Here is an interesting problem I had working on piece of industrial equipment. The customer had some conducted emissions failures at another EMC lab and needed some help resolving them.
The lessons from fixing this problem was that the first thought is not always the correct one, and that sometimes, all you need is a bit of green-and-yellow earth wire!
A block diagram of the system is shown below with the major components shaded.
An industrial power supply feeds power to the controller (a custom PCB connected to a Raspberry Pi) and to the power measurement board (measures the power consumed by the load).
Conducted emissions on both the Ethernet port and the AC mains port on the power measurement board were both dominated by a low frequency hump around 700kHz.
Notice how the shape or profile of the emissions is almost identical. To my mind, this points towards a single component in the system causing the same noise to be seen everywhere.
The first thing I wanted to do was to simplify the test setup as much as possible. I replaced the industrial power supply (often designed for Class A emissions performance) with my trusty Thandar TS3022S adjustable linear bench supply.
The idea here was to eliminate the industrial power supply from my inquiries.
Wow, what a big difference!
So the conclusion here is that the industrial power supply DC output is very noisy, that this noise is propagating through the system, and manifesting as conducted emissions on the outputs via a variety of coupling paths.
Differential Mode Filtering
Because conducted emissions noise in this lower frequency range tends to be differential in nature (+ve relative to -ve), my first thought was to implement a differential mode filter on the output of the power supply.
I’ve got a little filter prototype board that I use in situations like this. This pi filter was made up from two Panasonic FC series 470uF, 25V on either side of a Wurth 33uH iron powder inductor.
Unfortunately it did nothing to the emissions!
Could it be Common Mode?
This sounds like a obvious question to ask in hindsight. Most EMC problems are common mode in nature, I’m just used to thinking about LF conducted emissions as a differential mode problem.
Let’s try a common mode mains filter on the output of the power supply to see if this is indeed the case.
That’s much better! It looks like the problem was common mode noise after all.
This Time It Was Actually A Good Idea…
Common mode noise in this instance is current on both the DC output lines together. But, as I point out in one of my talks, current flows in a loop and always returns to the source. So where is this common mode current returning to? What is it’s reference?
Our common mode emissions measurements are being made in relation to the metalwork of our screened room test setup which is connected to the AC mains Protective Earth (PE).
The AC mains line to each LISN contains a PE connection and, inside the LISN, this is connected directly to the floor of the chamber.
Logically then, connecting the DC negative to the PE on the power supply will provide a shorter path for this common mode noise from the power supply.
Will this have the desired effect on emissions?
Yes. Yes it does.
Ooooooh, bloomin’ common mode noise. Not just for the higher frequencies but lower ones too!
This was a fun half day project fixing this particular problem. Much nicer to be able to recommend a low cost cable assembly than £$€ 20 worth of filter block.
If you’ve got any EMC problems then give me a call, I’d be happy to help.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2021/03/1-lf-common-mode-noise-system-overview-.png4031046James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2021-03-09 16:51:522021-03-16 08:39:07Low Frequency, Common Mode, Conducted Emissions
It’s a little known fact that Brian Wilson from the Beach boys was an environmental test engineer at Big Corp before pop stardom called him away from his first career.
He quickly realised that the hit parade wasn’t ready for ballads about Bellcore or ditties about humidity and switched to songs about the West Coast, cars and surfing. What could have been eh?
What is true however, is that whatever product it is that we make, at some point we have to release it from our development lab and into the big bad world. This raises the question: how well is it going to survive?
Why Perform Vibration Tests Anyway?
As we know, there are known knowns, known unknowns, unknown knowns and unknown unknowns…. y’ know?
“But James,” you cry, “surely I could just send my product via DHL / UPS / least favourite courier service (delete as appropriate) and achieve the same effect as testing?!”.
Well that’s very true, but I’m sure it’s going to be quite difficult to calibrate Dave the delivery driver, Vinnie the Van, and Winston the warehouse man for a consistent acceleration profile.
Testing is a simulation of what can happen in the big bad world. Like all simulations, it is a sensible average of many different situations. I’m not sure how these standards were originally derived but I’m sure more thought went into them than just kicking a development sample around the car park.
