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

2nd April 2023/in Article /by James Pawson

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.

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.

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!

What is a DC Power Port? – EMC Explained

2nd April 2023/in Article /by James Pawson

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

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

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.

https://global-sei.com/technology/tr/bn75/pdf/75-25.pdf

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.

https://blelighting.co.uk/product/central-battery-units

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.

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

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

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)

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

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.

Quick Reference – Immunity Test Steps and Test Time

17th January 2021/in How To /by James Pawson

Here is a quick reference for common EMC immunity tests for a given start frequency, stop frequency, and step size. Outputs are number of steps and time per test.

Test	EN IEC 61000-4-6 (Conducted)	EN IEC 61000-4-3 (Radiated)
Start Frequency	150 kHz	80 MHz	1 GHz	1 GHz
Stop Frequency	80 MHz	1 GHZ	2.7 GHz	6 GHz
Step Size	1%	1%	1%	1%
Number of Steps	633	255	101	182
Time per step	3	3	3	3
Time (seconds)	1899	765	303	546
Time (minutes)	31.7	12.8	5.1	9.1
Time (hours)	0.53	0.21	0.08	0.15

This is just the test time per setup.

For the Conducted RF Immunity test, this should be multiplied by the number of ports under test.

For Radiated RF Immunity, this figure needs to be multiplied by 2 x antenna polarities and 4 x turntable positions, meaning the overall time is 8 times larger.

The outcome here is that to do a full radiated immunity test, from 80 MHz to 6 GHz, takes 3 hours. And that doesn’t account for setup time or investigating failures.

Labview VI For Calculating EMC Immunity Test Frequency Steps and Test Time

17th January 2021/in Recent Work /by James Pawson

This replaces an Excel macro I’ve used in the past with something that can be used in a variety of types test software.

EDIT: now updated to allow addition of specific spot frequencies to the test frequency list

You can download Version 2 here.

EMC Immunity Testing EUT Monitoring Software

11th November 2020/in News, Testing /by James Pawson

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.

iso 7637-2 pulse 1 vs iec 61000-4-5 waveform comparison

IEC Surge/EFT Generators for ISO 7637-2 Automotive Pre-Compliance

29th June 2020/in Article /by James Pawson

Intro

Like most long articles, this started off as a short one. It all stemmed from a customer question:

“We had some issues using a LED driver that could not cope with load dump and volt spikes. Do you have any provisional tests that could determine the circuit reliability? It doesn’t have to be to [ISO 7637-2]”

The ISO 7637-2 standard defines automotive conducted transient test pulses on vehicle power lines (12V or 24V). It is called up by standards including:

UNECE Regulation 10.06 for E-marking
EN 50498 (aftermarket automotive equipment)
ISO 13766-1 (earth-moving and building construction machinery)

I don’t have an ISO 7637-2 pulse generator (edit: I do now!). Automotive surge generators are less commonly found in many EMC test labs due to their more specialised nature.

Systems are available to hire; budget for €/£1000/week for a generator that will cover Pulses 1, 2 and 3. They are also available to buy new; expect to pay around €/£15k. If you need to cover pulse 4 then this will increase the costs yet again, mostly for the bipolar amplifier.

But, like most EMC test labs, I do have an IEC 61000-4-4 (EFT) and IEC 61000-4-5 (Surge) generator capable of 1.2/50us and 10/700us pulses.

Question: Could I use the IEC generator to simulate the surge pulses from the ISO generator?

This question comes with caveats:

The aim here is pre-compliance / confidence testing with the tools available. Not to replace the ISO 7637-2 tests entirely.
We are only looking at the potentially destructive Pulses 1, 2a, 3a and 3b.

Unit 3 Compliance can perform pre-compliance and full CE Marking testing to EN 50498. We can also perform pre-compliance testing for many of the R10 tests for E marked products.

Please get in touch for a chat if this is of interest.

