How Do You Design Transformers and Inductors for Medical Devices?

Published by West Coast Magnetics, July 2026, based on a 2018 interview with our CEO Weyman Lundquist in Manufacturing & Design Today (available for download below); content reviewed and confirmed current as of publication.

Medical-device magnetics require stricter isolation, tighter dimensional control, and more demanding documentation than most other markets: patient safety, FDA traceability, and ISO 13485 compliance are built into every design decision, not treated as afterthoughts.

Isolation transformer: a transformer whose primary and secondary are galvanically separated, so that no direct current path exists between the power source and the patient or clinician side of a circuit. In medical devices the isolation requirement often includes a very low leakage-current limit across the transformer in addition to a high-voltage hipot rating.

ISO 13485: the international quality management system standard for medical device design and manufacture, incorporating requirements for risk management, traceability, and documentation in support of FDA and other regulatory submissions.

What Are the Design Constraints for Transformers in Medical Devices?

One key area is patient and clinician isolation from the power source. Isolation transformers are an integral part of many medical devices, and isolation requirements typically have stringent specifications that often require very low levels of current leakage across the transformer. Meeting that leakage limit drives decisions about core geometry, winding topology, insulation material, and the physical clearance between primary and secondary.

Beyond leakage current, medical transformers must be designed to support high-voltage hipot testing. A standard commercial core-and-bobbin combination that is adequate for the power level may not have the winding window geometry needed to accommodate the required separation and insulation for a high isolation voltage. That mismatch between what a catalog part can support and what the application requires is exactly the kind of design problem WCM’s engineering team evaluates before committing to a geometry.

When standard components don’t fit your needs, our teams will engineer a solution: re-evaluating the core geometry, specifying the winding construction, and selecting insulation materials that can meet both the hipot requirement and the finished-part dimensions your assembly demands.

What Is a Rotating Transformer and Where Is It Used?

Rotating transformers transfer power across a moving interface. Applications include catheters, adjustable hand-held medical devices, and boring tools, where one part of the transformer moves relative to the other and the electrical connection must be maintained without physical contact.

The engineering challenges in a rotating transformer are different from those in a static design. The choice of core and winding construction and the dimensional tolerances are uniquely critical, because power must be transferred efficiently across a moving interface. The air gap between the two halves of the transformer creates an inherent decoupling between primary and secondary: unlike a statically assembled transformer where the gap can be minimized and controlled at assembly, a rotating design must maintain efficient energy transfer across a gap that varies with motion and must be tight enough to meet the coupling requirements across the full range of movement.

Tight dimensional tolerances are therefore not a manufacturing preference but a functional requirement. A tolerance stack-up that would be acceptable in a static assembly can produce performance variation in a rotating design that falls outside the application’s operating envelope.

How Are RF Inductors Specified for Medical Devices?

RF energy is used in many medical device applications, and tuned inductors are typically part of the design, particularly where power transfer or generation is required. Medical RF inductors must meet requirements that differ from those in industrial or communications applications:

• Self-resonant frequency (SRF): the inductor must have a very high SRF so that its useful inductive behavior extends through the operating frequency without the part becoming capacitive early. WCM’s catalog includes tuned air coil inductors for RF power delivery in medical devices above 5 MHz, with pre-tuning tolerance of 2% or better.
• Tight coupling: impedance-matching transformers and RF inductors for power transfer require tight coupling between windings. Parasitic effects dominate at RF frequencies, so the design is often specified in terms of impedance matching properties, not simply turns ratio.
• Inductance stability: inductance must be controlled tightly and must remain stable with core excitation and temperature. A value that drifts with power level or operating temperature destabilizes the tuned circuit and can affect the medical function the inductor is part of.

When patient isolation is also required in an RF application, the design simultaneously manages a high SRF, tight coupling, and a high isolation voltage: three constraints that pull in different directions and require careful selection of core material, winding geometry, and insulation. WCM’s IM001 impedance-match transformer, used for surgical RF power delivery at 25 MHz, is one example of a part built to hold that combination together.

What Does ISO 13485 Certification Mean for a Magnetics Supplier?

ISO 13485 represents the requirements for a comprehensive quality management system for the design and manufacture of medical devices. Because WCM components are incorporated into medical devices, being ISO 13485 certified means we support our customers in traceability and documentation for their FDA submissions.

ISO 13485 controls the work environment to ensure product safety and focuses on risk management throughout the entire product life cycle. Risk management must be implemented not only during design and development, but also across manufacturing and in any activities associated with design changes. A change to a winding construction, insulation specification, or core material is not just a production event: it is a controlled activity with traceability requirements, and its effect on product safety must be evaluated and documented.

West Coast Magnetics holds ISO 13485:2016 certification (issued by ABS Quality Evaluations, Inc.) covering the design and manufacture of standard and custom magnetic components and RF filter assemblies for OEM manufacturers, including the medical device industry. That certification covers both our Stockton, CA and Tecate, Mexico facilities.

For customers building toward FDA submissions, that certification is a supplier qualification input. The traceability chain from incoming material through design history records, manufacturing records, and test records supports the documentation an OEM needs for its regulatory file.

How Did WCM Redesign a 50 W, 5,000 Vac Isolation Transformer?

A customer requested a low-voltage, 50-watt SMPS transformer requiring 5,000 Vac of isolation and had selected a catalog item core and bobbin typical for that power level for their preliminary design. WCM’s engineering team reviewed the design and concluded that the core and bobbin geometry would not support the 5,000 Vac hypot requirement: the winding window and the physical separation it could accommodate were insufficient for the insulation stack needed to pass the hipot test.

WCM redesigned the transformer in a WCM410-10 geometry using triple Teflon-insulated litz wire. The redesign produced a transformer that was compact, inexpensive, and easily supported the 5,000 Vac hypot requirement.

That combination of design choices is worth unpacking. Triple Teflon-insulated litz provides three layers of fluoropolymer insulation on each strand: Teflon’s dielectric strength supports a very high insulation voltage across a thin wall, which allows the winding window to carry the required isolation in a compact form factor without the insulation consuming so much space that the part becomes impractically large. The WCM410-10 geometry, with its cores and bobbins suitable for medical-grade isolation, provided the physical framework to assemble that winding construction reliably in production. For background on how litz wire strand diameter and count are selected for a given operating frequency, see our litz wire winding design guide. For the underlying SMPS transformer design considerations that shape geometry selection, see SMPS transformer design and winding and conductor selection.

WCM’s 410 series transformers have been designed to meet medical device requirements for patient isolation, and the WCM410-60MG is the series’ dedicated medical-grade isolation variant, with litz wire windings and support for up to 2 kW.

Designing Custom Magnetics for Your Medical Application

Medical devices impose a combination of requirements that few catalog parts are built to meet simultaneously: low leakage current, high isolation voltage, tight dimensional tolerances, stable performance with temperature and excitation, and a supply chain with ISO 13485 traceability from design through manufacture. Those requirements tend to converge on custom magnetics.

West Coast Magnetics has provided custom isolation transformers, rotating transformers, and RF inductors for medical device OEMs, with ISO 13485:2016 certification across our facilities, NIST-calibrated test equipment, and a quality management system built to support FDA documentation requirements. We have completed more than 500 designs for the medical device industry and are a qualified supplier to 30 key medical OEMs.

When the isolation spec exceeds what a catalog part can support, the geometry needs to fit a constrained envelope, or the RF application requires tight inductance control and a very high SRF, our engineering team starts from your constraints and builds the part to meet them. Contact WCM to discuss your medical magnetics requirement.

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