An introduction to magnetic components

As electronics becomes more prevalent in our daily lives, most users are aware of only a few component types. An internet search will bring up semiconductors, microprocessors and transistors, but little about the magnetic components that are essential to make those devices function.

Even talking to electronic engineers, the humble inductor and transformer is considered to be very low 'tech'. In actuality, there is a great deal of technology and know-how applied to the design and construction of these passive components.

From both technology and user viewpoints, magnetics can be broken down into four main categories; low frequency, high frequency, isolated and non-isolated. Custom designs are often required to meet specific electrical and physical parameters. The importance of maximising material efficiency in accordance with standards drive transformer design engineers. In the field of wound components the main innovation is driven by materials research.

Low frequency is typically considered to be 50 or 500Hz and connected to the 220 to 240V AC single phase mains input in Europe, or 115V AC mains input in the Americas. Applications include line filtering, motor drives, uninterruptable power supplies (UPSs), pumping, conveyor systems, HVAC equipment, linear power supplies and electricity metering.

The use of high frequency magnetics became more popular with the introduction of high efficiency switched mode power supplies (SMPS).Initial frequencies were approximately 16kHz, just above the human hearing limit, but now can be MHz. These types of power supplies are used predominantly now to charge mobile devices and switching regulating and protecting LEDs, TVs, computers, communications equipment and even electric cars.

Non-isolated magnetics consist of inductors used to reduce electric noise or briefly store energy, filters and transformers to 'step-up' or 'step-down' AC voltages. A step-down transformer, for example, would be used to take a 400V AC (415V AC in the UK) input and reduce it down to 230V AC.

Where human contact with electricity is possible, for instance with a laptop power supply, isolated transformers are used to avoid electric shock.The mains (primary) circuitry is separated from the secondary side using a transformer.Internally, the transformer windings would have one or more layers of insulation, which could consist of the plastic bobbin or insulation tape.One of the most severe applications is for products used in the medical industry, where the barrier uses triple insulation and/or a spacing of several millimetres.Toroidal isolation transformers are often used to provide additional protection where the electronics may come in direct contact with the patient.

Magnetic components consist of wire or foil wound either onto a core, bobbin or mandrel (for air cored inductors) and may or may not have insulation.For high volume production of very simple components, automation is possible, although the majority of transformers and inductors are quite labour intensive.

Low frequency magnetics typically use steel or iron laminations for the core material. These can be in the form of E and I shapes and are interleaved around the bobbin and winding. Alternatively a toroidal core can be used, where grain-orientated silicon iron is wound to form a doughnut shape. The wire and insulation is then wound around the core. This process is more time consuming, but because of the core shape, these types of transformers have very low stray magnetic fields and are more efficient, leading to a smaller overall size.

High frequency magnetics normally use a ferrite or powered iron material for the core, which is available in many shapes. The more complex transformer design and construction is offset by standards applied but also size, impedance, leakage and creepage distances. Usually the higher the rated frequency the smaller the inductor is. To overcome this, multi-strand wire is used, or foil which has to be insulated with tape.In some cases the primary winding will be split into two separate windings, 'sandwiching' the secondary to improve magnetic coupling. TIW (triple insulated wire) is often used to guarantee a better performance in Hi-pot tests.

Although machines are used to wind the wire onto a bobbin, manual assembly is still required to add insulating tape between the windings, remove the wire coating for terminations, add sleeves to flying leads to protect the integrity of any safety barriers and fit the cores.

Depending on the application, transformers and inductors may be subject to safety certification. In this case independent test houses like VDE, CENELEC, UL and CSA will examine the construction and perform electrical testing. It is important to be equipped to process acceptance tests and to simulate transformers in operating conditions to ensure the part does not overheat in normal operation or when subjected to short circuits on the windings. Type tests (for example thermal or surge tests) need to be agreed with the final client. Ambient and operating temperature of the part will also determine the choice insulation class; 130ºC (B), 155ºC (F) and 180ºC (H) are typical temperature ratings.

Transformer design standards always include 'safe margins', for example a transformer design in class H must have a maximal operating temperature of 125ºC in the winding. This is because the design engineer class H allows +40ºC ambient temperature +15ºC safe margin imposed by the regulation.