Types of Transformers for Power Electronics Applications

Created: February 3, 2022
Updated: October 10, 2024

Any power system that requires precise conversion, regulation, and safety through isolation will use transformers as power coupling elements. Until you’ve had to engineer a power conversion system, you probably haven’t looked deeply at the different types of transformers available for these devices. Both AC-DC and DC-DC switching converters make use of transformers to provide galvanically isolated power conversion while ensuring safety, but this requires selecting the correct transformer that can handle the system’s power and safety demands.

In this article, we’ll look at some characteristics of the common types of transformers used in power electronics, particularly board-mount transformers for AC-DC and DC-DC conversion. In addition to the basic types of transformers, we’ll look at some of the main specifications on these components as these will limit applicability for a given transformer. Finally, if you can’t find an off-the-shelf transformer for your system, then you’ll need to engineer a transformer for your system.

Common Types of Transformers and Specifications

Transformers are inductive components with a magnetic core material whose primary function is to convert an input signal to an output at (ideally) a different voltage/current depending on the turns ratio of the primary and secondary coils. Despite their apparently simple construction, transformers are complex components that have many important specifications.

Important Transformer Specifications

Different types of transformers and applications will put greater focus on certain specifications. Some of the main transformer specifications include:

DC winding resistance - The winding resistance influences how magnetics components will heat up during operation, particularly in high power conversion systems where a lot of power can be lost as Joule heating.

Switching frequency limit - When used in power conversion and regulation, transformers will have some switching frequency limit that is determined by their parasitics and average power handling capabilities. Typical values for high current planar magnetics are on the order of 100 kHz to 1 MHz. There may also be a duty cycle limit as this will determine the average power dissipation.

Winding capacitances - The inter-winding capacitance and the intra-winding capacitance will determine how noise can couple across the transformer coils, as well as how an ESD pulse could propagate through the coils. It also determines the limits of switching frequencies in power regulators; lower capacitances correspond to higher switching frequency limits.

Leakage inductance - This is the primary mechanism by which the magnetic field strength will be reduced during operation. The magnetic field will not be fully confined in the coil of a magnetic component, so there will be imperfect transfer of magnetic flux between the primary and secondary coils.

Pri-sec isolation - The isolation between coils is listed as a voltage value (either pulsed or DC). This is a measure of breakdown resistance between the coils. The isolation voltage can reach kV levels in some transformers. This is important for safety in isolated power systems as this will determine the level of galvanic isolation the component can provide.

Mounting style - Board mounting methods include tab mounting, SMD placement, or through-hole mounting. The mounting style will determine a transformer’s ability to withstand mechanical vibration during operation.

Cooling method - Some transformers, such as planar transformers, can be difficult to cool via forced airflow because they are so compact. A cooling method may be specified so that heat can be dissipated through the package into a heat sink or into the board. Some transformers can be mounted directly to an enclosure to provide maximum heat dissipation.

In total, these specifications will determine the voltage and current limitations imposed on the component. They will also limit the applicability of the transformer to specific designs, thus we categorize different types of transformers based on electrical application rather than power levels. These points, as well as the isolation rating, should be compared with safety regulations or industry standards to ensure a design can meet safety targets.

Types of Transformers

The construction, packaging, and geometry of a transformer will determine how it is categorized. Different types of transformers will have different operating characteristics as well as construction (e.g., autotransformers). Most transformers are core-type or shell-type, as shown in the graphic below. This will determine their level of UL compliance, as well as their operation at higher frequencies.

Power Transformers

Technically all transformers will convert power, but power transformers are specifically designed for mains power conversion. These transformers are designed to provide high-efficiency power conversion between input voltage levels. These components are primarily used in AC-AC power conversion (single-phase or 3-phase) at line frequencies with ratings reaching hundreds of VA or kVA levels. Frequency limits are low for these components as they do not need to operate at the switching frequencies found in DC-DC converters.

