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with satisfactory operation; however, three layers provide greater density and better control of interference from the power switching lines. Should a four-layer board be required for other circuitry, the use of separate layers for ground and power planes, a layer for switching signals and a layer for analog signals would allow the most straightforward layout of the PWS745 system. Critical consideration should go to minimizing electromagnetic radiation from the power switch- ing lines T-To and T-To. The dynamic component of the current is supplied by the bypass capacitor on the VD pin of the transformer. The high frequency AC current flows through the transformer, To, returning in the T pin, passing through the MOSFET and exiting through the COM pin back to the bypass capacitor. This current path defines a magnetic loop which transmits a magnetic field. The magnetic field lines reinforce at the center ofthe loop, while the field lines from opposite points of the loop oppose each other outside the loop. Cancellation ofthe magnetic radiation occurs when the loop is collapsed to two tightly spaced parallel line seg- ments, each carrying the same current in the opposite direc- tion. All of the current in the ground or power plane will flow directly under the T-To traces because this is the path ofleast inductance or impedance. Another potential problem with the T-To lines is electric field radiation. Here, the power plane is effective in terminating most ofthe field lines because of its proximity. Additional shielding can be ob- tained by running ground trace(s) along the T-To lines, facilitating a minimum loop area for the transformer's cen- ter-tap bypass capacitor.