Secondary ground

That would have brought the Drain end of the Primary winding right next to the safety barrier, with the Secondary ground end (which is usually connected to the chassis) directly across the isolation boundary. With this winding arrangement, we would have a healthy dose of CM noise injected directly into the chassis/earth. 

Figure 11-10 AC Coupling of Secondary Ground to Protective Earth at Power Supply End by Means of any 0.1 (X.F Disc Ceramic Capacitor... 

Recommendation 1 (see Figure 11-10) To avoid allowing the CM noise to flow down the cables before it closes its loop, place a disc ceramic capacitor between Secondary ground and PE ground. This need not be a safety-approved Y-capacitor. 

Recommendation 3 (see Figure 11-12) Acommon EMI trick on commercial power supplies is to place two safety-approved Y-capacitors between the Primary and Secondary grounds. Some suggest a better place for this is from the HVDC rail to Secondary ground. But sometimes... 

Figure 11-12 Two Safety-approved Y-capacitors from Secondary Ground to Primary Ground to Flelp Complete the CM Noise Loop Close to the Power Stage... 

Recommendation 8 (see Figure 11-17) A good high-frequency AC connection between Secondary ground and the enclosure is recommended at the output end. A thick bunch of braided copper helps. 

In high-voltage applications (e.g. off-line power supplies), it may in fact be required by law, to provide electrical isolation between a hazardous input voltage level and user-accessible ( safe ) output terminals of the power supply. Therefore there is a primary ground at the input side of the transformer, and a separate secondary ground on the output side. 

Line-to-earth capacitors are called Y-capacitors. Since Y-caps are critical in terms of having the potential to cause electrocution if they fail, approved Y-caps are typically impulse-tested up to 5 kV peak. Note that Y-caps used anywhere on the primary side (in off-line applications) must always be safety approved. Depending on the location in the power supply, we may even need two Y-caps in series (basically corresponding to double insulation). However, sometimes we can also find Y-caps placed between the secondary ground and earth/enclosure (for EMI suppression purposes). In this position, it is usually acceptable to use any ordinary 500 V ac rated capacitor (unapproved). 

Note In off-line power supplies, for better EMI suppression, we may decide to place Y-caps from the rectified dc rails (either one or both) to earth. So sometimes we place Y-caps from primary ground to secondary ground (usually connected to earth ground), or from the HVDC (high voltage dc) rail to secondary ground. In either of these positions, a Y1 capacitor (or two Y2 capacitors in series) may be required. 

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