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conductive contamination underneath surface mount isolated power supply causing line to earth surge failure(marked up photo)

Surge Test Failure Due to PCB Manufacturing Process

We recently had a piece of customer equipment fail the IEC 61000-4-5 surge test at 2kV line-to-earth. There was a loud crack of an electrical arc forming, the unit stopped responding to communications and was making a hissing/squealing noise.

To give it the appropriate technical term, this was “A Bad Thing”.

Using the thermal camera we quickly found several hot components all on the 3V3 supply line that we supposed had been damaged by the surge. The hissing noise was the DC/DC converter in a cycle of burst mode trying to supply too much current before shutting down.

However these were all secondary side components on the isolated part of the system. How did the surge get across the safety barrier? The designer was using correctly rated parts and the PCB creepage distances were dimensioned correctly.

As part of the fault diagnosis process, we used our hot air solder rework tools to remove one of the isolated power supplies providing a low voltage supply to the AC mains monitoring circuitry. Underneath we found this:

 

conductive contamination underneath surface mount isolated power supply causing line to earth surge failure(marked up photo)

 

The samples had been hand soldered by the customer, unfortunately leaving a large amount of solder paste underneath the power supply.

Whilst this was not a short circuit across the safety barrier it did reduce the creepage distance significantly. When a 2kV surge (1.2/50us, 12 ohms) was applied from AC mains to earthed secondary this pollution was enough to cause an arc to form and into the 3V3 supply pin (centre right).

This voltage was enough to fry several components on the 3V3 line, rendering the board inoperative.

 

Lessons Learned

  • Hand soldering prototypes is OK provided you take great care in the process and cleaning the board afterwards
  • Professionally manufactured boards will generally avoid this issue
  • Apply a line-to-earth safety test on your AC mains powered products to check your samples
  • We are going to start a policy of performing a line-to-earth safety test on all AC mains powered products coming into the lab for testing from now on to try and catch problems like this.

 

 

A Bag of Water.

This is a very useful analogy to use when considering an EMC emissions problem, particularly true for radiated emissions in the (often problematic) 30MHz to 1GHz band.

 

Lets get squeezing.

Many of you will have experienced this before. Making a change to an emitting structure inside the equipment by changing the electrical connection between two points results in some emissions going down and some going up.

radiated emissions plot

Then you make another change and this has the opposite effect.

This is like squeezing our bag of water. We can move the water around in the bag much like we can emissions around in the spectrum. The harder we press down in one area, the more it pops up in another.

Emission goes up.

Emission goes down.

 

Reducing the volume

But unless we reduce the amount of water in the bag we will nearly always have a problem. The water is incompressible and it just finds new places to appear.

To achieve this in an EMC context we need to reduce the overall energy in the system.

This could be achieved either by keeping the energy controlled on a PCB away from the radiating structure or by adding lossy components (filters, ferrites, etc) to reduce the amount of energy coupling into the radiating structure.

Changing grounding and bonding within a system without reducing the energy is going to be an exercise in frustration and probably wasted time. Better to address the problem at source where possible.

 

Caveats inbound

There will always be a requirement for us to have to try and achieve the goal of “shaping” our bag of water to fit the radiated emissions limits.

A good example is a manufacturer that has already built a production run of units and needs a quick fix to get them onto the market.

Whilst this is often achievable, there are often significant rework / modification costs involved.

There is also the question of repeatability and consistency. If small changes in bonding of parts can make a large difference to emissions, how can you guarantee that each unit will be compliant? Testing multiple samples can help. As can having good production inspection points during the manufacturing process.

But common mode noise is a slippery customer and these kind of fixes should only ever be considered as temporary pending design changes to address the root cause of the issue.

 

A small plug.

Help is available.

We are really good at this kind of work

We’ve been through the cycle many many times with many many different products.

Using Unit 3 Compliance to help with your emissions problems gets you access to our years of accumulated experience.

Our on site test lab allows us to have a rapid cycle time between analysis of a problem on the bench, developing a fix, and testing in the chamber.

 

Hope this was interesting!

James

Low Frequency, Common Mode, Conducted Emissions

Here is an interesting problem I had working on piece of industrial equipment. The customer had some conducted emissions failures at another EMC lab and needed some help resolving them.

The lessons from fixing this problem was that the first thought is not always the correct one, and that sometimes, all you need is a bit of green-and-yellow earth wire!

 

Outline

A block diagram of the system is shown below with the major components shaded.

An industrial power supply feeds power to the controller (a custom PCB connected to a Raspberry Pi) and to the power measurement board (measures the power consumed by the load).

 

 

Conducted emissions on both the Ethernet port and the AC mains port on the power measurement board were both dominated by a low frequency hump around 700kHz.

