Overvoltage protection device selection

[logicnibble]

Hi!

I've been searching and reading a lots of papers about choosing the right overvoltage protection devices.

A good explaining article is:

**broken link removed**

I want to protect:
- Power supply output (12V powered from the board to external devices)

I want to protect from:
-ESD (TVS diode recommended)
-Lightning (MOV recommended)
-Inductive load switching (MOV recommended)
-More protection the better...

So I have now this dilema: TVS diode or MOV?

TVS Diode:
-faster response (1ps).

MOV:
-higher current and power capacity.

Which one must I use?
Can I use both in parallel? I guess not since they have different breakdown voltages, and the one that has the lowest will be working but the other won't?!
Or can I use a low power TVS diode in parallel with a high power MOV?

Thanks!

 

[dknguyen]

What are you REALLY trying to protect against? That ends up deciding what you use since MOVs and TVS diodes are used in different scenarios. It's impractical to try and protect against everything. ie. Does your 12V power supply output ever going to need lightning protection?

The unipolar and bipolar thing also come into play here. A bipolar tying a unipolar line to ground won't protect the line from a negative voltage. A unipolar would though.

It can also depends on what circuitry is actually at the output of your voltage supply. The circuitry might not be very sensitive at all.

In a general case, I personally go with extra big TVS diodes. At $10 a pop for some of the larger ones you don't want to be piling them up. You can also use fast devices with a higher breakdown voltage and then slower, more powerful devices with a lower breakdown voltage so the fast ones react first but then the slower ones still eventually become active. But really, a fuse and TVS diode are probably all you need.

 

[logicnibble]

OK, the circuits I want to protect are:

- 12V: LM317 + 22uF (using PTC fuse)
- open drain MOSFETs output
- backup battery connection (using PTC fuse)
- transformer secundary (using MOV)

The main goal is to pass electromagnetic compatibility tests (from CE marking), which involves the EN61000, EN55022 and EN50130 norms.
So protection against:
-ESD
-Radiated imunity
-EFT/B
-Surges
-Conducted RF
-Power frequency magnetic
-voltage dips and interruptions

These tests don't apply to all circuits, but most of them.


About the protection devices, I've read about the new MLV which are varistors with faster response time (<500ps against <25ns for regular varistors) and higher power for smaller smd packages. The only disadvantages to TVS diodes are bigger clamping voltage and still slower response time (1ps for TVS diode).
Size matters, so those big TVS diodes (they have to be big to support higher power) are not suitable for small boards.

 

[Optikon]

OK, the circuits I want to protect are:

- 12V: LM317 + 22uF (using PTC fuse)
- open drain MOSFETs output
- backup battery connection (using PTC fuse)
- transformer secundary (using MOV)

The main goal is to pass electromagnetic compatibility tests (from CE marking), which involves the EN61000, EN55022 and EN50130 norms.
So protection against:
-ESD
-Radiated imunity
-EFT/B
-Surges
-Conducted RF
-Power frequency magnetic
-voltage dips and interruptions

These tests don't apply to all circuits, but most of them.


About the protection devices, I've read about the new MLV which are varistors with faster response time (<500ps against <25ns for regular varistors) and higher power for smaller smd packages. The only disadvantages to TVS diodes are bigger clamping voltage and still slower response time (1ps for TVS diode).
Size matters, so those big TVS diodes (they have to be big to support higher power) are not suitable for small boards.

For passing these compliance tests, the first line of defense is your PCB layout. Make sure that is correct first and foremost. Some good ESD protection can be developed by simply how you layout your PCB.. ditto for surge protection, immunity and emissions. EFT & voltage dips etc.. those are mostly testing the power supplies.

Fuses & TVS diodes go a long way.. fuses are only needed for "sustained" overloads / overvoltages.. none of those tests you mentioned are sustained. so a fuse is not needed to help pass any of those tests. Place those proctecion devices in locations wehre they will work the best. Sometimes that is tough to decide. As a general rule, proetction devices placed right at the connectors/interfaces is a good start. But more importantly, is possibly the return impedance from that point..

Use good grounding schemes (planes and or star) where appropriate and you wont have much trouble. Dont forget about the PCB layout.. I would say that is as important if not more so than choosing and placing the correct protection device.

 

[logicnibble]

For passing these compliance tests, the first line of defense is your PCB layout. Make sure that is correct first and foremost. Some good ESD protection can be developed by simply how you layout your PCB.. ditto for surge protection, immunity and emissions. EFT & voltage dips etc.. those are mostly testing the power supplies.

Fuses & TVS diodes go a long way.. fuses are only needed for "sustained" overloads / overvoltages.. none of those tests you mentioned are sustained. so a fuse is not needed to help pass any of those tests. Place those proctecion devices in locations wehre they will work the best. Sometimes that is tough to decide. As a general rule, proetction devices placed right at the connectors/interfaces is a good start. But more importantly, is possibly the return impedance from that point..

Use good grounding schemes (planes and or star) where appropriate and you wont have much trouble. Dont forget about the PCB layout.. I would say that is as important if not more so than choosing and placing the correct protection device.

Yes, you're right, I've just had it on my mind.

Back to the original post, Can I use TVS diodes in parallel with varistores?
With this I mean to use TVS diodes for fast response and varistor for high energy protection, but I don't know if they are suitable to work together.

Any help?

 

[jcesaree]

HI, I have the same problem as you, but as far as I know we cant use MOV for IEC..

what did you do?