What to expect from adjusting the suggested EMI filter and surge protection on a DC/DC power supply's application note. 61000-4-5

Question

I am trying to make the CQB50W12-72S12 120VAC input capable by adding a bridge rectifier into the design that is already called out in the application note (app note and data sheet found here: https://www.cincon.com/productdetail/CQB50W12.html).

See picture of what I have drawn up in altium:

Because it is AC powered now I figure I need line to earth surge protection in order to get the same or better ratings for 61000-4-5. I looked up some littelfuse application notes on surge protection and saw that you can add impedance between surge clamping components for better coordination. I am going to start building what I have in LTspice but would like to get someone else's eyes on it for a sanity check.

I think that the mov's will help protect the bridge rectifier and remaining surge will be clamped by the TVS diodes that were recommended by the datasheet. I am just trying to achieve the same 2kV line to line rating and 4kV line to earth rating that the application note declares without completely messing up the EMI filtering.

Answer 1

Because it is AC powered now I figure I need line to earth surge protection in order to get the same or better ratings for 61000-4-5.

I don't think you can get better immunity as the converter module already complies with 61000-4-5 Level-4 which is the toughest level already (unless a higher one is defined with Level-X by the manufacturer or yourself). Of course, this doesn't mean that you should rely on the converter module's surge protection so it's good to implement a separate protection for the AC-input-side components.

^{The datasheet on p.3 says the converter module complies with 61000-4-5. But the app note on p.14 (under Safety & EMC) mentions the requirement of external components in order for the converter module to comply with 61000-4-5.}

I think that the mov's will help protect the bridge rectifier and remaining surge will be clamped by the TVS diodes that were recommended by the datasheet. I am just trying to achieve the same 2kV line to line rating and 4kV line to earth rating that the application note declares without completely messing up the EMI filtering.

The idea of placing TVS diodes for common- (D1, D2) and differential-mode (D3) surges is correct, however, the selection of the components doesn't seem to be. I don't know if you have a grasp of 61000-4-5's details, but apparently, you missed the fact that the currents during a surge, depending on the class/level of course, can reach hundreds of Amps.

What you need to do first is to check the standard and the required class/level for your application, then pick diodes having correct reverse stand-off and clamping voltages, and most importantly, an I_PP rating (for 8/20 us) of higher than the class/level requirement. TVS diodes for surge immunity/protection generally mention what standards they are compatible with. This can be a good starting point. For example, if you have a look at the datasheet or product highlights of SMDJ180A you'll see that it's not compatible with 61000-4-5 as its peak pulse current rating is only 51 Amps.

As for placing MOVs for common-mode surge (across L-PE and N-PE), as Tim pointed out in his comment, you need to be very careful because, since the MOV will always be under mains stress (assuming N and PE have the same potential), a leakage current will flow through it to the earth which can cause self-heating (if not selected properly) and eventually initiating positive feedback and possibly destruction (i.e. heat causes higher leakage current, therefore higher self-heating, so higher self-heating causes even higher leakage current which causes even higher self-heating. This goes on until destruction). This may result in shock or even fire. Also, you may want to place another fuse on the N-side, plus you may want to cover the TVSs with nonflammable material.