Trolling for Parts

There comes a dreaded moment in the realization of any electrical design: the component selection. That's when the unconstrained analytical phase must give way to reality.

Up until that point, calculations are not constrained by availability and can produce any value of resistance, inductance, or capacitance. The conclusion of the analytical phase is the culmination of countless hours spent considering input voltage variations, timing relationships, power dissipation, temperature, output voltage ripple, power factor, current harmonics, phase angle, loop stability, voltage spike, maximum current, and any number of other constraints. The circuit design is at a point where everything works great (theoretically) under all conditions with those ideal components. Now comes the time when the rubber hits the road.

The first practical consideration is the package type and power dissipation of the electronic device. Will the design use surface-mount technology, through-hole technology, or some hybrid configuration? What is the component's power dissipation? Ideal inductors and capacitors do not dissipate power, but the same is not true of real components. Inductors and transformers have appreciable wire resistance that results in power dissipation and internal heating. Some capacitors likewise have leakage current.

Construction and tradeoffs

Next to consider is the construction method. There are numerous common methods for fabricating most electronic components, including resistors, capacitors, magnetics, and semiconductors. The sundry construction methods typically optimize some performance features of the device over others, or they differentiate one supplier from another.

Resistors, for example, can be carbon composition, carbon film, ceramic, metal film, metal foil, metal oxide film, thin film, thick film, or wire-wound. Each method optimizes different performance factors: stability with respect to temperature or time, tolerance, reliability, pulse power dissipation, continuous power dissipation, etc.

The same is true of capacitors. There are many different construction methods available, such as aluminum electrolytic, ceramic, film, mica, PTFE, and tantalum, to name a few. They optimize various performance features of the device, such as working voltage, capacitance, ripple current, value tolerance, stability over time and temperature, and reliability.

With inductors, the construction methods will relate to the type of core materials, the core configuration, the dielectric, the winding method, and whether the component is shielded.

The search

Finally, the design is ready for the actual part selection. The process can seem daunting, perhaps even overwhelming at times. I find the best method is to start with a good parametric search engine. For most parts, I often start with Digi-Key. Its search engine is easy to use and fairly comprehensive. Mouser and Newark also good parametric search engines for electronic components. I'm sure there are others, as well. I use these generic parametric search engines for integrated circuits, resistors, capacitors, semiconductors, fuses, MOVs, etc.

For inductors, even though the ubiquitous parametric search engines offer magnetics, I tend to gravitate to Coilcraft. It specializes in magnetics and has one of the best parametric search engines, coupled with good inventory, delivery, and a liberal sampling policy. The website is well organized into sections corresponding to applications. I am frequently looking for power magnetics and EMI/RFI/LC filter magnetics. These applications have their own section on the website. LED driver electronics often use inductors as the main power transfer mechanism between the input line and the LED load. These are typically the larger components capable of absorbing, storing, and transferring the energy to drive the load.

Next in line is the input filter. Input filters pass the desirable frequencies (for example, from the power sources, such as DC, 50 Hz, 60 Hz, or 400 Hz) and block undesirable frequencies. The undesirable frequencies are usually higher frequencies related to the switching power supply.

Reality sets in

Alas, even with the best parametric search engines, some of the calculated values are often not realistic, or at least not for the desired price, quantity, and delivery. What happens then? It's back to the drawing board for some fine tuning. The electrical design is often the result of back-and-forth iterations between the theoretical and the practical.