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Do I need to EMC test a pre-approved power supply? – EMC Explained

One of the most common questions we get asked when we send an EMC Test Plan / quotation to our customers is along the lines of:

 

“Our equipment is powered from a pre-approved CE marked power supply so we don’t need to do any AC mains EMC testing… right?”

 

If a power supply has already been EMC tested (if it has a CE or UKCA mark you would hope that this was the case) then it is a fair question – why should we retest it?

Adding AC mains specific tests into the EMC Test Plan adds time and therefore cost, something that some of our customers would like to avoid. For smaller businesses, the cost of assessment for EMC might be one of the largest external costs incurred on a project.

The main assumption driving this question is that EMC emissions – the noise that is coming out of the power supply and either back onto the AC mains or radiated from the power supply – is the only EMC problem we have to worry about. It’s the main one, but not the only one.

The pre-approved power supply will have been tested for immunity, but only the immunity performance of the power supply itself, not the equipment that it is powering.

Some noise will get through the power supply and into the equipment being powered. How does your product respond to this noise?

Also, how low are the AC mains conducted emissions from the power supply? Have you seen a test report? How reputable is the vendor?

Testing is the most reliable way to find out.

 

Our Recommendations

We generally recommend to our customers that they perform all of the applicable tests to the product.

(What, a test lab recommending testing? I’m shocked!).

Firstly, the tests are called up in the EMC standards, and for CE/UKCA marking, testing to a Harmonised Standard gets you a “Presumption of Conformity” to the requirements of the Directives – a pass without any further Risk Assessment or justification on your part.

Deciding not to perform the testing puts the responsibility on you to assess the remaining EMC risks. If you needed us to do this assessment for you or advise on it, the cost of a few hours of consultancy time would be equivalent to just doing the tests in the first place.

Secondly, EMC performance is often dictated by parasitic capacitances and inductances, component values that are not on the datasheet or intentionally designed into the product. Even knowing their magnitude does not give a good understanding of how they will interact. Testing allows us to measure their interaction under standardized conditions.

 

 

Risk Assessment Factors

As discussed above, our recommendation is always to perform testing on applicable ports, the AC mains port included.

If you are worried about costs or time taken for testing, then you might decide to omit some of the specific tests. The below table outlines some of the factors you may wish to consider when making this decision.

The more items that apply from the Risk Increasing Factors column, the less strong your argument becomes for not carrying out testing.

 

Risk Reducing Factors Risk Increasing Factors
Class II power supply (un-earthed)

 

Class I power supply (earthed)

Especially if the DC negative of the power supply output is connected to Protective Earth in the system.

Power supply comes from reputable vendor (e.g. Meanwell, XP Power, Recom, Traco, TDK Lambda, Puls, etc) Power supply comes from cheap or from far east supplier
Power supply external to product Power supply internal to product
No analogue or sensitive circuitry Analogue circuitry e.g. audio, 0-10V I/O, 4-20mA I/O

Sensitive, low level signals e.g. thermocouple, RTD

No other long (>3m) cables connected to equipment One or more long (>3m) cables connected to equipment
Main use in Basic (residential, commercial) EM environment Flexible use, could be used in Light Industrial or Industrial EM environments

 

If you are at all unsure then you should test the AC mains port with your intended production power supply.

For the ultimate in performance, or if the equipment is for flexible use (could be powered from an AC/DC supply or from a distributed DC power supply) then we would recommend treating the DC power input to your product as a signal port with a length greater than 3m.

This would then call up Conducted RF Immunity (EN 61000-4-6) and Electrical Fast Transient (EFT, EN 61000-4-4) testing to the power port at the appropriate levels for the end EM environment (e.g. Basic or Industrial)

One step further would be to apply line-to-line and line-to-earth surges to the DC input, assuming that the design already contains a transient surge voltage suppressing element like a TVS diode or an MOV.

Let’s take a look at some of the technical justification behind the selection of these items.

 

AC Mains Port vs DC Power Port

If you typically derive your equipment power from an AC mains power supply, then it is unlikely that you will fall under the DC Power Port classification.

The term DC Power Port in EMC terms means a very specific classification of port. We discuss this in some length in this article.

 

Power supplies do not always meet the regulations

A scenario that we have experienced on several occasions: the power supplies that end up with our customers or in our test lab are not the same as the ones in the manufacturer supplied EMC test report.

 

Another customer had similar problems on  power supply that they had received samples of in that the EMC performance varied wildly. In this case the clue was that the weight of the two samples was significantly different.

