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

[ENRICOMIGLIORE]

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

 

[electricuwe]

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.

 

[ENRICOMIGLIORE]

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

 

[schnell]

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

 

[schnell]

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