Development Case Study of a Custom PCB Power Supply for POS Terminals | Compatibility Design with Existing Power Supplies, PFC, and Momentary Power Interruption Sequence

A full custom development case study of a board-mounted power supply for POS terminals. In replacing an existing board power supply from another manufacturer, specifications were finalized on the premise of ensuring compatibility in terms of mechanical, electrical, connection, and operational aspects. This article organizes and presents the key points that required close coordination, including the integration of PFC, tolerance for the printer's repetitive peak loads, a stepwise shutdown sequence during momentary power outages, and a board design that allows for conformal coating.

We want to replace the current board-mounted power supply without changing the system side -- For design engineers considering a custom power supply

When switching the power supply of equipment already on the market, the difficulty is not in finding a replacement power supply. The key question is how to define "operates in the same way as before."

It is not enough for the output voltage and current to match. The connector pin assignment, communication of control signals, shutdown procedures in the event of a momentary power failure, and peak load tolerance must also be considered. If even one of these differs from the current product, modifications will be required on the system side, whether in software or on the board. As long as the current product is operating without issues, the party deciding on the change will bear the responsibility, which often leads to postponing the decision to switch.

This case study describes the development of a custom power supply (board-type switching power supply) for retail terminals (POS terminals), replacing an existing product from another manufacturer, with compatibility with the current power supply as a prerequisite. The equipment is installed in an outdoor enclosure, and the power supply is mounted inside the housing. Below, we explain the key points that required detailed alignment during the process.

A custom power supply is designed specifically in shape, capacity, and characteristics to match the customer's equipment. Whether modifying a standard product is sufficient or a new design is required depends on how much compatibility is demanded.

1. Document the Details of "Compatibility with the Current Model" Before Starting

The only prerequisite from the customer was that compatibility with the current power supply was mandatory. Taking this statement at face value often leads to misunderstandings surfacing in the later stages of design. Before preparing a quotation, we confirm compatibility by dividing it into the following four categories.

In this case, you provided a list of the current model numbers and pin assignments for the AC input, DC output, and fan connectors. In addition, we received confirmation that "equivalent connectors are acceptable for each part." When component supply is unstable, proceeding with a design while fixing the model number to a single manufacturer can cause schedule delays solely due to procurement issues. Defining in advance the acceptable range of equivalent parts is also effective in reducing supply risks after mass production begins.

Often overlooked point: The mounting side of the connectors (whether consolidated on one side or distributed on both sides) is sometimes not specified in electrical documentation. However, since it directly affects cable routing and the assembly process, changing it later will require reworking the PCB layout. Placement constraints should be confirmed during the initial hearing.

2. Shutdown Sequence Agreement -- How to Respond to Instantaneous Power Interruptions

The most critical point to finalize in this case was the behavior when the AC input is interrupted. For equipment that handles payments and records, the problem is not the power loss itself, but rather the system not being notified in advance that it is about to shut down.

The customer specified the following step-by-step operation:

This "absorption time" and the "time to maintain output after advance notice" are directly linked to the design of the power supply's hold-up capacity. It is necessary to satisfy the relationship among the primary-side capacitor capacity, the secondary-side load conditions, and the response time of the detection circuit, so even a difference of a few milliseconds can change the component configuration. A single line in the specification can impact both cost and board area.

Similarly, for the startup side, we defined how to handle the remote control signal. We separated the system that starts up simultaneously with AC input from the system that starts up after receiving a control signal from the set side, and configured it so that the main output is not enabled unless the control signal is supplied. Regarding the signal voltage level, we adopted input specifications with sufficient margin in consideration of future backward compatibility.

Items to confirm: A control signal specification is defined only when the following three points are clarified: "which side outputs it (power supply side or set side)," "whether active is High or Low," and "what the voltage level is." In this case, similar to the cooling fan rotation detection signal, there were signals whose logic had not been determined because the fan model selection was not yet finalized. In such cases, it should be decided before starting whether to design a circuit configuration that supports either logic or to incorporate the timing of finalization into the project schedule.

③ Peak Current Is Determined by the "Waveform," Not the "Maximum Value"

POS terminals are equipped with receipt printers. Each time printing occurs, the thermal head and motor draw current, causing peaks far exceeding the value expected from average power consumption to repeatedly occur at short intervals.

