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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

EUT Monitoring Hardware

For Equipment Under Test (EUT) monitoring during EMC tests we’ve adapted a Digilent Analog Discovery 2, added an input filter board, and enclosed in a nice case from Lincoln Binns.

This connects to the (rapidly evolving) Monitor-o-Matic 8000 software mentioned in a previous post.

This adaptor and software is going to be used this week during testing of a piece of industrial equipment. This test adaptor will be monitoring both 4-20mA and relay outputs (using the in built power supplies to generate the voltage on one pin of the relay contacts and the digital inputs to monitor the other pin).

 

Analog Discovery 2

The AD2 is a very versatile piece of kit with a good balance of analogue and digital input and output for a reasonable price. It includes

  • 2 x 14-bit, 30MHz differential scope channels
  • 2 x 14-bit, 10MHz waveform generators
  • 16 x digital logic I/O with pattern generation and logic analyser
  • 2 x programmable power supplies

 

Future Plans

We’re going to be monitoring how well this device performs for monitoring during testing. We may need to add extra filtering beyond that already fitted such as optical isolation for the digital inputs.

We may also look at fitting a battery and a USB to fibre optic converter for fully isolated measurements in a variety of EMC environments.

 

 

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

 

 

 

EMC Immunity Testing EUT Monitoring Software

One of the hardest parts of EMC immunity testing is monitoring EUT (Equipment Under Test) performance. Not that it is hard-as-in-complicated but it is hard-as-in-difficult.

Concentrating on a display of figures scrolling past looking for small deviations in one or two characters sounds easy, but try doing it for a couple of hours straight whilst doing Radiated RF Immunity testing and you will be fighting an itch to defocus, stare off into the distance or check the news on your phone.

Go on, ask me how I know  😉

Not ideal when you only have a short (think a few seconds) window to catch potential problems or if you have multiple screens to monitor.

 

Introducing the Monitor-o-Matic 8000

To remedy this and improve the quality of our testing we’ve written a simple application in LabView to handle logging and display of data captured from the EUT during testing.

 

 

Specifications

  • COM Serial input to monitoring PC from EUT. all standard serial port baud rates and configurations supported
  • Use USB to RS-232 or RS-485 adaptors to connect serial port to EUT
  • Extract values / parameters from data stream
  • Plot numeric values on graph
  • Record min and max values seen during test to determine if EUT meets appropriate performance categories
  • Logging of all data during test (all data will be made available as part of any immunity testing carried out at U3C for post testing analysis)
  • Alerts/alarms for data that exceeds defined performance limits. These can be set to latch on in case of problems to prevent missed alarms

 

Use Requirements

1) EUT has the ability to output serial debug ASCII text data for all key parameters like

  • analogue sensors (e.g. temperature, pressure, humidity, light, voltage, current, etc)
  • digital I/O values (e.g. High/Low, True/False)or system status
  • raw digital values read from other parts of EUT
  • checksums from memory
  • whatever other parameters that you need to monitor to ensure the EUT is working as intended during the tests

2) Format could be human readable text, comma delimited, JSON, XML… whatever gets the job done for you. So long as the values are extractable from the text using regular expressions we can log and plot the data.

3) These can either be output as a continuous stream of data that the MoM8000 software will parse, or the EUT could require separate commands to read each parameter. If you can send us an example serial output ahead of time we can get the software setup before your arrival so that no testing time is wasted during setup.

4) We also need to know what performance limits you might have (e.g. temperature deviation of +/- 0.5C) so that we can enter the appropriate limits. This notification is key as it lets us quickly evaluate EUT performance to the Immunity Criteria (A/B/C) in the appropriate standard.

 

Future Additions

We’ll be adding extra functionality to this software over time when we develop new requirements. This includes:

  • Subscribe to MQTT topics on local or remote server
  • Read HTTP data
  • Read text data file on local network
  • Tighter integration of test equipment and software to speed up EMC tests

Discuss with us in advance if you have a special requirement for testing and we will do our best to accommodate you.

Compromise EFT Test Setup

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

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

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

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

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

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

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

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

Networking Equipment – EMC Radiated Emissions Problem Solving

I had an email from a customer that I’m working on some design consultancy work with, saying that one of their prototype products was having some radiated emissions problems at an accredited lab. Could I take a look?

Absolutely, EMC radiated emissions problem solving is my favourite part of the job! Ironically, it is usually the customers least favourite part!

Thankfully I had a slot free the next week so they bundled their kit into the car for the long drive “Up North” from their base in the South West of the UK.

After some tinkering, the equipment was set up in the chamber for some radiated emissions work. The first scan confirmed the problem levels and frequencies that had been observed at the other laboratory.

The problem areas from their last scan were at 35MHz, 80-90MHz and a broad band between 150MHz and 220MHz.

