Power Circuit PCB Layout and Routing: 48V, Soft-Start, SMPS, and LDO

Power supply circuits establish the electrical and thermal foundation of a PCB. Their layout and routing determine whether the board delivers stable voltage under load, keeps ripple and switching noise under control, survives hot-swap and surge events, and meets safety spacing requirements. Unlike low-speed signal traces, power paths carry large currents, occupy wide copper areas, and often require clear separation between high-current regions, analog control circuits, and sensitive interfaces. This article covers the layout and routing of common power circuits, including -48 V input circuits, secondary power modules and power ICs, soft-start circuits, switching power supplies, and linear low-dropout (LDO) regulators. It focuses on the practical decisions made during placement, copper routing, plane partitioning, via placement, spacing, and thermal design.

Highlights:

Route the full power path with copper width that satisfies current-carrying requirements and is at least 50 mil; keep high-current loops short and use copper pours for the switch section and main current path.
Place input and output filter capacitors close to the relevant pins, apply protection before filtering at the -48 V interface, and maintain 2 to 3 mm spacing between -48 V copper and other nets on both trace and plane layers.
Separate analog control ground from high-current ground, use a remote single-point ground, and reserve keep-out and thermal copper areas that support the chosen heatsink or PCB heat-dissipation method.

Common Circuit Layout and Routing

48V Power Supply Circuit

The -48 V power supply circuit requires careful attention to power flow, protection, filtering, current capacity, and safety spacing. Its layout and routing should follow these design considerations:

Arrange the circuit by functional block so that the power flow is clear and input and output routing do not cross.
Place protection before filtering. The protection channel trace width should be at least 50 mil.
Keep each functional block relatively concentrated and compact. Do not allow crossing or misalignment. For example, place the capacitor on the CASE pin of the power module close to the CASE pin, and make the connection from the CASE pin to the capacitor short and thick.
The entire power path routing, or copper pour, must have a width that meets the current-carrying requirement and should be at least 50 mil.
Where safety requirements permit, route -48 V and BGND in parallel and adjacent to each other. Copper on adjacent layers, or splits on adjacent power and ground planes, is preferred.
From the -48 V/BGND input to the input side of the DC/DC converter, clear all power and ground planes except for the corresponding -48 V/BGND/PGND planes. The interface power area should contain no other routing. In particular, no trace or plane may pass under the common-mode choke.
Traces, vias, and copper pours of non -48 V nets must be kept 2 to 3 mm away from the traces, vias, and copper pours of the -48 V net. The same spacing applies on plane layers.
Place the VCC output filter circuit close to the DC/DC output.

The -48 V power supply schematic is shown below.

Figure 41 | Schematic diagram of the -48V power supply circuit Design considerations for the -48V power supply circuit:

Figure 41 | Schematic diagram of the -48V power supply circuit — Design considerations for the -48V power supply circuit:

At the PCB level, these constraints translate directly into copper geometry and clearance rules. The protection channel is widened first, the power path is routed with adequate copper, and the -48 V region is kept clear of unrelated traces and planes. The 2 to 3 mm spacing requirement affects plane clearings, via placement, and copper pour boundaries, while the common-mode choke area requires an explicit keep-out. The corresponding PCB layout is shown below.

Figure 42 | PCB diagram of the -48V power supply circuit

Secondary Power Modules and Power ICs

A secondary power module or power IC must have a filter capacitor at its input. Provided thermal design allows, place this capacitor close to the input end. It serves three purposes:

It reduces noise generated inside the power supply that is fed back to the input side.
It prevents input voltage oscillation when the module input wiring is long. Such oscillation can produce voltage spikes several times the input voltage, causing unstable output at best and fatal damage to the module at worst.
If an abnormal transient voltage appears at the module input, the capacitor suppresses the brief transient.

A secondary power module or power IC must also have a filter capacitor at its output. Provided thermal design allows, place this capacitor close to the output end. This capacitor provides the following benefits:

It reduces output ripple.
It improves the dynamic performance of the module when the load changes.
It improves certain module characteristics, such as the startup waveform and system stability.
After the module output is turned off, the output voltage can be maintained for a period so that the load circuit can complete certain operations, such as saving data.

Soft-Start Circuit

The input of a board power supply usually has a large capacitor, and higher power generally means larger capacitance. Without limiting measures, power-up or live hot-swap charging of these capacitors produces a large surge current. This current can cause arcing between connector pins, generate other interference, or blow the fuse. The input therefore requires a soft-start circuit.

Soft-start circuits can be built with discrete devices or implemented with dedicated ICs. The soft-start schematic is shown below.

Figure 43 | Schematic diagram of the soft-start circuit

For a discrete soft-start circuit, place the components compactly and keep the main current path smooth. During PCB routing process, give priority to the width of the main path, keep the routing smooth, and minimize bends. Other connections should also use thick traces. D2 protects Q1 promptly, so place D2 as close as possible to the gate and source of Q1, and make its traces as thick as possible. Dedicated soft-start ICs are available from many vendors; consult the corresponding datasheet for layout guidance.

Switching Power Supply

A switching power supply mainly includes an input line filter, an input rectifier and filter, a converter, an output filter, a control circuit, and a protection circuit. The circuit schematic is shown below.