Conclusions (TLDR)

ISO Pulse 1

IEC 10/700 pulse generator can be used as a close substitution for a 12V system
For a 24V system the 10/700 pulse is not as good a match. Follow the flowchart to select the test compromise and set the surge voltage based on the values in the tables.

ISO Pulse 2a

Not a good match, recommend a compromise between current and energy as shown in these tables

ISO Pulse 3a, 3b

IEC EFT generator is a good match and can be substituted for ISO pulse 3a and 3b

Pulse Parameter Comparison

Comparing the pulse widths and impedances against each other gives a mixed picture.

For Pulse 1, neither waveform is a great match with both of the ISO pulses having a longer pulse width than the 10/700 generator. Whilst the 24V bus pulse has a much higher impedance, this could be corrected with an additional series resistor in the IEC generator output.

For Pulse 2a, the 1.2/50us IEC generator appears to be an excellent match.

For Pulse 3a and 3b, the 5/50ns EFT generator is pretty close but the width of the ISO pulse is three times bigger.

However, as we shall see below, this approach is incorrect as it does not tell the whole story.

Pulse Width Definition

The problem comes from how the pulse widths are defined in the standards. Let’s take the comparison between ISO Pulse 1 to IEC 10/700 comparison as an example.

e can see that the ISO pulse width is defined at the 10% crossing point, whereas the IEC pulse width is defined at the 50% crossing point.

This is not helpful.

How do we compare a ISO 1000us @ 10% with a IEC 700us @ 50% waveform?

Open Circuit Ideal Waveform Comparison

I found some information over on the PSCAD website that showed the equation for the waveshape (from IEC 61000-4-5)…

…along with some Matlab optimised coefficients for alpha, beta and k.

From the PSCAD website “Standard Surge Waveforms” https://www.pscad.com/webhelp/Master_Library_Models/CSMF/Surge_Generators/Wavelet_Transformation_(WT).htm

ISO 7637-2:2011 gives the equation for the falling edge only of the pulse waveform. It also states that “The influence of the rise time is not taken into account (tr << td), which is allowed for all pulses specified in this part of ISO 7637”

Modelling Notes

After watching a Numberphile video on coronavirus infection curve modelling I decided to give Geogebra a try for modelling these waveforms. It’s quite a useful graphing calculator package, much more powerful than I’ll ever need to use.

I also modified the equation for the IEC waveshape equation to take into account the generator and load impedances by taking the first term of the ISO equation and adding it to the start of the IEC equation.

A required surge voltage of 1V was used for simple direct comparison.

Review of Waveform Comparisons

For Pulse 1 we can see that the 10/700 IEC waveform is actually a really good match for Pulse 1 for a 12V bus.

The same cannot be said for the 24V bus requirement. Some further thinking is required here.

The 55 ohm impedance for the 24V version of the pulse is the 15 ohm 10/700 generator natural impedance with a series 40R resistor in addition.

Despite Pulse 2 looking like a good comparison initially, the modelling shows that it is actually a very poor match.

For Pulse 3, the IEC EFT generator is a very good match and should be able to be used without any issue

Dealing With Pulse 1 (24V) and Pulse 2a

How could we go about compensating for the poor match between Pulse 1 (24V) and 10/700 IEC and between Pulse 2a and 1.2/50 IEC?

We need to ask ourselves: are we more interested in the peak voltage & current or pulse energy?

To answer this, first we need to understand the power input design of the Equipment Under Test (EUT)

EUT Design Assessment

It is useful to establish the following EUT design parameters:

Is there a discrete reverse protection diode? What is the Vrrm and Trr rating (reverse recovery time) of this part?
What is the maximum clamping voltage of the TVS diode and can the downstream circuitry survive this voltage?

It is important to remember that Pulse 1 is a negative going pulse caused by the disconnection of a large inductive load in parallel on the vehicle power bus. If the EUT has a reverse protection diode fitted then it’s Vrrm and Trr will change the effect of the test on the EUT.

W2AEW has a good video on diode reverse recovery time over on YouTube.

It is also important to test at full current consumption if a reverse recovery diode is present as this will affect recovery time and therefore surge performance.

