EMC Certification is not just a rubber stamp by the test lab!

“Do You Do Certification?”

“Do you do certification or just pre-testing?”
“Can you certify our products?”
“Can you do EMC testing even though you aren’t accredited?”

The concept of “certification” is an interesting and, judging by these real customer enquiries that we’ve received, a confusing aspect of EMC testing.

The short answer to the questions above is “no, you do, in a way, most of the time, for most things” but like most short answers it isn’t particularly helpful.

To help clear this up, lets have a quick look at declaration vs certification for EMC testing to CE marking (EMCD & RED) and “FCC” CFR 47 Part 15B, lab accreditation and the operating philosophy of Unit 3 Compliance.

In summary:

  1. Unit 3 Compliance test results are valid for a wide range of regulatory approvals
  2. In the context of the CE Mark, there is no such thing as a ‘CE certificate’ or a ‘CE certification’ process
  3. You (the manufacturer) “self certifies” or rather you legally Declare your product to be compliant with the EU Directives
  4. For the USA (FCC) certification does exist but it depends on the product. Many products are exempt from certification.

 

Unit 3 Compliance Test Results Validity

Regulatory Regime

Product Type

Unit 3 Compliance can be used for testing?

CE Marking

EMC or Radio Equipment Directive

Yes

FCC

Unintentional Radiators (Part 15B)

Yes

FCC Unintentional Radiators with FCC Approved Radio Module

Yes

FCC

Intentional Radiators (Part 15C)

Pre-compliance only.

Accredited laboratory required for final test

 

 

CE Marking

When CE marking for selling products in the EU, most electronic products are going to be covered by either the EMC Directive (2014/30/EU) or the Radio Equipment Directive (2014/53/EU). The latter refers to the wording of the EMC Directive anyway.

In all cases, the manufacturer “self certifies” by assessing the product (usually to a Harmonised Standard) and then producing and signing a Declaration of Conformity (a legal document) to confirm that their product meets the Essential Requirements of the Directives in question.

Note that the directive requires the manufacturer to “assess” the product. It doesn’t specifically require testing of a product. However, by testing the product to Harmonised Standards, you gain a “Presumption of Conformity” to the requirements of the Directive.

However, testing is the best way to determine performance; EMC behaviour is largely dictated by parasitic components that are not generally present on the design documents.

It is then up to the manufacturer to ensure that all future products remain compliant through control of production.

Try searching either of these Directives for the following:

  • certificate
  • certification
  • accredited
  • accreditation

and you will find that these words are only used in relation to a Notified Body (NB) or an EU Type Examination Certificate provided by such a body. This approach is only mandatory for a narrow range of products or applications (e.g. where no Harmonised Standard exists for the Radio part of the equipment).

Similarly, there is no requirement to use an ISO 17025 accredited laboratory for any of the assessment activities. Accreditation is managed in the UK by UKAS and as such are sometimes referred to as “UKAS accredited laboratories”. This also includes testing submitted to a Notified Body to support an EU Type Examination Certificate process.

In summary:

  • There is no “certification” of products for CE marking
  • Using an accredited laboratory is not mandatory for CE marking
  • Whilst not strictly required, testing is definitely the best way to determine EMC performance

 

 

FCC

When seeking to comply with the “FCC” requirements of CFR 47 Part 15 for sale of products into the USA, we need to consider the type of product we are making and fit it into one of these categories.

  • Unintentional Radiators are products that can generate RF energy but are not designed to radiate it. Essentially, a product that does not contain a radio (like Bluetooth or Wi-Fi). Examples would be a power supply, a desktop PC, etc. (defined in 15.13 (z))
  • Intentional Radiators (defined in 15.13 (o)) are products that intentionally emit RF (e.g. mobile phone, Wi-Fi router)

A complicating factor are Radio modules that have undergone a Modular Approval process (15.212). This is an easy way to add radio functionality to your product. These have already been reviewed by a Telecommunications Certification Body (TCB) and approved by the FCC.