Figure 44 | Circuit schematic diagram of the switching power supply

On a board that receives DC power from an off-board source, the input line filter and input rectifier are often omitted.

Main Circuit

The input filter removes noise present on the power grid and also prevents noise generated by the local circuit from being fed back to the public grid.
The rectifier and filter convert the AC mains directly into relatively smooth DC for the next conversion stage.
The inverter converts the rectified DC into high-frequency AC. This is the core of a high-frequency switching power supply. The higher the frequency, the smaller the ratio of volume and weight to output power.
The output rectifier and filter provide a stable and reliable DC source for the load.

Control Circuit

The control circuit samples the output, compares it with a set reference, and then controls the inverter by changing its frequency or pulse width to stabilize the output. It also uses data from the detection circuit, which is evaluated by the protection circuit, to provide various protection functions for the complete unit.

Detection Circuit

The detection circuit supplies operating parameters to the protection circuit and also provides data for display instruments.

Modulation Methods

Switching power supplies use three modulation methods:

Pulse width modulation (PWM). The switching period is constant, and the duty cycle is changed by varying the pulse width.
Pulse frequency modulation (PFM). The on-pulse width is constant, and the duty cycle is changed by varying the switching frequency.
Hybrid modulation. The on-pulse width and switching frequency are both variable and can change independently. This is a combination of the two methods above.

PCB Design Principles for Switching Power Supplies

Keep the layout as compact as possible, and make the routing as short and thick as possible.
Consider the high-current path and its current-carrying capability.
Distinguish the AC paths to reduce noise.
Keep the common-ground sampling feedback for high-current input and output, and the modulation output, away from the inductor and noisy areas.
The location of the module on the board is a strong source of EMI radiation. Place it away from clocks, interfaces, and other sensitive devices.
Identify the core devices of each block in the schematic: input rectifier (optional), input filter, switch transistor, control circuit, and output filter.
- Switch transistor: use a compact layout, account for the high-current path, connect the input and output grounds directly, and minimize the loop area.
- Input filter: place it close to the switch transistor so that the high current is filtered before it enters the switch transistor.
- Output filter: place it close to the switch transistor so that the high current is filtered before it enters the board plane.
- Control circuit sampling: place the sampling resistor between the output filter and the comparison circuit. During layout, keep the sampling circuit as close as possible to the chip pins and close to the comparison circuit.
- Control circuit comparison: place the comparison circuit close to the control chip.
- Control circuit filter network: place the capacitors as close as possible to the corresponding pins.
- Switch transistor section: keep it as short and thick as possible, generally by using a copper pour.
- Input and output filtering: pay attention to the number and position of vias on the power plane. Place these vias after the filter capacitors.
- Control circuit sampling: this is an analog signal, and the sampling point should be after the output filter. If both current sampling and voltage sampling are present, route them as a tightly coupled differential pair. Keep the sampling lines short to reduce the area exposed to interference.
- Control circuit modulation output: this is an analog signal. Do not route long traces under the switch transistor, and keep them away from high-current power and ground areas.
Connect the input and output grounds with a large copper area.
The control circuit ground is analog ground. Keep it separate from the high-current ground and connect it at a single remote point.

The switching power supply schematic and PCB layout are shown below.

Figure 45 | Schematic diagram of the switching power supply

Figure 46 | PCB diagram of the switching power supply

DC/DC Converter Circuits (LDO)

A linear voltage regulator adjusts the output voltage by dissipating excess energy internally. Its advantages are a simple circuit, convenient use, and low cost. Its disadvantages are high self-heating and low power conversion efficiency.

If the regulator itself dissipates a large amount of power, select a through-hole package, add a suitable heatsink, and keep the ground leads as short and thick as possible to reduce heat loss and the effect of lead inductance.

The power dissipation of a linear voltage regulator is calculated as:

Power dissipation = voltage differential × load current = (Vi - Vo) × Io

Because the device generates substantial heat, the hardware design may add a heatsink or use the PCB copper for heat dissipation. During PCB design, confirm with the hardware engineer which cooling method is used. On some power ICs, the thermal tab is connected to the chip ground pin, while on others it is connected to the input or output pin. If this is overlooked, the input or output can be shorted to ground. A keep-out area should be provided on the PCB surface; otherwise, the heatsink may scrape the board after assembly and cause a short circuit. When a metal enclosure and PCB copper are used for heat dissipation, also confirm whether connection to the ground layer is permitted.

Linear voltage regulators can be divided by output voltage into fixed regulators and adjustable regulators.

For a fixed-output regulator, place the input and output capacitors close to the pins during layout, and use copper pour for routing. The fixed-output LDO schematic is shown below.

Figure 47 | Schematic diagram of the linear LDO circuit

For an adjustable-output regulator, place the input and output capacitors close to the pins during layout. Place the adjustment resistors nearby, use copper pour for the input and output paths, and use thick traces for the control signal. The adjustable-output LDO schematic is shown below.

Figure 48 | Schematic diagram of the LDO circuit with adjustable output voltage

The PCB layout for the adjustable-output LDO is shown below.

Figure 49 | PCB diagram of the LDO circuit with adjustable output voltage

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