(If not actual thought, then at least a lot of tea and biscuits at the committee meetings, and that’s good enough for me)
Testing it primarily about mitigating risks, both of known risks and unknown risks. Whilst you might not be able to envisage a situation where your product is subject to a 1g, 30Hz vibration, real life might have other plans.
Primarily we are trying to quantify our knowns and unknowns.
Common Questions, Regular Requests
A version of one of these questions crops up every month or so.
“I want to do vibration testing, is there a standard that I can use?”
“What level of vibration do I need?”
“Do I need to do a random vibration test or is a swept test OK?”
“How do I simulate something being driven around in the back of a van?”
Vibration standards are quite often customer driven with a defined procurement specification. How close those specifications are to real life conditions depends on how much research that organisation has done or if they just pulled the numbers out of a hat. Often there is no way to get at the data or decisions that led to those choices and one must take them at face value.
Sometimes serious research goes into these levels, with car manufacturers employing multichannel data recorders and taking the latest model out for a test drive whilst getting tangled in accelerometer cables (safely I’m sure). This ends up being proprietary data so good luck getting your greasy mitts on it unless you happen to be working for them.
For the rest of us without a big budget how do we even make a start? Just go an buy every single BSI standard on vibration testing? Oh wait, we said without a big budget…
In the absence of any customer specification to work to, a good place to start in determining required levels and profiles is with the ETSI Environmental Engineering series of standards.
It’s a Splendid Smorgasbord of Shaky Situations
(or a Buffet of Battering if you will…)
The root standard ETSI EN 300 019-1-0 v2.1.2 (most recent at the time of writing) gives an introduction to the “Environmental conditions and environmental tests for telecommunications equipment”. Since this could easily be used to describe most modern electronic equipment then it is widely applicable.
This incredibly useful series of standards are provided for download free of charge, a much more welcome approach to the usual way that standards are sold at high prices.
Not only that but they don’t just cover vibration. They cover the expected environment in terms of
Temperature and rate of change of temperature
Solar and Heat radiation
Moving air speed (wind)
Weather conditions (driving rain, icing, etc)
Biological conditions (mold, rats, animals)
Concentrations of chemicals
Vibration and shock levels
That’s a pretty comprehensive list! The root standard contains a useful table that enables easy selection of a sub part appropriate for your product.
Each Class has two standards that relate to it:
Part 1 (suffix -1-x) specifies the expected environmental conditions for the situations (storage, transportation, in use) and locations (underground, on a ship, etc)
Part 2 (suffix -2-x) specifies the recommended test levels and methods/standards for each class.
This table has quick links to each of the Part 1 and Part 2 standards for the various classes for reference:
Part 1 – Environment Definition
Part 2 – Test Specification and Methods
Root Standard with background and general definitions
“Yes Robin, but how do we apply them to our (bat) product?”
Let’s invent an imaginary product.
The Monitor-o-Matic 9000 is a battery powered environmental monitor for indoor and outdoor locations. It gets periodically transported around site in the back of a van (maybe not always in its nice Peli case, naughty naughty)
In this instance we could reasonably apply:
Storage – the equipment is often kept in the van when not in use which can get pretty hot and cold. So we’ll apply “Class 1.2 Weather protected, not temperature controlled storage locations”
Transportation – with a bumpy van, and the M-o-M 9000 not always stored in its box means we’ll pick the harshest tests of “Class 2.3 Public transportation”
Usage – Non-Weather Protected Locations could be characterised as locations “…where transmitted vibrations are experienced from machines or passing vehicles. Higher level shocks may be experienced e.g. from adjacent machines.” We’ll choose “Class 4.1E: Non-weather protected locations – extended”
So lets pull out only the vibration requirements from each of these (bear in mind they also call up temperature, humidity, chemical resistance and other factors)
Unsurprisingly (some might say shockingly), transportation forms the biggest risk of vibration and shock to the product. We’ve now got some test levels to work with.
Note that some of the standards don’s account for the product being dropped from height. The storage standard does have a static load test simulating things being stored on top of the product.
