HS+LS output EMF capability

Heard back from Contactor manufacturer,  We can have an EMF clamping diode rated at no less than 50V if required.  What is your recommendation?

This is a very common dilemma when controlling relays and contactors. A regular diode with low clamping voltage causes the contactor to open 4-5 times slower than normal, burning up the contacts.

I often use a diode+zener diode in that case. It protects the drive module and does not slow down the contactor opening significantly.

https://www.te.com/en/products/relays-and-contactors/electromechanical-relays/intersection/coil-suppression-relay-life.html?tab=pgp-story

Thanks Pierre,
1) Not a major in electronics, but why use a Zener rather than a clamping diode specifically designed to absorb the voltage spike?
2) But the real question is what voltage limit should we use.  the relay manufacturer says 55V is the minimum they will allow, 150V is the default internally.  I do not seem to find a Specification for the MC43-FS in general and more specifically when outputs are used in the LS+HS scheme.

A problem with designs based on zener diodes is that the clamping voltage is often higher than the level where the protection in the highside switch cuts the transient. 

If the lowest you can get to is 55 V as you say, it will not have any effect in protecting this DOUT. 

For this application I'd recommend looking at the MC4x DOUT HS+LS maximum allowable load inductance without external clamping diode. Maybe you could do without it.  

Also see  Coil Suppression on MC4x Outputs / Hardware / IQAN

Hi Gustav, 

thank you for your comment and redirect.  the problem with the link you refer to is the lack of actual real value we can use.  What would be the maximum  inductance allowed?  I do understand the Inductance on its own is not the factor as most likely the energy to released is a square function of the system voltage.  Is my only option to test and see If it does not destroy the output?  The hardware is a bit too expensive for this.
I understand the output are rated at 32 V maximum I am assuming this is continuous. But there must be an allowance for overvoltage spikes?  I would assume that the outputs are tested against some IEC standards for overvoltage?  What standard is being used? 

Gustav,

After a lot of research, I found that according to the manual the unit complies to ISO7637-2 (appendix A pg109):

I have extremely limited knowledge on the true meaning of those test.  But it seems that the definition of the pulses for that test are not always consistent and are manufacturer defined.  Do we know the test used for the validation?  do they they apply to the LS and HS output too?  
  

Regarding that questions, it is worth pointing out that ISO 7637-2 is on power supply. As you are looking at transients on I/O, that is ISO 7637-3. 

These are all important tests, but it does not take away the need to design the electric system to avoid transients from large inductive coils. 

The idea is, if the contactor manufacturer says you may have a standard clamping diode rated at 50V, you can measure how long time it takes for the contactor to open with that configuration.

Then you make a test with for example a 12V zener in series with a standard rectifier diode and measure the time it takes to open the contactor. Use the lowest you zener voltage you can get away with and see if this is agreeable with the limitations of the IQAN output circuit.
The opening time will be faster than when using only a zener diode.