Specifications for Custom Transformer Design
Transformers appear in most power systems, both large and small. It is often the case that a transformer selected from a distributor will not hit form factor or power specifications. This is often the case in systems requiring non-standard voltage levels, or in low-profile systems. Many reference designs might also include custom transformers as this is the only way for the design to hit its form factor and power output targets.
If you have determined you need a custom transformer, how should you get started with a custom design? We’ll outline an approach for developing specifications for custom transformers, as well as some design options for custom transformers based on readily available sub-components. Once you have this information, you can approach a transformer manufacturer and start the process for building your custom component at volume.
How to Start Custom Transformer Designs
The first step in custom transformer design involves selection of a packaging style. The packaging style will drive the overall form factor of the transformer, and depending on the resulting size of the component, it may force the component to use through-hole mounting or screw mounting to the PCB. While higher power transformers will require physically larger transformers, a given packaging type can typically be scaled up to higher power.
Three common packaging types for transformers are core-type, shell-type, and toroidal. Core-type transformers are the simplest type of transformer, but they may have high leakage and large footprint for a given power rating.
A simple alternative to core-type packaging is shell-type packaging. This packaging type is more common than core-type as it can offer comparable power density, but often in a more compact footprint. These transformers can have less leakage with higher efficiency as well due to the core material surrounding most of the space around the windings. These can mount to a PCB typically as through-hole components, and larger components may require screw-mounting.
The third common type of packaging for a transformer is toroidal, similar to a toroidal inductor. These can be designed with very high power ratings and low leakage, although they may require horizontal mounting on a substrate once they are too large. These are also relatively simple to manufacture as many toroidal inductor cores are available; the same cores could be used for a toroidal transformer.
Most custom designs will fall into one of these three categories as cores are readily available. The remaining challenge is to hit an isolation target, current limit (DC resistance) target, and operating temperature target without compromising on form factor. The important list of design specifications for custom transformers is listed below.
Important Transformer Specifications
Different types of transformers and applications will put greater focus on certain specifications. Some of the main transformer specifications include:
Primary coil inductance - The input coil will have a particular inductance value, which will determine its switching characteristics when used in switching regulators. This will determine output voltage and ripple in, for example, isolated bridge converters and LLC resonant converters.
DC winding resistance - The winding resistance will create heat dissipation at DC, and thus it will determine a current limit that can be specified in the primary and/or secondary coils in the transformer.
Primary-secondary isolation - An isolation rating should be specified between coils, either as a pulsed value, an AC value, or a DC voltage. The specification effectively describes the breakdown resistance between the two coils and typically reaches kV levels. This is very important in galvanically isolated power systems operating at high voltage.
Leakages (winding capacitances and inductance) - The winding capacitance and leakage inductance are both parasitics that influence how signal is carried along the windings. Winding capacitance limits the useful frequency of the component, and the leakage inductance limits the coupling efficiency for the magnetic field below 100%.
For most designers working on custom components, designing to balance every specification can be quite difficult, particularly when trying to hit targets on parasitics. Instead, it’s best to leverage your component manufacturer’s experience to ensure a custom component can hit your specifications.
When In Doubt, Work With Your Manufacturer
Some transformer manufacturers provide engineering services for customers. They can work with you to create a custom design and put it into volume manufacturing as required. They can also use their existing component line to create a modular design, which may carry lower costs and lead times than a totally custom component.
If it is determined that this route should be pursued, then the best approach is to find the closest comparable transformer from your manufacturer. You can then ask the manufacturer to work with you to develop a custom component that hits similar specs. They may be able to re-engineer the existing design so that it can perform to your new requirements.
Once you’re ready to include your custom transformer in electrical simulations, 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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