Build or Buy 2: Evaluating the resources

2023/06/14
On the topic “Build or Buy?” [1], we took a fresh, holistic view on perspectives for addressing this difficult, yet common question that system designers and board engineers must make all the time.

1. Surveying the landscape

In a previous whitepaper on the topic “Build or Buy?” [1], we took a fresh, holistic view on perspectives for addressing this difficult, yet common question that system designers and board engineers must make all the time. We identified many factors (technical, logistical, business, and otherwise) that need to be considered to enable a designer to initiate their own process for evaluating these many factors and forge a plan most amenable to their situation. We also presented the pros and cons to consider and compare when trying to streamline the variety of factors involved in application design.

In this whitepaper, we shall focus more in-depth on specific factors within the evaluation process and their success rates prior to development. Namely, a focus on evaluating external power vendor resources, determining your own organization’s/team’s flexibility to meet your approach, and advice on how to drive that approach with team buy-ins are presented in this instance.

One factor to consider is a power vendor’s modular portfolio - pick a power vendor’s website and review the products they have that you feel may be a good fit (as close as possible) to what you are looking for. Bear in mind that the key objective of this exercise is not to cross your fingers and hope to strike gold with the perfect, commercial-off-the-shelf (COTS) part that matches all your design requirements.

Naturally, that can be a great happenstance too, but what you should really be looking for in most scenarios are the vendor’s capabilities.
This can be investigated through product listings, discussions about product spaces and industrial markets covered, and perhaps even case studies or historical background information on real, demonstrated solutions to real problems.

Vendors present this kind of content in a mix of formats, which include, but are not limited to, data sheets (a.k.a. specification or spec sheets), certifications, application notes, whitepaper, webinars, product/marketing overview documents, design guides, evaluation modules, etc.
If you go a level deeper, then you may also want to consider the human resources in addition to the capital expenditure (CAPEX) resources. What is the level of experience of the team? How long have they been working together as a team? How long have they been working specifically in the application space for your solution?

Many of these questions may seem trivial, but in practice, one tends to learn that this minutia can make all the difference in the world when deciding on the “Build or Buy?” question, as well as facilitating the selection of a vendor when going external.

This applies far more to full-custom or semi-custom developments, but should not be disregarded for COTS solutions either. Just because you are buying a fully-designed, qualified part does not mean:

  1. It was designed/qualified for your exact application.
  2. You will get the support/resources you need to properly integrate it into your system.
  3. You will get the support/resources you need to adequately address a quality/reliability and/or field failure issue.
Below we see an example of a product matrix listing from a leading power supply company’s website, which is a very common way to quickly get the “bird’s eye view” of the vendor’s offerings and compare the key specifications of numerous power solutions. This commonly comes with some filter options to help narrow things down.

A well-organized, informational website that is easy to navigate and is intuitive in helping you quickly converge on what you are looking for can be your first, unofficial indicator of the type of experience you can expect from a power vendor.



Fig. 1: Power vendor product selection tool website screenshot, courtesy of RECOM Power [2]


Product data sheets should be easy to come by without needing to jump through many hoops of steps such as clicking from page to page and/or having to submit a great deal of information. Ideally, the data sheets are accompanied using plenty of design examples, application notes, usage in target applications, and other handy design tools (e.g., calculators and simulators).

If you are forced to select a single item to evaluate a vendor by over anything else (before even talking to them), then that would be the data sheets. When properly scrutinized, a data sheet can tell you, or at the very least clue you into, what kind of vendor you can expect when working with this development partner.

Some common data sheet items and some of the implied messaging (direct or otherwise) are summarized in the table below.

