Considerations for Choosing a Custom Design for Your Power Supply Project

The power supply world is complex and ever-changing. A wide variety of design approaches are needed to meet diverse application requirements, ever-increasing energy efficiency standards, and an array of applicable regulations.

Although RECOM provides a broad range of standard products, sometimes an application calls for something more: i.e., a customized approach.

Several decades ago, designing a power supply was a relatively straightforward task. The power world was linear, and the front end of a typical power supply consisted of a transformer feeding a full-wave bridge rectifier into a large-value filter capacitor.

Such a linear design is simple, reliable, and if an unregulated power supply meets your requirements, quite efficient, too. Add regulation, though, and the efficiency plummets. Linear regulators, as is well known, control the output voltage by dropping it across the pass element (the power transistor), which dissipates copious amounts of heat and results in an efficiency as low as 60%.

Switching power topologies changed all that. Chopping up the incoming DC voltage into a high-frequency switched voltage and using that to produce the desired output voltage and current allows the power transistors to spend most of their time in the efficient “on” or “off” states and minimizes power losses.

There are many ways to architect a switching power supply; over time, new switching topologies proliferated like weeds, each one with advantages and drawbacks for a given application. Available options now include the boost, buck, buck-boost, Cuk, flyback, forward, full-bridge, half-bridge, push-pull, SEPIC, synchronous buck, two-switch forward, Weinberg, and zeta topologies.

Not only that, increasing-stringent regulations mandate the addition of a front power factor correction (PFC) stage to most AC-DC designs. Here again, the quest for higher efficiency has driven an increase in design complexity, from early passive PFCs to the conventional boost converter, and more recently the totem pole topology.

Plus, there has been a proliferation of power devices: bipolar transistors are long gone, replaced by a variety of MOSFET technologies, and silicon has been joined by silicon carbide (SiC) and gallium nitride (GaN).

Switching power supplies today operate at frequencies in the MHz range and can achieve efficiencies of well over 90%. Unfortunately, this increased performance has raised the bar considerably for the designer, who now must be familiar with a wide range of techniques: high-frequency magnetics, thermal management, electromagnetic compatibility (EMC), switching transistor technology, printed circuit board layout, digital and analog control theory, and more.

DC/DC converters provide a specific voltage and isolation between input and output. They can be used for many different reasons and provide all of the voltages used by electronic components in one single board. The isolation within the DC/DC converter allows the design engineer to comply with applicable safety regulations and solve issues such as interferences and failure protection. DC/DC converters also offer a flexible and clean solution to Distributed Power Architecture systems.

A DC/DC converter module is designed to meet one or more of the following requirements:

• to match the secondary load to the primary power supply
• to provide isolation between primary and secondary circuits
• to provide protection against the effects of faults, short circuits, or overheating
• to simplify compliance with safety, performance, or EMC legislation.

RECOM’s unregulated DC/DC converters are typically used in general purpose power isolation and voltage matching applications and feature a full industrial operating temperature range of -40°C up to +85°C without derating.

RECOM’s regulated DC/DC converters are designed for industrial applications and can drive high capacitive loads and operate over a broad temperature range without derating. They provide stable output voltages when the input voltage suddenly changes.

Some of the key DC/DC converter specifications are:

• Input voltage range over which the converter can maintain functional performance over the operating temperature range at full load.
• Load regulation: the change in output voltage over the specified change in output load, usually specified as a percentage of the nominal output voltage
• Line regulation: The change in output voltage for a given change in input voltage, expressed as percentages.
• Output voltage accuracy: the proximity of the output voltage to the specified nominal value.
• Input and output ripple and noise: The amount of voltage drop at the input, or output between switching cycles, including high-frequency noise caused by converter switching spikes.
• Input to output isolation: The dielectric breakdown strength test between input and output circuits
• Insulation Resistance: The resistance between input and output circuits.
• Efficiency at full load: The ratio of power delivered from the device to power supplied to the device when the part is operating under 100% load conditions at 25°C.
• Temperature drift: The change in voltage, expressed as a percentage of the nominal, per degree change in ambient temperature. This parameter is related to several other temperature dependent parameters, mainly internal component drift.
• Switching frequency: The nominal frequency of operation of the switching circuit inside the DC-DC converter. The ripple observed on the input and output pins is usually twice the switching frequency, due to full wave rectification and the push-pull configuration of the driver circuit.
• No load power consumption: This is a measure of the switching circuits power consumption; it is determined with zero output load and is a limiting factor for the total efficiency of the device.
• Isolation capacitance: The input to output coupling capacitance. This is not actually a capacitor, but the parasitic capacitive coupling between the transformer primary and secondary windings.
• Mean Time Between Failure (MTBF): RECOM uses MIL-HDBK-217F standard for calculation of MTBF values for +25°C as well as for max. operating temperature and 100% load.

AC/DC converters need to be safe, cost effective, energy efficient, reliable and significantly smaller than their predecessors. New RECOM AC/DC designs focus on no-load and light-load efficiency performance to meet the latest regulatory requirements; the AC/DC converter must operate efficiently over the entire load range: from no load conditions, through light load, to full load operation.

