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I have a circuit that I want to protect against surges and eventual peaks (e.g. indirect lightning surges) and I was thinking about TVS diodes or MOV.

The situation is:

-My circuit input is 48V that is stepped down to 12V with a MP24943

-The 48V comes from a standard converter (that I bought) connected on the AC line.

-The maximum voltage accepted by the MP24943 is 60V (datasheet)

-I want to protect the MP24943 and what is after it from surges but I couldn't find any TVS Diodes or MOVs that meet the requirements (for 48V the clamp voltage is something around 75V)

I have already read this, and I couln't find any of the Bourns ICs mentioned here in Brazil (specific components are very hard or impossible to find), and the most cost effective solution for me would still be MOVs or TVS, is there any way I can use them in my circuit? Or to find a replacement for them?

I tried Zener Diodes but no success too, the BZD27C Series (datasheet) for example still have a Clamping voltage too high for me.

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  • \$\begingroup\$ Why do you think your 48 volt supply is not dependable? Most have over voltage protection (not the same as a surge). To fail the feed back loop inside it would have to fail in an odd way. \$\endgroup\$ – Sparky256 Feb 19 '18 at 20:24
  • \$\begingroup\$ Indeed it should be well protected, however it failed already about two times since I started using it and all the power supplies in the price range I can use operate in similar ways. It is rare but it happens, the power supply is cheap but a better one won't fit in the space I have to put it, I just want a circuit to be sure to protect the main circuit, since it is more expensive. \$\endgroup\$ – Augusto Mattos Feb 19 '18 at 20:42
  • \$\begingroup\$ How does that 48 volt power supply fail? Does the output go up or down in voltage? \$\endgroup\$ – Sparky256 Feb 19 '18 at 20:45
  • \$\begingroup\$ It burns out, sometimes shorted sometimes not, in any way it stops working, but the rest of the circuit after the power supply, which I am trying to protect is hit with the same surge and burns too. \$\endgroup\$ – Augusto Mattos Feb 19 '18 at 20:55
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Well first of all I'd make sure I bought a 48V supply that was guaranteed NOT to go out of some tolerance range that you can accept.

Otherwise you would be better to add a circuit that shuts off the power to your regulator if the source goes above the range you deem acceptable. If the supply is going to fail out of spec, a simple TVS or Zener will not last long.

The design in this answer to another question may be more to your liking. The circuit there is of course for 5V but if you understand it, you can modify it to whatever voltage you need to limit to. Watch you do not drop the gate of the MOSFET below its limit though.

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  • \$\begingroup\$ Thank you for your answer Trevor. The power supply I am using will only go out of the specification if a surge hits it, and I want to protect the circuit after it. The power supply have some protection but I want to do one extra myself. As for the design you linked, is it fast enough to stop a surge? I really don't know about time of reaction of a circuit and if it can stop surges as fast as a TVS diode. \$\endgroup\$ – Augusto Mattos Feb 19 '18 at 20:39
  • \$\begingroup\$ @AugustoMattos the circuit indicated is as fast as the transistors you chose. \$\endgroup\$ – Trevor_G Feb 19 '18 at 21:12
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    \$\begingroup\$ @AugustoMattos adding a suitable power filtering LCL network would also be prudent if you have a serious fast spike issue. \$\endgroup\$ – Trevor_G Feb 19 '18 at 21:19
  • \$\begingroup\$ The circuit is fast enough (a few nanoseconds) but it wil never sustain a surge from a lightning. The FET will burn and the circuit will be shorted. Even if the FET is rated 20A or more, the voltage will destroy it. Its gate will not make it through. By comparison, a MOV can sustain several thousands of A and infinite voltage. \$\endgroup\$ – Fredled Feb 19 '18 at 21:47
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    \$\begingroup\$ @Fredled NOTHING will sustain a direct surge from lightning. But I agree if spikes is the issue other protection is warranted, but OPs question more indicated he is worried about surges killing the 48V supply and then allowing higher rail through. At least that's how I read it. He needs both may be the real answer. \$\endgroup\$ – Trevor_G Feb 19 '18 at 22:11
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The clamping voltage is always much higher than the working voltage and you must always select a working voltage slightly above your effective usage voltage. If you want to protect 48V, select a 50V MOV or above. If you want to protect 12V, select 14 or 15V MOV. (In your case I would protect for both voltages, before and after it's stepped down) Don't worry if the clamping voltage is way higher, it's normal to avoid false shorts. TVS diodes are for data lines. MOV are much more robust. Take the highest peak current value. Zeners are much too weak (but can help to some extent if there isn't anything else).

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  • \$\begingroup\$ You may also consider the same surge protection on the main line. \$\endgroup\$ – Fredled Feb 19 '18 at 21:50
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Good power supplies need great protection from PLT or power line transients that sometimes exceed the IEC standard test.

The energy in the transient can either be shunted on secondary which draws more stored energy or filtered by series high (L) impedance in the primary, which draws less power.

I recall in mid'80's we need high-rel supplies which were custom designed by , Brown and Hammond , to our specs, which included higher standards for PLT. They choose to implement 2 stage line filters which consist of a higher inductance CM choke and a differential choke with twin Y caps as in a PI filter. They also included MOV's and one design also had a primary fused gas tube.

I suggest you investigate the weak link and add one of the above solutions. If this is a commercial design, you either need to have or rent, borrow or make your own PLT generator.

If this is an open frame 1U high 48V supply, I would recommend Lambda's having qualified them in the past. ( vs PowerOne) Both were about 70 cents per watt.

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