Switch Mode Power Supply: a varistor between EARTH and the secondary's GROUND

[ENRICOMIGLIORE]

Hi,

I'm developing a 20-watt switch mode power supply.

The PWM controller is STM's Viper 27. So far everything is going fine, the prototype works pretty well under light and full load.

At the moment the PSU is in our oven at 70 Degrees

I noticed on a competitor's power supply the presence of a Disc
Varistor (56 Volt) between EARTH and the GROUND of the secondary.

This is quite surprinsig for me.

Can anybody explain me the reason of that choice?

Thanks in advance,
Enrico

 

[rob_croxford]

Varistors are used to protect circuits against excessive transient voltages by shunting the current created by the high voltage away from the sensitive components.

The function of the varistor is to conduct larger amounts of current when the voltage is excessive.

 

[(*steve*)]

Maybe it's there to ensure an ELCB triggers if L and N are reversed.

Normally N and E are tied together where power entered a premise and any voltage between them is due to neutral (or ground) currents -- typically you would not see more than a couple of volts.

Without an ELCB (or even with one) L and N reversed would lead to a situation where you have a race to see whether the varistor or the fuse/breaker trips/fails first.

 

[ENRICOMIGLIORE]

Hi,

> Maybe it's there to ensure an ELCB triggers if L and N are reversed.
No, because all Power Supplies in the world have a Diode Bridge across the L and N.
After the Common Mode Filter you find a diode bridge.

I suspect that the varistor is required some the safety regulations.

Enrico

 

[(*steve*)]

Oh, I totally misread your question.

 

[ENRICOMIGLIORE]

Oh, I totally misread your question.

No problem

Enrico