If you’ve ever worked with power electronics—whether designing a switch-mode power supply (SMPS) for a consumer laptop, a fast-charging station for an electric vehicle, or a compact solar inverter—you’ve probably overlooked the unassuming component that makes those compact, efficient systems possible: the high frequency transformer. Unlike their bulky, low-frequency cousins, these transformers operate at frequencies ranging from tens of kilohertz to several megahertz, cutting size, weight, and core losses dramatically. But here’s the thing: their performance, safety, and reliability depend entirely on getting insulation right. As a high frequency transformer supplier that’s specialized in this space for over a decade, I’ve seen firsthand how cutting corners on insulation leads to catastrophic failures, field returns, and lost revenue for our clients. So today, I want to break down exactly what the insulation requirements for these components are, why they’re non-negotiable, and what our team does to hit those standards. High Frequency Transformer
First, let’s ground this in why high frequency insulation is different from anything you’d see in a line-frequency transformer. At 50/60Hz, insulation has to survive long-term voltage stress but doesn’t have to deal with the rapid, steep voltage spikes that come with switching at high frequencies. Those spikes—called dv/dt transients—are a big problem. When a MOSFET or IGBT switches on and off quickly, it generates a voltage pulse that can rise and fall in nanoseconds. Those pulses don’t just carry high voltage; they also produce electric fields that are concentrated at the edges of the transformer’s windings. If the insulation between turns, layers, or primary and secondary windings is too thin, or not rated for these rapid transients, it will break down prematurely—often without warning.
Let’s start with the most basic insulation tier: turn-to-turn insulation. Each winding of a high frequency transformer is made of hundreds or thousands of individual wire turns, each coated in a thin insulating enamel. But at high frequencies, the electric field between adjacent turns isn’t uniform. The turns at the end of a winding experience higher peak voltage than those in the middle, because they’re at the edge of the winding’s electric field. That means the enamel alone isn’t always enough for high voltage applications. For our transformers designed for 1kW to 10kW SMPS, we typically specify a double-build enamel (two coats of insulation) or even a polyamide-imide (PAI) coating, which has a dielectric strength of around 200V per micrometer—far higher than standard polyester enamel. For higher-power units (10kW and up), we often add a thin layer of Kapton polyimide film between winding layers, because Kapton can handle 1,000V per micrometer and also provides mechanical stability to prevent winding movement during thermal cycling.
Next, layer-to-layer insulation. A typical high frequency transformer winding is split into multiple layers to minimize skin effect and proximity effect losses—two big enemies of efficiency at high frequencies. Each layer is separated from the next, and the insulation between these layers has to handle not just the operating voltage, but also the transient voltage from switching. I once worked with a client who designed a 50kW EV on-board charger transformer and used only enamel between layers. After 100 hours of operation, 12% of the units failed because the layer insulation broke down when the MOSFETs switched at 100kHz, generating a 500V transient in 10 nanoseconds. We redesigned the layer insulation with 25 micrometers of Kapton and a 10 micrometer layer of epoxy resin between layers, and the failure rate dropped to less than 0.1%. That’s the difference between understanding the specific stresses of high frequency operation and guessing at insulation requirements.
Then there’s the most critical tier for safety: primary-to-secondary (P2S) insulation. This is non-negotiable for any transformer that isolates a low-voltage secondary (like the 12V side of a laptop charger) from a high-voltage primary (like the 120V or 240V grid side). Here, the requirements aren’t just about withstanding voltage—they’re also about meeting regulatory standards like IEC 60950 for IT equipment, IEC 61558 for power supply transformers, or UL 1584 for safety in the US. These standards mandate three key parameters for P2S insulation: creepage distance, clearance distance, and dielectric withstand voltage.
Creepage is the shortest path along the surface of the insulation between the primary and secondary terminals, while clearance is the shortest air gap between those terminals. At high frequencies, you might think air gaps are more vulnerable, but in reality, creepage is often the limiting factor, especially in environments with high humidity or dust. For a 400V primary SMPS transformer used in a data center, IEC 60950 requires a minimum creepage distance of 8mm for a pollution degree 2 environment (which is standard for most indoor commercial applications). That’s almost twice the distance you’d need for a line-frequency transformer of the same voltage, because high frequency voltage gradients can cause surface tracking—small, conductive paths of carbon that form on the insulation over time, eventually leading to a short. As a supplier, we calculate creepage based on not just voltage, but also pollution degree, altitude, and expected operating lifetime—something that many new design engineers overlook.