 

System Overview

The system was housed inside a nice aluminium case that was being used for CPU heatsinking and environmental protection as well as EMC shielding. A rough diagram of the internals shows a main PCB with a large CPU / memory block in the centre and a variety of cables leaving the PCB and the casing.

The main power cable housing also had two debug connections inside the same housing that weren’t being used in the field but were available for updating software and such like.

rwns overview diagram

As is so often the case, this product was in it’s final stages of the development life cycle, meaning that no major design changes were possible. These EMC problems would have to be resolved using easy to fit additional components. Thankfully I have plenty of things in stock to try out.

 

Emissions Analysis

There are two important characteristics about these emissions that show us where to look

  1. They are predominantly broadband, an indication of analogue noise e.g. DC/DC converter / power supply. Sometimes this broadband noise is generated by digital switching but this can be less common.
  2. They are all low in frequency, where large or long structures are the most efficient antennae. This usually means cables.

So power noise and cables…. hmmm…. any good ideas?

 

OK Kids, Let’s Take a Look at the Cables.

In a very sensible move by the designer, both the DC power and Ethernet cables had some common mode filtering on the PCB.

Ethernet magnetics have common mode chokes built into the transformer stack which reduces the noise emitted and increases the susceptibility performance of Ethernet despite the often unshielded twisted pair cables used.

The caveat is that once the cables have left the magnetics that they must be protected from other interference sources. Noise coupling on to these lines is going to be heading straight out of the enclosure using these lines as the antenna. Similarly, if common mode noise gets onto the centre-tap of the output side of the magnetics then this can also cause similar issues.

I have experienced system noise coupling on internally routed Ethernet cables before and it nearly always results in lots of low frequency emissions.

The power cable had a small surface mount Murata filter in place with excellent attenuation at the frequencies of interest.

murata filter characteristics and equivalent circuit

Both the Ethernet and power cables pass through the shielded enclosure with no connection or filtering to the case. In bypassing the quite nice Faraday cage of the enclosure, any noise current on these lines will inevitably appear as radiated emissions and be picked up by the receive antenna..

Now to find out some more info.

 

Radiated Emissions Experiments

First, unplugging the Ethernet cable dropped the emissions significantly from 30MHz to 120MHz.

Secondly, some messing around with ferrite cores on the power cable reduced the 150MHz to 220MHz hump down to sensible levels.

This left a single peak at 270MHz that was traced to noise using the coaxial RF cables to the antenna to radiate.

Lets look at each of the points in a bit more detail:

 

Ethernet

The only practical method of dealing with the Ethernet emissions was to change the bulkhead connector to a metallic screened version and the external cable to a SSTP (Screened Shielded Twisted Pair) type of cable. No exciting analysis here I’m afraid.

 

Details of the Power Cable Noise Coupling

The most interesting coupling mechanism was happening inside the un-screened bulkhead power connector. Thanks to the power filter on the PCB, there was very little noise being conducted back down the cable from this line. However, the debug connections to the CPU are picking up all kinds of noise and carrying that noise to the connector.

rwns cable coupling close up

Disconnecting and bundling the debug cables near the connector cuts the radiated emissions down to next to nothing.

What’s most interesting is that the capacitive coupling region between the power cable and the internal debug cables is so small. The connector is only 20mm long and the cables run parallel with each other for barely any distance. And yet there is enough noise current being coupled onto these lines that it causes a radiated emissions problem.

 

Details of the RF Antennae Noise Coupling

By the time that all of the cables had been filtered or removed, there remained just one emission at 270MHz that was failing the Class B limit. An investigation with RF current probes showed a lack of noise on the main output cables listed above, even when they were screened or filtered appropriately.

A wander round the enclosure with an electric near field probe and spectrum analyser showed a spike in emissions near the RF antenna housing on the side of the EUT.

 

rwns antenna spurious

Checking the antenna feed cables showed them connected to the PCB pretty centrally. Disconnecting the coaxial cables from their mating halves dropped the emissions down to the noise floor.

Even though the noise isn’t in-band for the antennae themselves, they still perform well enough to radiate the noise and cause an emissions problem.

 

Summary of Fixes Applied

The below diagram shows the fixes applied to the EUT to achieve a Class B pass.

rwns applied modifications

Firstly, a fully screened metal bulkhead Ethernet connector was chosen for use with a shielded cable. This isn’t ideal from the installation point of view but is ultimately unavoidable without more significant modifications to the EUT.

Secondly, a Wurth ferrite was equipped around all three of the cables connected to the power bulkhead connector. As detailed above, it is necessary to put the ferrite around all three cables and not just the power to reduce the noise entering the capacitive coupling region around the connector.

Thirdly, a small ferrite was placed around each of the UFL cables at the point at which the antenna cables left the housing. This is a fairly common modification for radiated emissions, one I’ve employed several times before, and there are numerous suppliers of ferrites of various lengths with just the right inside diameter for the type of thin coaxial cable used with UFL connectors.

