Posts

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.

 

 

 

ukca mark

UKCA Mark Updates

Because of the United Kingdom leaving the EU, the CE Mark will no longer be recognised as demonstrating conformity with UK legislation.

Instead the CE Mark will be replaced by the UKCA mark (UK Conformity Assessed) which will be required to sell your products in the UK. This mark can coexist with the CE mark on the same label.

The transition period starts this coming January 2021 and UKCA marks become mandatory for the UK on 1 Jan 2022.

Whilst it sounds like a year in enough time to get everything in order think back to university and how much time you had to finish your dissertation – am I right? Start sooner rather than later, especially if you have multiple products.

unit 3 compliance ce mark to ukca mark transition

This applies to goods sold (“placed on the market” to use the correct term) in England, Scotland and Wales. Northern Ireland will still require CE marking due to the Irish border.

 

Action Stations for UKCA

You will need to create a new “UK Declaration of Conformity” similar to the EU Declaration of Conformity (which you will still need for CE marking). Contact me if you need a template. If you’ve been a customer and we’ve performed CE marking testing for you then we’ll be sending out UK DoC templates for your products before the end of this year.

The EU Technical Documentation that I’m sure you keep up to date for all your products will need an additional section with references to the UK Statutory Instruments (equivalent to the Directives) and Designated Standards. Let me know if you need some help with this.

Add the UKCA mark to your product label. You can find image files on the gov.uk website. It must be at least 5mm high.

It can be applied as a temporary label until 1 January 2023 after which it must be “permanently attached” in the same fashion as you currently apply the CE mark.

The product, or documentation where this is not possible, must have the manufacturer’s name and UK address shown. If the manufacturer is outside the UK, this must be the importer’s address.

 

UK Manufacturers Selling to EU

You are now a “3rd country” and will need an EU Sales Office (assuming you don’t already have one) whose address and contact details will need to go on the EU Declaration of Conformity. Various companies offer an “EU Authorised Representative Service” which can be found with a little searching.

If you use a UK based Notified Body, they will probably have already been in touch to discuss what is happening with your compliance certification. If not, get in touch with them sharpish and ask about your compliance status.

 

Key Dates

1st January 2021

UKCA becomes valid and can be placed on electrical / electronic products to demonstrate conformity with UK legislation.

CE mark enters transition period but is still valid for 12 months.

This transition period applies if you currently self declare CE compliance using an EU Declaration of Conformity (the vast majority of products do this).

 

1st January 2022

CE mark ceases to be valid in the UK.

UKCA mark becomes mandatory.

 

Legal Eagles

The EU directives relating to CE marking are already UK law. SI 2019 No. 696 will modify the below SIs (and more) to add UKCA marking and change the terminology. All compliance documentation must refer to these Statutory Instruments instead of the EU Directives.

Notified Bodies become Approved Bodies.

Harmonised Standards become Designated Standards and use the BS prefix (e.g. BS EN, BS ETSI EN). No list of Designated Standards is available yet, this is likely going to be published around 1 Jan 2021 where the list gets transposed from existing standards.

Most standards change at a slow pace so we’ll have to wait and see how quickly changes to the IEC, CENELEC and ETSI standards filter through to the UK standards list. Certainly no massive changes in technical requirements will happen overnight.

 

References

Guidance: Placing manufactured goods on the market in Great Britain from 1 January 2021 (Gov.uk)

Guidance: Guidance Using the UKCA mark from 1 January 2021 (Gov.uk)

UKCA information from the clever chaps over at Conformance.co.uk

 

 

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, including CE marking and FCC
  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

 

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.

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

 

 

 

By James Heilman, MD - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=34056919

Ventilator Projects and EMC Testing (EN 60601-1-2:2014)

Summary

If you haven’t already, check out part 1 of this blog Part 1: Rapidly Manufactured Ventilator (RMV) Projects and EMC Regulations

We’re going to take a look at the EMC requirements for RMVs, consider some of the risks posed by EMC and propose some methods of mitigating them.

Probably the biggest EMC risks are Radiated RF Immunity and ESD due to their higher than normal test levels.

If you need any fast turnaround design support and testing services for your Rapidly Manufactured Ventilator project then get in touch.

