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iso 7637-2 pulse 1 vs iec 61000-4-5 waveform comparison

IEC Surge/EFT Generators for ISO 7637-2 Automotive Pre-Compliance

Intro

Like most long articles, this started off as a short one. It all stemmed from a customer question:

 

“We had some issues using a LED driver that could not cope with load dump and volt spikes. Do you have any provisional tests that could determine the circuit reliability? It doesn’t have to be to [ISO 7637-2]”

 

The ISO 7637-2 standard defines automotive conducted transient test pulses on vehicle power lines (12V or 24V). It is called up by standards including:

  • UNECE Regulation 10.06 for E-marking
  • EN 50498 (aftermarket automotive equipment)
  • ISO 13766-1 (earth-moving and building construction machinery)

I don’t have an ISO 7637-2 pulse generator (edit: I do now!). Automotive surge generators are less commonly found in many EMC test labs due to their more specialised nature.

Systems are available to hire; budget for €/£1000/week for a generator that will cover Pulses 1, 2 and 3. They are also available to buy new;  expect to pay around €/£15k. If you need to cover pulse 4 then this will increase the costs yet again, mostly for the bipolar amplifier.

But, like most EMC test labs, I do have an IEC 61000-4-4 (EFT) and IEC 61000-4-5 (Surge) generator capable of 1.2/50us and 10/700us pulses.

 

Question: Could I use the IEC generator to simulate the surge pulses from the ISO generator?

 

This question comes with caveats:

  1. The aim here is pre-compliance / confidence testing with the tools available. Not to replace the ISO 7637-2 tests entirely.
  2. We are only looking at the potentially destructive Pulses 1, 2a, 3a and 3b.

 

Unit 3 Compliance can perform pre-compliance and full CE Marking testing to EN 50498. We can also perform pre-compliance testing for many of the R10 tests for E marked products.

Please get in touch for a chat if this is of interest.

 

Conclusions (TLDR)

ISO Pulse 1

  • IEC 10/700 pulse generator can be used as a close substitution for a 12V system
  • For a 24V system the 10/700 pulse is not as good a match. Follow the flowchart to select the test compromise and set the surge voltage based on the values in the tables.

iec 10-700 for pulse 1 24V surge voltage selection flowchart

iso pulse 1 24V vs iec 10-700 Best Compromise

iso pulse 1 24V vs iec 10-700 Best Compromise actual voltages and currents

ISO Pulse 2a

  • Not a good match, recommend a compromise between current and energy as shown in these tables

iso pulse 2a vs iec 1.2-50 Best Compromise

iso pulse 2a vs iec 1.2-50 Best Compromise actual voltages and currents

ISO Pulse 3a, 3b

  • IEC EFT generator is a good match and can be substituted for ISO pulse 3a and 3b

 

Pulse Parameter Comparison

Comparing the pulse widths and impedances against each other gives a mixed picture.

For Pulse 1, neither waveform is a great match with both of the ISO pulses having a longer pulse width than the 10/700 generator. Whilst the 24V bus pulse has a much higher impedance, this could be corrected with an additional series resistor in the IEC  generator output.

comparison table - iso 7637-2 pulse 1 to IEC 61000-4-5 10-700

For Pulse 2a, the 1.2/50us IEC generator appears to be an excellent match.

comparison table - iso 7637-2 pulse 2a to IEC 61000-4-5 1.2-50

For Pulse 3a and 3b, the 5/50ns EFT generator is pretty close but the width of the ISO pulse is three times bigger.

comparison table - iso 7637-2 pulse 3a 3b to IEC 61000-4-4 eft 5-50n

 

However, as we shall see below, this approach is incorrect as it does not tell the whole story.

 

Pulse Width Definition

The problem comes from how the pulse widths are defined in the standards. Let’s take the comparison between ISO Pulse 1 to IEC 10/700 comparison as an example.

e can see that the ISO pulse width is defined at the 10% crossing point, whereas the IEC pulse width is defined at the 50% crossing point.

iso 7637-2 pulse 1 vs iec 61000-4-5 waveform comparison

This is not helpful.

How do we compare a ISO 1000us @ 10% with a IEC 700us @ 50% waveform?

 

Open Circuit Ideal Waveform Comparison

I found some information over on the PSCAD website that showed the equation for the waveshape (from IEC 61000-4-5)…

exponential surge waveform formula…along with some Matlab optimised coefficients for alpha, beta and k.