 

Results

Closing Thoughts

Any time your cable passes through a shielded enclosure with no RF termination at that point, you can pretty much guarantee its going to need some filtering.

Nothing particularly in depth in this analysis of the EUT, but I did find the coupling in and around the power connector particularly interesting.

At the end of the day, the best outcome was a happier customer with a path forward for their product.

 

Useful Test Adapters for EMC Testing and Electronics Development

Working in an EMC test lab means I get to see all kinds of equipment. No two devices are ever the same so I have to make up / adapt cables to interface various devices. If you work with a wide range of products or just want a bit more versatility in your lab then read on.

I have no affiliation to any of these products, I just use them a lot.

Clever Little Boxes

These versatile little test adapters from Clever Little Boxes are great for being able to quickly hook up one thing to another. As you can see they come in all shapes and sizes. I’ve got a box full of various ones, including the ones shown in the above photo.

Go Bananas

The ubiquitous 4mm “banana” plug and socket is super common on power supplies and other kinds of test equipment. They give a surprisingly low resistance connection for their size which, along with their simplicity, goes a long way to explaining their popularity.

If you’ve ever made up a cable assembly with standard connectors then you know they can be a pain. That’s why I really like these connectors that have a spring loaded gate that accepts a bare wire up to 2.5mm^2.

I’ve just got the standard red and black colours to keep things simple. These work well when paired with a set of crocodile clips

Get Me a Crocodile Sandwich…

I really like to pair these crocodile clips with the 4mm connectors above for super versatile connections to anything big like metal frames or enclosures of equipment.

Hook and Spring

Big numb adapters get a bit crowded when trying to connect onto individual connector pins or component legs. That’s where these teeny spring clips come in. I’ve often ended up with one of my development boards looking like an electronic porcupine with these stuck all over them!

 

Something More Permanent for Sir?

If I’m wiring up anything using mains voltages that I want to be a bit more permanent and safe then my go to are these spring terminal blocks from Wago. They are like choc block terminal strips with the main exception that these are not rubbish. Rated at 32A they can accept much larger wires that you would think and the spring clips retain the wires with a remorseless grip.

They come in multiple ways although I tend to use 2, 3 and 5 by default. Best of all they are ridiculously cheap. Just don’t get your thumbnail caught underneath the orange lever when it clicks down otherwise you’ll be using some language that is distinctly NSFW.

So “be prepared” (Scout motto) and happy testing.

James

 

p.s. don’t get me started on the adaptor vs adapter debate.

 

radiated emissions plot

RS-232 to USB Converters – EMC Problems Part Two

A while ago, I wrote about EMC immunity problems with USB to serial converters and how it was easy to fix with a small 100pF capacitor to ground on the TXD and RXD lines for a bit of filtering. Well, now I’ve found the opposite problem of EMC radiated emissions failures caused by these periodically problematic products.

In this case it appears to be harmonics of the 48MHz internal clock of a SiLabs CP2102 being conducted out of the converter on the TXD and RXD pins.

These little boards are generally used as development tools in a laboratory setting but there’s nothing to stop this IC or module being integrated into a product where these problems would manifest themselves.

The below plot shows the radiated emissionsbefore (light blue) and after (red). This module was connected to it’s host by 10cm unshielded wires, not an unreasonable application by any means.

radiated emissions plot

And what was the fix? Yep, you guessed it, some 0603 100pF capacitors on the output pins to ground. I bet that would help with immunity too! 😉

Busy, and a Birthday

It has been a very busy few months at Unit 3 Compliance; it feels like the chamber turntable hasn’t stopped spinning. There has been a wide range of products through the door from prosthetics to video wall controllers, from high spec IoT products to motion sensors, from lighting power supplies to RF amplifiers. I really love the variety of work!

I’ve also had some safety assessment work to carry out on which is always interesting. Disassembling mains transformers to measure the creepage distances inside is fascinating, getting out the angle grinder to hack the laminations apart just adds to the fun.

There have also been a fair amount of design reviews and general consulting work in between. To be able to work with customers right at the start of the project is invaluable as it sets them on the right path without having to find problems further down the line.

I’ve found lots of interesting nuggets of EMC information during this process that I’m looking forward to sharing with you in some future blog posts once I get time to sit down and write them up.

I managed to escape down to Lincoln to speak at the Open Source Hardware User Group oshcamp18 meetup on the subject of EMC testing. The delegates came up with lots of good questions at the end and the audience participation (see below slide) of the talk went down well. Higher! Lower!

play your compliance cards right!

Good to see reconnect with some old contacts and make some new ones. The other talks were very interesting also, lots of good work going on in the open source hardware field at the moment.

Lastly, and it snuck past without me spotting it, Unit 3 Compliance had it’s first birthday. It’s been a whole year since I got the keys to the unit. In that time, and with lots of help, it has gone from this:

the empty unit

Via this:

To this:

And finally this:

 

Here’s to the next 20 years of compliance, I hope to see you on the way.