Summary
The physical PCB layout is the final step in any switching power supply design. Improper layout can increase electromagnetic interference and cause unstable power operation. The following sections analyze key considerations for each step of the layout and routing process.
From Schematic to PCB
Typical design flow: define component parameters → import schematic netlist → set design rules → manual placement → manual routing → verify design → review → generate CAM output.
Design Parameters
Adjacent trace spacing must meet electrical safety requirements and should be as wide as practical for assembly and manufacturing. Minimum spacing must accommodate the expected voltage. When routing is not dense, increase signal trace spacing where possible. For signals with large high/low level differences, keep traces short and increase spacing. A common default is 8 mil trace spacing.
Maintain a distance of at least 1 mm from the edge of the board to the edge of pad holes to avoid pad damage during processing. When a thin trace connects to a pad, use a teardrop-shaped transition between pad and trace to reduce the risk of pad lifting or connection failure.
Component Placement
Even if the schematic is correct, a poor PCB layout can reduce product reliability. Closely spaced parallel traces can introduce signal delay and cause reflections at transmission line terminations. Poor power and ground routing can introduce interference and degrade performance. Every switching power supply contains four main current loops:
- Power switch current loop
- Output rectifier current loop
- Input signal source current loop
- Output load current loop
Input circuits charge the input capacitors with a near-DC current; filter capacitors provide broadband energy storage. Output filter capacitors store high-frequency energy from the output rectifier and smooth DC energy delivered to the load. Therefore, the capacitor terminals for input and output filters are critical. Input and output filter capacitors should connect directly to the corresponding power paths. If connections between input/output loops and the switching/rectifier loops do not connect at the capacitor terminals, AC energy may be radiated from the filter capacitors into the environment.
The switching current loop and rectifier AC loop carry high-amplitude, rapid currents with significant harmonic content, often much higher in frequency than the switching fundamental. Peak amplitudes can reach several times the DC input/output current, and transition times are typically on the order of 50 ns. These two loops are the main sources of electromagnetic interference, so route them before other PCB traces. For each loop, place the three principal elements — filter capacitor, power switch or rectifier, and inductor or transformer — close to each other and adjust positions to minimize current path lengths.
A recommended placement sequence for switching power supplies follows the electrical design:
- Place the transformer
- Lay out the power switch current loop
- Lay out the output rectifier current loop
- Connect the control circuitry to the switching power circuit
- Design the input current loop and input filter
- Design the output load loop and output filter
When arranging components by functional block, follow these principles:
- Consider PCB size first. Too large a board increases trace lengths and impedance, reduces noise immunity, and raises cost; too small a board impairs heat dissipation and increases crosstalk. Prefer rectangular boards with an aspect ratio near 3:2 or 4:3. Keep components near the board edge at least 2 mm from the edge.
- Allow adequate spacing for soldering; avoid overly dense placement.
- Place components around the core element of each functional block. Arrange parts evenly, neatly, and compactly to minimize and shorten leads and connections. Place decoupling capacitors as close as possible to device VCC pins.
- For circuits operating at high frequency, consider parasitic distribution between components. Arrange components in parallel where possible for manufacturability and consistent performance.
- Arrange functional blocks according to signal flow to keep signal directions consistent.
- Ensure good routability. When moving components, consider the resulting jumpers and place connected components together.
- Minimize loop areas to suppress radiated interference from the switching power supply.
Routing for Switching Power Supplies
Routing in switching supplies carries high-frequency signals. Any PCB trace can act as an antenna; trace length and width affect impedance and inductance and therefore frequency response. Even DC traces can couple into nearby traces and convert into RF signals, causing circuit issues or radiated interference. Keep all traces that carry AC currents as short and wide as possible; consequently, place components connected by these traces close together. Trace length is proportional to the effective inductance and impedance; trace width is inversely related to inductance and impedance. Longer traces support lower-frequency electromagnetic radiation and tend to radiate more RF energy. Increase power trace width according to current to reduce loop resistance.
Route power and ground traces so their directions match the current flow to enhance noise immunity. Ground is the reference for the four current loops in a switching supply and plays a critical role in controlling interference. Mixing different grounds can destabilize the supply. Key ground design considerations:
1. Single-point grounding where appropriate
Typically, the common terminal of filter capacitors should be the only connection to the heavy AC ground. Keep connection points of the same circuit level close together and connect the level's filter capacitors to that level's ground point. Different parts of the circuit carry varying return currents; impedance in the return paths can change local ground potentials and introduce interference. In switching supplies, ground loop effects often dominate interference, so a single-point ground is preferred. For example, tie the ground returns of the power switch loop components to a single ground node, and similarly tie the ground returns of output rectifier loop components to the ground terminal of their filter capacitor. If a strict single-point ground is not possible, connect the common ground areas through two diodes or a small resistor, or concentrate connections on a localized copper plane.
2. Use wide ground traces
Narrow ground traces develop larger voltage drops as currents change, which can destabilize timing signals and reduce noise immunity. Use short, wide traces for high-current grounds; widen power and ground traces wherever possible. A good priority is: ground trace > power trace > signal trace. If possible, make ground traces wider than 3 mm, or use large copper areas as ground planes. Connect unused copper areas to ground to increase ground copper area.
Global routing guidelines:
- Routing orientation: where possible, orient component placement and routing to match the schematic diagram orientation. This aids in production testing, debugging, and repair, provided electrical and mechanical constraints are respected.
- Minimize sharp corners and sudden trace width changes. Keep corner angles at or above 90 degrees and prefer simple, clear routing.
- Avoid trace crossings. For potential crossings, use routing under or around other component leads or use wire jumpers as needed. On single-sided boards, through-hole components on the top side and surface-mount on the bottom may overlap in placement, but avoid overlapping pads.
Input and output ground connection
For low-voltage DC-DC converters where output voltage feedback is returned to the primary side, both sides should share a common reference ground. After providing copper pour for input and output grounds separately, tie them together to form a common ground.
Design Review and Checks
After routing, thoroughly check that the routed design follows the established rules and that those rules match PCB manufacturing constraints. Verify clearances between traces, between traces and component pads, between traces and vias, pad-to-via, and via-to-via to ensure manufacturability. Confirm power and ground trace widths and identify areas where ground copper can be widened.
Notes: Some issues may be acceptable to ignore, for example when part of a connector outline lies outside the board outline and spacing checks flag an error. After each routing or via modification, refill copper pours.
Review the design against a PCB checklist, including design rules, layer definitions, trace widths, spacings, pad and via settings. Pay particular attention to component placement, power and ground networks, high-speed clock routing and shielding, and the placement and connection of decoupling capacitors.
Design Output
When generating fabrication data, consider the following:
- Required output layers typically include routing (bottom/top), silkscreen (top and bottom), solder mask (top/bottom), and drill layer(s). Also generate the NC Drill file.
- When setting layer content for silkscreen, include the board outline, text, and lines, but do not select part-type-only options.
- When generating drill files, use the PCB tool default drill settings unless a different setting is specifically required.