 

 

These power supplies were almost identical on the outside but significantly different on the inside. Same manufacturer and model number, different components. Imagine the conversation:

“I’d like to order some HM-A132 power supplies please”

“Certainly sir, which ones?”

“Erm…”

 

This is mostly related to cheaper power supplies sourced from China. We often see significantly different results to those shown in the manufacturer test report.

The worrying thing is if changes like this are being made on the basis of EMC, what changes are being made that affect Electrical Safety that are going unchecked? We can check that for you as well.

 

Cable Routing

If your power supply is integrated into your equipment then there is the possibility of noise on the AC mains cable coupling onto other nearby cables.

It is also possible for noise to couple (both to and from) components connected to the AC mains and internal system components. This could be an emissions (noise getting out) or an immunity (noise getting in) risk.

 

This is particularly likely if you are using slotted trunking and mixing AC mains cabling in with other cables.

 

 

 

This is less important for an external power supply like a laptop type charger or a plug top power supply as the AC mains cable remains outside of the equipment enclosure.

 

 

Power Supply Common Mode Impedance

Electrical noise inevitably gets coupled onto the AC mains bus. Normally this noise is coupled onto the AC mains Common Mode. This means all the lines together in relation to a high frequency “ground” reference plane.

The noise current through the power supply and equipment will flow something like this:

The noise reaching the equipment will have been attenuated by the Common Mode impedance of the power supply and the currents diverted through the parasitic capacitance of the power supply relative to the HF ground reference plane used in the tests.

Crucially, some noise still gets through to the power supply and will flow through the product. The magnitude of this current can be estimated or measured but relies on electrical parameters that are not on the power supply datasheet.

It is this noise current that we are interested in. How does it affect your product? The only way to find out is to perform testing.

 

Class I vs Class II Power Supply CM Impedance

The construction of a typical switch mode AC/DC power supply is broadly similar across a wide range of topologies. One of the main EMC variations results from if the power supply is Class I (earthed) or Class II (unearthed).

 

Class II

A Class II power supply relies on Double or Reinforced insulation between Live parts and user accessible secondary low voltage parts for Electrical Safety. There is no connection to Protective Earth. This kind of power supply is usually identifiable by:

  • the square-in-a-square double insulation symbol (IEC 60417 symbol # 5172)
  • a plastic earth pin on a UK mains plug (technical name is an ISOD or Insulated Shutter Opening Device)
  • An IEC C8 “figure-8” AC mains inlet socket with just two pins

 

Looking at the typical internal structure of a Class II AC/DC SMPS we can see that the components providing Common Mode noise attenuation are

  • the inductive common mode filter (Lcm)
  • the components across the safety isolation barrier, transformer Tx and class Y capacitor Cy

The value of parasitic parallel capacitance of the choke or transformer (or wanted series capacitance of Cy) will reduce the impedance ( Xc = 1 / [ 2 * pi * f * C ] ) and allow more noise current to flow at higher frequencies.

This capacitance is usually a low value to prevent too high a touch / leakage current to flow which would compromise Electrical Safety.

However, at EMC frequencies of MHz and higher this presents a much lower impedance allowing noise currents to flow through the cable.

 

block diagram of a class II power supply showing EMC immunity noise current through the power supply

 

Because current always flows in a loop, and because current always returns to the source, to close this common mode current loop we need to have return currents flowing. We usually think of these coupling capacitively onto a nearby metallic element like a nearby metal structure.

In the test lab we simulate this with a nearby metal plate but in real life this could take a number of forms (building steelwork, conductive cable trays, other wiring).

 

 

Class I (Or Class II with Functional Earth) Power Supplies

With a Class I power supply, the Protective Earth is connected to accessible metalwork for Electrical Safety reasons (prevention of electric shock). Basic insulation (or higher) is required between the live parts and user accessible secondary parts.

Possibly the protective Earth is also connected to DC negative somewhere in the system as well.

A Class II with Functional Earth power supply is similar from an EMC point of view but very different from an Electrical Safety point of view. In this case, the Earth is connected for functional reasons (reducing noise or EMC emissions) but the power supply still relies on Double or Reinforced insulation for safety.

This isn’t a very common power supply topology choice, so I was surprised to see it marked on my laptop charger power supply.

 

 

In both cases, when we apply common mode noise to the AC mains input (L+N+E) then the Protective Earth conductor allows the noise to bypass the common mode impedance of the power supply. It is for this reason that we view the use of a Class I earthed power supply as a higher EMC risk for immunity reasons.

 

block diagram of a class I power supply showing EMC immunity noise current through the power supply

 

 

How this noise couples into the rest of the equipment, its magnitude, and how it affects it depends massively on the construction of the equipment. Again, testing is the best way to determine this.

