INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Flyback: a varistor between EARTH and the secondary's GROUND

Flyback: a varistor between EARTH and the secondary's GROUND

(OP)
Hi,

I have to design an isolated flyback power supply of 22 W.

At the moment I'm testing the performances of my competitor's power supply.

I noticed that on this power supply there is a Zinc Disc Varistor (56 Volt) between EARTH and the GROUND of the secondary.

This is quite surprinsig for me.

Does anybody know the reason of that choice?

Thanks in advance,
Enrico Migliore

RE: Flyback: a varistor between EARTH and the secondary's GROUND

I can only assume that this might be a workaround for not being able to provide reinforced isolation in the transformer (assuming the varistor would shunt dangerous voltages to earth if the transformer isolation fails).

I have doubts if this is an acceptable method.

RE: Flyback: a varistor between EARTH and the secondary's GROUND

(OP)
That's an interesting answer.


I have a different guess but I'm not sure this is the correct answer to my initial question.


1) In flyback power supplies, the secondary's ground is connected to the primary ground through an Y1 capacitor which withstands 4000 VAC.

The typical value of this capacitor is 2.2 nF.


2) I'm buying this from TDK: CD12-E2GA222MYNS


3) The Y1 capacitor dumps the fluctuations of the secondary's ground and helps to pass the conducted EMI tests.
In fact, the fluctuations of the secondary's ground draw displacement currents from the ground reference metal plane which is placed under the table of wood.

The Device Under Test is capacitive coupled to the ground reference plane.

Common mode currents are displacements currents.


4) The Y1 capacitor is just able to filter AC currents, therefore the DC voltage of the secondary's ground can ramp up to any voltage (in principle).


5) If someone touches the secondary's ground can get an electric shock.


Enrico Migliore

RE: Flyback: a varistor between EARTH and the secondary's GROUND

The ZDV must be to stop the secondary ground from  floating up to high potentials with respect to earth ground.

What you have found is either an industrial secret, as its not common, .............or a mistake

 

RE: Flyback: a varistor between EARTH and the secondary's GROUND

The ZDV must be to stop the secondary ground from  floating up to high potentials with respect to earth ground.

What you have found is either an industrial secret, as its not common, .............or a mistake by the designer

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close