Where do you put the MOV in an offline SMPS?

[grizedale]

Hello,

In an offline SMPS, should the MOV be put just before the mains rectifier, of should it go just after the mains fuse?

..Basically, which side of the Mains EMI filter should the MOV be placed?

Ive heard some say that it should go on the line side of the EMI filter, so that it protects the X caps more.

....others say that the action of the MOV is improved if it comes "after" the EMI filter, because the filter slows the voltage rise across the MOV, giving the MOV more time to do its work properly.

 

[Mehrshad74]

I put MOVs just after main fuse because EMI filters may fail with voltage surges.
But Select the fuse and MOV size carefully.

 

[grizedale]

i thought the idea was that the MOV was mainly to protect the electrolytic capacitors at the input?

...so the MOV should come after the mains emi filter, since the mains filter then slows up the rising edge of any mains transient, and allows the MOV more time to clamp it properly?

.....if the mov is before the emi filter, then the mov doesnt clamp the transient so well, and there is resultant ringing in the emi filter which produces harmful high voltage spikes across the electrolytic capacitors?

.....X caps can handle high voltage spikes and so the mov isnt really to protect them.

I know of one particular showers company, who have x cap based input filters but NO MOVs AT ALL, and they dont get problems, and they are plugged in 24/7 365 days a year......after the emi filter is a mains transformer in that case.


So anyway, back to the point, MOV should really come just before the mains rectifier -you agree now?

 

[Mehrshad74]

I dont agree that MOVs Are for protecting AL Caps.
In a 100kW 3phase Drive there is 10000uf/800V cap that can withstand highest surges with very little voltage change But still you will see MOVs.
MOVs have high Amper surge capability like 20KA because they are the very first stage of the circuit.
An EMI filter has limited isolation in its winding and limited Bm for the core so a high voltage surge can destroy it.
ofcourse in most cases puting them after EMI filter will work because there are MOVs on the other equipments that limit the surges and that is why that even a MOVless circuit may work fine for a long period...

---------- Post added at 17:42 ---------- Previous post was at 17:38 ----------

right now I checked a MEANWELL 15v/65w power supply and its MOV was placed before EMI filter and after FUSE and NTC.

 

[grizedale]

OK Thanks, but i can assure you that the equipment that i speak of, is connected directly to the mains, and there are no MOVs in the live line that it is connected to.

.......these units are sold in 100's of thousands across UK, and are connected to the mains 24/7......they use a mains transformer, but before that, there is a EMI filter with two X caps and a common mode choke........they have absolutely no MOV and no failures are reported.

Ill leave it to your imagination what equipment i speak of......but its highly likely that you probably use this type of equipment......though maybe some houselholds dont use them as there is an alternative.

 

[ea.arun]

i feel if we use multiple stages of EMI filter consisting Common mode choke and X we can put the MOV after the EMI filter.....i came across some like that with 3 levels of Common mode and X cap and then comes the MOV which will slow down the transients significantly and stress will be less due to the multiple inductors in the path....
but normal designs they use MOV after the fuse(before the filter) i feel....

 

[treez]

And surely if a mov is placed after the emi filter, then the mov will wear out because every time the psu is turned on , the ringing in the emi filter will qet quenched by the mov, which will wear it out?

The ringing in the emi filter is between the leakage inductance fo the common mode choke and the x capacitors.