 

Conclusion

Power supplies and the equipment they power are not perfect and can have varying EMC performance depending on how you connect them and how the equipment is designed.

It isn’t always easy to estimate how likely EMC issues are, even for experienced engineers and problem like us at Unit 3 Compliance. It is for this reason that we would always recommend testing to characterize the unknown EMC performance.

If you do decide to omit some testing, then the Risk Reducing or Increasing Factors above should help with that decision.

Again, we hope that this guide was useful to you in some way. Get in touch with us if you have any thoughts, questions, observations, or (obviously) a need for EMC or Electrical Safety testing.

All the best!

 

 

 

sketch showing dc power distributed around a building on busbars to a vriety of loads, and with a battery bank. There is an AC/DC charger for the batteries.

What is a DC Power Port? – EMC Explained

Everyone knows what a DC power port is, right? It’s this…

sketch showing an ac/dc adaptor and a piece of equipment with a dc power input - this is classified as a signal port for emc purposes

It’s got DC power on it, and it is a port on the equipment. DC. Power. Port.

Not in the context of EMC I’m afraid. Despite the similar name, the EMC definition for a DC Power Port (from the IEC / EN standards) is very different.

The DC Power Port is unfortunately mis-named. A better term would be “DC Mains Port” to indicate how similar it is in construction and EMC requirements to its counterpart “AC Mains Port”.

In this guide we will refer to it in this guide as a DC power/mains port and look at:

  • The EMC definition of a “DC Power Port”
  • The EMC implications of classifying a port as a “DC Power Port”
  • Examples of a DC Power/Mains Port
  • Examples of NOT a DC Power/Mains Port

Any port that doesn’t meet ALL of the definitions of a DC Power Port is just classed as a Signal Port, albeit one that happens to carry DC power.

Those key parameters are:

Criteria Met?
Local supply in a site / building / infrastructure? ???
Flexible use by different types of equipment? ???
Supply independent from AC mains? ???

 

Definition

The definitions in the Generic EMC standards of EN 61000-6-1 (immunity) and EN 61000-6-3 (emissions) lays out what a DC Power/Mains Port is:

 

EN 61000-6-3:2007+A1:2011, Clause 3.8

“d.c. power network

local electricity supply network in the infrastructure of a certain site or building intended for flexible use by one or more different types of equipment and guaranteeing continuous power supply independently from the conditions of the public mains network

NOTE Connection to a remote local battery is not regarded as a DC power network, if such a link comprises only power supply for a single piece of equipment.”

 

Let’s break out the key terms to understand the definition:

 

“…local electricity supply network in the infrastructure of a certain site or building…”

 

This suggests something wiring that is built into or spreads around a large area. A good example is the way that AC mains wiring is distributed around a building. Imagine this carrying DC instead of AC.

Typical cable lengths are probably around 10m or longer. Longer cables means they can act as antennae for low frequencies (longer wavelength). So we need to be concerned with power supply noise from our equipment on these cables that could radiated from them.

Longer cables will also pick up lower frequency common mode disturbances (conducted RF and surge) and present a larger surface for capacitive coupling of fast transients (EFT).

 

“…for flexible use by one or more different types of equipment…”

 

Use of the word flexible implies ease of use and simple connection to this power distribution system. Perhaps a common power connector (similar in nature to an AC mains plug) is used, or an agreed connector standard.

A DC Power/Mains bus that requires tools and time to connect to (example a fire alarm wired with Mineral Insulated Copper Clad (MICC) or “pyro” cable) might not meet the definition of “flexible” in terms of “ease of connection”. Nevertheless it would be flexible in terms of connection of different types of equipment (sounders, detectors, etc.)

 

“…guaranteeing continuous power supply independently from the conditions of the public mains network…”

 

The likely scenarios here are:

  • A “DC UPS” system where a bank of batteries are kept topped up by an AC mains charger
  • A DC micro-grid system where power is generated from sources like solar power

 

Importantly

1) Any port that doesn’t meet ALL of these definitions is just classed as a Signal Port, albeit one that happens to carry DC power.

2) Any piece of equipment connecting to this DC power supply is classified as a “DC Power Port” regardless of whether it supplies or consumes the power

 

EMC Tests Required for a DC Mains/Power Port

The classification of a port as a DC Power/Mains Port invites extra EMC testing to be applied.

