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In an isolated DC-DC converter datasheet (link), the Y-capacitor recommended is 270 pF. How is this value arrived at? And would the intended use of the Y-cap be achieved if a higher value like 1nF is used?

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  • \$\begingroup\$ Most of Y-capacitor values are found experimentally, but if you already have a complete application note for a complete converter, what makes you question this value out of all the ones listed? \$\endgroup\$
    – winny
    Commented May 26 at 8:28
  • \$\begingroup\$ Inter-winding capacitance and switching frequency play a role. I’ve had the instance where the ‘same’ spec converters from different manufacturers had different switching frequencies and transformer capacitance that required different Y cap values. \$\endgroup\$
    – Kartman
    Commented May 26 at 13:08

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A typical isolated DC-DC converter has two contradictory requirements: -

  • Common mode noise on the output wires needs to be kept low
  • Isolation impedance at high frequencies needs to be kept high

So, CM noise on the output can be kept low by using the CY capacitor but, maintaining a high isolation impedance at high frequencies cannot be achieved using the CY capacitor.

So, it's a compromise.

would the intended use of the Y-cap be achieved if a higher value like 1nF is used?

If you don't care about high isolation impedance then use a bigger CY capacitor. However, there are many, many applications that cannot tolerate anything more than a few picofarads of barrier capacitance.

How is this value arrived at?

The 270 pF capacitor is enough to "pass" output conducted emissions for the product but, if your application doesn't care about CM noise then don't bother with CY.

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