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 [email protected] 

Published on 04 Sep 2020 by Rebecca Charles

What is a capacitor ?

▲ WHAT IS A CAPACITOR ?   Definition from the dictionnary: « Capacitor »: Device able to accumulate charges and to relaunch them in a very short time.    > See our capacitors in catalog    What it really is: It's an electrical component made of 2 conductive armatures (called electrodes) separated by an isolating layer. Its main property is to store electrical charges on its armatures. There is a direct link between the voltage put on the capacitor and the value of the charge at the armatures This coefficient C, the capacitance, is the value caracterizing mathematically the capacitors. As we can identifie a direct link between U and I in the capacitor we can caracterize it as a dipole this way: 3 main dipoles : In the physical reality:   Contacts with the PCB (terminations) + Other internal contact suh as the metallic contacts, or the physical internal resistivity of the used materials. → Resistance in the circuit   Other losses due mainly to the leads → inductive effect in the circuit. Example of possible caracterization: That’s why the Esr is always written « at a certain frequency »  which should be the resonnance frequency. This is also why the capacitors have frequency optimal ranges. The higher the resonance frequency is, the higher the frequencies are withstanded by the capacitor.   In terms of energy Efficient energy is Ec.   In reality E= Ec+Er+Ei with: Ec = Energy due to the ideal capacitor Er = Energy to the ESR Ei = Energy due to the leakage.   So Er and Ei are caracterized by heating (Joule effect). So even if that’s not always a key paramter, the lower the esr the better it is for the circuit.                                                                                                                                                                               If the capacitor is polarized : If the capacitor is not polarized : > See our capacitors in catalog   ▲ MAIN CARACTERISTICS 1) Voltages (V) 2) Capacitance of Capacitor   3) Capacitance / volume 4) Tan Delta / ESR   5) Price of the function Whatever the function, the price of capacitors is important ! A cheap function does not mean a cheap product: 10 caps at 10€ is less expensive than 1 cap at 50€ …    > See our capacitors in catalog   ▲ FINAL OVERVIEW   Technology                                         Benefits of capacitors                          Constrains  Aluminum The least expensive The highest energy density Polarized Difficulties in storage High ESR and tan Delta Lowest temperature range  Ceramic   Ideal for high frequencies The biggest range of values (CAPA voltage)   Highest Price of the functions Low energy density Low values of capacitance  Film Highest ripple curents Highest voltages Lowest ESR and tan Delta Most expensive Lowest energy density High price of the function  Tantalum Lower ESR than aluminum, Good energy density and price of the function Polarized Solid can burn High ESR and tan Delta       > See our capacitors in catalog