Provided an approved module is installed into your equipment in line with the OEM instructions then your responsibilities as manufacturer are to verify that the combination of Unintentional Radiator and Radio Module do not infringe any radiated emissions limits.

15.101 shows the paths available (SDoC or Certification) for different types of Unintentional Radiator.

2.906 Self Declaration of Conformity can take place in any test laboratory whereas 2.907 Certification has to take place in an FCC registered laboratory (must me nationally accredited to ISO 17025). In all cases the provisions of 2.948 measurement facilities apply.

 

 

Accreditation

Accreditation of laboratories is a slightly different subject. Accreditation is a method by which the test procedures of a test laboratory are verified by an independent 3rd party (e.g. UKAS in the UK) to be compliant with ISO 17025.

Similar to ISO 9001, ISO 17025 is a quality management system that demonstrates a laboratory is operated to a certain standard. 17025 also extends this quality system to the tests being carried out where the individual test procedures and personnel are checked by an external assessor.

This is useful to demonstrate competence of the lab to their customers. It also demonstrates (but does not guarantee) the quality of test results have met a certain agreed basic standard. Some manufacturers choose to always use accredited laboratories for their testing for a variety of reasons e.g. their quality policy might dictate it.

 

At Unit 3 Compliance, we choose not to be an accredited laboratory.

Accreditation costs a lot of time and money in fees, inspections and internal paperwork. This cost ultimately gets passed on to the customer. By remaining un-accredited we can keep our fees around 33% less than an accredited laboratory.

Many accredited labs subscribe to a business model of employing multiple technicians to perform the day to day testing whilst retaining a couple of engineers for consultancy and compliance paperwork. The operation can end up as a bit of a sausage factory – seeking to have a full calendar of testing and turning the handle as quickly as possible.

The fallout from this is that test reports often take a a back seat and are delivered weeks after the testing has been completed and in the event of problems you might not have time in the relevant test area to perform diagnosis of the problem before you are hurried out for the next customers’ scheduled test to take place.

Most people at the test lab have been working in that environment for most of their working lives. This makes them very capable at performing the tests but their lack of experience with product design means the staff are frequently not as capable of

You might get informal suggestions of “try and improve the shielding” or “you need a ferrite on that” but beyond that the likelihood of getting good quality problem solving advice is low.

I certainly don’t want to tar all accredited labs with the same brush. There are good labs and good engineers out there. However with some labs it can be pot luck whether you get Technician A (interested, helpful, keen, knowledgeable) or Technician B (uninterested, jobsworth, clock watching).

Whilst many accredited labs do have experienced personnel on site, getting access to them in a time or cost sensitive manner is often hard. Because of the requirements of accreditation and the need for impartiality, many labs run their consultancy services as a separate division within the company. Sometimes they aren’t even in the same building as the test lab! Inevitably these services have to be accessed outside of the test cycle leading to delays.

 

comparison of emc lab capabilities

 

How we operate

 

Unit 3 Compliance is not a sausage factory. Our motivation is doing interesting work and solving challenging problems for people who care about their products.

We are significantly cheaper than an accredited lab, putting EMC testing within the budget of startups and smaller businesses. It also makes it more economical for medium to larger companies to run ongoing quality control checks, product cost down exercises and experiments.

We have a strong product design background, particularly in design for EMC. We can suggest, trial and optimise EMC fixes during the test process rather than send you back to base to figure it out for yourself. These fixes take into account the nature, volume and cost of the product – there’s not one fix that is suitable for all applications.

First time EMC pass rates are generally low. Of all the products that I’ve tested, less than 20% have passed first time. Many of those passed because we reviewed their design first from an EMC perspective and made suggestions for improving the design.

Because we have a strong background in fixing EMC problems and not just testing, we can resolve your EMC problems faster than anyone else. This is not an idle boast but something we genuinely believe. Every problem we fix makes us faster and better next time and this compounding experience is available to you.

We turn every test session into a miniature EMC class, explaining the tests, why we perform things the way we do and how it sits into the larger framework of standards, directives and compliance. We work hard to acquire our experience and love to share it with our customers.