Datasheet item Potential Interpretations
LINE/LOAD REGULATION, RIPPLE & NOISE
  • The bare minimum for a power supply should be in a table of specs with min./nom./max. values. Ideally, it should also include pertinent test conditions (such as ambient temperature).
  • A comprehensive document will have many curves/waveforms demonstrating performance over full line/load/temperature over the full operating range.
  • DIFFERENTIATORS: Curves cover a line/load range consistent with typical application/environmental usage (0-100% load), or at least the full, operating temperature range.
EFFICIENCY
  • This is commonly considered to be one of the most important metrics. The bare minimum for a power supply should be in a table of specs with min./nom./max. values. Ideally, it should also include pertinent test conditions (such as loading process for each specified point).
  • A comprehensive document will have many curves/waveforms for a handful of common Vin, Vout scenarios demonstrating performance over the full load range.
  • DIFFERENTIATORS: Curves cover a line/load range consistent with typical application/environmental usage and does not try to cut off any of load range (especially in 0-20% range). Multi-modal curves are commonly an indicator of a supply supporting more than one operating mode (e.g., pulse-skipping mode at <10% load transition point, multi-phase solutions, etc.).
OPERATING/SWITCHING, FREQUENCY
  • The bare minimum for a power supply should be in a table of specs with min./nom./max. values. Ideally, it should also include pertinent test conditions (such as ambient temperature).
  • A comprehensive document will define if frequency is fixed/variable and provide curves/waveforms demonstrating performance over the full operating range (particularly if variable).
  • DIFFERENTIATORS: Any frequency-dependent features, whether it be load-based changes to frequency (e.g., resonant topologies) or steps to mitigate EMI (e.g., spread-spectrum clocking). A clear red flag is if one cannot even tell if the switching supply supports a fixed or variable switching frequency and/or the vendor cannot respond when prompted.
PROTECTION CIRCUITS
  • This can be highly variable given the amount of potential protection features (or not). The bare minimum for a power supply should be in a table of specs with min./nom./max. values. Ideally, it should also include pertinent test conditions (as appropriate). Such specs should include fault/comparator thresholds, response times (inc. any appropriate hysteresis), and actions taken (e.g., latching, non-latching, and hiccup mode).
  • A comprehensive document will define the protection method circuits with descriptions, equations, and functional block diagrams.
  • DIFFERENTIATORS: Curves/waveforms demonstrating action under specific conditions (identifying said conditions), detailed explanations of actions taken (e.g., flow diagram of state machine for both SW and HW), and perhaps even some thoughts/suggestions on alternative applications/uses. The best documents will also provide potential failure modes along with the risks/impacts of such faults. This is an area in which vendors are typically very risk-averse and not forthcoming about potential failure scenarios, but when pressed with “What if I did this…?” or “Does the warranty cover if I can damage the converter by shorting X and Y?” or similar questions, comprehensive answers start to materialize quickly.
INPUT/OUTPUT CAPACITANCE (INC. FILTERING)
  • The bare minimum for a power supply should be in a table of specs with min./nom./max. values, but with lots of variance here due to the many purposes caps serve. A min. and/or max. amount of cap may be required for loop stability, but a ripple spec can have dependency on both internal and external factors (i.e., internal EMC choke, ripple reduction due to external transients). Related ripple/life spec requirements should be tabled as well.
  • A comprehensive document will have a section(s) dedicated to input and/or output cap requirements. For EMC purposes, this can be in the form of specifying filters or merely compliance levels.
  • DIFFERENTIATORS: Detailed calculations/requirements for capacitance, discussion/input on different types of caps, and impedance curves (big bonus). The best documents will also provide some tradeoffs between cap type, size, and other ratings to allow for optimization of height/footprint/reliability/cost.
SAFETY/ISOLATION (INC. CERTIFICATIONS)
  • This can be highly variable given how diverse application/industry-specific safety standards can be. At the very least, compliance with a certain standard should be stated, as well as to what degree (often many classes/levels of compliance). For power supplies, this should include a bare minimum of EMC and compliance with a functional standard related to safety spacing/isolation. Pay special attention to regional/industrial requirements.
  • The bare minimum for a power supply requiring/claiming isolation should specify the max. isolation level and withstand time in a table of specs. Ideally, the isolation test parameters/ grade (inc. resistance and cap) and/or standard are referenced.
  • A comprehensive document will share test data or at least provide some referral/link/contact to where such data can be found.
  • DIFFERENTIATORS: An isolation voltage figure (say 3kV) which does not specify whether to use AC or DC or for how long is meaningless. Similarly, a mention in the data sheet that a particular standard is met without stating which edition or amendment is covered is also less than helpful. There is also a big difference between “designed to meet” and “certified” when compliance to a standard or norm is claimed. A good data sheet will provide measured data from compliance testing, particularly if taken for different levels of compliance (e.g., EMI curve for both class A & B, conducted & radiated emissions) to indicate how well the requirements are met. The best documents will also provide guidance (mostly related to layout) on how to optimize/maintain compliance to standards (as appropriate according to application/ industry). A team’s response to questions about any marginal-pass data will surely yield valuable learnings.
QUALITY/RELIABILITY DATA
  • This can be highly variable given just how many standards/guidelines are involved in determining compliance. A typical minimum level for this is to specify compliance to a standard (e.g., MIL-HDBK-217F, IPC-9592). A mean time between failure (MTBF) specs will commonly be specified. Note that MTBF calculations can be very easily manipulated by choosing different environmental conditions, so do not select a part based on MTBF hours alone.
  • A comprehensive document will include test levels (typically max. for shock, vibration, and powerline immunity) and details of the test setup. Figures are provided for both calculated and demonstrated MTBF.
  • DIFFERENTIATORS: The best documents will share detailed data from tests, but that is rare. On the other hand, access to a quality/reliability resource and/or ability to discuss results and changes made to mitigate test failures can be some of the most fruitful additions possible. As referenced below, providing thermal data is very helpful here.
MECHANICAL DATA (Not including thermal issues)
  • The bare minimum mechanical data for a power supply include mechanical dimensions, weight, and a declaration of material conformity (e.g., RoHS, WEEE) and flammability ratings (e.g., UL94).
  • A comprehensive document will have a mechanical diagram with an isometric view of the supply and identification of all input/output connections (electrical or otherwise). A recommended footprint and soldering profile info should be provided for any mountable components.
  • DIFFERENTIATORS: The best documents will include full pinout information for any connectors, any data appropriate for orientation (e.g., Pin 1 marker), and mechanical tolerancing info. They will also include details on packaging (dimensions and storage info).
THERMAL DATA (Not including form-factor issues)
  • The bare minimum for a power supply, should be in a table of specs with min./nom./max. values of supported temperature ranges in both operating and non-operating (e.g., shelf life) environments. This is typically accompanied by a max. relative humidity (RH) spec as well.
  • A comprehensive document will have thermal derate curves that demonstrate how output can be limited (but still viable) at the extremes of the supported temp range.
  • DIFFERENTIATORS: The best documents will include thermal profiles (typically an IR-camera thermal map) both sides of the main system PCA(s), to identify specific component hotspots and contrast again different loading levels and airflow profiles and/or cooling solutions.
Table 1: Power Supply Data Sheet Information & Interpretation


As information that is missing from the data sheet can tell you a great deal too, be sure to question why something may or may not be present. These questions, while sometimes awkward, can help clue you quickly into to how well the product performs, if there were any nuances or cause for concern (inc. supply issues and field bugs), how well-organized the vendor is, how transparently they communicate, and how well the team seems to communicate amongst themselves.

When performing this analysis across multiple vendors and starting to get a survey of the market, make sure to listen to the data created by your analysis. Are there many same/similar solutions to what you are looking for on the market? There are likely good reasons behind this, and they can give you hints as to ...

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