 

AC Mains

Ethernet

Notice how the shape or profile of the emissions is almost identical. To my mind, this points towards a single component in the system causing the same noise to be seen everywhere.

 

Simplify First

The first thing I wanted to do was to simplify the test setup as much as possible. I replaced the industrial power supply (often designed for Class A emissions performance) with my trusty Thandar TS3022S adjustable linear bench supply.

The idea here was to eliminate the industrial power supply from my inquiries.

 

 

Wow, what a big difference!

 

So the conclusion here is that the industrial power supply DC output is very noisy, that this noise is propagating through the system, and manifesting as conducted emissions on the outputs via a variety of coupling paths.

 

Differential Mode Filtering

Because conducted emissions noise in this lower frequency range tends to be differential in nature (+ve relative to -ve), my first thought was to implement a differential mode filter on the output of the power supply.

 

 

I’ve got a little filter prototype board that I use in situations like this. This pi filter was made up from two Panasonic FC series 470uF, 25V on either side of a Wurth 33uH iron powder inductor.

 

 

Unfortunately it did nothing to the emissions!

 

Could it be Common Mode?

This sounds like a obvious question to ask in hindsight. Most EMC problems are common mode in nature, I’m just used to thinking about LF conducted emissions as a differential mode problem.

Let’s try a common mode mains filter on the output of the power supply to see if this is indeed the case.

 

 

That’s much better! It looks like the problem was common mode noise after all.

 

This Time It Was Actually A Good Idea…

Common mode noise in this instance is current on both the DC output lines together. But, as I point out in one of my talks, current flows in a loop and always returns to the source. So where is this common mode current returning to? What is it’s reference?

Our common mode emissions measurements are being made in relation to the metalwork of our screened room test setup which is connected to the AC mains Protective Earth (PE).

The AC mains line to each LISN contains a PE connection and, inside the LISN, this is connected directly to the floor of the chamber.

Logically then, connecting the DC negative to the PE on the power supply will provide a shorter path for this common mode noise from the power supply.

 

 

Will this have the desired effect on emissions?

Yes. Yes it does.

AC Mains

Ethernet

 

Conclusion

Ooooooh, bloomin’ common mode noise. Not just for the higher frequencies but lower ones too!

This was a fun half day project fixing this particular problem. Much nicer to be able to recommend a low cost cable assembly than £$€ 20 worth of filter block.

If you’ve got any EMC problems then give me a call, I’d be happy to help.

 

 

dc dc converter emissions before and after with notes on sources

Li-Ion Battery Charger DC/DC Converter – Radiated Emissions Problem Solving

I had a challenging EMC problem solving project in the lab this week.

A customer making a miniaturised 4 cm^3 buck-boost DC/DC converter for Li-Ion battery charging was having radiated emissions issues. The small size meant that adding common mode chokes to filter the input and output connections wasn’t practicable so a more in depth investigation was required.

 

How bad is it?

Here are the emissions for the EUT without any modifications. The green reference trace is the AC/DC mains power supply being used to power the EUT. It is failing the Class B limit (blue) by some margin.

unmodified dc dc converter radiated emissions

Initial Isolation and Investigation

To investigate the emission radiation source (not the cause yet), I placed large clip on ferrite cores around the DC input cable and the battery output cable to reduce emissions directly from the cables.

 

dc dc converter with ferrites on cables

This improves some of the frequencies but not all of them. If the radiation was entirely cable related then this would have dropped the emissions significantly. As it hasn’t, we can conclude that the majority of the emissions are coming from the PCB.

The three peaks we’ll focus on are 180MHz, 300MHz and 500MHz.

Next step is to turn on the spectrum analyser and break out the near field probes. I’ve got a selection of commercial and home made probes but the ones I keep coming back to are the give-away probe cards that I have on my exhibition stand at trade shows.

 

Switching Noise Investigation

The location of the emissions for the 180MHz and 300MHz emissions was initially puzzling. Mostly it was centred around the drain of Q1. If we consider the operation of the circuit, Q1 is turned on permanently in boost mode with Q3 acting as the normal switching element and Q4 acting as a synchronous rectifier. Where is this switching noise coming from?

 

buck boost in boost mode

 

Those of you familiar with synchronous switching converter operation will be shouting at the screen right now. Of course, the answer is bootstrapping.

The high side N channel MOSFETs Q1 and Q4 need a gate voltage higher than their source voltage + their threshold voltage to turn on. In this kind of circuit, this voltage is derived from the switching node via a bootstrap circuit.

This explanation on bootstrap circuit operation from Rohm saves me from re-inventing the wheel.

