TABLE OF CONTENTS Suppose I take the greatest gift known to the world of switching power supplies, and ask you to build a high power AC-DC power supply without it? Wouldn't you feel really challenged? Because if you did, you wouldn't be alone! Many AC-DC commercial power supplies are still built on cheap single-sided CEM1 or CEM3 PCB laminate and therefore can't afford the luxury of the magical ground plane. Bereft of that dependable ally, routing requires specialized skills, developed through years of experience. If anything, it makes you keenly aware of all that the ground plane accomplishes for you in multilayer boards, without you even being aware of it.
Let us take the popular 384x family for the purpose of illustrating some key routing principles in AC-DC power supplies.
The first thing we should be conscious of in laying out our design is that such ICs always have a current sense resistor connected to the Source lead. So it is not possible to minimize the physical distance between the Source and the Mosfet ground. We will certainly use noninductive sense resistors and so on, but there will still be some bounce left. To avoid aggravating the situation further, we need to at least minimize the bounce between the IC ground and the Mosfet ground. We also have to remember that the Fet will usually be a certain distance away from the IC, on a heatsink, but we still want to minimize Gate drive trace impedances...
