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 LOW PASS FILTERS

Low-pass filter solutions are mainly used for EMI suppression in electronics systems. Exxelia Technologies (ex-Eurofarad), part of Exxelia Group, has developed several ranges of miniature filters with different low-pass configuration (C, L, Pi, T, 2xPi, 2xL and 2xT) mainly intended to protect electronic equipment from interferences. Exxelia Technologies produces sophisticated filters assembling Exxelia Group’s manufactured ceramic capacitors (X7R/ NPO) with ferrite inductors or winding cores in a shielding case. This solution’s main benefits are performance, reliability and optimal traceability. Considering a filter in a shielding case implies a good metallic package to insure high shielding performances with attenuation up to 10 GHz. Among options, Exxelia offers glass sealing, steel or kovar package using tin, silver or gold plating treatments to withstand any thermally or mechanically challenged applicaiton. Exxelia offers innovative EMI suppression filter solutions providing great shielding performance including the FC030 feedthrough mounted on shielding enclosure and FCM030 series designed to prevent EMI on printed circuit board. FC030 series is feedthrough filter allowing to prevent not only EMI conduction but also EMI radiation on power supply or data signals designs up to 200V. FC030 insertion loss can be 20dB at 1MHz to reach 70dB up to 10 GHz. FC030 series is extremely performant on low frequencies applications. On the other hand, FC030 can offer very low capacitance values starting from 5pF allowing to protect high bandwidth data signals. Operating temperature from -55C° up to +175C°. FC030 series is ESA qualified.   FCM030 features same design and performances’ as FC030 and is intended for surface mount devices. FCM030 is packaged in full metallic silver plated allowing optimal contact with ground plane that improves the interferences flow to the ground. The series particularly fit for amplifiers, radars, sensitive HMI, accurate measuring.

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