Choosing an Internal AC‑DC Power Supply for Your Product

A common requirement in product development is selecting an internal AC‑DC power supply capable of delivering from a few watts up to several hundred watts.

Why choose an internal supply?

Consumers dislike bulky in‑line adapters, and as power densities increase, many wall‑wart units now fit inside the plug itself. Using an external supply also keeps high voltages away from the end‑product, simplifying safety certification.

However, cable length can introduce voltage drop, requiring additional regulation inside the device. External adapters also lack “smart” features such as sleep‑mode shutdown or dynamic voltage adjustment, and they complicate EMI compliance because the cable and adapter must be tested as part of the final system. For these reasons, many manufacturers opt to mount the adapter internally to streamline safety and EMI certification.

At higher power levels, or when control and advanced functionality are needed, an internal or “equipment” power supply is preferred. The decision should be made early in development, as late selection often leads to space constraints, cost increases, or the need for a custom design.

Safety, EMC, and environmental compliance are paramount

A built‑in supply must meet voltage and current requirements, but must also comply with the relevant safety, EMC, and environmental standards for its application—industrial, household, medical, building automation, rail, military, etc. New hazard‑based safety standards further require that the supply prevents misuse, which an internal module can guarantee by eliminating the possibility of an inappropriate external adapter.

Mechanical considerations

Beyond shape and size, connectors, mounting style, and cooling strategy must match the product’s layout. Open‑frame supplies are low‑cost and common, often with optional covers for technician access. DIN‑rail modules are widely used in panel wiring. Both types typically feature screw terminals or Molex‑style plug‑in connectors for AC input and output. The wiring, fuses, switches, and chassis connectors must be appropriately rated and certified.

Because AC input cables can pick up interference inside the product, a certified filter may be required close to the inlet. Grounding is critical: if the power module is unplugged, the chassis must remain grounded to avoid a live wire fault. Ground connections should not be pluggable unless unplugging simultaneously removes all live connections, or they must be secured with a permanent fixture and anti‑vibration measures.

Careful inlet fusing is essential

When a chassis connector supplies the AC input, a fuse must be placed at the inlet. The fuse should protect the internal supply while allowing the external AC cable to carry its rated current. Fuse coordination—ensuring that the inlet fuse opens before any upstream fuse—prevents simultaneous disconnection of multiple circuits (see Figure 2).

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Figure 2: Fuse values in a system should be “coordinated”: value Fuse 1 > Fuse 2 > Fuse 3. (Source CUI)

When evaluating AC connection options, consider overall system cost, not just the cheapest open‑frame module. An under‑qualified supply may necessitate additional filtering, driving up the total cost.

Cooling strategy determines supply type

Fans, natural convection, or baseplate cooling are the primary options. Fan‑cooled units are smaller but may be unsuitable in noisy or hard‑to‑replace environments. When using a fan, define inlet and exhaust paths to avoid dead spots, especially when other system fans are present. Manufacturers such as CUI provide airflow recommendations in their datasheets (see Figure 3).

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Figure 3: Recommendations for fan size, direction, and distance should be considered to avoid air “dead spots”. (Source CUI)

Convection‑cooled supplies are sensitive to orientation and must be positioned away from other heat sources. Baseplate‑cooled units eliminate uncertainty in the heat path but require a cold wall and robust mounting. When sizing a supply, compare continuous and peak power. An inexpensive unit with a high surge rating can often meet intermittent load requirements.

Leverage expert guidance

With the breadth of available internal supplies, the optimal choice depends on safety, compliance, cooling, mechanical fit, and cost. Identifying the supply type early—aligned with standards, application, and integration constraints—reduces risk and keeps the project on schedule.

Related Contents:

• Selecting an external power supply
• How USB Type‑C can reduce consumer power adapter confusion – and cut global e‑waste
• Combining power and data wires, Part 1
• USB Type‑C and power delivery 101 – Power delivery protocol
• Using wired data connections to power demanding IoT devices
• Untangling in‑vehicle wireless power design challenges

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