 

Port Length Conducted

Emissions

EN 61000-4-4

EFT

EN 61000-4-6

Conducted RF

EN 61000-4-5

Surge

DC mains/power Any YES YES YES YES
Signal (with DC) <3m NO NO NO NO
Signal (with DC) >3m and <30m NO YES YES NO
Signal (with DC) >30m NO YES YES YES

 

Almost inevitably, unless the equipment has been explicitly designed as a DC Mains/Power port, there will likely be EMC test failures.

Conducted emissions invariably fails the limits. Usually the first system component after the input power connector are a series of DC/DC buck converters to change the input voltage down to levels that are needed in the system.

Buck converters suffer from noisy input nodes because of the high dI/dt requirements of the switching transistors. This needs to be mitigated through good quality high frequency decoupling and can cause noise at 20MHz upwards. Common mode chokes in the DC input may be required to mitigate this noise.

At lower frequencies, there will be current draw from the supply at the switching frequency of the DC/DC and at it’s harmonics. Unless low impedance electrolytics and a differential mode filter (usually an inductor in the 2.2uH to 10uH range forming a pi-filter) are used, the emissions from the port will fail the average limits in the 150kHz to 1MHz range.

DC Mains/Power also requires the addition of the surge test in both line-to-line (DC+ to DC-) and line-to-earth (DC+ and DC- together relative to Earth) coupling modes.

The line-to-line surge of 500V (commercial/light industrial EM environments) or 1kV (industrial EM environments) with a 2 ohm source impedance is capable of damaging the first switching transistor it comes across on the DC line unless a Transient Voltage Suppressor (TVS) is employed between DC+ and DC-.

The line-to-earth test with a series impedance of 42 ohms (not the 12 ohms as used for the AC mains port test) tests the insulation of any isolated power supply and depends heavily on how (or indeed if) a Protective Earth connection is made within the system.

 

Examples of A DC Power/Mains Port

The sketch below tries to capture a typical DC Mains/Power port application

sketch showing dc power distributed around a building on busbars to a vriety of loads, and with a battery bank. There is an AC/DC charger for the batteries.

 

Criteria Met?
Local supply in site / building / infrastructure? Yes
Flexible use by different types of equipment? Yes
Supply independent from AC mains Yes

 

Specific examples include:

 

Telecoms

48V distribution around telecoms switching / data centers to power the equipment and to provide low levels of power to handsets in a Plain Ordinary Telephone Service (POTS)

 

Computing Data Centres

Large data centre and cloud computing providers like Facebook, Microsoft, Google, and Amazon are moving away from traditional DC>AC UPS systems and towards DC power distribution (380V, 200V, 48V depending on standards) to servers and other electrical loads.

The efficiency savings from not having to convert from AC power to DC in every load, multiplied by the number of loads makes for significant energy efficiency savings and heat reduction – some of the biggest costs for such facilities.

In addition, the DC to AC conversion loss in the UPS from battery DC voltage to AC voltage is removed. Instead there are just the batteries connected to the DC power bus.

 

Electricity Substations

Battery Tripping Units (BTU) are used to power monitoring and control equipment in electricity substations. The LV AC mains supply to the substation equipment (derived from the HV or MV feed) is considered to be an “auxiliary” supply. Control of the equipment is a requirement even if this power is not present. Common DC voltages are 220V, 110V, 48V, 36V, 24V.

 

DC Micro-Grid

Local power generation from renewable sources like Solar PV might be distributed around a power generating plant or a local area.

 

Emergency Lighting Central Battery Units

There is a requirement in Building Regulations to have fire exit emergency lighting powered separately so that in the event of a power cut the building occupants can find their way out of the building safely.

In smaller buildings this is usually achieved using emergency lighting with independent battery backup. However in larger buildings, a Central Battery Unit is used to provide power (and often control / monitoring functionality) to emergency lights spread throughout the structure.

The combination of data and DC power blurs the lines between a DC mains/power port and a Wired Network port. Both call up conducted emissions tests and similar levels of immunity.

 

Fire Alarm System

DC power is passed to different critical components of the fire alarm system (e.g. smoke / fire detectors, displays, alarm sounders) in a loop system from a central control panel.

sketch showing the connection of fire alarm components to a central panel - emc dc power port example 2

Criteria Met?
Local supply in site / building / infrastructure? Yes
Flexible use by different types of equipment? Yes [1]
Supply independent from AC mains Yes

 

[1] May be difficult to connect to and reconfigure but certainly flexible in terms of variety of equipment that could be connected

Interestingly, the EMC product family standard that deals with fire, security, and social alarms (EN 50130-4) only focuses on emissions from the AC mains port with no mention of DC power outputs. Since other standards address EMC requirements for DC Power Ports, including the Generic EN 61000-6-x series mentioned above, we have a path to bring in these requirements to the EMC Test Plan as part of the EMC Risk Assessment.

