Author Topic: Place MOV before or after CM choke?  (Read 135 times)

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Online Circlotron

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Place MOV before or after CM choke?
« on: Yesterday at 10:20:07 AM »
Several sites where we have a piece of equipment installed have suffered blowups in the 150W SMPS. We are putting it down to mains spikes. They already have a 12mm diameter round 275V MOV fitted but for these sites we are going to instead fit a 250V 26mm square MOV. Lots more meat!

As for placement, the existing MOV is across the line just after the fuse and thermistor. Following that is an X cap, CM choke, another X cap then the rectifier bridge. Would it be better to place the new MOV after the CM choke so that any resistance in the CM choke helps reduces the clamping current in the MOV and consequently reduces the max clamping voltage, or is this a totally bad idea?
 

Offline Hero999

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Re: Place MOV before or after CM choke?
« Reply #1 on: Yesterday at 10:26:36 AM »
I wouldn't have thought it would make any difference. The choke isn't going to limit the current that much and if the impedance of the MOV is low enough that it does, you've got worse things to worry about.
 

Online jbb

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Re: Place MOV before or after CM choke?
« Reply #2 on: Today at 06:08:34 AM »
Several sites where we have a piece of equipment installed have suffered blowups in the 150W SMPS.  We are putting it down to mains spikes.

(Maybe you've done this already...) If you can collect and analyse the failed equipment you might get some good feedback.  Sometimes a small change can solve big problems.

Also, if you talk to your local EMC test house, they can help you test mains surge to the appropriate standards (sorry, I forget the appropriate numbers...) in a repeatable fashion.  The standards are generally designed to encompass real-world misfortunes and could be quite helpful for finding the weak points without failures in the field annoying users. (Note: sanity of standards is not guaranteed...)

fit a 250V 26mm square MOV

What's your line voltage? 'Cause it would seem like the equipment may be subject to 264V high line conditions in some parts of Australia (note, this info could be out of date).  Also note that sometimes MOVs need a lot of derating of their voltage rating - I suggest you check the data sheet and look for application notes from the MOV manufacturer.

Then you have to look at the maximum clamping voltage of the MOV and make sure that voltage won't mess anything up.

The joy of the EMC standards is that they provide test waveforms (event suggested schematics) that can be simulated in SPICE, which can save you some trouble.

 

Offline treez

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Re: Place MOV before or after CM choke?
« Reply #3 on: Today at 06:47:05 AM »
You may need a mains impedance tester...this is where you can test if the mains is inductive or capacitive...if inductive then you can get spike problems......if capacitive then less likely...underground cables are usually capacitive, overhead lines usually inductive.

Mains transients are cuased most often by fault currents in heavy equipment getting interrupted by a fuse blowing or circuit braker acting..........you can get 100's of amps flowing in the mains, then it suddenly has nowhere to flow to when the fuse blows, so an ovevoltage spike results due to the breaking of an inductive current flowing in the stray inductance of the mains......if this is bad enough, then its unlikely that anything cost effective that you could put inside a power supply would be any  use.   :horse:

You may need an external transient protection module on the line.   :phew:   

Having said that, for smaller spikes, a big electrolitic cap after the pfc stage can act to absorb smaller transients.

Movs die after so many quenches.    :horse:  :scared: 

 8)

 

Online Circlotron

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Re: Place MOV before or after CM choke?
« Reply #4 on: Today at 08:08:04 AM »
This piece of equipment with the SMPS we don’t actually manufacture, it is made in Europe and we just use it as part of something larger that we actually do build. Seeing it is only happening at several locations and we have several thousand out there running with no problem, another approach I was thinking of is to simply put a low value inductor in series with the mains supply to the unit. I got a 100VA transformer and stripped off all the windings then threaded through the stack 20 turns of mains rated plastic insulated wire, half on one side of the bobbin for active and the other half for neutral. Phased the two windings as differential mode, not common mode. At full load the AC voltage loses about 20 volts off the peaks, so at spike frequency hopefully it should be very effective. Probably only trying it out in real life will tell.
 


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