meanwell power supply comparative radiated emissions

Meanwell Power Supply Radiated Emissions Investigation

We’ve been doing pre-compliance scans on a customer’s product and helping them overcome some interesting radiated emissions problems (spoiler, 3rd party display module. Again.)

Now that it is back in the lab for it’s final EMC measurements we suddenly found that we were measuring an extra 10dB of noise at 170MHz.

Hang on… it was passing during the pre-compliance measurements last month… what is going on?

During the pre-compliance measurements, we only saw this peak when we were powering the EUT from the provided open frame mains power supply. So we removed it for some investigations.

Using our Tekbox TBCP2-750 current probe and Signal Hound BB60C spectrum analyser, we measured all of the cables connected to the power supply.

 

On the AC mains supply:

current probe on ac mains input

On the DC power output cables:

current probe on dc power output

and on the 12V auxiliary fan power cable

current probe on 12v aux fan output

 

What we measured was a bit of a surprise:

meanwell power supply comparative radiated emissions

 

Of all the cables we expected to have a problem with, the low power 12V fan cable was not our first candidate. It looked to be carrying the most noise at 170MHz so we did what every good EMC engineer does – put a ferrite on it!

 

 

Now it meets Class A with a 5dB margin at that frequency.

Upon further investigation, a second fan had appeared inside the equipment since our pre-compliance measurements. The engineer had mentioned improving the temperatures within the product but we hadn’t opened it up to verify if any changes had been made.

The cable routing for the new fan was undefined, allowing it to lie across the power supply, or next to other components depending on how it was assembled. This appeared to be the cause of variability that we had observed in our testing.

 

Takeaways

One of the key rules of EMC troubleshooting is to change only one thing at once, and be careful that you are only changing one thing. Reassembling the unit with different fan cable position accounted for some of the variability in emissions performance.

Don’t assume that just because you are using a pre-approved component that it will automatically pass when integrated into your system. Having worked with many Meanwell power supplies of all different flavours over the years this is only the second time we’ve had any significant issues with one.

 

Bye for now,

James

 

 

 

MPS Presentation on DC/DC Converter Myths

I wanted to share this excellent and useful presentation by Francesc Estragués Matamala from Monolithic Power Systems.

This addresses many myths about DC/DC converter layout and I love tackling EMC myths!

I’ve been wanting to do a study like this for a while now so thanks to MPS for doing this.

Make the time to watch this.

 

 

Spring or Star Washers for Earthing Stud?

Introduction

This article started with a simple question: what is the correct washer to use to secure a ring crimp terminal on a threaded bolt?

I have seen either spring/split washers or internal/external star washers being used.

I asked on LinkedIn and found some good advice, some received wisdom and “we’ve always done it this way”, but not much in the way of citeable standards or references from technical authorities.

 

Goals of Earthing / Bonding System

The fundamental goals of the electrical fixings in the protective earthing / bonding system are:

  • Provide (at assembly and maintain during use) a low resistance, potentially high current (tens/hundreds of Amps depending on supply) electrical contact
  • Not corrode or loosen under the normal environmental operating conditions to the point where the resistance goes out of specification

In reality there are many factors that one could worry about:

some considerations for protective earthing conductor connection to metalwork

 

Common Safety Standard Review

A review of the more frequently used safety standards for electronic products yields the following clauses.

Standard Clause Clause Text
EN 62368:2014 4.6.1 “parts fixed by means of screws or nuts provided with self-locking washers or other means of locking are not liable to become loose or detached
NOTE Spring washers and the like can provide satisfactory locking”
EN 61010-1:2010 6.5.2.2 c) “Screw connections shall be secured against loosening”
EN 61010-1:2013 6.5.2.3 k) “The contact pressure required for a bonding connection shall not be capable of being reduced by deformation of materials forming part of the connection”
EN 60335-1:2012 28.4 “Screws and nuts that make a mechanical connection between different parts of the appliance shall be secured against loosening if they also make electrical connections or connections providing earthing continuity. This requirement does not apply to screws in the earthing circuit if at least two screws are used for the connection or if an alternative earthing circuit is provided.
NOTE 1 Spring washers, lock washers and crown type locks as part of the screw head are means that may provide satisfactory security.”
EN 60335-1:2013 27.2 “27.2 The clamping means of earthing terminals shall be adequately secured against accidental loosening.”
EN 60730-1:2016 9.3.6 Clamping means of earthing terminals for external conductors shall be adequately locked against accidental loosening.
EN 60730-1:2017 11.2.2 “parts fixed by screws or nuts provided with a locking washer are regarded as not liable to become loose”

(note clause not specifically related to earthing)

 

Typical Locking Fixings

The below image shows a variety of locking methods that I would consider acceptable for this purpose.

a table showing locking nuts and washers

Conclusions

The standards are not prescriptive about the type of locking washer to be used.