Similarly the transportation test has the same static load requirements but adds free fall, toppling and rolling into the mix.
Questions to Ask Yourself
We normally ask ourselves what the expected use conditions are for a product. As with anything like this, we must also ask “what are the foreseeable misuse conditions?“.
Confession: I’ve Been Seeing Other Standards
This is just a simple guide to using the ETSI standards to make sensible design decisions in the absence of any other good information. However, many standards codify vibration and shock requirements, making the decisions a little easier.
EN 54 for Fire Alarm components has some quite challenging requirements for alarm components. In one shock test I performed for a customer on a smoke alarm the unit pinged straight off it’s base at the lowest shock level, necessitating some further work on the mounting (it subsequently passed no problems).
Military, Automotive and Aerospace sectors will definitely have their own vibration requirements and will commonly be clearly specified as part of the procurement specification.
Anyway, I hope you find this useful. Get in touch if you need some further guidance or need to ask any questions.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2019/01/300-019-1-0-environmental-and-vibration-table.png12361262James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-11-03 22:28:482020-11-03 22:35:16How Do I Choose Vibration Tests For My Product?
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.
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”
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.
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.
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:
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.
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.
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.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/06/iso-7637-2-pulse-1-vs-iec-61000-4-5-waveform-comparison.png8861931James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-06-29 19:36:022020-10-19 08:10:37IEC Surge/EFT Generators for ISO 7637-2 Automotive Pre-Compliance
The LVD a.c. voltage limits are defined in terms of rms. From 2014/35/EU
This Directive shall apply to electrical equipment designed for use with a voltage rating of between 50 and 1 000 V for alternating current and between 75 and 1 500 V for direct current
So that’s 50Vrms and 1000Vrms.
A customer was asking about a low current (sub 1mA) 40Vac rms source and if the LVD applied.
This voltage is less than the 50Vrms threshold mentioned above so would be technically exempt from the LVD. Unless of course the equipment contained a radio module in which case the RED makes the LVD applies with no lower voltage limit.
I think that there is still a risk that needs to be assessed here.
Just because you are exempt from the directive doesn’t mean you are exempt from making your product as safe as possible.
Looking at the main table from 62368-1 for categorising shock risk, 40Vrms/50Hz means it is an ES2 hazard. The voltage source would have to be under this limit for normal operation, abnormal operation (e.g. blocked vent, stalled motor, controls set incorrectly) and single fault (open/short circuit) conditions.
This means, even though the LVD is not strictly not applicable, it would be wise to put in a Basic Safeguard between the user and the exposed voltage.
Additional: The provisions of the General Product Safety Directive (2001/95/EC) would apply to any product falling outside of any specific safety standard. The Harmonised Standards for this Directive include EN 60065 and EN 60950-1. Since both of these have been superseded by EN 62368-1 it would be reasonable to use this standard instead.
Thanks to Charlie Blackham from Sulis Consultants for the tip.
This safeguard could be an enclosure, insulation, an interlock or barrier.
Instructions or PPE aren’t sufficient as they are considered supplementary safeguards.
But what about the current limits?
That’s just considering the voltage source purely from a voltage perspective. If it can’t drive enough current into a 2000 ohm load for more than 2 seconds to form a hazard then that might change the classification.
This current is measured using one of the appropriate networks from EN 60990 such as the one below
But I know what I’m doing…
The requirement for safeguards depends on if you classify the user as a normal person or an instructed person
Skilled person > instructed person > normal person
18.104.22.168 instructed person person instructed or supervised by a skilled person as to energy sources and who can
responsibly use equipment safeguards and precautionary safeguards with respect to those
ordinary person person who is neither a skilled person nor an instructed person
skilled person person with relevant education or experience to enable him or her to identify hazards and to
take appropriate actions to reduce the risks of injury to themselves and others
The level of safeguard required between the user and the ES2 hazard is defined in EN 62368-1
For a normal person we must use a basic safeguard
But for an instructed person we may use a precautionary safeguard
A Precautionary safegard (defined in 0.5.5.3) could take the form of instructions or training, but the addition of warning stickers, PPE could also be considered part of this.
