Technical Article

Modified Standard, Semi-Custom and Full-Custom Designs

March 03, 2022 by Steve Roberts

RECOM offers over 25,000 standard portfolio DC/DC and AC/DC products, but still at times, a non-standard converter is needed. This article explores the difference between modified standard, semi-custom and custom power converters.

One of the reasons RECOM has the broadest power converter portfolio in the industry is the sheer number of variations available within our standard product range. If we take a classic 5W isolated and regulated DC/DC converter in a DIP24 case, the REC5 series (for example), then the datasheet explains that it has five input and nine output voltage range options, including single and dual outputs. There are three different isolation voltage options from 2kV up to 6kV and three different pinning options to make it a pin-compatible alternative to the competition, including a version with a remote on-off pin. Additionally, the board mounting pins can be through-hole or SMT. The case material can be plastic or metal. Finally, the series is available with either tube or tape-and-reel packaging options. Overall, for this one DC/DC converter series, there are well over two thousand permutations as standard!


Figure 1. Simplified shunt regulator circuit (in this circuit, Vset would be Vref(1+(R1/R2)) + Voptodiode). Image used courtesy of Bodo’s Power Systems


Modified standard power converter

It would seem that all possible customer requests would be covered with such a wide portfolio of catalog products, but still, sometimes specific characteristics requested by customers are not included in these options. An example would be different pin lengths (shortened or over-long) or a 5.7V output voltage instead of the usual 5V output voltage so that an OR-ing diode could be included in the output circuit. Such requests are very easy to implement and fall under the umbrella of ‘modified standard’ products.

To understand why a modified output voltage is so simple, it helps to look at the most common way of regulating the output voltage in a DC/DC or AC/DC converter – an output shunt regulator based on a precision programmable reference IC (Figure 1).

On each switching cycle, the output voltage rises until the shunt regulator set voltage, Vset, is exceeded, upon which the optocoupler is energized and the cycle reset. Although very simple, this circuit accurately regulates the output voltage for both load and line voltage variations. The switching duty cycle and hence output voltage is decided by combining the turns ratio of the transformer, the Vref voltage (typically 1.22V) and the two programming resistors R1 and R2. Modifying this converter to offer 5.7V instead of 5.0V would require fitting alternative R1 or R2 values to create a different programming divider ratio, a simple modification that could be carried out at little to no extra cost. RECOM has its own SMD assembly lines, so creating a batch of converters with a modified output voltage would mean changing only the production SOP and the laser-printed label.

We can also optimize the choice of component values to fit the specific requirements of the customer’s end application, rather than relying on a standard converter. If we take an unregulated DC/DC converter as an example, then the output voltage depends on both load and input voltage. Typically, we adjust the component values so that the nominal output voltage is delivered with the nominal input voltage and 80% load, as shown in the diagram of the output voltage tolerance envelope (Figure 2).


Figure 2. Example of an unregulated converter voltage tolerance diagram (R1SX series). Image used courtesy of Bodo’s Power Systems


This standard configuration fulfills the most common requirements for the output voltage tolerance for unregulated DC/DC converters, which typically operate at slightly less than full load. The compromise is that the output voltage is lower at full load and higher at low loads. Should a customer demand that the output voltage rise to less than +7% under low load conditions, then we can modify the design to keep the output voltage under, say, +5% by modifying the component values so that the Vnom voltage is delivered at 70% load instead of 80%, effectively shifting the tolerance envelope downwards. The disadvantage would be a lower output voltage at full load but may not matter in this particular application.

However, the amount of output voltage modification that can be achieved by optimizing the component values to better match the requirements of the end application is limited. The nominal output voltage depends on the turns ratio of the transformer, so if the required output voltage deviates too far away from the standard, then a modified transformer with an adjusted turns ratio could be required. This would entail not only a custom transformer part number but also a re-certification of the safety standards. We will now move away from a modified standard converter to describe a ‘semi-custom’ design.


Semi-custom power converter

The boundary between a semi-custom and a modified standard power converter is not always obvious, so a useful definition is whether safety re-certification is needed. Modifications that influence safety-critical components or separations – such as the transformer construction, choice of optocoupler or Y-capacitor, or creepage or clearance distances – will almost certainly require a new certification process. However, the safety test reports typically allow a ±10% variation in the output voltage and current rating to accommodate production tolerances, leaving some scope for flexibility. The input voltage rating also has at least a ±20% tolerance and often more. If the required modification can respect these tolerance bands, then safety re-certification may not be required. The same applies if the original safety test report lists ‘alternate construction’ options for certain components or the potting materials. We have customer applications where silicone potting material is unacceptable, either because of the possibility of outgassing, which could affect other parts or because silicone could emit smoke in a fire, so although silicone is an excellent encapsulation material with high thermal conductivity and good adhesion, we could offer these customers epoxy potting as a modified standard.

It may seem surprising, but the label design and text are considered part of the safety documentation and are very carefully controlled by the certifying bodies. We regularly receive requests for a custom label design with, for example, the customer’s own brand name and part number instead of RECOM’s standard label. Although physically the two parts would be identical, we would need to apply for a copy certificate with the new label, part number and customer logo to manufacture this modified part with valid safety certification. However, we have experience in handling such requests and we strive to make the process as fast, simple and unbureaucratic as possible.

Once we enter the realm of semi-custom power converters, then we can offer much more flexibility than simply tweaking the specifications, such as using alternative PCB layouts to change the shape of the converter or matching the pinout to replace a competitor’s part that has been rendered obsolete. The changes do not need to be physical; we have made semi-custom converters that use higher-rated components to extend the operating temperature range or harder epoxy to allow the converter to meet specific environmental conditions. All of these modifications are based on standard designs, so although a semi-custom product meets individual customer requirements for form, fit or function, it can still be manufactured quickly and cheaply on our standard production lines with a short time-to-market.