In this case, what the customer provided was not numerical data but the current waveform measured on the actual printer. In addition to the peak value, it shows the proportion of time (duty) that the peak is sustained. The point at which overcurrent protection should operate must be determined with these repetitive conditions taken into account.

If the protection point is set based only on the peak value, two problems can occur. If the protection point is set too low, the power supply will shut down during normal printing operations; if set too high, it may fail to detect abnormalities that should be protected against. We determined the setting to withstand overcurrent with an equivalent duty cycle and peak, using both the protection operating point of the current power supply and the measured waveform as reference.

We also offer power supplies designed to handle such repetitive peak loads as part of our standard lineup. Even in the case of full custom designs, we can directly apply the circuit design and evaluation expertise accumulated from our standard products, enabling a higher degree of design reliability compared to starting from scratch.

View the list of peak load compatible AC/DC Power Adapter

4. Implementation Environment Conditions -- Application of Moisture-Proof Coating and Undetermined Cooling Method

The requirement specifications stated "outdoor installation." The design changes significantly depending on whether waterproofing and dustproofing are required for the board power supply itself or ensured on the enclosure side. Upon confirmation, the policy was to secure waterproofing and dustproofing with the enclosure, and that the power supply would only need to be a standalone board mounted inside the enclosure.

However, additional conditions based on actual operation were introduced here. Because there is a possibility that a moisture-proof coating (coating material) may be applied to the board surface, we were asked to eliminate any factors that could hinder this at the design stage. Based on market experience, in outdoor equipment, even if countermeasures are taken at the enclosure level, there are cases where only the power supply section ends up receiving additional moisture protection treatment.

When assuming the application of a moisture-proof coating, there are points that should be considered in advance, such as components with moving parts, semi-fixed resistors for adjustment, and ensuring heat dissipation paths. If the decision to apply the coating is made after mass production begins, component selection may have to be redone. Being informed at the stage of "it might be applied" was effective in preventing rework.

Regarding cooling, the customer was still considering whether to install a fan on the equipment side. Therefore, we were given the condition that the thermal design must be viable even without a fan. If component placement is optimized on the assumption of forced air cooling, the temperature margin will be insufficient once the configuration changes to one without a fan. When the cooling method is not yet determined, it is necessary to design based on the stricter conditions.

5. Schedule and Initial Cost Considerations

Custom power supply development cannot be scheduled on the assumption that prototyping will be completed in a single iteration. We proceed through the following stages.

When presenting a schedule to customers, we clearly separate the timing for delivering samples and the timing for starting material procurement, rather than referring only to the development completion date. This is because, when working backward from the mass production start date, the latter has the greater impact. If the material lead time is known, we can design in advance the stage at which a preliminary order is required to stay on schedule.

Regarding initial costs, we present an estimate that includes expenses anticipated to be necessary for development, such as PCB molds, films, and jigs. If costs are added item by item later, the total amount may change during internal approval processes, potentially halting decision-making. If additional costs become necessary during development, we address them separately at that time.

Handling of exchange rates: When comparing multiple suppliers, if quotations are mixed between yen and foreign currencies, the basis for comparison becomes inconsistent. Upon request, we can also provide quotations in foreign currencies. Please consult with us during the comparison stage.

Summary of Development Case

Do you have any of these concerns?

Since our founding in 1989, we have operated as a manufacturer of AC/DC Power Adapter and switching power supplies, handling everything in-house from design and manufacturing to post-mass-production after-sales support. For custom power supplies, we support consultations from small lots with outputs ranging from 3W to 1.5kW. We also offer board-type open-frame power supplies as part of our standard lineup, allowing us to assist in determining whether a modified standard product will suffice or if a new design is required.

Even if the specifications are not yet finalized, we can review documentation of your current product and organize compatibility considerations for proposal.

Contact us here

▶ View the scope and process of custom power supply support
▶ View the list of full custom power supply development case studies
▶ View the list of open-frame switching power supplies (board type)

*This case study is published within the scope of the customer's consent. Company name/Products name/Model number/Quantity/Price/Specific numerical specifications are not disclosed.
*The content described is as of the time of consideration, and actual specifications and schedules vary by project.