This is why I like EN 62368-1 over some of the older safety standards (I’m looking at you, 60950)
Rather than a prescriptive “thou shalt use 2.5mm clearance or be smote verily” it helps and guides you through all the steps into understanding why or how a requirement is derived.
Also the companion EN 62368-2 explanatory document contains even more background and context. I wouldn’t recommend applying -1 without having -2 to hand.
Stay SAFE kids.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/06/table4.png3781237James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-06-18 11:02:402020-06-26 08:49:23LVD Voltage Limits are RMS + Thoughts on Marginal Voltages
“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.
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.
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
There are two big risks to the immunity performance: Radiated RF Immunity and ESD.
Radiated RF Immunity
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:
Test Level (V/m)
18 Hz pulse, 50%
FM +/- 5kHz dev. 1kHz sine
710, 745, 780
217 Hz pulse, 50%
810, 870, 930
18 Hz pulse, 50%
1720, 1845, 1970
217 Hz pulse, 50%
217 Hz pulse, 50%
5240, 5500, 5785
217 Hz pulse, 50%
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:
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.
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 ideas to mitigate this interference include
Keep traces/connection as short as possible between sensors and ADC
If you can mount them all on the same circuit board then do so
This circuit board will have one layer dedicated to a solid ground plane fill over the entire plane. All ground pins
Cables = antennas that are good at receiving the interference. Minimise use of cables where possible.
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.
Decouple the supply lines to the pressure sensor well
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.
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.
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.
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.
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.
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
Radiated Emissions, Class A, 30MHz to 1GHz (EN 55011)
Mains Conducted Emissions, Class A, 150kHz to 30MHz (EN 55011)
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
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.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/04/BIPAP.jpg7981024James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-04-18 15:55:522020-04-18 16:05:00Ventilator Projects and EMC Testing (EN 60601-1-2:2014)
1. CE marking probably won’t be necessary for UK for temporary ventilators 2. Still need to apply to MHRA for permission to use in clinical setting 3. This will likely involve the supply of test data to MHRA 4. The requirement for EMC testing is currently To Be Confirmed 5. U3C will give any ventilator design a pro bono EMC design review
In the current Covid-19 pandemic there are many project teams around the world coming up with designs for ventilators. The potential is for many such devices to be required.
The problem that this article identifies is that for normal medical devices, the length of time taken to get products approved is defined in terms of months. This is especially true for ventilators which are, according to several sources, a Class IIb device requiring a robust quality system and Notified Body approval.
The current requirement is for devices to be delivered within weeks.
The procedure covered under the “Exceptional use of non-CE marked medical devices” involves applying to the MHRA directly for a pass to supply this product for use without CE marking. This request will have to include data that shows compliance with the test criteria in Appendix B of the MHRA specification.
Page 11: “3. When the current emergency has passed these devices will NOT be usable for routine care unless they have been CE marked through the Medical Device Regulations. The device must display a prominent indelible label to this effect.“
Page 24: “EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”
Note that the requirement for EMC testing remains TBC = To Be Confirmed. I strongly feel that at least some testing should take place, but functionality of the ventilator has to come first.
Next article will be a breakdown of the EMC test requirements called up in EN 60601-1-2:2014. I’ll also highlight some risk areas that designers of these ventilators will need to to bear in mind.
If anyone has any need for pro bono EMC design review services for ventilator projects or needs some EMC testing turning around quickly then get in touch.
I had an email from a customer that I’m working on some design consultancy work with, saying that one of their prototype products was having some radiated emissions problems at an accredited lab. Could I take a look?
Absolutely, EMC radiated emissions problem solving is my favourite part of the job! Ironically, it is usually the customers least favourite part!
Thankfully I had a slot free the next week so they bundled their kit into the car for the long drive “Up North” from their base in the South West of the UK.
After some tinkering, the equipment was set up in the chamber for some radiated emissions work. The first scan confirmed the problem levels and frequencies that had been observed at the other laboratory.
The problem areas from their last scan were at 35MHz, 80-90MHz and a broad band between 150MHz and 220MHz.
