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I used this link to generate a 5VDC (1A) from the 220Vac power source, the circuit itself works perfectly fine but i want to make sure that the overvoltage on the AC line won't damage my board.

I searched the web and noticed that for protecting my board against the overvoltage on the AC line i should use the MOV device connected between the Line and Neutral.

In addition i noticed that there is a device called bi-directional TVS diode used to protect the device from the voltage spikes and it connects to the AC line in parallel just like the MOV.

The problem is that I'm not familiar with the TVS devices and i don't know which TVS diode is suitable for my application.

Please give me some suggestion for the suitable bi-directional TVS diode. I also attached the transformer-less power supply design for reference. enter image description here

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    \$\begingroup\$ I would personally be much more worried about galvanic isolation \$\endgroup\$ – PlasmaHH Oct 31 '16 at 9:13
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    \$\begingroup\$ not as good as google can \$\endgroup\$ – PlasmaHH Oct 31 '16 at 9:15
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    \$\begingroup\$ Why don't you ask the guy who designed it what to use? After all, the design link you supplied shows a schematic with very little information about the transistors and resistor power levels. Knowing what to protect tells you how to protect it. I also agree with @PlasmaHH and you might get more help if you state what the target load is. \$\endgroup\$ – Andy aka Oct 31 '16 at 9:19
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    \$\begingroup\$ The circuit in itself is a good idea (if no galvanic isolation is required, indeed). However, more than overvoltages on mains, what is likely to destroy it is the fact that you repetitively overdrive the gate voltage of T2. It seems to go as high as the ~45V you provide as input of the LM2575. Also, I think you are really pushing the limits of the LM2575 input voltage (unless you use the HV version). \$\endgroup\$ – dim lost faith in SE Oct 31 '16 at 10:02
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    \$\begingroup\$ @MehdiHafezi NO!!!! Read my answer. Don't use this circuit at all. It is like you ask something, and then don't care about what the answers are. \$\endgroup\$ – Marcus Müller Oct 31 '16 at 10:50
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If you care about overvoltage protection, you shouldn't use this kind of circuit. It's dangerous.

The most likely part to fail in an overvoltage situation is T2, in which case there's full recitified 220VAC over D1, which will fail, so there's full rectified 220VAC over your 5V output, which will probably make D2 explode, moments before VD1 does the same (hopefully; diodes often fail in a mode that make them short circuits, thus shorting the power line, thus killing the fuse in your house, so lethal risk is limited to a short period of time).

Generally, I fully agree with PlasmaHH's comment that you shouldn't use this at all – there's no galvanic separation from grid to 5V, and that is never a good thing. Components might fail, and then you'd instantly have 220V at your 5V output, killing whatever (or worse: whoever) is there.

You won't find this kind of schematic in modern application notes on how to build off-line converters, simply because it

  1. lacks protection due to lacking galvanic isolation and hence, cannot be sold as commercial product, very often
  2. is not a very effective way of building such a thing, both cost- and power-wise.

To explain: The Left half of your schematic, ie. between your diode bridge VD1 and your buffer capacitor C1 is really just a very simple linear converter, wasting a lot of power.

It is a clutch employed by the designer of the circuit to work around the fact that, due to point 1. above, you typically can't find step down controllers specified for a > 100V input, 5V output range (you can definitely build those without any problem using a lot of existing chips, though it's harder than what the guy who wrote your link did).

For off-grid applications, you'd typically want some controller IC to not control a buck-architecture (the L1/D1 switching thing), but something that, long story short, switches one side of a transformer. That transformer will be the isolating part between your precious life (and not-as-precious board) and grid, and also doesn't waste energy like the T1 and T2 linear voltage controller (and hence, won't get as hot).

In that architecture, you can add overvoltage protection on the primary side very easily (overvoltage -> overcurrent-> fuse blows, for example) without risking the secondary side, because that can just leave out a few cycles until the overvoltage condition has cleared. Also, primary coils of such transformers are far less sensitive to overvoltage than T1/T2 are.


Conclusion

Don't take random blogs' circuits of the internet when there's grid voltage involved; life is too precious for that.

Manufacturers of ICs like the LM2575 do have websites with both application notes on how to build power supplies, and even more helpful, complete design guides where you essentially just type in "input: 220VAC, output 5V @ 1A", and get a board schematic, including a bill of materials, an analysis of how well everything operates, etc.

These schematics tend to be relatively complete, and include overvoltage protection (within bounds of what is to expect in a typical environment).

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    \$\begingroup\$ FWIW the input cct is not just a linear converter. When V out of bridge is less than about 40 Vdc C1 can be charged by mains. When Vdc in >= 40V D1 conducts and turns on T1 which clamps gs on T2 and turns off feed to C1. So cct draws power on valleys of Ac mains and typically does so for portion around 35-40V depending how much C1 dips between half cycles. So while power factor is attrocious power efficiency might be OK. However T2 Vgs rises to about Vd1 which should destroy T2. \$\endgroup\$ – Russell McMahon Oct 31 '16 at 11:57
  • \$\begingroup\$ @RussellMcMahon hm, true. It kind of is a net-synchronous SMPS, but uaaargggh... let's hope the Ugs rises fast enough to put T2 out of misery fast. \$\endgroup\$ – Marcus Müller Oct 31 '16 at 12:28
  • \$\begingroup\$ From my comments above on his question: " ... The circuit is very clever and "quite fun" BUT I would not touch it with a very very very long barge pole in the real world. ..." -> :-) :-(. Clever. Lethal. Lifespan uncertain. \$\endgroup\$ – Russell McMahon Oct 31 '16 at 12:48
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This is unsafe, use certified "AC overvoltage switch" that cuts off power automatically when voltage is too high.

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