Semi-custom power converter examples

In 2010, a US competitor abruptly stopped manufacturing a series of switching regulators, leaving their existing customer base ‘in the lurch.’ One customer approached us, asking if we could modify our standard R-78 series to be pin-and-function compatible with these end-of-life products, even though the format was completely different from our existing design. In only 12 weeks, we released the R-78T series with three different open-frame SMD pinning options, designed to be pin-compatible with these no longer available parts. The reason we could react so swiftly was that we did not need to start from ground zero – we already had extensive experience in designing and manufacturing switching regulator modules and could use this as a base for the semi-custom design.


Figure 3. The three different formats (/AC, /AL and /FC) of the R-78T series. Image used courtesy of Bodo’s Power Systems


Figure 4. The semi-custom RAC20-K/X6 with IEC 62477-1 certified OVC III. Image used courtesy of Bodo’s Power Systems


More recently, we received a request for a 20W board-mount AC/ DC module to be used as an auxiliary power supply in an EV charging station. As the charging station is hard-wired to the electricity supply, it is classed as an OVC III (Over Voltage Category III) installation and has to satisfy additional safety requirements concerning overvoltage surges, isolation ratings and creepage and clearance distances. We were able to manufacture a semi-custom product, the RAC20-K/X6 with a revised PCB layout and transformer construction to meet the IEC 62477-1, OVC III requirements with 4kVAC/1 minute rated reinforced isolation instead of the standard 3kVAC/1 minute rating. This feat was achieved without changing the dimensions, pinout or other important specifications such as the wide input voltage range, wide operating temperature range and <40mW no-load power consumption.

With many semi-custom designs, other customers also need similar parts, and they can become standard off-the-shelf options in our portfolio.


Full-custom power converter

A full-custom power converter differs from the modified standard or semi-custom variants because it is not based on an existing product, but it obviously uses proven and tested building blocks and existing production infrastructure where appropriate. Typical requirements that would make a custom power converter project viable would be that no existing product can fulfill the need, that the costs (R&D, testing and production) are viable for both partners, and that the project volume justifies the investment in resources.

The typical custom power converter project stages are shown in Figure 5:


Figure 5. Custom power converter project stages. Image used courtesy of Bodo’s Power Systems


Project Proposal: The initial request from the customer detailing the technical requirements of the custom power converter (functional specifications, performance requirements, technical standards and physical dimensions) as well as the commercial factors (budget, expected volume, target cost, etc.).

Project Evaluation: Analysis of the technical requirements (suitability, manufacturability, resource availability, etc.). Is the project viable?

Project Plan: The project plan details the project schedule, resource allocation and milestones that need to be achieved at each stage of the project.


Type Customization level Complexity Speed Added Cost
Modified Standard Product restricted Simple Several weeks Low or no added cost
Semi-Custom flexible, but limited medium A few months Low NRE costs, but may involve re-certification fees.
Full Custom open complex Several months Product-dependent, but NRE charges and new certification costs are significant
Figure 6. Summary of the advantages and disadvantages of modified standard, semi-custom and full custom power converters.


Prototype: Prototypes will be built to verify compliance with the functional specifications. A change in specifications at this stage is common. RECOM understands that even when the initial custom power converter requirements are clear and well-defined, the specifications may need to be occasionally updated or further modified, usually reacting to changes in the market. This is normal. As long as the communication channels between the customer and the RECOM design team are open, we aim to be as flexible as our customers.

After approval of the initial prototypes, the next stage is the manufacture of pre-production prototypes (at least 50 units) so that statistically valid performance and stress testing can be conducted.

DVT/PVT: The design verification tests and production verification tests are critical to the success of the project. RECOM needs to provide datasheets and test documentation that will inspire in the customer a high level of confidence that the custom design is fit for purpose and will function reliably under all foreseeable operational and fault conditions. To do this, RECOM has an automated test lab with networked test stations to allow rapid testing and evaluation, whether it be electrical testing, environmental testing in climate chambers, reliability testing (electrical and thermal stress tests, safe operating area tests, long term soak tests, shock and vibration, etc.) or production tolerance testing.

Production and Certification Plan: As soon as the DVT testing is advanced enough to initiate a design freeze, then the production plan can be started, entailing purchasing of the necessary components from primary and second sources and ordering the custom housing, printing and packaging. The production-ready prototypes can be sent off for agency approval (UL, IEC, EN, CB Reports, etc.).

Production: An assembly line will be set up to manufacture, test and ship the final product to the customer.

Change Management: As mentioned previously, change is a fact of life and needs to be managed. Technical standards are updated on average every three years, and sometimes this requires a modification to the custom design to stay compliant with any new regulations. Changes to the availability of key components may require notification to, and agreement from, the customer, with a Product Changer Notice (PCN). The Product Life Cycle (PLC) documentation is maintained over the lifetime of the product until the eventual end-of-life decision by the customer.


As can be seen, by this brief introduction, a RECOM power converter can be offered as a modified standard, semi-custom, or full-custom product to the customer in several ways. Each variant has advantages and disadvantages (Figure 6), so the simplest way to decide is to establish a dialog with us so we can advise and work together with you to offer the optimum solution.

To facilitate this, RECOM has set up four dedicated e-mail addresses for customization requests. Someone from RECOM will then contact you to discuss your requirements in more detail.


This article originally appeared in Bodo’s Power Systems magazine.