From the PSCAD website “Standard Surge Waveforms” https://www.pscad.com/webhelp/Master_Library_Models/CSMF/Surge_Generators/Wavelet_Transformation_(WT).htm

 

ISO 7637-2:2011 gives the equation for the falling edge only of the pulse waveform. It also states that “The influence of the rise time is not taken into account (tr << td), which is allowed for all pulses specified in this part of ISO 7637

iso 7637-2 pulse shape equation

 

Modelling Notes

After watching a Numberphile video on coronavirus infection curve modelling I decided to give Geogebra a try for modelling these waveforms. It’s quite a useful graphing calculator package, much more powerful than I’ll ever need to use.

I also modified the equation for the IEC waveshape equation to take into account the generator and load impedances by taking the first term of the ISO equation and adding it to the start of the IEC equation.

A required surge voltage of 1V was used for simple direct comparison.

 

Pulse 1 (12V) vs IEC 10/700us

Geogebra Link

ISO 7637-2 (Pulse 1, 12V) vs IEC 61000-4-5 (10_700) geogebra

Pulse 1 (24V) vs IEC 10/700us

Geogebra Link

ISO 7637-2 (Pulse 1, 24V) vs IEC 61000-4-5 (10_700) geogebra

Pulse 2a vs IEC 1.2/50us

Geogebra Link

ISO 7637-2 (Pulse 2a) vs IEC 61000-4-5 (1.2_50) geogebra

Pulse 3a/3b vs IEC 5/50ns

Geogebra Link

ISO 7637-2 (Pulse 3a_b) vs IEC 61000-4-4 (5_50ns) geogebra

 

Review of Waveform Comparisons

For Pulse 1 we can see that the 10/700 IEC waveform is actually a really good match for Pulse 1 for a 12V bus.

The same cannot be said for the 24V bus requirement. Some further thinking is required here.

The 55 ohm impedance for the 24V version of the pulse is the 15 ohm 10/700 generator natural impedance with a series 40R resistor in addition.

comparison table - iso 7637-2 pulse 1 to IEC 61000-4-5 10-700 - GEOGEBRA RESULTS

Despite Pulse 2 looking like a good comparison initially, the modelling shows that it is actually a very poor match.

comparison table - iso 7637-2 pulse 2a to IEC 61000-4-5 1.2-50 - GEOGEBRA RESULTS

For Pulse 3, the IEC EFT generator is a very good match and should be able to be used without any issue

comparison table - iso 7637-2 pulse 3a 3b to IEC 61000-4-4 eft 5-50n GEOGEBRA RESULTS

 

Dealing With Pulse 1 (24V) and Pulse 2a

How could we go about compensating for the poor match between Pulse 1 (24V) and 10/700 IEC and between Pulse 2a and 1.2/50 IEC?

We need to ask ourselves: are we more interested in the peak voltage & current or pulse energy?

To answer this, first we need to understand the power input design of the Equipment Under Test (EUT)

 

EUT Design Assessment

It is useful to establish the following EUT design parameters:

  • Is there a discrete reverse protection diode? What is the Vrrm and Trr rating (reverse recovery time) of this part?
  • What is the maximum clamping voltage of the TVS diode and can the downstream circuitry survive this voltage?

vehicle power input protection circuit

It is important to remember that Pulse 1 is a negative going pulse caused by the disconnection of a large inductive load in parallel on the vehicle power bus. If the EUT has a reverse protection diode fitted then it’s Vrrm and Trr will change the effect of the test on the EUT.

W2AEW has a good video on diode reverse recovery time over on YouTube.

It is also important to test at full current consumption if a reverse recovery diode is present as this will affect recovery time and therefore surge performance.

 

EUT Surge Suppression

The assumption is that we are testing an EUT that contains some basic low voltage electronics of some kind. The extension of this assumption is that it has some kind of surge suppression component connected across the power inputs.

This could be a Metal Oxide Varistor (MOV) or a Transient Voltage Suppression Device (TVS). These have a non-linear impedance with voltage and will restrict or “clamp” the input voltage to a defined level. Perhaps a component like a SMBJ26CA-TR.

This clamping voltage is dictated by the impedance of the part when conducting. This would be a diode-like VI curve for a TVS or the current-dependant resistor of a MOV.

Peak current is dictated by available peak voltage and generator impedance. So we need to be interested in the peak current to ensure that the correct clamping voltage is met.

Also, because the MOV or TVS absorbs some of the pulse energy internally, these components will have a datasheet rating for pulse energy. Exceeding this could cause significant damage to the part and affect its capability to handle future surges.

 

Pulse 1 Peak Voltage & Current or Pulse Energy?

Our main tools for adjusting an IEC pulse to suit an ISO pulse are:

  • Peak voltage
  • Series impedance

The surge generator has an easily adjustable peak voltage through the control panel or software so this is the main method that will be used.

The Peak voltage is a significant consideration if the system has the reverse protection diode but the compromise test will depend on it’s voltage rating.