EUT Surge Suppression

The assumption is that we are testing an EUT that contains some basic low voltage electronics of some kind. The extension of this assumption is that it has some kind of surge suppression component connected across the power inputs.

This could be a Metal Oxide Varistor (MOV) or a Transient Voltage Suppression Device (TVS). These have a non-linear impedance with voltage and will restrict or “clamp” the input voltage to a defined level. Perhaps a component like a SMBJ26CA-TR.

This clamping voltage is dictated by the impedance of the part when conducting. This would be a diode-like VI curve for a TVS or the current-dependant resistor of a MOV.

Peak current is dictated by available peak voltage and generator impedance. So we need to be interested in the peak current to ensure that the correct clamping voltage is met.

Also, because the MOV or TVS absorbs some of the pulse energy internally, these components will have a datasheet rating for pulse energy. Exceeding this could cause significant damage to the part and affect its capability to handle future surges.

Pulse 1 Peak Voltage & Current or Pulse Energy?

Our main tools for adjusting an IEC pulse to suit an ISO pulse are:

Peak voltage
Series impedance

The surge generator has an easily adjustable peak voltage through the control panel or software so this is the main method that will be used.

The Peak voltage is a significant consideration if the system has the reverse protection diode but the compromise test will depend on it’s voltage rating.

I’ve produced a flowchart to help selection of the right test level for using IEC 10/700 instead of ISO Pulse 1

Pulse 1 Best Compromise Voltage

I ran some more simulations in Geogebra adjusting the ratio between the IEC and ISO peak voltages and tabulated the results.

The best compromise is to minimise the total difference between current and voltage when expressed as ratios. This works out at a V_iec or around 0.6 * V_iso.

This yields the following test voltages, peak currents and pulse energies for the different severity levels.

Sidebar

It is interesting that the series impedance for the 24V version of ISO Pulse 1 is up at 50 ohms. This higher impedance implies that the surge expected in such a system would be induced from a parallel adjacent cable in a wiring loom rather than something directly connected to the ignition switch / inductive load circuit directly.

Pulse 2a Best Compromise Voltage

Same approach as for Pulse 1

Test Practicalities & Further Compromises

Pulse 1 Power Disconnection

The waveform for Pulse 1 shows a synchronised disconnection from the DC supply and application of the surge voltage. Since this is not easily done without

It is the surge pulse that will cause the damage rather than the momentary disconnection of voltage therefore, for these compromise tests, this is being ignored.

Coupling/Decoupling Network Requirements

The CDN inside the IEC test generator for mains coupling is adequate for the task of decoupling but the options inside my KeyTek ECAT test generator preclude the coupling of the 10/700 waveform. Instead, some creative front panel wiring with banana plugs will be required.

Since this CDN is designed for decoupling of surge and EFT impulses from the mains, I’m sure it will adequately protect the 12V linear power supply being used and also prevent the power connection from unduly affecting the test.

In may case, input is through a 16A IEC mains plug/socket but it is easy to make an adaptor. Output is via a BS1363 socket or, more convieniently, 4mm banana plugs.

The End.

This took way longer to research and write that I was hoping. Something in the order of three days of work was spent going backwards and forwards, thinking about it whilst doing DIY at home (nearly painting the cat as a result) and half listening to Tiger King on the TV.

I’m quite pleased with the result and I hope this eventually proves useful to someone.

Simple RF Current Transformer for EMC / EMI Investigation

30th September 2018/in Article /by James Pawson

This post contains some background info related to the video I posted on YouTube on how to make a simple RF current transformer, a great tool for debugging EMC / EMI issues such as radiated emissions from cables, or tracing conducted RF immunity noise paths.

RF current transformers (or probes) are commercially available products from places like Fischer CC or Solar Electronics and they work really well, have specified bandwidth and power handling characteristics, built in shielding, robust case, etc.

They also cost a few hundred £$€ each which, if you are on a budget like most people, represents a significant investment for a individual or small laboratory. However, this one can be built very cheaply; most labs will have a development kit with some clip on ferrite cores, if not the core I used only costs £5 from RS.

