On behalf of Anthony Armstrong: Referring to the schematic on the left side of slide 5, during the on cycle with M1 closed and M2 open, the AC current follows the solid blue loop. During the off cycle, with M1 open and M2 closed, the AC current follows the green dotted loop. It is common to have difficulty grasping that the loop producing the highest EMI is not the solid blue nor the dotted green. Only in the dotted red loop flows a fully switched AC current, switched from the zero to I peak and back to zero. We commonly refer to the dotted red loop as a hot loop since it has the highest AC and EMI energy – hence the red lightning bold shown inside.
It is the high di/dt and parasitic inductance in the switcher “hot” loop that causes electro-magnetic noise and switch ringing – highlighted on the right-hand side of this foil on the scope image. To reduce EMI and improve functionality, you need to reduce the radiating effect of the dotted red loop as much as possible. If we could reduce the PC-board area of the dotted red loop to zero and buy an ideal capacitor with zero impedance, the problem would be solved. However, in the real world, an engineer must find the optimal compromise!