The regulatory environment for AC/DC and DC/DC converters includes several aspects of their operation.

Power supplies generate and handle potentially lethal voltages and currents, so achieving the appropriate safety certification is a critical component of a power supply design.

Excellent electromagnetic compatibility (EMC) performance is a key requirement for switching converters to ensure that any high-frequency noise does not affect nearby components. In Europe, the EN5022 standard governs both conducted and radiated emissions. EN5022 has different standards for Class A devices that are designed for use in commercial, industrial, or business environments, and Class B devices that are intended for use in the home. The equivalent standard in the US is FCC Part 15.

As part of efforts to make better use of natural resources and reduce waste, governments around the world are implementing energy efficiency regulations. For external power supplies, these regulations mandate levels of performance under a range of operating conditions, including no-load power draw and efficiencies at various load levels.

The US, China, EU, Australia, Canada, and other countries all have regulations in place, often following the framework established by the US Department of Energy (DoE). The DoE’s Level VI is the current standard.

There are numerous other efficiency standards. For example, the 80 PLUS program is widely used to rate power supplies used in PCs and servers. There are six levels. An 80 PLUS Titanium power supply, the highest level, is rated for 90% efficiency at as low as 10% load, 92% efficiency at 20% load, 94% efficiency at 50% load, and 90% efficiency at 100% load. The 80 PLUS certification also rates the power factor of a power supply: Titanium requires a power factor correction (PFC) of .95 at 20% load.

Many industries mandate that power supply designs meet additional requirements that exceed the ones discussed above.

Power supplies for medical use, for example, must comply with the ES/IEC/EN 60601-1 3rd Ed. that is more stringent than guidelines for commercial or industrial appliances. Higher safety standards and lower electromagnetic interference are required, providing EMC Compliance.

The power supply market is ever-changing. Regulatory standards are continually increasing, as are the expectations of customers. Designs must accommodate these new demands, so new technologies are regularly appearing.

Some recent and emerging trends in power supply design include the increasing replacement of silicon MOSFETs and IGBTs by wide bandgap devices made from compound semiconductors such as silicon carbide (SiC) and gallium nitride (GaN), which have fundamental advantages over silicon for power applications; new topologies such as the totem pole PFC that are made possible by WBG devices: and new packaging technologies, especially 3-dimensional packages that internal components for higher power density and improved EMC performance.

RECOM offers over 25,000 standard portfolio DC/DC and AC/DC products, but despite the large number of off-the-shelf products, a particular application may call for something more, from a slight change in the specifications of an existing power supply to a one-of-a-kind design. In other words, a customized design.

There are three levels of customized designs: a modified standard product; a semi-custom design; and a full custom design.

The lowest level of customization is to take an existing standard product and change it in some way. RECOM and PCS catalog products can often be modified to accommodate simple customer change requests. Examples of such modifications might be:

• A change to the output voltage, say 5.7V vs. the standard 5V, to allow the addition of an OR-ing diode to the output
• A simple pinout change
• A change of encapsulation material
• A longer or shorter pin length

Such requests are very easy to implement and qualify as modified standard products. In many cases, the existing certifications for safety and EMC remain valid, saving significant costs and time. RECOM has manufactured many ‘modified standards’ in the past, so a product might already be available for a particular set of requirements.

RECOM‘s RHV2 and RHV3 are a family of standard DC/DC converters with reinforced insulation certified to withstand 20kVDC or 12kVAC isolation voltage. Applications include high voltage industrial applications such as hard vacuum systems, electrostatic filters and x-ray generators.

A customer had a high voltage application for the RHV2-0505S/R20 which needed even more input/output pin separation than the 30.5mm offered as standard. The solution was to change the secondary side pinout, eliminating pin 14 and reassigning pin 15 as –Vout to increase the creepage and clearance distance by a further 2.54mm to more than 33mm (see below).

As we had included a pin reassignment option in our test report specifications, the customer was able to still use our existing IEC/EN 61010-1 certification (Safety requirements for electrical equipment for measurement, control, and laboratory use).



The modification was relatively simple, requiring only a change to the internal PCB and the Standard Operation Procedures (SOP), so the modified standard part was made available at the normal cost price, with no overhead for this customer request.

A semi-custom design is the next stage of customization. The boundary between a semi-custom and a modified standard power converter is not always obvious, so a useful definition is whether the change will require a safety re-certification.

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.

Often, a customer specification can be met using an existing ‘platform’ design that has the advantage of proven performance and reliability in the field. This is a more economical approach than a full custom, and product safety assurance and EMC certification are simplified, reducing the risk, and accelerating the time to the market. Existing in-house stock components, tooling, and manufacturing processes may be used, resulting in a cost-effective product.

The customer produces a high-frequency generator for endoscopic- and laparoscopic surgery. The system delivers every common form of energy used in surgery today (monopolar, bipolar, ultrasonic, and advanced bipolar), making it unique in the medical market.

