HVDC rail

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... 

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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