If you’d like to benefit from this then get in touch.

 

 

 

References

 

ESD Latch Up Behaviour in Diodes Inc. Power Switch Parts

A new customer came to me with their product that was having problems during testing at another laboratory. There were radiated emissions problems (mostly solved with improvements to the ground plane scheme on the PCB) and a very interesting (and challenging) ESD problem which I’ll cover in this blog.

Here was the device exhibiting the problem, a Diodes Inc AP22802AW5-7 “power distribution load switch”. Input VBAT from a stick of AA batteries, SW_PWR from a rotary switch, and output to the rest of the circuit.

Problem outline

The ESD problem was described by the customer:

The EUT stopped working when 4kV contact discharges were applied on discharge point shown. I removed the batteries and I put them [in] again and there was not any response from the sample (no otuput and the green LED remained OFF).

[A second sample] was then tested with the same result, although this time not on the first discharge

Upon inspection both devices had failed due to the load switch (AP22802AW5-7Diodes), with one failing open and one failing short and both becoming very warm.

ESD diode placed on input and output of load switch (with no effect)

ESD diodes placed on all [discharge points] (with no effect)

ESD diode places on VCC close to pullup resistors for [discharge points] with no effect

First thing first was to get the product set up on the ESD table (with a bit of added blur to protect the innocent).

It was very easy to re-create the problem observed at the original test lab with the second contact discharge to the EUT exposed contact point causing the unit to shut down.

In each case, the power switch was failing low resistance from IN to GND. The initial theory was that the device was being damaged by the high voltage punching through the silicon layers leaving a conductive path.

 

Eliminate the possible

I made a series of experiments to determine the coupling path into the problematic device. Working on the principle that, because of the 15cm distance between discharge point and problem device, that conduction might have been the problem.

  • Capacitors on Vin and EN
  • plus disconnect EN line
  • plus ferrite beads and capacitors on Vin, Vout and EN
  • plus local TVS diodes on pins of device
  • plus ferrite beads in series with [EUT input] lines

Whilst none of these experiments were successful they certainly helped eliminate conduction as the coupling path.

Because of the very high frequency content of the ESD pulse, capacitive coupling is likely going to be the dominant coupling method. Whilst it could couple into the device directly, there was more opportunity for the pulse to couple into the traces connected to the device first. Filtering the inputs eliminates two coupling possibilities

 

Change of sample

The PCB was starting to get a bit tired from the repeated hot air SMT de-soldering and re-soldering so I swapped to another supplied sample. To be able to operate the unit out of the casing I swapped to a linear DC bench supply instead of the AA batteries.

This proved to be an interesting mode as it allowed me to kill the power quickly. The next set of experiments were in an attempt to reduce the effect of capacitive coupling to the problem device.

  • Improved ground stitching / connection
  • Changing supply voltage
  • Indirect HCP discharge – not to EUT but to the Horizontal Coupling Plane albeit with a vertical ESD gun to increase capacitive coupling to EUT.
  • Reduction of coupling into Vin terminal by removing components and copper
  • Addition of copper foil shield over the top of the device

 

Failure mode discovery

Setting the current limit on the DC supply to a fairly low value (about 20% higher than nominal current draw) was a good idea.

When applying the ESD strikes the supply went into foldback as the EUT power input went low resistance. I discovered that quickly turning off the power and then turning it back on effectively reset the failure mode of the device. This proved to be repeatable over several discharges: zap – foldback – power cycle – EUT OK.

What silicon component behaves like this? A thyristor.

This is a phenomena known as “latch up” where the parasitic thyristor structure present in the CMOS process fires due to over voltage… such as an ESD strike for instance!

Because the device is only small the power dissipation caused by the battery short circuit current is enough to “pop” the device through overheating.

 

Out of circuit testing

Whilst it doesn’t get used very often, my Sony Tektronix 370 curve tracer is perfect for testing components like this.

(not mine, picture From CAE Online)

Here’s the VI curve of an undamaged device. It’s a bipolar voltage between VIN and GND. On the left of centre is the standard forward biased body diode. On the right is the reverse biased breakdown of around 8V.