Special requirements for this design included:

• Wide range input
• Limited available space
• Medical approval
• High reliability

The solution proposed was a modified RACM1200-48SAV/ENC that could handle 2.2kW peak load versus the 1.2kW of the standard product. Several design modifications were needed, including doubling the bulk capacitance, changing the inrush circuit, and substituting upgraded switching components. In addition, the mechanical design had to be changed.

A full custom power converter differs from a modified standard or semi-custom design because it is not based on an existing product, but is designed specifically for the required application. That said, the design team typically does not begin with a blank sheet of paper; instead, the team uses proven and tested building blocks and existing production infrastructure where appropriate.



Fig 2: shows the stages of a typical custom design project. These stages are:

• Project Proposal: The initial request from the customer detailing the technical requirements of the custom power converter (functional specifications, performance requirements, technical standards, physical dimensions) as well as the commercial factors (budget, expected volume, target cost, etc.)
• Project Evaluation: Analysis of the technical requirements (suitability, manufacturability, resources availability, etc.). Is the project viable? Typical requirements that would make a custom power converter project viable would be that no existing product can fulfil the need, that the costs (R&D, testing and production) are viable for both sides of the partnership, and that the project volume justifies the investment in resources.
• Project Plan: The project plan details the project schedule, resource allocation and milestones that need to be reached at each stage of the project.
• Prototype: Prototypes will be built to verify compliance with the functional specifications. It is not uncommon for the specifications to change at this stage. 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 carried out.
• 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 give 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.RECOM has an automated test lab with networked test stations to allow rapid testing and evaluation of electrical performance over the full set of environmental conditions. Reliability testing includes electrical and thermal stress tests, safe operating area tests, long term soak tests, shock and vibration tests, as well as production tolerance testing.
• Production and Certification Plan: As soon as the DVT testing is advanced enough to initiate a design freeze, 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: Change is a fact of life and needs to be managed over the life of the project. Technical standards are updated on average every three years and sometimes this requires a modification to the custom design to stay compliant to any new regulations. Changes to the availability of key components may require notification to, and agreement from the customer, with a Product Change 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.

Full custom designs can range from sub-1W to several kilowatts by engineering teams in Austria, Italy, Taiwan, and China, depending on the individual specification. RECOM’s sister company PCS in Italy has expertise in custom high-power single- and three-phase AC/DCs, DC/DCs, battery chargers/conditioners, PFC front ends, and inverters. These can be designed for any desired market—industrial, medical, energy, aerospace, rail, and military COTS. State-of-the-art design techniques are used for high power density and high efficiency, with the lowest cost. Safety certification can be arranged to meet all the common standards. EMC compliance can also be realized with the pre-compliance testing performed using our in-house test chambers, and we can arrange for a third-party EMC certification.

During the last couple of years, global interest in methanol or hydrogen as an alternative fuel has drastically increased. The customer provides fuel cell solutions for mobile and stationary applications for a wide variety of industries around the world. They required a system that would charge a battery from a fuel cell with high maximum efficiency and a power level of around 7000 Watt. Other requirements included input current tracking (ICT), liquid cooling, and a CAN Interface.

An in-house customer design solution with parallel operation of several PCB-modules was functional, but its efficiency of around 92% was not acceptable and required a difficult and complex layout design to process currents of up to 220A while still performing ICT.

The proposed solution was the PCS Model SD7008. This non-isolating converter concept is based on a parallel cascaded, overlapping buck-boost topology that generates an adjustable output voltage which is lower, equal to, or higher than the widely-varying input voltage. Achieving an efficiency level of >97% this solution can be equipped with the requested input current tracking and the customer demand of liquid cooling and CAN-Interface.

Based on the existing switching topology concept, PCS can generate the needed design with improved efficiency by using the latest generation of semiconductors and implementing a liquid cooler in a very short time. The interface will be “upgraded” to a CAN J1939 Standard interface. The project timeline is approximately 12-14 weeks, meeting the customer’s schedule as well as the target price.

When considering a non-standard design, consider that each option has advantages and disadvantages, as shown in the table below.

In addition to its broad range of standard products, RECOM also manufactures full-custom, semi-custom and modified-standard products. Our engineering teams in China, Taiwan, Austria, and Italy have extensive experience in product development and can design new products from the ‘ground up’ to increase the volume of production.

Alternatively, semi-custom-designs and modifications to existing products are offered with very competitive pricing. Modification costs are kept low because RECOM’s automated production lines offer benefits due to its economies of scale through various methods such as by reusing existing tooling and production equipment with access to our extensive on-site component stock.

RECOM also has an extensive library of platform designs to call on, particularly through our Italian sister company PCS, who specializes in fast-turnaround, high-power and full-custom solutions. These proven platform designs can often be readily adapted for other applications without necessarily incurring extensive development or certification costs.

RECOM’s customization and standard product modification service includes low- to high-power AC/DCs and DC/DCs, battery chargers and conditioners, AC inverters, PFC front ends, and much, much more. All major application areas are covered – including industrial, medical, energy, aerospace, mobility, and military COTS. According to customer requirements, fully customized and semi-custom products can also be supplied with the necessary safety and EMC certifications.

With so many options, the simplest way to decide is



so that we can work together with you to offer the optimum solution.