Even though Q1 is nominally on all the time it still needs to perform a switching operation with Q2 to charge up the bootstrap capacitor powering it’s gate driver circuit.

Checking the datasheet, this switching operation takes just 100ns. That’s very fast indeed and explains the source of our switching noise!

The same bootstrap operation is happening to provide the drive voltage for Q4 but because the boost node is continuously switching this voltage is being provided without such a short switching event.

Due to space constraints it wasn’t easy, but I managed to get the microscope out and modify the board to accept a small but high current ferrite bead in series with Q1 drain.

 

 

500MHz Emission

It didn’t take long to narrow down the 500MHz emissions to the boost output diode D1 with a large amount of ringing on the cathode.

The interesting thing about this diode is that it is only conducting for a very brief period in the dead-time between Q3 turning off and Q4 turning on. Dead time between these parts is set at 75ns, again a very short time period. Good for reducing switching losses, disadvantageous for EMC emission.

 

dead time turn on for the parallel boost diode

 

The part selected for this was a slightly electrically over-rated 40V 1A, SMB packaged part with a reasonable capacitance. Switching 1 amp of current through this part for only a brief period of time before shorting it out and discharging the diode capacitance was causing the ringing to occur.

A ferrite bead was added in series to damp this as the customer wasn’t too keen to head down the rabbit hole of investigating specifying a lower capacitance rated new diode or looking at whether the diode could have been removed altogether at the expense of slightly higher power dissipation in Q4.

Interestingly, this is what the emissions looked like with the diode removed but still with the lower frequency emissions present from the input transistor drain. Note the wideband reduction in emissions above 300MHz.

 

dc dc converter emissions plot with the parallel diode d1 removed from the circuit

Solution

With both of the ferrite beads in place the emissions profile of the EUT was reduced to meet the Class B limits. With more time the peak at 160MHz could be investigated and further reduced but project time pressures and the customer understandably wanting a “good enough” result meant we concluded this investigation here.

 

dc dc converter emissions before and after with notes on sources

The end.

DC/DC converters are often provide a challenging EMC opponent when it comes to radiated emissions. I was glad of the opportunity to work on this project and provide a successful result for the customer. This is the kind of work that I love.

The advantage of being an EMC-consultant-with-a-test-lab combined is that this kind of work can be compressed into hours of work rather than days/weeks oscillating between your lab and the test lab. Problems Fixed Fast!

I hope you found this piece useful, get in touch via the usual channels if you have any questions.

Cheers,

James

 

 

 

HDMI? More like HDM-WHY? Thoughts on Cable Shield Grounding

Ladies and gentlemen, I present this week’s episode of “Crimes Against Cables”

 

Example 1: “I had some leftover components to use”

I’ve seen plenty of interesting EMC “solutions” over the last several years to deal with radiation from cables.

A common one is to separate the shield ground from the signal ground with some combination of components (beads, capacitors, resistors). This approach appears to be particularly common on industrial touch screen display modules for some reason.

poor USB cable grounding suggestions

 

This is (in 99% of cases) a bad idea. I’m not sure what you are hoping to achieve by this and, probably, neither are you 😉

In fact I dedicated a small part of a recent talk to discussing grounds and grounding – you might want to check it out.

 

Example 2: How to Break a Shield

Another notable poor example was an otherwise well crafted piece of military equipment. Shielded connectors and cables all over, it looked like it would be survive some serious electromagnetic abuse (as anything being tested to MIL-STD-461 should).

However,

insulating plastic insert bad idea

insulating plastic insert cross section detail

 

This ends up being not only an emissions problem but an immunity one as well as the cables are just as capable of conducting noise into the shielded case.

This sort of thing can be solved with something like an EESeal type component or by a secondary external screen over the entire assembly.

 

Example 3: Plastic Fantastic

I’ve even seen ferrite cores that were just a moulded plastic lump to appear like cores. Maybe it was a “special” plastic? I never found out, it didn’t help the emissions either.

vga cable ferrite just plastic

But this next one was a first even for me.

 

Example 4 – The Strangest Decision Yet

I was performing a full set of EMC tests to EN 55032 and EN 55035 for a customer. The product had a HDMI interface so obviously there were radiated emissions problems.

The first step as a diagnostic was to use some copper tape to make a connection between the connector shell and the metal back plate – the anodised chassis and EMI gasket material provided was not making a good contact.

This didn’t help so I buzzed the connection with the multimeter to make sure I had some continuity and… nothing.

No connection between the connector shell and PCB Ground.

OK, so there must be a capacitor in series with the shield connection. Fetch the capacitance meter and… 1.2pF.

The board designer had neglected to connect the shield of the HDMI to PCB Ground. It’s a new one for me!