Now for a damaged device. In this case the current changes quickly for a small applied voltage and there is no non-linear characteristic. Essentially, a short circuit.

Turning up the maximum voltage that the curve tracer can apply and dialling down the series impedance allowed me to simulate the over voltage fault condition and create a latch up condition. This latch up wasn’t permanent due to the bipolar sine wave nature of the curve tracer applied voltage.

However turning up the voltage enough to cause excess power dissipation inside the device did result in the same failure mode using the curve tracer.

 

Summary

I have never encountered a device that is this unusually sensitive to ESD events before. A nearby 2kV discharge on the PCB top layer ground plane was enough to cause the latch up condition.

I noted in the report to the customer that this device had been changed to “not recommend for new designs” by Diodes Inc. I wonder if they identified this condition in the device and withdrew it for that reason.

The customer resolved the issue by replacing the device with a different part and we all lived happily ever after.

The end.

 

 

 

Standards Update for Bluetooth/Zigbee/WLAN Radios

There’s a new v2.2.2 revision of ETSI EN 300 328 – the 2.4GHz radio standard for Bluetooth, Zigbee and WLAN/Wi-Fi.

The main difference appears to be an increase in the Receiver Blocking requirements in 4.3.1.12. Blocking signal power increases from -53dBm to -34dBm.

Transition starts on 6th August 2020. The V2.1.1 standard ceases to give a Presumption of Conformity on 6th August 2021.

What does this mean for your products?

If you use a CE marked Radio module in your product then get onto your manufacturer and ask what their plans are to demonstrate compliance to this requirement.

If you have your own radio solution on board then you should investigate additional testing to determine if you are compliant or not. Unit 3 Compliance can assist with testing if required, get in touch to see how we can help.

You’ll also need to update your Technical Documentation with details of the new standard.

References:

Overview of changes (Laird)

Overview of changes (Element)

Old standard v2.1.1

New standard v2.2.2

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.

 

 

 

a roll of Wurth Elektronik copper tape - the scoundrels last resort?

So You Want To Be An EMC Engineer?

 

“Abandon hope all ye who enter here”

– Sign above the door on any EMC lab.

 

I’ve been asked a couple of times for career advice in relation to EMC. How do I get into EMC in the first place? How do I progress, perhaps moving from testing to design? Where should I take my career?

I’m generally sceptical about people who offer career advice. Much advice tends to be parochial “do this and you will succeed”. It is based entirely on what the person giving the advice thinks you should do (even if they never did it themselves.

Everyone’s upbringing and experience is so different there is no “one size fits all” approach to any career.

I can only share what I have done.

Maybe it will help.

 

Pre-Flight Check # 1: Make sure you are in the right career

Too many people are guided into careers like doctor, lawyer, engineer that might not be the best fit for them.

Make sure that engineering is right for you.

If you aren’t sure (and that’s OK) then writers like Tim Urban (career advice featuring the Yearning Octopus and your mum in disguise – long read but worthwhile) or James Altucher have lots of thought provoking advice for you.

I think being an engineer is more of a vocation than a job. If you cut most engineers through the middle it will say ENGINEER like a stick of Blackpool rock (a very British analogy). The chances are, if you are reading this, you are already in this category.

 

Pre-Flight Check # 2: Be honest about your reasons for wanting to get into EMC

Why are you wanting to get into the world of EMC?

Wanting something impressive on your CV? Think it might be a good way to get to that promotion you’ve been after? Probably will, but if these are your only reasons then you might be frustrated by the learning curve associated with the field.

One good answer is “it sounds really interesting.” If these are your thoughts then you are not wrong. I think it is one of the most fascinating fields of electronics.

In my case I was cheesed off with working in project management where I was spending less time with my soldering iron and more time in bullshit meetings. An opportunity for an EMC engineer came up in the organisation I worked for and without even thinking about it too deeply I said “I’ll do it”.

Best snap decision ever!

