# Help in assessing the input mains circuit integrity

I've been working on designing a (220-380 VAC) 3/4 wire phase energy meter and in terms of metrology operation, things were working very well quite similar in terms of metrology if compared to a commercial unit. Now I'm no expert in mains power electronics circuit design so I opted to integrate an off the shelf AC/DC flyback power supply, which when integrated in the meter worked well in lab conditions. I've followed and implemented the mains Input protection circuit going into PSU which is specified by the PSU manufacturer but to be honest I feel helpless as I have no idea how to calculate/know how and why the values are specified in this stage, I need to be able to know how much energy/ voltage and the where bouts of the circuit operates in a surge, fast transient and ESD situation since I plan on doing a pre-compliance test and one of the tests I'm concerned with is if the input protection fails then I have not backup plan so I would really appreciate if someone would kindly share his insights and knowledge on how to pre-simulate or calculate the values of the input circuit below in order to pass the following test conditions below as well.

• Schematic of the mains input circuitry with the power Supply

• Test 1

• Test 2

• Test 3

• Some information on the MOV used on each line (230 VAC

My questions :

1. How do you calculate/simulate or choose the correct MOV model/value for the above application to pass the test conditions mentioned above?
2. Will the current input stage of the power supply survive the tests mentioned above? ​

Final Note: I come from an Electronics and Communications background and i have done a reasonable amount of work in low voltage (analog, digital) board designs but I'm no expert in this subject but I'm willing to learn. I know how to simulate how an AC signal going in to a bridge rectifier and analyse the output; you know basic stuff that most of us learned during undergrad, but the (fast transients, etc) on an AC mains situation, I've never worked with such case. I'm having a hard time on how to figure out how to model and simulate the MOV in the simulation programs (LTSpice, Proteus ISIS & TI TINA).

I need to be able to know how much energy/ voltage and the where bouts of the circuit operates in a surge , fast transient and ESD situation

and

How do you calculate/simulate or choose the correct MOV model/value for the above application to pass the test conditions mentioned above ?

You need to model the way in which the surges, bursts and ESD are applied to your live and neutral connections. There will be an interface circuit for all three and, these circuits should be locatable on-line. For instance, indirect lightning surge testing is available because I've researched it myself to uncover whether some TVS diodes (that I was planning on using for a particular job) were man-enough to withstand the surges.

So, do some digging and uncover those circuits - they won't be too tricky - you can easily model the waveforms with straight lines ramps of voltage; it doesn't need to be exact - reasonable approximations to the waveforms are good enough but, the important thing to model is the surge/burst/discharge source impedance because this tells you how much current is fed to the protection device when it is protecting.

In my surge example above, I had to deal with surge voltages at a peak of 4,000 volts sourced through a 2 Ω series resistor. In other words, the peak current could be up to 2,000 amps. So, I modelled it and simulated the surge protection circuit and determined how much peak energy I might subject my TVS to. Then I looked at the data sheet for the TVS and worked out whether I had a comfortable enough margin.

if the input protection fails then I have not backup plan

Your only back-up plan is diligence - have you got a comfortable enough margin. I mean; if you don't simulate and get a feel for things then you have no idea whether the protection you fit is anything like suitable so, do the right thing and model each threat in terms of voltage waveform and output impedance and model each protection device to see that you don't exceed the peak energy handling in the data sheet (and add a comfort margin of 1.5).

Will the current Input stage of the Power Supply survive the Tests mentioned above ?

You can be sure if your modelling and simulation suggest not, then you need to improve things. But, of course, the real test is the lab test. My surge test (EN 61000-4-5) was a pass if that gives you any confidence in things but, I probably spent two solid weeks with a simulator.

You have to know the maximum Vrms that the MOV is subject to.

Go to digikey.com pick a MOV that has VAC larger by 20% than Vrms.

When filtering the results in digikey.com, be informed that VAC is the RMS voltage.


See here for example:

https://www.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_ciii_datasheet.pdf.pdf

Take a look here as well:

Varistors in three phase application