What?! There is a capacitor in my transformer?

Get a quick tutorial from James Tabbi, our Deputy Vice President of Exxelia's Magnetics Business Units, explaining what's interwinding capacitance


Exxelia recently designed an auxiliary transformer for a spacecraft application, where interwinding capacitance was of concern to the customer.  The controller chip they were using in their power supply was noted to be “rather sensitive to excess capacitance.”

Exxelia has also supplied thousands of driver transformers for use in a subsystem of the AN/TPQ-53 Radar System in which interwinding capacitance within the toroidal windings is held to a very demanding tolerance.

But what is interwinding capacitance? 

Capacitance in a transformer winding cannot be avoided. The voltage difference between turns, between winding layers and from windings to the core, creates “parasitic” capacitances in the transformer circuit.  These capacitances are shown as Cp, Cs, and Cw in this schematic diagram of an electronic transformer “equivalent circuit.”

Interwinding and distributed capacitance occur in transformers due to the physical separation of, and electrostatic coupling between, different turns of wire. In general, the capacitance presents itself between the different layers within a winding and between the outside layer of one winding and the inside layer of the next.  

In conventional magnetics, interwinding capacitance is a function of coil configuration – the geometry of adjacent conductors and separating dielectric media. Specifically, it is directly proportional to the shared surface area of the windings (shown in green and red below), the dielectric constant of the insulator between the windings (shown in gray below), and is inversely proportional to the separation distance through the dielectric media.

           

In high-frequency transformer design, leakage inductance and capacitance are often competing design requirements since the beneficial parameters that provide low leakage inductance also tend to increase the interwinding capacitance.

Excessive capacitance can cause undesirable common-mode noise transmission between transformer windings or between transformer windings and core or another ground connection.

Exxelia can assist with these design challenges when creating products that have to deal with interwinding capacitance, for all types of magnetic components.  

Important coil configuration design considerations must be made when capacitive coupling causes unacceptable signal transmission (for example, common-mode noise transmission or undesirable spurious ringing on a high voltage output).  Windings may be configured in a way that reduces the dV/dt voltages induced across dielectric media. Conductive screen(s) tied to preferred potential(s) can also be added between adjacent windings to reduce transmission.

If you’d like to learn more about interwinding capacitance or would like to discuss your specific magnetics needs, contact us sales.usa@exxelia.com 

Published on 04 Sep 2020 by Rebecca Charles

Exxelia to showcase advanced Microwave Capacitors and Ferrites at IMS Microwave week 2024

Exxelia will be showcasing their Super High-Q series of RF ceramic capacitors. These capacitors feature ultra-low ESR and can operate at voltages up to 1500V. They are available in sizes ranging from 0402 to 1210, offering capacitance values from 0.1pF to 1 000pF. They are ideal for advanced RF applications such as power amplifiers, mixers, filters and matching networks.  Exxelia also features advanced High-Power High-Q series, designed for lower frequency applications with power handling up to 7200V and capacitance value up to 10nF. These capacitors are available in NPO and P90 dielectrics, offering exceptional performance with low ESR and ESL, making them ideal for the most demanding application. Those CP (P90) and CL (NP0) series are ideal for defense communication (VHF/UHF), medical MR body coils, and RF generators for plasma, semiconductor manufacturing, and extreme ultraviolet (EuV) lithography.    Exxelia will also exhibit its comprehensive range of materials & tuning components, with a special focus on its cutting-edge microwave ferrite products. These advanced components are engineered to meet the rigorous demands of modern microwave applications, providing unparalleled performance and reliability.    As a highlight of our presence at IMS Microwave Week, Exxelia will be pre-launching a new High-epsilon microwave ferrite. This new High-Epsilon Ferrite is featuring a permittivity of 21 and is available in two versions (YK21 and DK21) to address all power applications. It represents a significant advancement in ferrite technology, engineered to enhance the performance of high-frequency radar systems, especially in Active Electronically Scanned Array (AESA) radar applications. This new component delivers exceptional magnetic properties and stability, crucial for the optimal performance of isolator and circulator devices.       Key Features and Benefits about the new high-epsilon ferrite YK21 and DK21:  High permittivity : 21 Two power levels available  : ∆Hk = 4.5 and ∆Hk = 8.5 Wide range of Ms : 1250 to 1900 Gauss Frequency applications up to X band (12GHz). Combination with Exxelia dielectric range for Ferrite Dielectric Assembly (FDA) Short leadtime of 8-10 weeks for evaluation.                                        Meet Us at IMS Microwave Week 2024   We invite you to visit us at IMS Microwave Week 2024, from June 16-21 in Washington, D.C., at booth 2108. Our team of experts will be available to discuss the capabilities of the High-Epsilon Ferrite YK21-DK21 and how it can enhance your high-frequency applications.