I’ve produced a flowchart to help selection of the right test level for using IEC 10/700 instead of ISO Pulse 1

iec 10-700 for pulse 1 24V surge voltage selection flowchart

 

 

Pulse 1 Best Compromise Voltage

I ran some more simulations in Geogebra adjusting the ratio between the IEC and ISO peak voltages and tabulated the results.

ISO 7637-2 (Pulse 1, 24V) vs IEC 61000-4-5 (10_700) Matched Pulse Energy

iso pulse 1 24V vs iec 10-700 Best Compromise

The best compromise is to minimise the total difference between current and voltage when expressed as ratios. This works out at a V_iec or around 0.6 * V_iso.

This yields the following test voltages, peak currents and pulse energies for the different severity levels.

iso pulse 1 24V vs iec 10-700 Best Compromise actual voltages and currents

 

Sidebar

It is interesting that the series impedance for the 24V version of ISO Pulse 1 is up at 50 ohms. This higher impedance implies that the surge expected in such a system would be induced from a parallel adjacent cable in a wiring loom rather than something directly connected to the ignition switch / inductive load circuit directly.

 

Pulse 2a Best Compromise Voltage

Same approach as for Pulse 1

iso pulse 2a vs iec 1.2-50 Best Compromise

iso pulse 2a vs iec 1.2-50 Best Compromise actual voltages and currents

 

 

Test Practicalities & Further Compromises

Pulse 1 Power Disconnection

The waveform for Pulse 1 shows a synchronised disconnection from the DC supply and application of the surge voltage. Since this is not easily done without

It is the surge pulse that will cause the damage rather than the momentary disconnection of voltage therefore, for these compromise tests, this is being ignored.

 

Coupling/Decoupling Network Requirements

The CDN inside the IEC test generator for mains coupling is adequate for the task of decoupling but the options inside my KeyTek ECAT test generator preclude the coupling of the 10/700 waveform. Instead, some creative front panel wiring with banana plugs will be required.

Since this CDN is designed for decoupling of surge and EFT impulses from the mains, I’m sure it will adequately protect the 12V linear power supply being used and also prevent the power connection from unduly affecting the test.

In may case, input is through a 16A IEC mains plug/socket but it is easy to make an adaptor. Output is via a BS1363 socket or, more convieniently, 4mm banana plugs.

 

 

The End.

This took way longer to research and write that I was hoping. Something in the order of three days of work was spent going backwards and forwards, thinking about it whilst doing DIY at home (nearly painting the cat as a result) and half listening to Tiger King on the TV.

I’m quite pleased with the result and I hope this eventually proves useful to someone.

 

 

 

Simple RF Current Transformer for EMC / EMI Investigation

This post contains some background info related to the video I posted on YouTube on how to make a simple RF current transformer, a great tool for debugging EMC / EMI issues such as radiated emissions from cables, or tracing conducted RF immunity noise paths.

RF current transformers (or probes) are commercially available products from places like Fischer CC or Solar Electronics and they work really well, have specified bandwidth and power handling characteristics, built in shielding, robust case, etc.

They also cost a few hundred £$€ each which, if you are on a budget like most people, represents a significant investment for a individual or small laboratory. However, this one can be built very cheaply; most labs will have a development kit with some clip on ferrite cores, if not the core I used only costs £5 from RS.

DIY Current Probe

I’m a big fan of making my own test adaptors and equipment as its a great way to really understand how things work and the compromises in any design. As such I decided to share how I go about making this kind of really useful tool.

It’s primary use is for A-B comparison work; measuring the current, performing a modification and then measuring the current to see the improvement.

It is to be stressed that my version is a crude but effective piece of equipment and does not replace a well designed commercial product. There’s a time and a place to invest in quality equipment and one should use engineering judgement on when that is. For instance, measuring the RF current accurately is definitely a job for a properly designed and characterised device.

If you want to explore RF current transformers in more detail then there is plenty of info on Google, but these links are useful places to start.

Some of the design compromises involved in this low cost approach include:

Core Losses / Insertion Loss

The ferrite material in these cores is specifically designed to be lossy at the frequencies of interest, which will result in a lower reading than a higher bandwidth core and a reduction in the amount of noise on the cable downstream from the noise source. This can in some cases mask the effect you are trying to measure. The commercially available products use low loss, high bandwidth ferrite cores.

A high insertion loss also makes these parts more unsuitable for injecting noise into circuits for immunity testing. they can be calibrated for this task using a simple test setup (to be covered later)

Secondary Turns

Number of secondary turns controls sensitivity but the more you add, the inter-winding capacitance increases, decreasing the bandwidth of the tool. I generally use 5 or 6 turns to start with but I do have a 20 turn part made with micro coax on a solid core which also helps to deal with…

Capacitive pickup

From the cable under test to the secondary winding. Normally a split shield (so that it doesn’t appear as a shorted turn) is built in to commercial products. Guess what, that’s easy to do on this with a spot of copper tape or foil.