Dielectric withstand testing is the final check for P2S insulation. Most standards require a 50Hz AC voltage test for 1 minute, but for high frequency transformers, we also perform a high-frequency dielectric test at the operating frequency to make sure the insulation doesn’t break down under real-world operating conditions. We once had a client send us a transformer that passed the 50Hz hipot test at 4,000V for 1 minute, but failed our 100kHz high-frequency hipot test at 2,500V. When we opened it up, we found tiny air pockets between the layer insulation that formed during the winding process—pockets that don’t matter for low-frequency voltage, but ionize under high-frequency AC, creating localized breakdown points. That’s why we don’t rely solely on standard hipot tests; we tailor them to the frequency our transformers will actually operate at.
Thermal insulation is another often-overlooked requirement for high frequency transformers. These components run hotter than low-frequency ones because of core and copper losses, and high temperatures degrade insulation over time. The En指数 (Exponential Ageing Index) of the insulation—how long it can operate at a given temperature before its dielectric strength drops by 50%—is critical here. For standard applications, we use Class F insulation (rated for 155°C continuous operation), but for high-power transformers operating at 100°C core temperature, we specify Class H insulation (180°C) or even Class C (220°C) for extreme environments like industrial motor drives. We also use insulation materials that have high thermal conductivity, like alumina-filled epoxy resins, to help dissipate heat away from the windings and extend insulation life. A mistake here might not cause a failure right away, but it can lead to a 50% reduction in transformer life, which means more returns for our clients and damage to their reputation.
One more thing that’s unique to high frequency transformers: the effect of insulation on parasitic capacitance. Every layer of insulation between primary and secondary creates a tiny capacitor, called interwinding capacitance. At high frequencies, this capacitance can cause common-mode noise to leak into the secondary side, which can interfere with sensitive electronics or cause electromagnetic interference (EMI). That’s why we use segmented windings—splitting the primary into two equal halves and placing the secondary in between—with insulation optimized to minimize interwinding capacitance. We also specify insulation materials with low dielectric constants (like polyimide, which has a dielectric constant of 3.4, compared to 5.0 for some epoxies) to keep that capacitance as low as possible without sacrificing insulation strength. For our automotive clients, who have strict EMI requirements per CISPR 25, this balance between insulation and parasitic capacitance is make-or-break.
As a high frequency transformer supplier, we don’t just source off-the-shelf insulation materials and stuff them into our transformers. Every design is customized to the client’s application: the operating frequency, power level, voltage rating, environment, and regulatory requirements all dictate the insulation specification. For example, a 500W laptop charger transformer operating at 65kHz has different turn-to-turn, layer, and P2S insulation needs than a 50kW solar inverter transformer operating at 150kHz, which is different from a 1MW industrial motor drive transformer operating at 20kHz. We work closely with our clients from the early design stages to test different insulation combinations, perform thermal cycling tests, dielectric withstand tests, and EMI testing to make sure the insulation meets all requirements before the transformer goes into production.
I’ve been in this industry long enough to know that insulation isn’t something you should cut corners on. I’ve seen transformers fail in the field because someone used enamel-only layer insulation for a 10kW SMPS, leading to a $1 million product recall for our client. I’ve worked with engineers who tried to save a few cents by using a lower-grade epoxy for P2S insulation, only to have 20% of units fail humidity testing six months later. That’s why we prioritize insulation as a core part of our design process, not an afterthought.
If you’re designing a system that uses high frequency transformers and you’re not sure if your insulation is up to par, or you’re looking for a supplier that will work with you to get it right, we’re here to help. We don’t just sell components; we provide custom insulation solutions tailored to your specific application, with full testing and compliance to all relevant standards. Whether you’re working on an SMPS, EV charger, solar inverter, or industrial drive, our team has the expertise to make sure your transformers are reliable, efficient, and safe.
High Frequency Transformer [1] McLyman, C. W. T. (2011). Transformer and Inductor Design Handbook. CRC Press.
[2] IEC 60950-1:2018, Information technology equipment – Safety – Part 1: General requirements.
[3] El-Hami, A., et al. (2005). “High-Frequency Insulation Requirements for Power Electronics Transformers.” IEEE Transactions on Power Electronics, vol. 20, no. 4, pp. 904-911.
[4] Klein, A. (2019). “Thermal Ageing of Insulation Materials in High Frequency Power Transformers.” International Journal of Electrical Power & Energy Systems, vol. 107, pp. 456-463.
[5] UL 1584:2020, Standard for Safety for Transformers, Power Units, Reactors, and Similar Products.
Dongguan Hensiron Electric Co., Ltd.
As one of the most professional high frequency transformer suppliers in China, we have world-leading production equipment and strong manufacturing capabilities. Please feel free to buy high quality high frequency transformer made in China here from our factory. Customized orders are welcome.
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