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Schaffner/Teseq NSG 5500 test system

New Automotive Test Capabilities ISO 7637-2

The best day is new equipment day 🙂

We are continuing to invest in our test capabilities. As such, the Unit 3 Compliance EMC test laboratory has just acquired a Schaffner (Teseq) NSG 5500 automotive surge/EFT test generator.

Schaffner NSG 5500 test systemWith this, we now have the capability to test your equipment to the ISO 7637-2 standard for automotive conducted transients.

The NSG 5500 will generate the ISO pulses 1, 2a, 3a and 3b, along with the Load Dump and Clamped Load Dump pulses 5a and 5b.

This gives us the capability to support your automotive product development to these standards:

  • EN 50498:2010 – Aftermarket electronics for vehicles – full testing for CE marking
  • CISPR 25 for non Immunity Related Function EUTs
  • UNECE R10.06 (pre-compliance)
  • ISO 13766-1:2018 Earth Moving Machinery (pre-compliance)
  • ISO 7637-2:2011 automotive conducted transients
  • ISO 16750-2:2012 automotive electrical loads (part)

 

Footnote:

Timing is a curious thing. Like two buses arriving simultaneously after a long wait I find things tend to cluster up. This acquisition occurred not long after publishing this blog post on how to test to the automotive standards without an automotive surge generator.

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.

 

 

 

Compromise EFT Test Setup

When the customer supplied cable isn’t long enough to fit inside the standard EFT/B capacitive clamp what do you do?

One answer, for pre-compliance testing, is to make your own clamp from aluminium foil cut to length and separated from the GRP by expanded foam blocks.

The capacitive clamp is not a sophisticated piece of test equipment and a close compromise can be achieved quickly with commonly available lab materials.

Details of a compromise EFT test setup using aluminium foil and foam blocks.

Making sure there is good contact to the GRP from the generator is important which is partly achieved by taping the cable down with some conductive adhesive aluminium tape.

Overall area of the injection plate is reduced by 25% from the standard capacitive clamp plate area. Therefore the injection voltage was increased by 25% to compensate for the reduced capacitance.

Safety warning: don’t touch the foil when the generator is running!

Obviously not good enough for exact testing to the standard but it is within the spirit of the test and will give some useful information.

EMC Immunity Issues with RS-232 to USB Converters

These little converters are super handy to interface between your modern PC or laptop and the simpler, lower technology RS-232 serial port used by many pieces of equipment for control or debug purposes. However, like any commodity item there are design compromises, including EMC ones, that you need to be aware of.

I was recently performing some Electrical Fast Transient (EFT) testing on a customers product and was surprised to observe it failing at quite a low level of injected transient of 200V. It appeared that the whole system crashed when the bursts were applied to any of the digital I/O ports.

Even more confusing was that I’d looked over the schematic and the port protection measures that they had implemented were very sensible with ferrite beads and diode clamps.

A pointer came from observing the front panel of the device with all of it’s indicator LEDs blinking away as if it was working properly. Yet the equipment under test (EUT) wasn’t responding to serial communications and the TeraTerm serial port software was still showing a connection.

Checking through the test setup, I theorised that the RS-232 to USB converter that I was using might be crashing or responding to the EFT pulse as a start bit to a frame. Despite being isolated with a Coupling/Decoupling Network (CDN), when a scope probe was added to the RXD line on the decoupled side of the CDN, a transient with 30V of pk-pk amplitude was visible when the EFT burst was applied.

I tried two other converters that I had in the lab and none of them were happy with this pulse and also refused to work correctly.

a selection of usb to serial converters

So I knocked up a small filter PCB with a pi filter on each line (RXD, TXD and 0V) consisting of 2 x 100pF capacitors and a ferrite bead. The non-line side of the caps was taken to the HF ground plane using some adhesive copper tape (the EMC scoundrel’s last resort!) to return the currents back to the generator and not into the converter.

EFT test setup showing flow of HF current and position of small filter

Success! No more interference and the converter works perfectly.

As an experiment (OK, I got slightly distracted by something interesting) I played around optimising the filter and managed to get it down to just two components – a 100pF capacitor on the TXD and RXD lines of the converter.

Now I know that these devices will be designed to the lowest price point but two 0402 capacitors is hardly breaking the bank! It does make you wonder how they managed to get through their own EMC testing, if at all.

Incidentally when this was later tested in the chamber it had some fairly strong 12MHz harmonics from the USB 1.2 data lines that only just squeaked under the limit line lending further weight to my suspicions of corner cutting and poor design!

So today’s lessons are:

  • Beware of cheap generic test adaptors and EMC issues caused by them – both immunity and radiated.
  • Consider your port filtering carefully. Many I/O interfaces can stand a small capacitor or filter adding to it and the benefits for EMC are significant. It gives a path for interfering signals to the local ground and will also improve your emissions too. The customer who’s product I was testing had such parts fitted; it passed the testing at 1kV EFT without issue (the spec is 0.5kV).
  • Using a fibre optic serial port adaptor would probably have helped here by increasing the common mode impedance of the connection (assuming of course it had been designed properly!)