Spring washers, lock washers and threaded fastener locking features are all valid approaches.

No washer is also an acceptable method provided there is a locking nut of some kind of suitable locking adhesive used.

Two independent fixings are considered to be acceptable in some standards.

 

Testing Testing Testing

In all cases, conformity with the standard is checked by inspection and/or appropriate testing. Testing is key.

Testing the protective earthing / bonding system includes measuring the resistance and/or measuring the current handling capability of the connections.

If you are the manufacturer and wanting to use a non standard fixing method then it may be acceptable. Any non-standard or atypical methods would need adding to the product compliance risk assessment.

The testing specified in the standard is the bare minimum and additional testing may be required to demonstrate that everything is indeed safe. Testing could include extended high humidity environmental testing to check for corrosion and representative vibration testing to make sure that loosening does not occur in use.

Selecting suitable environmental test levels for your product can be based on your experience as the manufacturer with typical operating environments, or perhaps using the ETSI EN 300 019 environmental engineering standards.

Of course, the simplest way is to just use a standard washer to reduce arguments.

 

 

Not Covered

Like all simple questions, there is a surprising amount of depth and possible considerations, including:

  • Corrosion, plating, passivation, surface oxide layers, dissimilar metals. This is a book in of itself!
  • Considering the current path. Using a locking washer with a small surface contact area in the primary current path can increase the resistance. Aiming for a larger surface area with a good quality connection would be optimum
  • Surface preparation: clearing paint, anodising, rust, or oxidation.
  • Minimum fixing size. Some standards call up a minimum 4mm diameter and number of threads engaged for certain types of screw fixings. This is not universal across all standards. If in doubt, selecting all threaded hardware to be at least M4 in diameter seems like a sensible option.

 

Your Thoughts

I would be very interested to hear of studies, standards, procedures, reports… indeed any published material that covers this topic of washers and fasteners specifically for electrical connection.

 

References & Links

  1. My original question posted on LinkedIn asking about washers
  2. NASA fastener design manual, page 10 has details of locking mechanisms
  3. The always amusing and informative AvE
  4. NordLock brand washers under the Junker test

 

 

 

One Ferrite Is Not Enough

This would be a great Bond film title…

“So Blofeld, do you expect me to talk?”

“No Mr. Bond, I expect you to solve this radiated emissions problem!”

* laser noises intensify *

 

I was doing some radiated emissions problem solving on a smart LCD module and found an issue that is not new but I haven’t encountered for a while.

In this case, the solution required two ferrites. One on the I/O cable to the module and one on the flexible cable between controller and LCD screen.

Adding only a single ferrite in some cases INCREASED the emissions rather than reducing them, presumably an effect where the addition of the ferrite changes the resonant frequency of either one leg or the entire antenna to the troublesome frequency at 192MHz.

This reinforces the approach of:

  1. Always add new fixes to existing fixes already implemented. Whilst it might be the fifth change that worked, it might not have worked without the previous four.
  2. Once the last fix is in place and validated as working only then can you try and figure out what combination is actually required to solve the problem

The last step can get very busy, particularly if there are a large number of modifications applied. It might only be worthwhile if some are particularly expensive or difficult for the customer to implement in production. Different fixes for different budgets!

 

self interference demo USB3 and 2.4GHz

2.4GHz Intra-System (or Self/Platform) Interference Demonstration

In this blog we are going to take a short look at noise and interference in the 2.4GHz band. Our example victim is a Zigbee controller and the sources are nearby USB3.0 devices and Wi-Fi sources.

 

Background

One of our customers makes these rather useful USB Zigbee Coordinator sticks, frequently used for controlling smart home or IoT devices like light bulbs.

These devices operate at 2.4GHz, a very crowded frequency band with Wi-Fi, Bluetooth and Zigbee all fighting for a narrow, congested slice of spectrum.

One of the common issues faced by users of this band is that of intra-system interference, sometimes referred to as “self” or “platform” interference. This is where components in the same system interfere with each other, primarily due to their proximity.

[Note: The counterpart to intra-system (within the system) in this context would be inter-system interference (between separate systems), which is what the conventional EMC test regime of radiated and conducted emissions and immunity seek to characterise.]