DIY Current Probe

I’m a big fan of making my own test adaptors and equipment as its a great way to really understand how things work and the compromises in any design. As such I decided to share how I go about making this kind of really useful tool.

It’s primary use is for A-B comparison work; measuring the current, performing a modification and then measuring the current to see the improvement.

It is to be stressed that my version is a crude but effective piece of equipment and does not replace a well designed commercial product. There’s a time and a place to invest in quality equipment and one should use engineering judgement on when that is. For instance, measuring the RF current accurately is definitely a job for a properly designed and characterised device.

If you want to explore RF current transformers in more detail then there is plenty of info on Google, but these links are useful places to start.

Some of the design compromises involved in this low cost approach include:

Core Losses / Insertion Loss

The ferrite material in these cores is specifically designed to be lossy at the frequencies of interest, which will result in a lower reading than a higher bandwidth core and a reduction in the amount of noise on the cable downstream from the noise source. This can in some cases mask the effect you are trying to measure. The commercially available products use low loss, high bandwidth ferrite cores.

A high insertion loss also makes these parts more unsuitable for injecting noise into circuits for immunity testing. they can be calibrated for this task using a simple test setup (to be covered later)

Secondary Turns

Number of secondary turns controls sensitivity but the more you add, the inter-winding capacitance increases, decreasing the bandwidth of the tool. I generally use 5 or 6 turns to start with but I do have a 20 turn part made with micro coax on a solid core which also helps to deal with…

Capacitive pickup

From the cable under test to the secondary winding. Normally a split shield (so that it doesn’t appear as a shorted turn) is built in to commercial products. Guess what, that’s easy to do on this with a spot of copper tape or foil.

Not as Robust

Although a well designed product, the plastic hinges and clips on the cores are not designed for repeated opening and closing. The Wurth Elektronik system of a special key to open and close the core is much more robust at the expense of having to keep a few keys to hand for when they inevitably go missing. However these parts are so cheap and quick to make that a broken clip on core is no real obstacle.

Future Videos

I’ll be following this video with some hints and tips on how to use these devices effectively for finding radiated emissions problems and for looking at conducted RF immunity issues. Stay tuned.

Video and Construction Errata

The sharp eyed of you will have spotted that I originally assembled the BNC connector on the core so that it covered the key-way to open the clamp. I rectified this but didn’t film the change.

Also, you can wrap the wire round the core without removing it from the housing but that means you don’t have a nice flat surface to affix the BNC connector to. It does make it easier to close the clamp however so make your choice.

conducted rf immunity calibration impedance and measurement voltages

When is a Test Level Not a Test Level?

14th March 2018/in Testing /by James Pawson

Answer: When you don’t read the standard properly!

I was verifying my EN 61000-4-6 conducted RF immunity test setup after the construction of some new test adaptors and acquisition of some new equipment when came across something that left me scratching my head. I figured it out eventually and updated my calibration procedure with a note but it did have me puzzled for an hour!

Like most conducted immunity signal generators, the one I use combines a signal generator and modulator with a power amplifier and some front panel controls/readouts for performing the basic functions. It also has an RF Input for calibrating Coupling/Decoupling Networks (CDNs) which measures the voltage at the 150/50 ohm calibration adaptor and sets the output voltage of the generator to the correct level. My generator has a LED bargraph display showing the level which provides a reassuring visual confirmation that everything is OK.

Confused by Conducted, Stumped by the Scope

Having calibrated my new CDN at 3V, since I had a scope within reach, I decided to run the test but monitor the output of the calibration adaptor with the scope to make sure it was all working OK.

I did not see the expected 3V level, instead the RMS measurement on the scope was 0.5V and the pk-pk was just over 1.5V. I checked my 50 ohm thru termination on the scope input and even swapped it for a different one. My second scope also read the same voltage so it clearly wasn’t the scope. Puzzling.

