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.
 |
| 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.
 |
| 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.
