Y-capacitor

Assume that a damping factor of 0.707 or greater is good and provides a -3dB attenuation at the corner frequency and does not produce noise due to ringing. Also assume that the input line impedance is 50 ohms since the regulatory agencies use an TISN test which make the line impedance equal this value. Calculate the values needed in the common-mode inductor and Y capacitors ... 

Sometimes the high-frequency attenuation is insufficient to meet the specifications and a third pole needs to be added to the EMI filter. This filter is typically a differential-mode filter and will share the Y capacitors from the common-mode filter. Its corner frequency is typically the same as the commonmode filter. This filter is made up of a separate choke on each power line, and is placed between the input rectifiers and the common-mode filter. 

Figure 11-11 Two Safety-approved Y-capacitors from Primary Ground to Protective Earth to Help Complete the CM Noise Loop Close to the Power Stage...

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 2 (see Figure 11-11) Two safety-approved Y-capacitors between the PE ground island and the Primary-side ground help return the noise routed into the PE ground trace by other Y-capacitors. 

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 11-16 Provision for the Possibility of Placing Two Y-capacitors Adjacent to the Bridge Rectifier... 

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 What were traditionally called X and Y capacitors are now more accurately called X2 and Y2 capacitors respectively. From the viewpoint of safety regulations (like impulse voltage rating etc.), the XI and Y1 are considered virtually equivalent to two X2 and Y2 capacitors in series, respectively. For example Y1 caps are impulse tested to 8 kV. Also, the original terms X-caps and Y-caps have recently started getting defaulted to refer to the more uncommonly used (higher voltage) XI and Y1 capacitors instead. 

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