I swapped the CDN for one that had been previously calibrated CDN and the lower than expected output voltage persists. Try turning up the generator voltage to 10V and I can’t even achieve 3V on the scope. Yet when I swap the connection from the scope to the RF generator it proclaims that yes, that is indeed the level that the generator says it is outputting.

Putting a BNC T-piece in series and monitoring the voltage with the RF input terminating the signal still achieves the same result. Is the generator RF input broken and reading the wrong voltage?

I checked the operating manual of the generator – the cal setup I’ve been using for years is correct. Then I carefully read the standard, focusing on the section that deals with calibration of the test adaptors. All became clear…

Open Circuit Voltage vs Loaded Voltage

EN 61000-4-6 specifies the test levels in terms of Uo, open circuit voltage. However the generator level setting part of the calibration is based on a measurement of Umr, the measured output voltage. This is a slightly simplified version of Figure 9 from the 2014 version of the standard showing the impedances of each part of the system.

Tucked away at the bottom of the calibration section is the formula that links the two together.

Uo = Umr / 6

Which yields the following values that the input of the generator or the scope should be looking to measure:

Test Level	Uo (Vrms)	Umr (Vrms)
1	1	0.167
2	3	0.5
3	10	1.67

For the measurement, the impedance of the decoupling part of the CDN is big enough that the termination of the AE port is not significant to the measurement, making most of the current flow through the EUT port network. You should be able to open or short the 150 ohm AE port termination and not see the measured output voltage change significantly.

By simplifying the above image and a bit of Ohms law you can clearly see that Umr is 1/6 of Uo.

Of course these are RMS voltages. If your scope that you are measuring with doesn’t have an RMS function then you’ll probably be measuring the peak to peak voltages. The conversion factor is:

Vpk-pk = Vrms x 2 x sqrt(2)

Which when added to the above table makes life a bit easier.

Test Level	Uo (Vrms)	Umr (Vrms)	Umr (Vpk-pk)
1	1	0.167	0.467
2	3	0.5	1.4
3	10	1.67	4.67

Panic Over

Armed with this new knowledge I revisited my calibrations to find that yes, everything was measuring correctly. The RF generator, being designed specifically for conducted RF immunity testing, takes care of the divide by 6 in it’s calculations.

As an ex-colleague was often heard to remark “every day is a school day” and today’s lesson was a good one. I hope this article saves you a bit of head scratching next time you are verifying your conducted RF immunity test setup.

Posts

Our Recommendations

Risk Assessment Factors

AC Mains Port vs DC Power Port

Power supplies do not always meet the regulations

Cable Routing

Power Supply Common Mode Impedance

Class I vs Class II Power Supply CM Impedance

Class II

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

Conclusion

Definition

EMC Tests Required for a DC Mains/Power Port

Examples of A DC Power/Mains Port

Telecoms

Computing Data Centres

Electricity Substations

DC Micro-Grid

Emergency Lighting Central Battery Units

Fire Alarm System

Examples of NOT DC Power/Mains Ports

AC/DC Power Adaptor

DC power distribution around a typical DIN rail electrical cabinet

Power over Ethernet (PoE)

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

Conclusion

Introducing the Monitor-o-Matic 8000

Specifications

Use Requirements

Future Additions

Intro

Conclusions (TLDR)

Pulse Parameter Comparison

Pulse Width Definition

Open Circuit Ideal Waveform Comparison

Modelling Notes

Review of Waveform Comparisons

Dealing With Pulse 1 (24V) and Pulse 2a

EUT Design Assessment

EUT Surge Suppression

Pulse 1 Peak Voltage & Current or Pulse Energy?

Pulse 1 Best Compromise Voltage

Sidebar

Pulse 2a Best Compromise Voltage

Test Practicalities & Further Compromises

Pulse 1 Power Disconnection

Coupling/Decoupling Network Requirements

The End.

DIY Current Probe

Core Losses / Insertion Loss

Secondary Turns

Capacitive pickup

Not as Robust

Future Videos

Video and Construction Errata

Social

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