The system was housed inside a nice aluminium case that was being used for CPU heatsinking and environmental protection as well as EMC shielding. A rough diagram of the internals shows a main PCB with a large CPU / memory block in the centre and a variety of cables leaving the PCB and the casing.
The main power cable housing also had two debug connections inside the same housing that weren’t being used in the field but were available for updating software and such like.
As is so often the case, this product was in it’s final stages of the development life cycle, meaning that no major design changes were possible. These EMC problems would have to be resolved using easy to fit additional components. Thankfully I have plenty of things in stock to try out.
There are two important characteristics about these emissions that show us where to look
They are predominantly broadband, an indication of analogue noise e.g. DC/DC converter / power supply. Sometimes this broadband noise is generated by digital switching but this can be less common.
They are all low in frequency, where large or long structures are the most efficient antennae. This usually means cables.
So power noise and cables…. hmmm…. any good ideas?
OK Kids, Let’s Take a Look at the Cables.
In a very sensible move by the designer, both the DC power and Ethernet cables had some common mode filtering on the PCB.
Ethernet magnetics have common mode chokes built into the transformer stack which reduces the noise emitted and increases the susceptibility performance of Ethernet despite the often unshielded twisted pair cables used.
The caveat is that once the cables have left the magnetics that they must be protected from other interference sources. Noise coupling on to these lines is going to be heading straight out of the enclosure using these lines as the antenna. Similarly, if common mode noise gets onto the centre-tap of the output side of the magnetics then this can also cause similar issues.
I have experienced system noise coupling on internally routed Ethernet cables before and it nearly always results in lots of low frequency emissions.
The power cable had a small surface mount Murata filter in place with excellent attenuation at the frequencies of interest.
Both the Ethernet and power cables pass through the shielded enclosure with no connection or filtering to the case. In bypassing the quite nice Faraday cage of the enclosure, any noise current on these lines will inevitably appear as radiated emissions and be picked up by the receive antenna..
Now to find out some more info.
Radiated Emissions Experiments
First, unplugging the Ethernet cable dropped the emissions significantly from 30MHz to 120MHz.
Secondly, some messing around with ferrite cores on the power cable reduced the 150MHz to 220MHz hump down to sensible levels.
This left a single peak at 270MHz that was traced to noise using the coaxial RF cables to the antenna to radiate.
Lets look at each of the points in a bit more detail:
The only practical method of dealing with the Ethernet emissions was to change the bulkhead connector to a metallic screened version and the external cable to a SSTP (Screened Shielded Twisted Pair) type of cable. No exciting analysis here I’m afraid.
Details of the Power Cable Noise Coupling
The most interesting coupling mechanism was happening inside the un-screened bulkhead power connector. Thanks to the power filter on the PCB, there was very little noise being conducted back down the cable from this line. However, the debug connections to the CPU are picking up all kinds of noise and carrying that noise to the connector.
Disconnecting and bundling the debug cables near the connector cuts the radiated emissions down to next to nothing.
What’s most interesting is that the capacitive coupling region between the power cable and the internal debug cables is so small. The connector is only 20mm long and the cables run parallel with each other for barely any distance. And yet there is enough noise current being coupled onto these lines that it causes a radiated emissions problem.
Details of the RF Antennae Noise Coupling
By the time that all of the cables had been filtered or removed, there remained just one emission at 270MHz that was failing the Class B limit. An investigation with RF current probes showed a lack of noise on the main output cables listed above, even when they were screened or filtered appropriately.
A wander round the enclosure with an electric near field probe and spectrum analyser showed a spike in emissions near the RF antenna housing on the side of the EUT.
Checking the antenna feed cables showed them connected to the PCB pretty centrally. Disconnecting the coaxial cables from their mating halves dropped the emissions down to the noise floor.
Even though the noise isn’t in-band for the antennae themselves, they still perform well enough to radiate the noise and cause an emissions problem.
Summary of Fixes Applied
The below diagram shows the fixes applied to the EUT to achieve a Class B pass.
Firstly, a fully screened metal bulkhead Ethernet connector was chosen for use with a shielded cable. This isn’t ideal from the installation point of view but is ultimately unavoidable without more significant modifications to the EUT.
