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

Innovative new products at Electronica 2022 - Booth# A5 - 339

Electronica will take place from November 15th to 18th in Munich, Germany. On our booth #A5 - 339, we will be displaying a bunch of innovative new ranges. EMBEDDED DIVISION - AVIATION / DEFENSE / SPACE   We are proud to show our latest capacitor technology innovation called MML® technology. Miniature Micro-LayerTM Film is a breakthrough in film capacitor technology, with an energy density up to 4 times superior to other film dielectrics. MMLTM innovative technology makes power capacitors smaller, lighter and compatible to higher temperatures. It is perfect for DC link, decoupling, charge-discharge applications. Alongside MML, we will also be exhibiting innovations in magnetics, allowing saving up to 40% weight and 20% volume while dissipating more power. You will also find our new aluminum electrolytic rectangular low profile capacitors named Cubisic SLP and HTLP, our MIL & DSCC tantalum capacitors (newly qualified to reliability level P) as well as our multi-outputs CCM transformers & Dual Active Bridge. But also our components on board the eVTOL.   RAIL & INDUSTRY DIVISION   It is with great joy that we will be exhibiting Alcon capacitors for the first time on our booth. Alcon manufactures film and aluminum electrolytic capacitors that are particularly suitable for locomotives and power electricity charging stations. We will also exhibit film and ceramic capacitors for signaling, and magnetics for rail tracking system, openable current transformer, TFPP transformer and resistors.   MEDICAL DIVISION   Our new Exxelia Micropen branch has an outstanding product offer for medical products that will be showcased at Electronica for the first time ever! Indeed, Exxelia Ohmcraft high voltage resistors are widely used into laboratory equipment such as mass spectrometers or thermocyclers. We will also be showing our Exxelia Micropen unique additive printing technology used extensively for medical disposable material such as endotracheal tubes or surgical ablation tools. You will discover our new range of film components from the Alcon family and our well-known non-magnetic High-Q capacitors for MRI imaging systems, as well as our low profile flyback transformers for implantable devices.