Onboard Charger PCB Engineering Example

Use distinct reviews for current flow and isolation

An onboard-charger PCB needs a coordinated electrical, thermal and mechanical definition. Heavy copper can address part of a conduction-loss problem, while switching-loop geometry, insulation boundaries and sensing paths require different decisions. Treating these subjects as one broad power-board requirement hides the reasons for the layout rules.

This is an illustrative board-level planning example. It is not a qualified charger, a construction standard or a procedure for energizing high-voltage hardware. Four layers and heavy outer copper may be considered as design inputs, but neither establishes the completed charger's power, efficiency or insulation performance.

Put the topology and boundaries on one drawing

The system brief should identify the actual conversion topology, input and output conditions, cooling arrangement, switching devices, magnetic components and control interfaces. Draw the boundaries between electrical domains, including auxiliary supplies, drivers, sensing and communication. Record the applicable insulation requirements and the person responsible for approving them.

TI's TIDM-02013 is a published example of a charger architecture combining PFC and an isolated DC/DC stage. It shows why an OBC discussion must identify the stages and interfaces. Its published power and test results apply to TI's reference configuration; they are not specifications for the hypothetical board discussed here.

A hypothetical copper-path loss comparison

Assume an identified conductor path, including its transitions, is provisionally modeled as 4 mΩ. At 12 A, conduction loss is I²R = 12² × 0.004 = 0.576 W. At 20 A with the same resistance, it is 20² × 0.004 = 1.6 W. Increasing current by a factor of 20 ÷ 12 therefore increases this calculated loss by approximately 2.78 times.

The assumed resistance is not a measured PCB value, and these currents are not allowable ratings. Temperature-dependent resistance, current distribution and joint losses are excluded. The calculation helps prioritize a path for analysis and measurement. It does not decide how much copper is sufficient or whether its heat can leave the enclosure.

Review fast loops separately from broad power areas

The UCC21520 data sheet discusses local bypassing and compact gate-current loops for that isolated driver. This supports a separate placement review around a gate driver and its switching devices. Follow the selected device and topology documentation rather than transferring its detailed values to another circuit.

In the board review, distinguish the switching commutation loop, gate-drive loop, low-frequency power path and measurement reference. Check whether a shorter high-current route creates a new coupling problem for the sensing circuit. A large copper area can reduce resistance while still creating undesirable coupling or placing stress on a connector.

Make the insulation drawing govern fabrication details

The design owner must specify the required clearances, creepage paths, slots, material conditions and inspection criteria under the applicable rules. Include component leads, exposed copper, mounting hardware and the final enclosure. Solder mask, coating and a laminate Tg designation must not be used as substitutes for a documented insulation design.

For a heavy-copper proposal, review finished copper by layer, etch allowances, pads, hole construction and the soldering process for power terminals. The thick-copper PCB page provides a starting point for that discussion. Any construction change affecting an electrical boundary needs the system engineer's assessment.

Assign verification evidence by responsibility

These are evidence categories, not supplied test settings. Bare-board electrical test, assembly inspection, converter performance and system safety assessment answer different questions and should not share a single unexplained pass label.

Keep manufacturing and system acceptance aligned

The controlled package should include fabrication files, stackup, assembly data, domain-boundary drawings, cooling interfaces and approved component alternatives. Include a responsibility table for supplied firmware, magnetic components, mechanical fixtures and each test. State which evidence will accompany the boards and which system checks remain with the integrator.

Begin with the topology, design stage and intended manufacturing scope. Send available files through the quotation request to define a project-specific review; an application name alone is insufficient to confirm a build.