This common problem is something that our customer knows all too well from helping their clients integrate these Zigbee products into the end application.

So, during a recent visit to our lab for some testing on a related product, we spent some time investigating this noise on a typical setup.

 

Demonstration Setup

The setup in the below image is common to many users with a Raspberry Pi Model B and lots of stuff plugged in to the USB ports. In this case, a Zigbee adaptor (black case) and an USB3.0 SSD in close proximity.

These parts, including the spectrum analyser, is part of the customers in-house electronics development laboratory.

 

self interference demo USB3 and 2.4GHz

 

The effects of USB3.0 on the 2.4GHz spectrum are well known. A good example is this 2012 paper from Intel which

For this demo, we used a near field capacitive probe and a 2.4GHz antenna to measure noise in the 2.4GHz to 2.5GHz band local to the Raspberry Pi.

This demonstrated the degradation of the noise floor with various levels of system activity including

  • Measurement of system noise floor
  • Presence of a USB3.0 SSD running a large file transfer using the dd Linux command
  • Activation of the Raspberry Pi internal Wi-Fi

The below image shows three traces under these different conditions.

 

spectrum of 2.4GHz band showing ambient noise, SSD noise and Wi-Fi emission

 

Experiment Conclusions

The conclusions we can draw about the in-band noise are:

  • Noise from the SSD raises the noise floor by approximately 10-20dB (a factor of x10 to x100)
  • The Wi-Fi transmission from the Pi is 40dB above the local noise floor. This will mask any received Zigbee signals from a remote transmitter.

 

In-Band vs Out-of-Band Sensitivity

Well designed radio systems are generally very robust to out-of-band interference i.e. anything outside of the narrow radio band that it is tuned to. For instance, a Zigbee radio system set to channel 20 (2.450GHz) will reject anything below 2.445GHz and above 2.455GHz.

 

Intra System Interference Diagnosis

Advice on diagnosing these issues is mostly outside the scope of this short blog. Differences in systems, components and ambient noise levels makes it impractical to offer guidance for all situations. However, some generic problem solving pointers are presented below.

A systematic approach to isolating the problem is required.

One of the primary rules of problem solving is to change only one thing at once and observe the effects.

In EMC terms, it is possible to change several things at once without realising it. Cable position, the specific port that a device is plugged into, location of nearby equipment and cables, even how firmly a connector is tightened will all make small differences that stack up. (Don’t use anything other than a torque spanner on those SMA connectors though!)

Another key rule is if you think something has made a difference, reverse the change and see if the problem re-occurs. Unless you can achieve consistency then you might be changing something else unintentionally, or the problem is caused by something outside of what you are changing.

Correlating the problem against time can help. Does it happen when something else happens (other devices on, or off, or switching, certain configurations, times of day, etc.) This can give clues.

Lastly, we should be looking for a significant step change in improvement to identify the issue. Phrases like “I think it made a bit of a difference but I’m not sure” indicates that we are dancing around the issue and not getting to the heart of it.

Ultimately, for a detailed understanding, the spectrum analyser is a key tool in gaining a proper grasp of this issue.

 

Solutions

The solutions to the problem are simple yet sometimes difficult – a technical balance needs to be struck.

Use of Ethernet rather than Wi-Fi on the Raspberry Pi.

It is not practicable to synchronise transmission from the Raspberry Pi Wi-Fi with that of the Zigbee stick. The simplest way of ensuring the Wi-Fi does not interrupt the Zigbee transmissions is to disable the Wi-Fi and provide network connectivity via Ethernet instead.

Depending on the installation this might not always be practicable but it certainly is more reliable.

 

Separation of components

Moving the antenna away from the noise source is usually the best way to achieve increased performance.

In this instance, placing the module at the end of a USB cable and away from other electronic items is a good start.

Another option that is not as ideal: a good quality SMA extension cable could be used to extend the antenna away from the problem area. This introduces loss into the RF channel, reducing signal quality.  Measurements made in our lab on a cheap extension cable from RS show a power reduction of 6.5dB at 2.4GHz for a 5m cable. This equates to a ratio of around 0.25 meaning we are broadcasting and receiving a quarter of the power we were before.

Also, it is still possible for the noise to couple onto the nearby module even without the antenna attached meaning the problem does not get entirely resolved.

 

Better quality components

Sourcing a bunch of cheap-as-possible parts from Amazon or eBay is likely to bring problems.