 

Pre-Flight Check # 3: You don’t have to be mad ^H^H^H enthusiastic to work here but it helps.

Whenever I solve an EMC problem I will generally do a little dance. It really floats my boat.

I’m lucky because I get to do what I love and people pay me. Most days I feel like I’ve won the lottery just for doing my day job.

If you don’t love the work (and it can be difficult) then its an excercise in frustration.

Try and follow what makes you want to dance in the middle of the lab. This is a fantastic lens for discovering what it is you are meant to be doing with your career.

 

General Skills: EMC is a Holistic Discipline

I spent the first 7 years of my electronics career working on…

  • power supply design
  • microcontroller coding
  • thermal CFD simulation and design
  • basic mechanical design
  • high speed digital design and test
  • system level architecture
  • cost sensitive design
  • project management

…before I became an EMC engineer. Before even realising I wanted to be an EMC engineer.

I still regularly use ALL these skills in my job as an EMC engineer.

Product design decisions made impact EMC performance.

EMC decisions impact product performance (and cost).

The two co-exist and cannot be separated.

Understanding the compromises of product design, the interaction between competing aspects (particularly cost!) is incredibly useful.

 

Go to the place least crowded / Leverage your existing skills

It might be that your team/employer/company has no EMC engineer. Take on that responsibility. This is what I ended up doing and now, 13 years later, I still love what I do.

Perhaps you have an EMC engineer colleague. Arrange to sit on their shoulder and talk to them. Ask lots of questions. Find out what area they don’t have time to work on or what problems they have. Work on that.

You are a member of an EMC team. Again, what areas do the team struggle with? What area consistently causes problems? No one is an expert on the finer points of widget calibration and the effects of temperature. Become that expert.

Find a niche (rhymes with quiche dammit) and fill it. You get to progress and inevitably find something else interesting to work on.

Follow your curiosity!

 

Get good at fixing EMC problems / make mistakes

Another fundamental truth of EMC is that There Will Be Problems.

Problems present a (usually) unique learning opportunity. Every problem I’ve solved has either taught me something or reinforced some piece of existing learning.

Spend a time in the test lab experimenting and getting an understanding of what works and what does not work.

All experiments are useful. Failed experiments or inconclusive data can help you refine your thinking.

This also leads on to mistakes. I make mistakes on a daily basis. They are usually small and easily correctable but sometimes they are bigger. Like the time I fried a piece of customers equipment by supplying 28V instead of 7.4V. Mistakes are hard teachers but you don’t forget the lesson in a hurry.

Importantly, people remember the mistake less than what you did to fix it. Own your mistakes.

 

Understand how HF current flows

In my opinion, this is the key to understanding EMC.

I recorded a presentation which might help your understanding but others have written about it before me and better (Henry Ott for instance).

Once you can visualise this you can understand the WHY behind so much of EMC.

 

Cultivate a Tolerance for Frustration

I would describe being an EMC engineer as alternately frustrating and elating.

You get better at dealing with the frustration of a problem and at solving it quicker.

Sometimes the scope of a problem is outside of your remit of available tools or skills to fix. Learn what you can and try and figure out a way forward.

 

Learn to automate

One of my favourite articles is Don’t Learn To Code, Learn To Automate.

EMC is no different to any other job, there will be repetitive tasks to perform.

Automating tests frees you up to work on other things and makes your work more consistent. Plus it gives you an opportunity to make a cup of tea whilst running a test. Maybe even a biscuit.

Automation doesn’t always go to plan or work out to be time efficient so pick your targets carefully.

 

Study Widely

Attend courses, webinars, lectures, presentations. Eventually some of it will sink in.

Sometimes you aren’t ready to grasp a piece of knowledge because you don’t have the existing framework for it to the idea to fit into.

Be wary of accepting everything at face value. Specific examples are sometimes presented without context or as globally applicable.

 

The learning never stops

I’m still trying to wrap my head around the intricacies of Power Distribution Network design, LabView coding for test automation and how antennas really work.

 

Share knowledge

Give a presentation to your colleagues about an EMC topic.