Not as Robust

Although a well designed product, the plastic hinges and clips on the cores are not designed for repeated opening and closing. The Wurth Elektronik system of a special key to open and close the core is much more robust at the expense of having to keep a few keys to hand for when they inevitably go missing. However these parts are so cheap and quick to make that a broken clip on core is no real obstacle.

Future Videos

I’ll be following this video with some hints and tips on how to use these devices effectively for finding radiated emissions problems and for looking at conducted RF immunity issues. Stay tuned.

Video and Construction Errata

The sharp eyed of you will have spotted that I originally assembled the BNC connector on the core so that it covered the key-way to open the clamp. I rectified this but didn’t film the change.

Also, you can wrap the wire round the core without removing it from the housing but that means you don’t have a nice flat surface to affix the BNC connector to. It does make it easier to close the clamp however so make your choice.

conducted rf immunity calibration impedance and measurement voltages

When is a Test Level Not a Test Level?

Answer: When you don’t read the standard properly!

I was verifying my EN 61000-4-6 conducted RF immunity test setup after the construction of some new test adaptors and acquisition of some new equipment when came across something that left me scratching my head. I figured it out eventually and updated my calibration procedure with a note but it did have me puzzled for an hour!

Like most conducted immunity signal generators, the one I use combines a signal generator and modulator with a power amplifier and some front panel controls/readouts for performing the basic functions. It also has an RF Input for calibrating Coupling/Decoupling Networks (CDNs) which measures the voltage at the 150/50 ohm calibration adaptor and sets the output voltage of the generator to the correct level. My generator has a LED bargraph display showing the level which provides a reassuring visual confirmation that everything is OK.

 

Confused by Conducted, Stumped by the Scope

Having calibrated my new CDN at 3V, since I had a scope within reach, I decided to run the test but monitor the output of the calibration adaptor with the scope to make sure it was all working OK.

I did not see the expected 3V level, instead the RMS measurement on the scope was 0.5V and the pk-pk was just over 1.5V. I checked my 50 ohm thru termination on the scope input and even swapped it for a different one. My second scope also read the same voltage so it clearly wasn’t the scope. Puzzling.

I swapped the CDN for one that had been previously calibrated CDN and the lower than expected output voltage persists. Try turning up the generator voltage to 10V and I can’t even achieve 3V on the scope. Yet when I swap the connection from the scope to the RF generator it proclaims that yes, that is indeed the level that the generator says it is outputting.

Putting a BNC T-piece in series and monitoring the voltage with the RF input terminating the signal still achieves the same result. Is the generator RF input broken and reading the wrong voltage?

I checked the operating manual of the generator – the cal setup I’ve been using for years is correct. Then I carefully read the standard, focusing on the section that deals with calibration of the test adaptors. All became clear…

 

Open Circuit Voltage vs Loaded Voltage

EN 61000-4-6 specifies the test levels in terms of Uo, open circuit voltage. However the generator level setting part of the calibration is based on a measurement of Umr, the measured output voltage. This is a slightly simplified version of Figure 9 from the 2014 version of the standard showing the impedances of each part of the system.

conducted rf immunity cdn calibration impedances

Tucked away at the bottom of the calibration section is the formula that links the two together.

Uo = Umr / 6

Which yields the following values that the input of the generator or the scope should be looking to measure:

Test LevelUo (Vrms)Umr (Vrms)
110.167
230.5
3101.67

For the measurement, the impedance of the decoupling part of the CDN is big enough that the termination of the AE port is not significant to the measurement, making most of the current flow through the EUT port network. You should be able to open or short the 150 ohm AE port termination and not see the measured output voltage change significantly.

By simplifying the above image and a bit of Ohms law you can clearly see that Umr is 1/6 of Uo.

conducted rf immunity calibration impedance and measurement voltages

Of course these are RMS voltages. If your scope that you are measuring with doesn’t have an RMS function then you’ll probably be measuring the peak to peak voltages. The conversion factor is:

Vpk-pk = Vrms x 2 x sqrt(2)

Which when added to the above table makes life a bit easier.

Test LevelUo (Vrms)Umr (Vrms)Umr (Vpk-pk)
110.1670.467
230.51.4
3101.674.67

 

Panic Over

Armed with this new knowledge I revisited my calibrations to find that yes, everything was measuring correctly. The RF generator, being designed specifically for conducted RF immunity testing, takes care of the divide by 6 in it’s calculations.

As an ex-colleague was often heard to remark “every day is a school day” and today’s lesson was a good one. I hope this article saves you a bit of head scratching next time you are verifying your conducted RF immunity test setup.