Background

As noted in the MHRA RMVS specification on page 24:

“EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”

So lets take a look at a the EMC tests that might be required for a typical Rapidly Manufactured Ventilator project.

This is with the view of meeting the Essential Performance / Basic Safety requirements of EN 60601 whilst addressing the highest risk items first. This is prioritising speed of testing instead of performing a belt and braces, test everything approach that would be the common approach for Medical Devices.

Emissions

These ventilators are going to be used in a hospital / clinical care environment under medical supervised use and not in a home environment.

60601-1-2 classifies a hospital as a Class A emissions environment for Radiated and Conducted emissions. This means that less time needs to be spent fighting to get the emissions below Class B.

Risk items to radiated emissions could include any brushed DC pump motors as these are notoriously noisy. Ferrite cores may be required around motor cables to mitigate this noise.

Following the design guidelines further down this article for any PCBs is recommended and will greatly assist with reducing EMC radiated emissions.

Most RMVs will be using an off the shelf power supply already approved to EN 60601 for Safety and EMC. AC Power conducted emissions should therefore look after itself and won’t be a significant worry for testing.

For Harmonic Distortion and Flicker, there is an interesting note in EN 60601-1-2 in Annex A

“It is assumed that ME EQUIPMENT and ME SYSTEMS used in hospitals (and large clinics) are not connected to the PUBLIC MAINS NETWORK.”

If this is the case, then Harmonics and Flicker requirements and tests need not apply as these only relate to the public mains network.

Immunity

Overview

Most ventilator systems have no external electrical ports apart from the power supply. They are mostly self contained units. This greatly speeds up and simplifies the testing, and reduces the risk of problems with Signal Input / Signal Output Ports (SIP/SOP Ports in the standard, analogous to a Signal Port from other EMC standards).

Mains borne interference (EFT, Surge, Conducted RF) should be handled by the EN 60601 pre-approved power supply without issue. It will still need checking but ultimately the risk is low.

Dips and Interrupts and the hold up time of the power supply is something that would need considering at the Risk Analysis level to derive the correct immunity criteria for each of the individual tests.

If this needs improving then selecting a slightly larger power supply than nominally required could help. More likely, additional bulk decoupling on the main power rail (e.g. 1000uF) will help maintain the system DC voltage under these conditions.

Immunity Performance Criteria

Caveat: This section is me thinking aloud as I have no domain specific knowledge for Risk Management and Medical Devices. I’m trying to approach this from a common sense perspective to aid anyone working on a RMV.

The function of the EMC Immunity tests for Medical Devices is to ensure that the Essential Functions continue to operate and that Basic Safety condition is maintained.

Normally the immunity performance criteria would be based on the type of EM phenomena being simulated in the test. This is normally Criteria A for continuous phenomena (radiated or conducted RF immunity) or Criteria B for momentary phenomena (ESD, EFT, Surge, Dips/Interrupts). Criteria C only tends to crop up for longer duration power interruptions.

In the case of a Medical Device, maintaining the Essential Performance is the key parameter. If a momentary EM phenomena causes this to happen then this is a major problem.

Therefore immunity performance criteria must be considered for the key function of the device as well as the duration of the EM phenomena.

Based on this thought process, a sensible starting point for the immunity criteria is:

Assume criteria A (unaffected performance) for:

  • Key function of assisting patient breathing for all tests. This includes momentary EM phenomena tests = ESD, EFT, Surge, Dips/Interrupts.
  • Non-critical functions under continuous EM phenomena tests = Radiated and Conducted RF Immunity

Assume Criteria B for:

  • Key function performance for this means that there should be a function in the RVM firmware that remembers its last current operating state and settings and that it starts up in that state from a power cycle. This creates a requirement for programmable non-volatile memory (some kind of EEPROM) in the RVM.
  • All momentary EM phenomena tests for non-critical functions e.g. display readout may temporarily distort or flicker so long as it recovers

Assume Criteria C for:

  • Non-critical functions from momentary power loss e.g. screen/display readout or setting

Immunity Risks

There are two big risks to the immunity performance: Radiated RF Immunity and ESD.

Radiated RF Immunity

Test Requirements

The basic requirement for radiated RF immunity is a flat 3V/m from 80MHz to 2.7GHz. So far so good, this is a fairly easy test to meet.

