• Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 14717 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Y Capacitors across Isolation Boundary in AC/DC converters

Parameshwara Bhat
Intellectual 290 points

Hello All,

I see in all Ref Designs of Isolated switching controllers, a Y cap connected from sec.ground to prim.ground.As I know,  it is used to return the stray voltages across primary-to-secondary coming from stray capacitance in the transformer. In some other forums, people have also referred to the need for returning the common mode noise from secondary to primary to reduce EMI/EMC.

But measuring on adaptors having these capacitors, I find  the secondary so called isolated DC  carrying a live AC Voltage of nearly half of Line Voltage. And this voltage does give shock if the secondary DC line is not connected to Earth Ground.So, as I see it,once you have this capacitor in place, there is no isolation. Also, as the electric shock is relieved only by connection to Earth Ground, a common mode noise can also pass to secondary side, defeating isolation. 

But I understand this is a Compliance requirement.

I want to learn why Authorities / Standards insist on this capacitive connection in a purportedly isolated circuit ?

Any document which explains how this capacitor reduces noise ?

If I choose to not mount it, then have I options to reduce noise, meet Compliance requirements ?

Thanks in advance,

Param.

  • TI__Mastermind 25570 points
    Hello Param

    Yes, there can be a voltage from primary to secondary but this will be at a very high source impedance so it isn't a shock hazard. This voltage generates what is sometimes called a touch current and there is lots of information available on the web on this topic.

    The Y capacitor provides a low impedance return path for currents generated by the switching voltages between the primary and secondary windings of the transformer. Without this low impedance path, the currents will return by other paths - normally through stray capacitances from secondary to ground or Live or Neutral - often bypassing the input filter and generating significantly increased amounts of conducted EMI.

    Different classes or applications will place different limits on the amount of Y capacitance from pri to sec, and also from Live/Neutral to Ground. Medical applications in particular place severe limits on touch currents.

    I don't think that the standards authorities insist on having a Y capacitor from primary to secondary but they do insist that the product meets the EMI requirements and the Y capacitor is a very effective way of reducing common mode EMI. They will insist that if a Y capacitor is used that it is appropriately rated - it has to be a 'proper' Y capacitor, not just a normal part which happens to have the required capacitance.

    This topic is discussed widely on the internet - for example at electronics.stackexchange.com/.../what-does-the-y-capacitor-in-a-smps-do

    If you don't use it then you can the transformer to minimise its primary to secondary capacitance. You can use internal screens or cancellation windings, but to be honest, adding a Y capacitor is the normal thing to do.

    Regards
    Colin