How to Select Magnetics for Power Electronics

Transformers, inductors, ferrites, chokes, line filters… the list of magnetic components that appear in power electronics is very long. Magnetic components provide the high inductance needed in certain high-power systems, but there are limitations in terms of the inductance and power density they can provide in power systems. Magnetics components are also important in terms of the noise they can suppress when used as filters or chokes, all of which are related to the core material and component packaging used in magnetics components.

In this article, we will look at some of the main specifications that designers should pay attention to when looking at magnetic components, particularly in power electronics. The specifications of these components will be a major limiting factor influencing system performance and noise. In some cases, you simply won’t find off-the-shelf components that will hit your magnetics specs, in which case you will need to locate a manufacturer that can produce a custom component.

Important Magnetics Specifications

Selection of magnetics typically involves two possible approaches: selection of off-the-shelf components, or design of a custom component. The range of off-the-shelf components is extensive, and most systems can be used with standard components found from electronics distributors. In most power systems, there are typically four specifications that are used to select magnetics.

Power Rating

The power ratings in a magnetic component refers to the amount of power they can handle, but it is not necessarily specified directly. The voltage and current being given to the component determine its operational capability, and this can be summarized in three possible factors:

• Winding resistance (DC)
• Maximum operating temperature
• Voltage and/or current rating

Typically, the winding resistance will be small as it is just the DC resistance of the wire making up the windings. Due to a winding resistance and temperature limit, there will typically be a nominal DC current limit or RMS AC current limit that is specified for a magnetic component. For power conversion, make sure your circuit will stay below these limits as they are an important element of safety standards, such as in IEC or UL standards requirements.

Form Factor

There are many possible form factors for magnetic components. The table below shows a set of standard form factors and functionality for inductors and transformers.

Technically, any of the above functions could be paired with any of the above form factors. Other magnetics, like ferrite chokes and plates, do not fit into the above set of packaging and function constraints, as they are deployed in specific locations in a system and their function is to block/filter noise.

Winding Inductance

In some power conversion systems, as well as generally for inductors, the winding inductance determines the power conversion efficiency and output voltage level from the system. Winding inductance determines how the component reacts to an input switching waveform, which is normally used in an isolated H-bridge switching converter, flyback converter, or similar isolated topology to set the secondary side voltage. For basic switching converters, inductance also determines the level of ripple observed on the output side of the system.

This high voltage flyback transformer is optimized to receive fast switching on the primary coil and provide high-efficiency power transfer to the secondary coil.

Frequency Rating

The frequency ratings in magnetics are determined by packaging, as well as the interaction of the core material with the magnetic field. Packaging will determine parasitics, which then determines leakage as a function of frequency.

The frequency rating might not be quoted in terms of a frequency, but it can be determined from another specification: leakage capacitance. This will determine the level of isolation between the primary and secondary coils in a transformer, and the resonant frequency of an inductor (this is due to winding capacitance).

No Suitable Magnetics? Design a Custom Component

Although there is a huge range of magnetic components on the market, not all of these will hit the multitude of design and operating specifications for a given system. This is most often the case with transformers operating at non-standard voltages, high power density inductors, and low-profile magnetics. When off-the-shelf components won’t work in a particular system, the solution may be to use a custom magnetic component.

Some component manufacturers have engineering divisions that can work with customers to design and manufacture custom transformers and inductors. Some materials manufacturers have the capability to produce custom magnetics shielding materials, such as ferrite plates. When you’re having trouble hitting a transformer spec with off-the-shelf components, the best approach is to find a component that comes closest to your spec and contact the manufacturer about producing a custom design.

When it’s time to design and simulate your power electronics systems with magnetic components, use the comprehensive set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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