Now the bad news. Table 9 gives a list of spot frequencies to be tested to simulate close range exposure to common wireless technology standards. The table is summarised here:

Frequency (MHz)ModulationTest Level (V/m)
38518 Hz pulse, 50%27
450FM +/- 5kHz dev.
1kHz sine
28
710, 745, 780217 Hz pulse, 50%9
810, 870, 930 18 Hz pulse, 50% 28
1720, 1845, 1970 217 Hz pulse, 50% 28
2450 217 Hz pulse, 50% 28
5240, 5500, 5785 217 Hz pulse, 50% 9

As you can see, this has testing up to 28V/m, a significantly higher field strength than 3V/m!

Risks to the EUT

This test loves to mess with analogue sensors. In the case of ventilators, the pressure sensors used frequently have an analogue output to a DAC on the CPU. This presents two risk areas:

  1. Demodulation of noise inside the pressure sensor amplifier. This takes the small transducer signals and amplifies it up to the output voltage. Noise demodulated here would cause the carrier to be superimposed on the pressure readings.
  2. The input of the ADC could be susceptible to noise picked up on the analogue voltage from the pressure sensor, even if the pressure sensor itself is unaffected. This will affect the readings.

Since the airflow and pressure sensors are a key component to the operation of the ventilator, these must be protected at all cost.

Design Recommendations

Design ideas to mitigate this interference include

  1. Keep traces/connection as short as possible between sensors and ADC
  2. If you can mount them all on the same circuit board then do so
  3. This circuit board will have one layer dedicated to a solid ground plane fill over the entire plane. All ground pins

    Check out my video presentation on PCB grounding and HF current flow.
  4. Cables = antennas that are good at receiving the interference. Minimise use of cables where possible.
  5. Figure out what your minimum bandwith requirements for airflow are and filter the signal appropriately. You probably won’t need to sample the airflow faster than 10kHz so put a low pass filter right next to the ADC input. Something like a 4k7 and a 1nF will give you a 3dB of 34kHz. This will reduce the risk of RF noise being demodulated by the ADC input.
  6. Decouple the supply lines to the pressure sensor well
  7. Add a small filter to the pressure sensor input, perhaps another RC filter as shown above. This will help prevent the pressure sensor from being affected by the test.
  8. It is possible that the pressure sensor will be directly affected by the radiated noise picked up by the sensor body itself and not by the traces. It would be prudent to provide a PCB footprint for a shielding can near the sensor. I have seen this effect on gas sensors in the past.

Risk Analysis

Assuming that the advice above is followed, the risk to the EUT is manageable.

One of the interesting features of Radiated RF Immunity testing is that of the Problem Band where most issues occur.

radiated rf immunity susceptibility characteristics

Most of the time, the problem band is in the 100MHz to 300MHz area (I’ll cover this in more detail in a future article). Cables tend to be the best antennae at these frequencies and, hopefully, our ventilator only has one cable of interest – the AC power cable. This has plenty of filtering for conducted emissions reduction which should handle this noise.

Probably the two biggest problem frequencies from the spot frequencies above are going to be 385 MHz and 450 MHz.

Then we are into the realms of direct pickup on internal signal cables and PCB traces at higher frequencies. If we’ve laid out our PCB well as highlighted above (short analogue traces, filtering, good ground plane, shielding provision) then this will help mitigate our risks.

ESD

Overview

The levels of ESD testing are almost twice that of the regular EMC standards with a requirement for 8kV contact and 15kV air discharges.

ESD is very good at upsetting digital systems and it has a particular fondness for edge triggered pins e.g. reset lines and interrupts.

Design Recommendations

If the reset line for the CPU controlling the RVM is shared with other digital circuit blocks or supervisory controllers then an RC low pass filter at the input to the CPU is highly recommended. This helps prevents unwanted resets.

Checking can be implemented in the Interrupt Service Routine to ensure that an interrupt condition actually exists, effectively de-bouncing the input.

Thankfully the Ingress Protection requirements for the RVM of IP22 and the requirement to provide flat, easily cleanable surfaces will probably dictate the use of some kind sealed membrane keyswitch panel. These have good ESD immunity as no direct contact discharge can take place on an switch where the plastic covering remains in place.

