Custom Power Supply Development for POS Terminal Boards | Compatibility Design with Existing Power Supplies, PFC, and Momentary Power Interruption Sequences

A full-custom power supply development case study for a POS terminal board. When replacing the existing board-mounted power supply from another manufacturer, specifications were finalized on the premise of ensuring mechanical, electrical, connection, and operational compatibility. This case study outlines the key issues that required coordination, including PFC integration, tolerance for repeated peak loads from the printer, a phased shutdown sequence during momentary power outages, and board design that accommodates conformal coating.

For Design Engineers Considering a Custom Power Supply: Replace the Existing Board-Mount Power Supply Without Modifying the Host Equipment

When switching the power supply in equipment already on the market, the challenge is not finding an alternative. It is determining how to define "operation identical to the current system."

Matching the output voltage and current alone is not enough. Connector pin assignments, control signal exchanges, shutdown procedures during momentary power outages, and peak-load capacity must all be considered. If even one of these differs from the existing product, modifications to the host equipment's software or circuit boards will be required. Since the existing product is operating without problems, the party deciding to make the change will assume responsibility, so the decision to switch is often postponed.

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 made by another manufacturer. The specifications were finalized with compatibility with the existing power supply as a prerequisite. The terminal is built into an outdoor enclosure, with the power supply mounted inside it. The following sections explain the key points that required coordination.

A custom power supply is specifically designed in terms of form factor, capacity, and characteristics to suit the customer's equipment. Whether modifying a standard product is sufficient or an entirely new design is required depends on the level of compatibility needed.

1. Document What "Compatible with the Current Product" Means Before Starting

The customer's sole prerequisite was that compatibility with the current power supply was mandatory. Taking this statement at face value can lead to discrepancies in understanding becoming apparent during the later stages of design. Before preparing a quotation, we divide compatibility into the following four categories for confirmation.

For this project, you provided a list of the current part numbers and pin assignments for the AC input, DC output, and fan connectors. You also confirmed that "equivalent parts may be used for each connector." During periods of unstable component supply, proceeding with a design that specifies parts from only one manufacturer can cause the schedule to shift solely due to procurement issues. Defining the acceptable range of equivalent parts in advance is also effective in reducing supply risks after mass production begins.

An easily overlooked point: The mounting side of the connectors, whether they are all placed on one side or distributed across both sides, may not be specified in the electrical specifications. However, because this directly affects cable routing and the assembly process, changing it later will require the PCB layout to be redone. Placement constraints should be confirmed during the initial requirements discussion.

2. Shutdown Sequence Requirements -- How the System Should Respond to Momentary Power Outages

The behavior when the AC input was interrupted required the most careful consideration in this project. For equipment that handles payments and records, the problem is not the loss of power itself, but rather that the host system is not notified in advance that power will be lost.

The customer specified the following phased operation.

This "absorption time" and the "time the output must be maintained after issuing a warning" directly affect the design of the power supply's hold-up capacity. The primary-side capacitor capacity, secondary-side load conditions, and detection circuit response time must all be coordinated to meet these requirements, so a change of only a few milliseconds can alter the component configuration. This is an item where a single line in the specification affects both cost and PCB area.

Similarly, we established how the remote control signals would be handled during startup. The rails that start up when AC power is applied are separated from those that start up only after receiving a control signal from the system side, and the main output remains disabled unless the control signal is supplied. To ensure future backward compatibility, the input specifications allow sufficient margin for the signal voltage levels.

Items to confirm:A control signal is not fully specified until all three of the following have been defined: "which side outputs it (the power supply side or the system side)," "whether it is active High or active Low," and "what its voltage level is." In this case, some signals, such as the cooling fan rotation detection signal, did not yet have defined logic because the fan model had not been selected. In such cases, before beginning work, either design the circuit to support both logic polarities or incorporate the expected decision date into the schedule.

3. Determine Peak Current by the "Waveform," Not the "Maximum Value"

POS terminals are equipped with receipt printers. Because the thermal head and motor draw current each time a receipt is printed, peaks far exceeding the values estimated from average power consumption occur repeatedly at short intervals.