Secondly, a Wurth ferrite was equipped around all three of the cables connected to the power bulkhead connector. As detailed above, it is necessary to put the ferrite around all three cables and not just the power to reduce the noise entering the capacitive coupling region around the connector.
Thirdly, a small ferrite was placed around each of the UFL cables at the point at which the antenna cables left the housing. This is a fairly common modification for radiated emissions, one I’ve employed several times before, and there are numerous suppliers of ferrites of various lengths with just the right inside diameter for the type of thin coaxial cable used with UFL connectors.
Any time your cable passes through a shielded enclosure with no RF termination at that point, you can pretty much guarantee its going to need some filtering.
Nothing particularly in depth in this analysis of the EUT, but I did find the coupling in and around the power connector particularly interesting.
At the end of the day, the best outcome was a happier customer with a path forward for their product.
I’ve always been impressed with Apple’s approach to reducing problems caused by EMC/EMI. Making top of the line technology in a compact case means minimising risk and maximising performance.
Let’s look at an example of well considered EMC design and why it is so useful.
Even the EMI shielding solutions are stylish
Because their products are charged at top dollar prices, they can afford to (or can’t afford NOT to) put in features like this.
The RAM on the new Mac Mini (thanks to iFixit for the great photos) has its own removable cage, secured to a PCB level counterpart with screws and, no doubt, a decent fit along the edges. What’s interesting is that this shielding system will have multiple functions.
Primarily, it will be used to reduce the EMC radiated emissions from the product. The Apple products I’ve had in my anechoic chamber have all been very quiet and this is why I hold Apple in some regard for their EMC design.
Apple will no doubt have tested their design with multiple RAM vendors to satisfy themselves that the design meets the requirements of international EMC standards.
However, were the user to install some non-Apple verified memory modules then the risk of emissions could increase. One can well imagine that Apple will have considered this in their EMC Risk Assessment.
The secondary benefit is more subtle. Take a look at this image.
Original image courtesy of iFixit, markup by author
The memory modules and their screening can are highlighted in red. Next to it, highlighted in green, is a smaller board level shielding and a UFL antenna connector. (There are another two connectors out of sight underneath the case)
That’s right, Apple have put the most noisy part of the system (RAM) right next to one of the most noise-sensitive (Wi-Fi). What?
This is not an uncommon problem, especially when trying to compress so much functionality into such a small space.
The Mac Mini is only 165mm square (that’s 6.5″ if you are watching in black and white). The case includes an integrated mains power supply making proximity between electromagnetically incompatible systems unavoidable.
Modern RAM speeds are fast and the Mac mini is no exception. Everymac lists the latest Core i7 model with a DDR4 memory speed of 2.66GHz. That’s uncomfortably close to the Wi-Fi operating band of 2.4 to 2.5GHz.
The interference spectra of a DRAM interface fundamental frequency is generally quite wide band.
If you turn on any form of Spread Spectrum Clocking (SSC) to reduce the peak energy then it can spread over tens or hundreds of MHz. Either way, that puts the edges of the memory fundamental in band for the 802.11 a/b/g/n/ac interface on the Mac mini.
The harmonic emissions of the memory are also prevalent and it’s easy for these to fall in-band of a wireless interface like Wi-Fi. For instance the second harmonic of 2.66GHz is at 5.32GHz in the channel 64/68 region for 5GHz Wi-Fi. Big problems.
Improve Performance? The Can Can.
The effect of in band interference on a Wi-Fi interface can be subtle.
At it’s most gentle, there’s a reduction in both performance and range. The modulation, coding type and channel width of the Wi-Fi sets the robustness of the interface to interference.
At the other end of the scale, whole channels can be blocked out entirely.
This intra-system, or platform level interference is pernicious and can be difficult to isolate and track down. Low noise floor spectrum real-time analysers are extremely useful tools here.
Ultimately, segregating the noise source from the receiver, is the only real solution. This can be achieved by physically separating the aggressor and victim (not possible here) or by shielding.