Explaining something complex to others in a simple fashion is the best marker as to how well you understand it.

I always spend lots of time on any talk I’m giving to try and make it as simple to understand as possible whilst still being useful.

 

Professional Accreditation

You may have the option of working towards accredited engineer status like the Chartered Engineer path through the IET here in the UK for example.

There are also the independent iNarte certifications which are particularly relevant for our field of work.

Some industry sectors or larger corporation might prefer you to have these qualifications. It certainly shows that you have achieved a certain level of competence and have been vetted to a certain extent by a 3rd party.

Find out what is expected or in your industry sector

I have no strong feelings either way on these professional qualifications. I investigated both whilst I was establishing Unit 3 Compliance and decided that I didn’t have the time to commit to them whilst I was setting up the business.

For me, there’s always something more impactful that I can be doing for my business than getting a piece of paper that might only make a small difference to one or two customers. I want to make a big difference for all my customers.

 

Connections and Groups

People to follow on LinkedIn

Groups on LinkedIn. Both of these are fairly active with some knowledgable members.

Other groups to join:

  • The IEEE EMC-PSTC email reflector is excellent with lots of good questions and answers on the subjects of EMC, safety and general compliance
  • IEEE EMC Society of UK and Ireland have bi yearly meetings
  • If you are in the UK, ICMA-TEL have a good email reflector with a diverse range of content including EMC, global market, safety, ROHS. Monthly meetings, mostly in the south of the UK.

 

Bonus: Copper tape is the scoundrel’s last resort

Useful as a diagnostic tool or emergency patch but not as a long term solution 😉

 

Fin.

Thanks for reading this far. If you have any ideas for what else could be included then drop me a mail.

That’s it from me. All the best on your journey.

.James

 

 

 

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.

LED Lights, the “Bulb Ban” and EMC / Radio Interference

I’ve just published a new article on LinkedIn titled LED Lights, the “Bulb Ban” and Resultant EMC / RFI Issues where I look at potential EMC effects from increased adoption of LED bulbs following recent EU legislation.

If you check it out I’d be interested to hear your thoughts. Please hit comment on the LinkedIn post.

Use of an LCD back panel as an image plane to reduce radiated emissions

EMC Radiated Emissions Fault Finding Case Study

I’m really happy to have one of my blog articles featured on Interference Technology.

Problem solving and fault finding EMC problems, especially radiated emissions, is one of my specialities and oddly enough is one of the facets of my job that I enjoy the most. After a successful exercise in helping a customer out with their product, getting the chance to write about it and share it with you is a real bonus.

Fixing radiated emissions is at it’s most challenging when the scope for modification to the unit are limited by the fact there are significant stock of PCBs or components that would require scrapping and redesign. Finding a way to use the existing stock was key in this example as the customer had significant time and money invested into the project. Thankfully I was able to help them out.

Head on over to Interference Technology and have a read through – I even put pictures in! Hopefully it will give you an idea of how I work and the sort of EMC issues that I can help you solve.

Case Study: Poor PC Board Layout Causes Radiated Emissions

 

Case Study: AC Mains Input EMC and Safety Troubleshooting

Many of the customers I deal with are technically savvy and extremely good at designing innovative and clever devices. I’m always learning something new every time I get a different product through the door. Unfortunately it isn’t practical or possible to be good at everything and EMC expertise, especially when it comes to fault finding and problem solving, can be hard to come by. This is where I come in.

I’ve been helping a good customer on a product that they’ve been working with that had some EMC troubles on a prototype design. It had originally been taken to a different test lab where they had performed a mains conducted emissions measurement showing a clear failure at low frequencies. There were a couple of other hard copy scans supplied where a capacitor value had been adjusted to try and improve the emissions but with no effect.

In need of some expertise, they got in touch.

