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

Posted by: ENRICOMIGLIORE on 11/26/2010

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

Comments

Usually a varistor is used for limiting the differential surge voltage, and it is
located between input lines.

If there is a variator between earth to ground, there must be a high surge voltage occured between earth to ground in this application

High,

> Usually a varistor is used for limiting the differential surge voltage,
> and it is located between input lines.
>
>
> If there is a variator between earth to ground, there must be a
> high surge voltage occured between earth to ground in this application

thank you for your answer.

I agree to what you say.

I just wanted to know if there's a regulation that states or suggests the use of a device that clamps possible surge voltages.

Enrico

The design probably assumes that Earth is connected to secondary ground in the intended application. The varistor ensures a low impedance between the output and ground and Earth is the power supply is used in an unintended application that does not short Earth to ground.

If the output voltage of the power supply is a low voltage (SELV), the output voltage is limited by low impedance and the ground is connected to Earth, then the insulation required between the Primary and Secondary is only Basic and not Double.