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 firstname.lastname@example.org
Exxelia à Space Tech Expo
ESA QPL Film Capacitors PM907S and PM948S are full series of Polyester Film Capacitors. PM907S products are suitable for voltages from 50V up to 1250V and offer capacitance values from 82nF up to 180μF. PM948S can be used from 50V up to 630V with capacitance values from 22nF up to 47μF. Both series can support extreme conditions with temperatures from -55°C to +125°C, and offer high energy density, low ESR & ESL and high RMS current. Film capacitor Series PM907S and PM948S are ESA QPL certified according to the ESCC Detail Specification No. 3006/025 and 3006/026 Smallest ESA QPL Ceramic Capacitor of the Market Exxelia ranges of low voltage MLCC for surface mounting, CEC19 and CNC19, have achieved the ESA Qualified Part List status under the criteria of the European Space Component Coordination’s (ESCC) 3009/042 and 3009/043 respectively. The 0402 size QPL-qualified parts are available from 10V to 25V, enabling substantial miniaturization and cost-saving. They are available either in the very stable NPO dielectric (type 1) or the high capacitance X7R (type 2). CEC and CNC series combine high capacitance values with high thermal and voltage stability. Versions with polymer terminations are also QPL-certified. Miniature EMI filters for space application Exxelia’s gold-plated glass-sealed EMI filters SFC030 is the solution of choice to protect several embedded power supplies and data lines. These small and reliable filters are available up to 5A @ 200V and 22nF. Available in a Kovar package, the better alternative to steel for enhanced temperature cycling resistance, all Exxelia’s EMI filters for space are ESA qualified according to ESCC 3008. Thanks to this extensive experience in stringent ESA testing, Exxelia is able to adapt the tests of any filter according to any need (example: SMD 20A chip, 0pF capacitor value for an arrays of filters).