USB PD 3.1) specification allows modern handheld devices—such as portable gaming consoles, field test equipment, and compact laptops—to draw up to 240 W of power over a single standardized cable interface (ranging up to 48 V at 5 A).

While high voltage reduces overall current requirements for a given wattage, routing 5 A continuously through a space-constrained handheld motherboard presents significant thermal and voltage-drop challenges. Poor trace sizing leads to localized copper heating, excessive VBUS voltage drop, and thermal degradation of adjacent battery charging circuitry.

Calculating Trace Widths for High-Current Power Paths

Trace width selection is governed by the allowable temperature rise of the PCB substrate (typically budgeted at a 10 to 20 degree Celsius rise above ambient) and the total copper weight of the layer.

When routing a 5 A USB PD VBUS rail on internal or external layers, implement the following design rules:

1. Copper Weight Selection: Standard digital PCBs use 1 oz/ft² copper (35 microns thickness). High-power handheld charging paths benefit significantly from specifying 2 oz/ft² copper (70 microns thickness) on outer power distribution layers. Doubling the copper thickness cuts the required trace width in half for the same temperature rise.
2. Minimum Trace Width Guidelines (for 5 A Continuous Load):

• External Layers (1 oz Copper): Requires a minimum trace width of approximately 3.0 mm to keep temperature rise under 10 degrees Celsius.
• External Layers (2 oz Copper): Requires a minimum trace width of approximately 1.5 mm for the same thermal performance.
• Internal Layers (1 oz Copper): Because internal layers are insulated by FR-4 dielectric glass and cannot radiate heat directly to air, internal traces require nearly double the width (approximately 5.5 mm to 6.0 mm) to carry 5 A safely.

Minimizing Voltage Drop and Parasitic Resistance

In addition to thermal safety, power routing must minimize IR voltage drop between the USB-C receptacle and the primary battery charger IC. A high line resistance causes the charger IC to detect false under-voltage conditions, triggering lower charging rates or negotiation drops.

To ensure low line resistance across the board:

• Use Solid Copper Polygon Pours: Avoid using simple traces for VBUS routing wherever possible. Instead, pour solid copper polygons extending directly from the USB-C connector pads to the input protection MOSFETs and charger IC pins.
• Stitch Power Planes Across Layers: If the VBUS rail must hop between top and bottom layers, use clusters of low-resistance power vias (0.5 mm drill diameter). A single standard via carries approximately 1.5 A to 2 A safely; place a minimum of four to six parallel vias at layer transition points to carry 5 A without bottlenecking current flow.
• Place Protection Components Near the Port: Position ESD protection diodes and reverse-voltage blocking MOSFETs as close as physically possible to the USB-C connector pins to minimize unshielded high-current trace lengths.