Mains Conducted Emissions Testing

I received the product and quickly set it up in our screened room to perform some EN 55014-1 conducted emissions measurements. Below you can see the first scan result, showing a failure of up to 10dB on the Quasi Peak detector. There’s clearly some room for improvement so let’s analyse the problem and see what we can do.

mains conducted emissions - before

Our starting point for the improvement work

Lower frequency mains conducted emissions are not uncommon and are usually caused by differential mode voltage noise. This is generated by current flowing through the impedance presented by the primary side bulk decoupling and switching circuit. The switching frequencies of the power supply controller are usually in the 30 kHz to 250 kHz range putting it (and it’s harmonics) right in this lower frequency (sub 1MHz) range for this test.

Improving differential mode noise can be done in a number of ways. Removing the noise at source is the approach I advocate, in this case this can be achieved by reducing the impedance of the rectified mains bulk decoupling capacitor. A review of the BOM showed that the units had been built with some general purpose electrolytic capacitors with a relatively high impedance. So the first thing that I did was to swap out these parts for ones from the Nichicon PW series of low impedance electrolytic capacitors.

after fitting low impedance bulk decoupling

Changing the electrolytics to a low impedance variety

The result: a big improvement on the QP measurements, bringing some of them down by around 10dB. The improvement on the Average detector readings was less pronounced, especially around 550 kHz where only a 3dB improvement was registered. It is likely that the HF impedance of the decoupling capacitor is still a problem. One option is to apply a suitably rated high frequency decoupling capacitor in parallel with the bulk decoupling capacitor. The other option is to improve the filtering on the AC mains input to prevent the noise from escaping back down the line.

Filtering for differential mode noise can be provided in several ways. The most common method is to make an LC filter from the leakage inductance of a common mode choke paired with a Class X safety capacitor between Live and Neutral. The leakage inductance is in the tens of micro-Henries whereas the common mode inductance is often a couple of magnitudes larger up in the tens of milli-Henries. Simplistically (there are other effects to consider) a 10uH leakage inductance paired with a 470nF capacitor will roll off frequencies above 100 kHz. Well, let’s try that!

now with added class X cap

Now with an additional 470nF Class X capacitor soldered across the mains input terminals

Performance is improved by around 5dB across a wide range of frequencies; indeed the improvement can be seen up to 15 MHz. This leaves a margin of around 2dB to the average limit line which is perhaps a bit close for comfort and I would generally recommend looking at a little more filtering to bring this down a bit further to allow for variations in production and tolerance of components. Options for further improvements could include a second Class X capacitor to form a pi filter but because of the low impedance of the differential mode noise this approach might not be as effective. Adding some inductance to form an LC filter with the bulk decoupling capacitor is another approach.

However this proved the case to the customer for a PCB redesign to make space for the larger bulk decoupling capacitors and at least one Class X capacitor.

Surge and Safety

Following on from this work, at the customers request, I carried out a full suite of EMC tests on the product to EN 55014-1 (emissions) and 55014-2 (immunity). One thing that I noticed was the sound of an electrical breakdown during the application of a differential mode surge test. Taking off the outer casing, I managed to catch the below arc on camera during a 1kV surge event.

Arcing caught on camera

Snap, crackle and pop.

The arc appeared around the resistor; desoldering and removing it from the PCB showed a couple of points where there was arcing between the resistor body and the trace running underneath it.

Arcind damage to the PCb to surface

Arcing evidence on the PCB

This problem has occurred because the resistor R1 is in series with the Live phase and the trace underneath is connected to the Neutral phase. When mounted flush to the PCB normally, the resistor has only its outer insulation between live and neutral. Reviewing the relevant electrical safety standard for the product requires a minimum clearance (air gap) for basic and functional insulation is 1.5mm. This can be achieved by standing the resistor up on spacers to keep it away from the PCB but then it starts to approach VDR1 and Q4 meaning a considered manufacturing approach is required. This was another incentive for redesigning the PCB.

The take-away lesson from this finding is to consider the Z axis / third dimension when reviewing a PCB as it can be easy to see things purely in two dimensions!

I hope you found this case study useful and that it has given you some tools with which you can improve your designs.

If you need some EMC fault finding expertise then get in touch: I’d be happy to help and I love a good challenge!