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Overvoltage in MOV at mains-ON of offline SMPS

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Flyback

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Page 12 of the following shows a 240W offline PSU schematic. The MOV is placed after the input LC filter.
Surely you agree that this is totally wrong?, and the MOV should appear after the input fuse?
After all, the input LC filter will ring up to a high voltage whenever the mains is applied at the instant of mains peak, and the MOV will end up quenching this ringing voltage…which will reduce its lifetime unnecessarily.
A MOV is simply to quench line overvoltage transients.

If it is required to damp down input LC filter overvoltage transients caused by ringing at application of mains at the instant of mains peak, then the filter inductors should be placed after the diode bridge and should feature diodes across them (cathode on input connector side)
Alternatively a TVS should be used following the mains input filer, though this is not recommended.

Please see page 12..
http://www.onsemi.com/pub_link/Collateral/DN05067-D.PDF

The schematic of page 12 is simulated here to show the overvoltage ring that the MOV would have to quench every time the SMPS is plugged in to the mains...
 

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ronsimpson

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I know the answer but I need to know what you are thinking first.
L5,L6 is a filter (transformer). L3,4 is a filter.
Question: How did you handle the leakage inductance of transformer L5L6? Did you use K1=.95 or are you using L3,L4?
Another way to ask that is: L4534=transformer or L3L4 = two parts and L5L6 is one part???
The answer is not easy found in spice. It is one of spice's problems.
 

Flyback

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L5 and L6 is a common mode choke....as you know the resonating current doesn't see them, the resonating current only sees the leakage of L5/L6 which is L(leak) = (1-k^2)*L5.
None of the other inductors are coupled.

I believe the MOV should be by the input connector, and the overvoltage ring should be dealt with by a TVS.
I believe that spice does show things accurately here. A real circuit like this really would ring up to the overvoltage shown
 

ronsimpson

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I think you have the phase on the filter transformer wrong.
The power line current flows through the leakage inductance (30uH) not the 3.5mH.
 

ronsimpson

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Also I think the transformer will saturate out at this level of VT or LI.
I don't know how to make the transformer turn into a piece of wire after (x) amount of VT.
When the transformer saturates this large amount of energy will not be stored on the core and thus will not kick up.
 

ronsimpson

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The point of D6 is to keep (more or less) this from happening. D6 shunts the inrush current around L7. If L7 sees the inrush current it will pump the output too high. To test that remove D6 and your input filters and this happens again.

I think your input filter will (in real life) saturate out and there will be much less over shoot. I have not seen this happen when D6 is in place.

I increased the resistance of the power line and the switch and that makes the simulation much faster. (slow computer)Also I added 100k ohm resistor across C4 to see things more clear.
 

Flyback

Well-Known Member
2R does indeed help, and that is my fault for not explaining that this is the input stage of a offline smps whose input current is 5.2A RMS.
As you know, such resistance for damping is not an option in this case, which I stupidly forgot to describe in full, my apologies for this.
 
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