Whatever user interface technology the RVM employs, this will be a key risk area for ESD. If this is on a separate PCB to the main controller, all interfaces will need some kind of filtering. A small capacitor to ground on each of the lines that goes to the keypad would be a good idea. 0603, 100pF usually works well here.

Lastly on the mechanical design, keeping the electronics well away from the enclosure seams will also reduce the risks of creepage of any discharge into the circuit board.

Summary Test Plan

Emissions

  • Radiated Emissions, Class A, 30MHz to 1GHz (EN 55011)
  • Mains Conducted Emissions, Class A, 150kHz to 30MHz (EN 55011)

Immunity

Text in bold is highlighted as a risk item.

  • ESD, (EN 61000-4-2), 8kV contact, 2/4/8/15kV air. Test to connectors as well.
  • Radiated RF Immunity (EN 61000-4-3)
    • 80MHz to 2.7GHz @ 3V/m
    • Various spot frequencies at up to 27V/m
  • EFT (EN 61000-4-4), AC Mains Port, 2kV
  • Surge (EN 61000-4-5), AC Mains Port, 1kV line-to-line, 2kV line-to-ground
  • Conducted RF Immunity (EN 61000-4-6), AC Mains Port, 3V/m (6V/m in ISM bands)
  • Dips and Interrupts (EN 61000-4-11), AC Mains Port, various

Conclusions

Not only has this article identified key EMC risks to Rapidly Manufactured Ventilators but also provided some design guidelines to dealing with the problems that might arise.

Some of the guidelines within might be useful to anyone designing a Medical Device. We haven’t covered the requirements for Patient Coupled Ports or SIP/SOP ports from an EMC perspective as they aren’t of too critical a concern for an RVM.

We can see how looking at the standard and pulling out the required tests can help us understand the risks involved in the design.

Experience of knowing how the tests will typically affect the EUT is the key to unlocking good design practices. In my case, this comes from having worked on many designs with problems and the learning that comes from fixing the issues that crop up.

Remember EMC test success comes from good EMC design. For a time critical RVM there is one chance to get it right – no do-overs!

I hope you found this article useful. See you when all this has calmed down.
All the best,
James.

By James Heilman, MD - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=34056919

Ventilator Projects and EMC Regulations (UK)

SUMMARY:

1. CE marking probably won’t be necessary for UK for temporary ventilators
2. Still need to apply to MHRA for permission to use in clinical setting
3. This will likely involve the supply of test data to MHRA
4. The requirement for EMC testing is currently To Be Confirmed
5. U3C will give any ventilator design a pro bono EMC design review

In the current Covid-19 pandemic there are many project teams around the world coming up with designs for ventilators. The potential is for many such devices to be required.

The problem that this article identifies is that for normal medical devices, the length of time taken to get products approved is defined in terms of months. This is especially true for ventilators which are, according to several sources, a Class IIb device requiring a robust quality system and Notified Body approval.

The current requirement is for devices to be delivered within weeks.

Recognising this, the MHRA in the UK have published a “Specification for ventilators to be used in UK hospitals during the coronavirus (COVID-19) outbreak.”

Interesting points from this document in relation to the Electromagnetic Compatibility (EMC) characteristics are:

Page 10: “2. It is not anticipated that devices will be CE marked and approval by the MHRA will be through the “Exceptional use of non-CE marked medical devices” route (https://www.gov.uk/guidance/exceptional-use-of-non-ce-marked-medical-devices)”

The procedure covered under the “Exceptional use of non-CE marked medical devices” involves applying to the MHRA directly for a pass to supply this product for use without CE marking. This request will have to include data that shows compliance with the test criteria in Appendix B of the MHRA specification.

Page 11:3. When the current emergency has passed these devices will NOT be usable for routine care unless they have been CE marked through the Medical Device Regulations. The device must display a prominent indelible label to this effect.

Page 24: “EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”

Note that the requirement for EMC testing remains TBC = To Be Confirmed. I strongly feel that at least some testing should take place, but functionality of the ventilator has to come first.

Next article will be a breakdown of the EMC test requirements called up in EN 60601-1-2:2014. I’ll also highlight some risk areas that designers of these ventilators will need to to bear in mind.

If anyone has any need for pro bono EMC design review services for ventilator projects or needs some EMC testing turning around quickly then get in touch.

hello@unit3compliance.co.uk

I hope you are all keeping well during these interesting times.

James