For some companies, the fallout in performance of a couple of Wi-Fi channels is no big deal.
If you are Apple however, then you can’t afford to have dissatisfied customers complaining about poor Wi-Fi speeds. As always, the EMC budget has to be congruent with the product budget and the desired performance.
The Last Line Of Defence
Check out the textured surface between the mounting holes for the lid (blue highlight on the above photo). That will be an EMI seal to ensure good contact between lid and case. Not only a nice touch but an important one.
The Wi-Fi antenna is mounted on the outside of the shield so this circular lid actually screens the antenna further from the noisy internal circuitry of the mini.
Well done Apple. I’d love to see your Wi-Fi range testing results… please?
A customer had purchased some power supplies from Amazon UK to get started with the development on their product. And why not? There are lots of cheap products available and everyone has a budget to meet. The chances are that they’ll get damaged, lost or broken anyway.
They were happy with the (perceived) quality of the PSU so approached the manufacturer directly for bulk pricing for volume production. However, the Amazon sample made it’s way to Unit 3 Compliance for EMC pre-compliance testing is where the fun began…
Externally, the only way to tell the difference between the compliant and non-compliant versions is a slight difference in the length of the barrel connector and a slightly different shape of strain relief grommet.
These devices are being marketed as the same device on the outside and yet are completely different on the inside!
I’ve not been able to subsequently find this exact power supply on Amazon but there are similar looking variants still available.
A Real Problem
Crucially, it’s not just EMC that is being sacrificed. This “race to the bottom” of extracting every last penny from products has more serious consequences.
At a previous employer, an inspection was performed on 50 power supplies (again, bought from Amazon) that one of the project teams had purchased for powering various development platforms within the company. This revealed some serious safety problems (creepage and clearance) resulting in the entire batch being quarantined and scrapped for recycling.
Another aspect to consider – if the manufacturer has two different, almost indistinguishable products then how does your supply chain guarantee that you will receive the correct one? What is to stop the manufacturer from swapping out the more expensive compliant power supply halfway through production?
The principle of caveat emptor still applies. Disingenuous product markings are being used to falsely indicate compliance.
What To Do?
The obvious way round this is only to buy small quantity power supplies from trusted suppliers. I know from working with other customers that suppliers like RS and Farnell / Element 14 take compliance seriously. Buying from these sources is more expensive financially but what price do you put on your own safety?
If you are relying on buying a pre-approved power supply always ask for the EMC and safety test reports and the Declaration of Conformity. A supplier who cannot readily supply these readily should be disregarded.
Compare the details in the reports with the physical sample in front of you. Especially for safety reports, photos of the unit are generally included, inside and out. Look for any differences between the two.
Differences in EMC performance are not obvious. The only way to be sure of the quoted performance is to perform some quick tests, conducted and radiated emissions being the two main ones.
How We Can Help.
Here at Unit 3 Compliance we can give you some peace of mind that your power supply isn’t going to cause you any issues. Some of the things we do include:
Provide full EMC testing for all off the shelf products
Electrical safety analysis and testing
Help you understand the technical and financial compromises between units
We can review test reports and compare to physical samples with an experienced eye
Every incoming customer power supply is given a HiPot test as standard to help catch any problems
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2018/10/power-supply-comparison-header-1.png308841James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2018-10-26 09:49:372021-09-12 07:56:46Off The Shelf and Non-Compliant Power Supplies (from Amazon)
I’m really happy to have one of my blog articles featured on Interference Technology.
Problem solving and fault finding EMC problems, especially radiated emissions, is one of my specialities and oddly enough is one of the facets of my job that I enjoy the most. After a successful exercise in helping a customer out with their product, getting the chance to write about it and share it with you is a real bonus.
Fixing radiated emissions is at it’s most challenging when the scope for modification to the unit are limited by the fact there are significant stock of PCBs or components that would require scrapping and redesign. Finding a way to use the existing stock was key in this example as the customer had significant time and money invested into the project. Thankfully I was able to help them out.
Head on over to Interference Technology and have a read through – I even put pictures in! Hopefully it will give you an idea of how I work and the sort of EMC issues that I can help you solve.