The addition of copper foil to bridge the connector pins to nearby solved the emissions problem but left me wondering why someone thought that was a good idea.

 

I’m going to leave you with this closing thought:

I’ve yet to come across an EMC problem where floating or not connecting a shield ground has improved the situation.

 

 

 

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.

 

 

 

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

 

 

 

When ESD Protection Gets Bypassed

ESD protection is essential to control the Electro-Static Discharge event from damaging sensitive circuitry within a product. But its location within the system needs to be considered carefully and is sometimes not obvious at the schematic level.

I’d like to share with you a great example of this that I found whilst working on a customer’s system. I probably wouldn’t have spotted this without testing but I will certainly have it in mind for future design reviews.

 

The EUT

In this instance, the Equipment Under Test is formed from a 2 part metal chassis consisting a large base and a hinged lid. On the lid there is a membrane keypad that interfaces via a Flat Flexible Cable (FFC) to the front panel PCB. There is a second ribbon cable from front panel PCB to CPU board carrying the button presses to the processor.

The ESD protection is on the front panel cable, next to the point where the unit is likely to be touched – the keys. So far, so good.

system under test showing front panel, esd protection and cables

The base and lid are connected elsewhere via the typical long piece of green and yellow wire for electrical safety purposes. The inductance of this connection (long wire, single point) means that it has minimal effect at the high frequencies present in an ESD waveform. Also, the case halves are separated by a rubber environmental seal meaning there is no contact around the edge of the case.

 

EUT + ESD = ???

So what happens when the EUT is subject to an ESD event? There is no discharge to the plastic membrane keypad on the top and discharges to the Vertical Coupling Plane don’t have any effect. However, when a discharge is made to the seam between the lid and base, something interesting happens.

Because of the conditions mentioned earlier (large seam with a significant, remote impedance connecting the lid to the base) the pulse is free to couple to the internal cable assembly as shown below.

Because the ESD protection is on the front panel display board it is unable to prevent the flow of high frequency current down the cable and into the CPU.

The effect of the discharge is to cause the entire system to reset and eventually the GPIO lines responsible for monitoring the front panel keys were damaged to the point of non functionality.

Analysis

On the face of it, the designers had acted sensibly; the ESD protection was right next to the interface that was likely to be touched by the user. However, the design of the case and the routing of the cable proved to be a problem – something that was not anticipated.

With the addition of some simple capacitive filtering or ESD protection at the point at which the cable enters the CPU board this problem was overcome.

 

Lessons

There are lessons for us all here that I would summarise as:

  1. Consider every cable as a risk, even internal ones
  2. Watch out for cables crossing enclosure seams or apertures where coupling is a risk. Not a dissimilar situation to a PCB trace crossing a split in a ground plane – and we all know how bad those can be, right?
  3. Consider how the PCBs and cables will be integrated within the system through a mechanical design review (with your EMC hat on)
  4. It doesn’t matter how well designed you think your system is, testing is necessary to find these problems

 

 

 

TWITL – Shield Prototyping for Sensitive Detectors

This Week In The Lab: prototyping a shielding can for some sensitive detectors.

The customer’s equipment contained some hazardous gas detectors. Despite a good circuit design, one of these sensors wasn’t too happy when tested at industrial 10V/m levels for radiated RF immunity.

EN 50270:2015 imposes some fairly tight limits on the allowed measurement deviation under immunity conditions (depending on the type of gas).

This “fabri-cobbled” shield proved to be a success and a good proof of concept for the customer to take their design forward.

Despite the less than ideal connection made to the PCB ground plane via the screws it was sufficient to achieve a pass.

copper shield for emc emi

 

stainless steel camera system

TWITL – Underwater Camera System Industrial EMC Testing

This Week In The Lab: a nicely engineered underwater camera and lighting system. All beautifully turned, milled and TIG welded stainless steel, this thing can go deep and withstand some rough treatment. It was seriously heavy!

stainless steel camera system

The exact installation environment wasn’t known. Since it was expected to be operated in harsh conditions we opted to test to the generic industrial standards EN 61000-6-2 for immunity and EN 61000-6-4 for emissions.

A Simple EMC Fix

Just one fix required: under 10V/m radiated RF immunity testing one of the positioning motors wasn’t responding to it’s control signals. The control from user to motor was all digital so interference on those lines was unlikely.

The fault finding process was relatively straightforward this time.

We quickly figured out that the problem lay with the optical sensor that detected the shaft position and set the end stops for the range of motion. It was being triggered by the noise which caused it to think that the shaft was simultaneously at both of its end positions.

A ferrite core around the cable and a decoupling/filtering capacitor on the sensor input to the controller stopped the noise from affecting performance.