Offline SMPS: Resistor across isolation barrier?

[cupoftea]

Hi,
For equipment powered by the mains, the signal circuitry generally runs off secondaries isolated from the
mains. However, during use, it often gets connected to other signal circuitry which, most often, has earth
ground as its ground. So therefore, when they are connected, you really want both of the signal
circuits to have grounds which are at the same potential
(ie, to both be at roughly earth potential).

..So is it best, where possible, for an offline SMPS that is supplying signal circuitry, to have say a >1MEG resistor
across its isolation barrier and connected to earth on the mains side?
(so that the secondary doesnt float off away from earth potential.)

Generally would you agree, that totally floating secondaries are not good, because they can float up (due to charge pumping in the stray C) to
overly high potentials?

 

[aliarifat794]

When the secondary side floats, it can be influenced by factors like capacitive coupling, leakage currents, and even electromagnetic interference (EMI). This can cause the secondary side to drift to unexpected potentials.

 

[KlausST]

Hi,

... and ESD.

Thus: A high ohmic resistor is recommended.

Klaus

 

[FvM]

Resistor surely doesn't help against EMI and capacitively couple interferences. Even DC leakage can't cause charging beyond voltage rating of the isolation barrier. ESD however can.

The resistor (e.g. 1 to 10 Mohm) must be rated for isolation working voltage and respective surge voltage level, e.g. 2.5 kV for reinforced mains insulation. Either a big THT or multiple series connected SMD parts.

 

[cupoftea]

The resistor (e.g. 1 to 10 Mohm) must be rated for isolation working voltage and respective surge voltage level, e.g. 2.5 kV for reinforced mains insulation. Either a big THT or multiple series connected SMD parts.

Thanks, that sounds a good way..also, would you say that the resistor concerned would best be connected from earth on the primary side, to the secondary side ground?

It seems to be a good point, because its often the case that one connects up to a PCB thats on the secondary side of an offline transformer, and one unfortunately gets a spark, since the isolated secondary side had unfortunately been induced up to a high potential with respect to earth ground.

 

[KlausST]

Thanks, that sounds a good way..also, would you say that the resistor concerned would best be connected from earth on the primary side, to the secondary side ground?

Tbh: I don´t see an alternative.

Klaus

 

[Easy peasy]

It is common in industry to put a 750V rated, 3W, 1 Meg ohm resistor from zero volt out to earth for LV power supplies to stop them floating up.

 

[cupoftea]

It is common in industry to put a 750V rated, 3W, 1 Meg ohm resistor from zero volt out to earth for LV power supplies to stop them floating up.

Thanks yes, i must admit i am trying to get our contractor to do it...but it is said to be unecessary, and the expense of the extra resistor, and also, they say if it is less voltage rated than 3.5kV, then it will be a problem and may break over on a transient and ruin the product in the field....and then we'll get customer failure returns?

I once worked on an offline isolated xenon flash product that charge a cap up to 500V and flashed it over.....they were worried there about induced voltage to the secondary side, and they actually had a thick insulated earth wire coming from pri side earth to sec side so as to earth ground the secondary.

 

[cupoftea]

It is common in industry to put a 750V rated, 3W, 1 Meg ohm resistor from zero volt out to earth for LV power supplies to stop them floating up.

Thanks, may i please ask if you mean offline SMPS with LV outputs?, eg <24V?

We have an offline SMPS, (three diode rectifier and 3 phase star supplied, no PFC) but only the three phases and Neutral come to the mains side connector (ie not an earth wire aswell) . But the PCB screws to an earthed backplane, and the secondary side of the PCB could take its earth connection from that. So would you say that we can just directly connect our secondary ground to the earth from the backplane?......or do you still prefer the 750V rated resistor?

Whichever way, i am quite certain that no-one would ever recommend a totally floating secondary for an offline SMPS?

I believe that if earth comes to the mains connector, (and there is no earthed backplane, say), then the earth obviously must come to the secondary side from that earth on the primary side. So in this case, would you agree to having a 750V resistor under the mains isolation transformer......and this resistor connected to earth on the primary side, and secondary ground on the secondary side? (ie the resistor is not rated to 2.5kv because its connected to earth on the primary side.?)

Mind you, suppose there is a lightning strike, and the local earth ground gets raised to say 2.5kv above "power station earth" (ie general average , overall earth potential) .......then there will be a huge voltage across the 750V rated resistor and it will blow?

 

[Easy peasy]

If the sec 0v goes up with the local earth - there is no issue as all things are at the same potential - you, the carpet, the local earth stake - and the phase and neutral.

 

[cupoftea]

Thanks, i thought the phase and neutral would/could be referenced to the earth at the power station/sub-station, which is many miles away potentially?
(ie an earth with potential very different than the lightning_struck local earth?)

If L, N & E are always referenced to the local earth, then how do we ever get common mode transients in the mains?