Using devices from big name manufacturers and buying from reputable sources helps. But, even reputable components are designed to a price point and can still cause problems if the other points in this blog are not taken into account.

USB cables can be a big source of the problem. Unshielded back shells (the part between cable screen and connector body) compromise the shielding to the point where their performance at high frequencies is equivalent to an unshielded cable.

The only way to tell if a cable is good quality is to perform an autopsy on the ends and check on the cable shielding

Remember that Pawson’s Law of Cable Quality states that the EMC performance is inversely proportional to the physical appearance. Braided covers, shiny plating, metal connector bodies, transparent mouldings etc are all indications of money spent on the OUTSIDE of the cable. EMC quality comes from the INSIDE and is not visible.

shiny usb cable vs boring usb cable

 

 

Hope this was useful! See you soon.

James

 

 

 

RCWL-0516 - board image from github.com-jdesbonnet

Compliance Assessment of a RWCL-0516 Doppler Radar Motion Detector

I’ve been helping a customer out with some EMC pre-compliance testing of their new domestic product which included a range of 3rd party modules.

One of these modules was an “RCWL-0516” 3GHz radar for motion detection. These modules are widely available but technical information is mostly reverse engineered by enthusiasts and hobbyists. The best collection of information seems to exist on this GitHub page.

RCWL-0516 - board image from github.com-jdesbonnet

The customer was very keen to use these devices but making some measurements and looking into the regulatory side meant that it got a Big Fat No from me.

 

EMC Radiated Emissions

Radiated emissions in the 1-6GHz band were in excess of the Average limit line by over 17dB.

RCWL-0516 - radiated emissions Class B domestic

This is normally OK for a radio system, as exceeding these limits is often required to achieve the desired range and operation. However this only works if there is a counter-part radio standard to refer to…

 

Analysis of the Regulatory Status of this device

  • No CE / UKCA marking applied to these devices – should not be sold in the EU / UK
  • No CE / UKCA marking Declaration of Conformity supplied by manufacturer – should not be sold in the EU / UK
  • No reference to technical standards used to assess the device to the Radio Equipment Directive
  • At present there are no radio standards published by ETSI for the use of this 3.1GHz band for this kind of application in the EU or UK.
  • This document from CEPT on the use of Short Range Devices gives more details about what radio bands can be used
  • 3.1GHz is not a Harmonised Frequency band. Instead, it is licensed, and operation is only permitted in some countries. The key to the table is at the bottom.

RCWL-0516 - CEPT radio band table

  • Even when taking this table into account, this band is only for UWB Location Tracking Systems.

RCWL-0516 - CEPT radio band table part 2

  • Following the documents further down the chain, the ECC/REC/(11)09 mentioned above refers to two documents:
    • TR 102 495-5 for use of Ultra Wide Band for location tracking operating in 3.4 to 4.8GHz. This device is not UWB and not operating entirely in this band.
    • ECC REPORT 120 requirements for UWB Detect-and-Avoid for operation in this band. This device has not such capabilities.
  • The only way that this radar device can be considered legal to operate is if it meets the Class B (domestic) emissions limits in the 1-6GHz band.
  • Currently this is not the case. With this example product, emissions will need to be reduced by 17dB or more to comply.
  • The oscillator used relies on parasitic components between PCB elements. Tolerance of components, PCB manufacturing tolerance, values over temperature means that frequency stable operation is not practicable.
  • From a regulatory standpoint, these devices should not be touched with a barge pole
  • Other motion detector products exist – I’ve not linked to any as I don’t want to unfairly endorse anything I’ve not investigated further or tested myself.

 

Summary

Anyway, I hope this clears up some of the questions about this device.

I can’t recommend using these devices at all. If you are going to use one of these then keep an eye out for interference with other systems. Don’t even bother if you want to make something that you can sell at the end of the process.

Cheap 3rd party modules like this are usually cheap for a reason.

Thanks to Charlie Blackham for pointing me in the right direction with the radio standards.

 

 

 

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.

 

 

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.

 

 

Choosing EMC/Radio Standards for CE/UKCA – Generic vs Specific

A short post prompted by a (summarised) request from a customer:

 

We’d like to test to the following standards for our CE/UKCA marking

– EN 61326-1 (Class B emissions, Industrial immunity)
– EN 61000-6-2 (Industrial Level Immunity)
– EN 61000-6-3 (Class B Emissions)

 

This customer is very compliance conscious, as their products end up in all kinds of harsh and hazardous environments where they are protecting the health and safety (and lives in many cases) of their customers.

