Tuesday, October 28, 2014

Electrostatic and Electromagnetic Shields in a Transformer

Article by Mike Horgan, Applications Engineer at Butler Winding

As high frequency switching power supplies, operating in the hundreds of kHz, have replaced 60Hz linear power supplies high frequency electrical noise on power lines has become more of a problem. In noise sensitive applications such as medical measuring equipment, audio gear, and telecommunication systems, an electrostatic shield can be used in the transformer to reduce high frequency electrical noise.

Electrostatic Foil Shield
Fig. 1: An Electrostatic foil shield with attached
ground wire & primary winding.
An electrostatic shield can be used in a transformer to reduce the common mode noise and transients. The shield consists of one winding placed between the primary and secondaries. Typically it is a one turn insulated copper foil with foil thicknesses around 0.005” and with a foil width equal to the bobbin width, see picture 1. One end is connected to ground via the green wire and the other end of the winding left as an open circuit. Similarly a single layer winding of relatively fine wire like #36 can be used as an electrostatic shield with one end of the winding connected to ground and the other end left as an open circuit. The use of an electrostatic shield for medical transformer applications is increasing.

An electrostatic shield suppresses noise by using its capacitive coupling. A transformer winding is very inductive and presents a high resistance to any high frequency noise inhibiting current flow. By installing an electrostatic shield a capacitive network is introduced allowing this high frequency noise current to flow to ground. A typical electrostatic shield gives about 60dB of common mode noise attenuation over the frequency range of 100Hz to 1MHz. Adding a second shield between the secondary and the core may increase this. Additionally a separate common mode inductor can be used to provide more noise attenuation.

Electromagnetic Enclosure
Fig. 2: An Electromagnetic enclosure (or can)
along with an assembled ferrite pot
Core Transformer.
While an electrostatic shield is used to attenuate high frequency conducted electrical noise an electromagnetic shield can attenuate radiated noise. An electromagnetic shield is typically a metal enclosure that envelopes the component or circuits being shielded. Picture 2 shows an electromagnetic shield for a ferrite pot core transformer. In addition to enclosing a transformer in an electromagnetic shield selecting a self shielding transformer core shape like the pot core as opposed to an EE core shape or a ferrite rod will help attenuate radiated electrical noise.