Because of the low frequencies, noise or ESD could couple across the gap in the transformer, so isolation may be low at high frequencies. One way to overcome this is to bridge the ground planes on each side of the primary and secondary coils with a safety capacitor (usually Y-type), where the capacitance is greater than the transformer’s parasitic capacitances. This directs noise away from sensitive circuits and back to a GND connection of your choosing by creating a low-impedance current path, how this can create a safety hazard at a power supply’s GND terminal if the GND noise currents are large.

Shielded Transformer

A shielded transformer has greater isolation as the core material and packaging provide additional shielding against RF noise. Specifically, this addresses high-frequency noise from the primary side (such as from mains power) and attempts to prevent it from passing to the secondary side via the component’s parasitics. The packaging also prevents greater transfer of surge/impulse voltages via the inter-winding capacitance.

Isolation Transformers

All transformers provide isolation, but an isolation transformer is intended to provide very high isolation values for low-power and moderate-speed data transfer tasks. They are also well-suited for low-voltage power supplies in commercial and industrial power systems. Some of the power and data applications where an isolation transformer will be used include:

Isolated serial data interfaces (RS-485, RS-422, and RS-232)
Isolated CAN interfaces
Isolated 4 – 20 mA AC current loops
Actuators and transducers
Isolated DAQ card systems
Other isolated bus interfaces
Low-power conversion at standard voltages (24 V, 48 V, etc.)

Isolation in these applications is intended to protect sensitive equipment from noise and ESD. These are typically lower current applications, so safety is less of a consideration, although isolation transformers do provide safety for any users interacting with equipment.

Switching Transformers

These devices are designed for use in AC-DC or DC-DC converters operating around kHz switching frequencies, such as flyback converters. In fact, switching transformers are sub-classified as flyback transformers, LLC transformers, or possibly some other name based on the circuit in which it will be used. The switching frequency of these components will be limited by their coil inductance, leakage inductance, and parasitics.

In addition, the coil inductance is important in resonant converters as the magnetizing inductance will determine the converter’s ability to function as either a buck or boost converter. This capability makes isolated LLC resonant converters very useful when high-precision tracking is needed alongside high power output. Some applications that use isolated bridge topologies are becoming more popular in fast charging applications, such as EVs.

RF Transformers and Audio Transformers

These components are not normally grouped together, but they do perform similar functions. These transformers provide power conversion for sinusoidal or modulate signals, just like other transformers. Their other function is to provide impedance matching to input and/or output ports on the device. The major difference between these components is their frequency rating; audio transformers are obviously limited to audio frequencies, while RF transformers can have bandwidth reaching as high as approximately 10 GHz. These components are also available as balun RF transformers.

Autotransformer

This type of transformer has electrically linked primary and secondary windings, where the two are separated by a tap along the body of the linked coil. Technically, any of the above types of transformers could be constructed as autotransformers, but these are normally used for power conversion (termed “power autotransformer”). Compared to a typical core-type and shell-type transformers, an autotransformer provides stronger coupling and will have lower leakage losses. For a given conversion level and inductance, they generally cost less and weigh less.

Power Electronics Continue to Evolve

During 2021, as more investment capital has flowed into innovative tech companies and electric vehicles are poised to become the norm, the industry is moving towards greater electrification and efficient power delivery at all levels. Power conversion systems for these applications need to deliver high currents at moderate voltages while providing isolation, which is exactly where transformers are ideal. Isolated power systems that provide precise regulation and high-efficiency power conversion can benefit from some of these additional components:

Unfortunately for some power systems, a standard off-the-shelf transformer may not be available for every design, and a designer will need to work with a contract manufacturer to produce custom transformers. Many reference designs for power products might use custom transformers, or they will recommend an off-the-shelf core material and coil former. These off-the-shelf options can still be assembled with an automated winding process, or custom winding can be engineered for novel power systems.

When you need to find any of the types of transformers shown above, use the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to up-to-date distributor pricing data, parts inventory, and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you need.

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