As such, it is understandable that they want to “throw the kitchen sink” at the EMC performance. Selecting Class B emissions and industrial immunity is a great way of demonstrating the robustness of your product in a wide range of electromagnetic environments.

So, why not quote all of the standards on the Declaration of Conformity (DoC)?

 

CE and UKCA

This article was originally written with CE in mind. It also applies to UKCA, just replace “Harmonised” with “Designated” as far as the standards go and you’ll be fine.

 

Guidance is Available

Thankfully the European Commission has published guidance on selecting Harmonised EMC and Radio standards for assessing the product to.

In each of these standards, a primacy or order of application, is given to the Harmonised Standards.

 

Guide for the EMCD (Directive 2014/30/EU)

4.3.2.2 Relevant harmonised standards

The selection of the relevant harmonised standards is the responsibility of the manufacturer.
When the manufacturer chooses to apply harmonised standards he shall select them in the following precedence order:

– Product-specific standards (if available)
– Product family standards (if available)
– Generic standards

Product-specific (family) standards are those written by ESO’s taking into account the environment, operating and loading conditions of the equipment and are considered the best to demonstrate to compliance to the Directive.

 

An example of a product specific standard would be EN 61326-2-6Electrical equipment for measurement, control and laboratory use – EMC requirements – Part 2-6: Particular requirements – In vitro diagnostic (IVD) medical equipment (IEC 61326-2-6:2012)”

These product specific standards often refer back to the root family standard, EN 61326-1 in this case.

Only if the manufacturer’s equipment does not fall into a product standard should the generic standards be applied.

 

Guide to the Radio Equipment Directive 2014/53/EU

5.2 Generic harmonised standards vs product specific harmonised standard

A manufacturer which has the intention to apply a harmonised standard for the conformity assessment of its products, has to apply in priority the product specific harmonised standard and only if this one is not available, the generic one, in order to benefit of presumption of conformity with the essential requirements of the RED.

 

Applying Multiple Standards

There are cases where applying several different Harmonised Standards could be the correct thing to do.

For example, if the equipment is a piece of measurement equipment that incorporates a lot of IT functionality (networking, data storage, PC control) then the manufacturer could decide to assess against EN 61326-1 for laboratory equipment and against EN 55032 for IT equipment. Both standards would appear in the test report and on the DoC.

 

Check Annex ZZ

One of the commonly overlooked Annexes (Annecies? Annecii?) is this one at the start of the standard. This details what Essential Requirements from the Directive are being covered by the standard.

Important: not all standards cover all Essential Requirements. You must check Annex ZZ carefully against them.

If you end up needing to apply more than one Harmonised Standard to a product to cover all of the Essential Requirements then you should state this on your Declaration of Conformity.

 

Presumption of Conformity

Remember that using Harmonised Standards (or Designated Standards for UKCA) gives you a “Presumption of Conformity” without further requirement to demonstrate compliance with the relevant directives/laws.

As this interesting piece on kan.de notes:

 

“Ultimately, the presumption of conformity is no more than a reversal of the burden of proof. This means that a product complying with the relevant [harmonised] standards may be challenged, for example by the market surveillance authority, only if actual evidence can be produced that the manufacturer has violated the requirements of the directives.”

 

Annex ZZ of a Harmonised Standard is your friend when it comes to understanding this link between the standards and the directives.

 

When the DoC Doesn’t Quite Cover It

This example of EN 61326-1 illustrates one of the problems of applying a Harmonised Standard that has multiple levels within it.

In this case, the EMC performance of equipment complying with EN 61326-1 could fall into one of six distinct categories.

Emissions

  • Class A (industrial)
  • Class B (domestic)

Immunity

  • Controlled (shielded and filtered environment)
  • Basic (domestic/commercial)
  • Industrial (heavy machinery)

On the face of it, a product tested to Class A / Controlled (poor EMC performance) can’t be distinguished from one that has passsed Class B/Industrial limits (excellent EMC performance).

What to do?

The way I suggest overcoming this and informing the end user a little more clearly about the performance of the product is to explicitly state in the DoC what levels the product was assessed against during any testing.

Example:

 

This equipment was assessed against the following Harmonised Standards:

 

– EN 61326-1:2013Electrical equipment for measurement, control and laboratory use – EMC requirements – Part 1: General requirements” (Class B emissions, Industrial Immunity)

 

I hope you enjoyed this short dive into standards land. It’s a nice place to visit but you wouldn’t want to live there!

Speak soon,

James

 

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