If using MICC / pyro cable, whilst the joints are required to be fireproof, there is no requirement for quality of termination for EMC purposes. Reliance on the shielding formed by the outside of the cable is contingent on a low impedance electrical termination which is not necessarily guaranteed.

 

 

 

Examples of NOT DC Power/Mains Ports

AC/DC Power Adaptor

sketch showing an ac/dc adaptor and a piece of equipment with a dc power input - this is classified as a signal port for emc purposes

Criteria Met?
Local supply in site / building / infrastructure? No
Flexible use by different types of equipment? No
Supply independent from AC mains No

 

In this event, the power bus with long cables is the AC mains interface that our AC/DC power supply plugs into (for non-UK readers: that is a UK AC mains plug).

The AC mains has all the EMC characteristics discussed above: long cables that can radiate noise (emissions) or have noise coupled onto them.

One question we get a lot is along the lines of:

“My product is powered from a pre-approved / CE marked power supply, so we don’t need to do any EMC testing on it… right?”

We’ve written a separate article to cover this interesting question.

 

DC power distribution around a typical DIN rail electrical cabinet

sketch showing typical dc power distribution around a DIN rail equipped electrical cabinet - again this would be classed as a signal port

Criteria Met?
Local supply in site / building / infrastructure? No
Flexible use by different types of equipment? Yes
Supply independent from AC mains No

 

In this example, the Load represents the equipment we are interested in. There is the probability of noise coupling onto the DC power cable from other equipment inside this cabinet. For example a large industrial machine would typically have contactors and large Variable Frequency Drives running close by.

If we think this could be the case then we would recommend testing Conducted RF immunity (61000-4-6) and EFT (61000-4-4) regardless of the anticipated maximum length of power supply cable.

This would form part of the EMC Risk Assessment for the equipment, an important part of the decision-making process for what EMC tests to apply. If you’ve not considered EMC Risk Assessments before then get in touch with us and we can help!

 

Power over Ethernet (PoE)

sketch showing an example power over ethernet distribution - these are classed as Wired Network Ports under EN 55032

 

Criteria Met?
Local supply in site / building / infrastructure? Yes
Flexible use by different types of equipment? Yes
Supply independent from AC mains No [1]

 

[1] Depends on the power source for the switch, it could come from a UPS for no-interruption requirements like security or network infrastructure.

Supplying DC power over an Ethernet cable is a thoroughly good idea. High speed data, enough power to run a simple device, all over cables approaching 100m in length? Sounds great!

Each port in a PoE switch will have power provided from a dedicated isolated power supply. This provides isolation (both in terms of EMC emissions and immunity) between different segments of the PoE network.

Despite the potentially long cables, it still doesn’t quite meet our criteria for a DC power port. However similar EMC requirements for a DC power port are called up by other standards:

  • EN 55032 (emissions of multimedia equipment) calls up a requirement for conducted emissions on wired network ports
  • IEEE 802.3 specifies a voltage isolation between Ethernet cabling and the circuit at each end of 1500Vac. This will often help (but not completely resolve) with the surge requirements
  • The surge test of EN 61000-4-5 is not applied line-to-line as the Ethernet lines are considered to be “symmetrical” in the language of this Basic standard. The tight coupling between the pairs in the cable and floating / isolated nature of the signaling means that coupling onto these cables generating line-to-line surges is considered unlikely. Only line-to-earth surges are applied.

 

Daisy chain of DC powered devices all running from the same bus

sketch showing a daisy chained series of DC powered loads - classified as a signal port

Criteria Met?
Local supply in site / building / infrastructure? No
Flexible use by different types of equipment? Yes
Supply independent from AC mains No

 

 

Conclusion

Hopefully this guide has cleared up some of the confusion about DC power ports in the context of EMC.

If you are unsure about whether your equipment falls into this classification then you can always contact us if you need help.

We generally advise that if you aren’t sure if your equipment could be used in this fashion then you should design and test your product as if they do apply. It is easier to “not-fit” or link out unwanted components than to try and add them in later.

 

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

 

 

 

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.

 

 

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.

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.

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.

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.

When ESD Protection Gets Bypassed

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

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

 

The EUT

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

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

system under test showing front panel, esd protection and cables

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

 

EUT + ESD = ???

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

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

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

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

Analysis

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

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

 

Lessons

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

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

 

 

 

stainless steel camera system

TWITL – Underwater Camera System Industrial EMC Testing

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

stainless steel camera system

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

A Simple EMC Fix

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

The fault finding process was relatively straightforward this time.

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

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