Eliminate the Guesswork in Controlling EMI

Jan 1, 2008 12:00 PM
By Majid Dadafshar and Jinho Choi, Field Application Engineers, Fairchild Semiconductor, San Jose, C

At 150 kHz, a second-order line filter can attenuate line-to-ground interference by 40 dB (100 times) with a corner frequency of 15 kHz. This leaves a total remaining attenuation requirement of 10¹ from drain to line.

Certain inherent elements within the power supply, such as transformer isolation and line inductance and capacitance, will attenuate the signal even before they reach the input filter line. If additional attenuation is needed, lowering the rise time of the main switch is an option given that the added switching losses are acceptable.

Another path for noise current is associated with the interwinding capacitance (C_WW) from the primary to secondary winding of the main transformer. This parasitic capacitance couples the switching harmonics into the ground plane along a similar path to that shown in Fig. 8.

Many switch-mode power-supply designers are at the mercy of the magnetic supplier and depend on the supplier's expertise for a good transformer design. So, in some cases, the power-supply designers might not fully understand the effects of transformer construction on emissions.

For any real transformer, there is a small capacitance linking the windings together; this capacitance is a function of the spacing and the dielectric used between windings. The size of this interwinding capacitance can be reduced by increasing the separation between the windings and by using low permittivity material to fill the space between the windings.

The number of layers used between the windings, their thicknesses and the material type can be designed to obtain a specific amount of capacitance and leakage inductance to lower emissions down to an acceptable level.

Power-supply designers should also note the potential impact of output rectifiers on the secondary side of the transformer. When these rectifiers change from forward- to reverse-bias states within the time frame of a few nanoseconds, they can produce spikes that extend the EMI envelope beyond the first 10 harmonics. That additional, higher-frequency noise can couple back through the main transformer capacitance, into the primary section, easily pass through the EMI filters and be radiated by the ac line cord.

Several factors have a direct influence on the inter-winding capacitances that provide paths to couple the harmonic noise frequencies to ground. One factor is the transformer winding technique, which could be a single layer, progressive or bank type of winding.

Another issue is the use of a shield, possibly fashioned from copper tape, in the primary section, secondary section or both. All of these factors also have a direct effect on the coupling between each winding and on leakage inductance, which can aggravate the high-frequency ringing during the main transistor's off time.

In general, the key to minimizing the conducted interference is to keep an eye on all interwinding capacitances, as well as capacitances from traces or wiring to ground. Designers should also minimize the inductive coupling from current-carrying conductors by reducing the loop area that the high-frequency noise has to travel through.

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