For this project, the customer provided us not with numerical values, but with current waveforms measured using the actual printer. In addition to the peak value, the waveforms show the duty cycle (Duty) for which the peak continues. The overcurrent protection threshold must be determined with these repetitive conditions taken into account.

Setting the protection threshold based only on the peak value can lead to two problems. If the threshold is set too low, the power supply will shut down during normal printing. If it is set too high, abnormalities that should be detected will be overlooked. Using both the protection trip point of the existing power supply and the measured waveforms as references, we configured the power supply to withstand overcurrent with an equivalent duty cycle and peak.

We also offer a standard lineup of power supplies designed to handle such repetitive peak loads. Even for fully custom designs, we can directly apply the circuit design and evaluation expertise accumulated through our standard products, improving design reliability compared with starting from scratch.

View the Lineup of Peak Load-Compatible AC/DC Power Adapter

4. Implementation Environment Requirements--Application of Conformal Coating and an Undetermined Cooling Method

The requirements specified "outdoor installation." The design would vary significantly depending on whether the board-mounted power supply itself needed to be waterproof and dustproof or whether protection would be provided by the enclosure. Upon confirmation, we were informed that waterproofing and dustproofing would be handled by the enclosure, and that the power supply could be supplied as a standalone board installed inside it.

However, an additional requirement based on actual operating conditions was introduced. Because a conformal coating may be applied to the surface of the board, any factors that could interfere with its application should be eliminated during the design stage. This request was based on market experience showing that, even when protective measures are implemented in the enclosure of outdoor equipment, conformal coating may ultimately be added only to the power supply section.

If conformal coating may be applied, certain factors must be considered in advance, including components with moving parts, semi-fixed resistors used for adjustment, and the maintenance of heat dissipation paths. If the decision to apply the coating is made after mass production begins, component selection may have to start over. Sharing this possibility while it was still at the "we may apply it" stage was effective in preventing rework.

Regarding cooling, the customer was still considering whether to install a fan in the equipment. We were therefore given the requirement that the thermal design must be viable even without a fan. If component placement were optimized on the assumption of forced-air cooling, the thermal margin would become insufficient if the configuration were later changed to one without a fan. When the cooling method has not yet been determined, the design must be based on the more demanding conditions.

5. Approach to Scheduling and Initial Costs

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 distinguish between the time required to complete development, when samples can be delivered, and when component procurement can begin. This is because the latter is what actually affects the schedule when working backward from the mass production start date. If the component procurement lead time is known, we can determine in advance when we need to receive a preliminary commitment to meet the deadline.

For initial costs, our quotations include anticipated development expenses, such as PCB tooling, films, and jigs. If costs are added item by item later, the total may change during the internal approval process, bringing the decision itself to a standstill. If additional expenses become necessary during development, we will discuss them with you individually at that time.

Regarding exchange rates: When comparing multiple suppliers, quotations presented in a mixture of yen and foreign currencies do not provide a consistent basis for comparison. Upon request, we can also provide quotations in foreign currencies. Please consult us during the comparison process.

Development Case Summary

Do You Have Any of These Concerns?

Since our founding in 1989, we have handled everything in-house, from design and manufacturing to post-production support, as a manufacturer of AC/DC Power Adapter and switching power supplies. For custom power supplies, we accept inquiries for outputs ranging from 3 W to 1.5 kW, including small-lot orders.Because our standard product lineup also includes open-frame PCB power supplies, we can help determine whether your requirements can be met by modifying a standard product or whether a new design is necessary.

Even if your specifications have not yet been finalized, we can review documentation for your current product, identify compatibility considerations, and propose a suitable solution.

Contact Us

▶ View the Scope and Process for Custom Power Supplies
▶ View Our Full-Custom Power Supply Development Projects
▶ View Our Open-Frame Switching Power Supplies (PCB Type)

*This case study is published with the customer's permission. The company name, Products name, model number, quantity, price, and specific numerical specifications have not been disclosed.
*The information provided reflects what was under consideration at the time. Actual specifications and schedules vary by project.