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Compromise EFT Test Setup

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

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

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

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

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

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

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

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

By James Heilman, MD - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=34056919

Ventilator Projects and EMC Testing (EN 60601-1-2:2014)

Summary

If you haven’t already, check out part 1 of this blog Part 1: Rapidly Manufactured Ventilator (RMV) Projects and EMC Regulations

We’re going to take a look at the EMC requirements for RMVs, consider some of the risks posed by EMC and propose some methods of mitigating them.

Probably the biggest EMC risks are Radiated RF Immunity and ESD due to their higher than normal test levels.

If you need any fast turnaround design support and testing services for your Rapidly Manufactured Ventilator project then get in touch.

Background

As noted in the MHRA RMVS specification on page 24:

“EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”

So lets take a look at a the EMC tests that might be required for a typical Rapidly Manufactured Ventilator project.

This is with the view of meeting the Essential Performance / Basic Safety requirements of EN 60601 whilst addressing the highest risk items first. This is prioritising speed of testing instead of performing a belt and braces, test everything approach that would be the common approach for Medical Devices.

Emissions

These ventilators are going to be used in a hospital / clinical care environment under medical supervised use and not in a home environment.

60601-1-2 classifies a hospital as a Class A emissions environment for Radiated and Conducted emissions. This means that less time needs to be spent fighting to get the emissions below Class B.

Risk items to radiated emissions could include any brushed DC pump motors as these are notoriously noisy. Ferrite cores may be required around motor cables to mitigate this noise.

Following the design guidelines further down this article for any PCBs is recommended and will greatly assist with reducing EMC radiated emissions.

Most RMVs will be using an off the shelf power supply already approved to EN 60601 for Safety and EMC. AC Power conducted emissions should therefore look after itself and won’t be a significant worry for testing.

For Harmonic Distortion and Flicker, there is an interesting note in EN 60601-1-2 in Annex A

“It is assumed that ME EQUIPMENT and ME SYSTEMS used in hospitals (and large clinics) are not connected to the PUBLIC MAINS NETWORK.”

If this is the case, then Harmonics and Flicker requirements and tests need not apply as these only relate to the public mains network.

Immunity

Overview

Most ventilator systems have no external electrical ports apart from the power supply. They are mostly self contained units. This greatly speeds up and simplifies the testing, and reduces the risk of problems with Signal Input / Signal Output Ports (SIP/SOP Ports in the standard, analogous to a Signal Port from other EMC standards).

Mains borne interference (EFT, Surge, Conducted RF) should be handled by the EN 60601 pre-approved power supply without issue. It will still need checking but ultimately the risk is low.

Dips and Interrupts and the hold up time of the power supply is something that would need considering at the Risk Analysis level to derive the correct immunity criteria for each of the individual tests.

If this needs improving then selecting a slightly larger power supply than nominally required could help. More likely, additional bulk decoupling on the main power rail (e.g. 1000uF) will help maintain the system DC voltage under these conditions.

Immunity Performance Criteria

Caveat: This section is me thinking aloud as I have no domain specific knowledge for Risk Management and Medical Devices. I’m trying to approach this from a common sense perspective to aid anyone working on a RMV.

The function of the EMC Immunity tests for Medical Devices is to ensure that the Essential Functions continue to operate and that Basic Safety condition is maintained.

Normally the immunity performance criteria would be based on the type of EM phenomena being simulated in the test. This is normally Criteria A for continuous phenomena (radiated or conducted RF immunity) or Criteria B for momentary phenomena (ESD, EFT, Surge, Dips/Interrupts). Criteria C only tends to crop up for longer duration power interruptions.

In the case of a Medical Device, maintaining the Essential Performance is the key parameter. If a momentary EM phenomena causes this to happen then this is a major problem.

Therefore immunity performance criteria must be considered for the key function of the device as well as the duration of the EM phenomena.

Based on this thought process, a sensible starting point for the immunity criteria is:

Assume criteria A (unaffected performance) for:

  • Key function of assisting patient breathing for all tests. This includes momentary EM phenomena tests = ESD, EFT, Surge, Dips/Interrupts.
  • Non-critical functions under continuous EM phenomena tests = Radiated and Conducted RF Immunity

Assume Criteria B for:

  • Key function performance for this means that there should be a function in the RVM firmware that remembers its last current operating state and settings and that it starts up in that state from a power cycle. This creates a requirement for programmable non-volatile memory (some kind of EEPROM) in the RVM.
  • All momentary EM phenomena tests for non-critical functions e.g. display readout may temporarily distort or flicker so long as it recovers

Assume Criteria C for:

  • Non-critical functions from momentary power loss e.g. screen/display readout or setting

Immunity Risks

There are two big risks to the immunity performance: Radiated RF Immunity and ESD.

Radiated RF Immunity

Test Requirements

The basic requirement for radiated RF immunity is a flat 3V/m from 80MHz to 2.7GHz. So far so good, this is a fairly easy test to meet.

Now the bad news. Table 9 gives a list of spot frequencies to be tested to simulate close range exposure to common wireless technology standards. The table is summarised here:

Frequency (MHz)ModulationTest Level (V/m)
38518 Hz pulse, 50%27
450FM +/- 5kHz dev.
1kHz sine
28
710, 745, 780217 Hz pulse, 50%9
810, 870, 930 18 Hz pulse, 50% 28
1720, 1845, 1970 217 Hz pulse, 50% 28
2450 217 Hz pulse, 50% 28
5240, 5500, 5785 217 Hz pulse, 50% 9

As you can see, this has testing up to 28V/m, a significantly higher field strength than 3V/m!

Risks to the EUT

This test loves to mess with analogue sensors. In the case of ventilators, the pressure sensors used frequently have an analogue output to a DAC on the CPU. This presents two risk areas:

  1. Demodulation of noise inside the pressure sensor amplifier. This takes the small transducer signals and amplifies it up to the output voltage. Noise demodulated here would cause the carrier to be superimposed on the pressure readings.
  2. The input of the ADC could be susceptible to noise picked up on the analogue voltage from the pressure sensor, even if the pressure sensor itself is unaffected. This will affect the readings.

Since the airflow and pressure sensors are a key component to the operation of the ventilator, these must be protected at all cost.

Design Recommendations

Design ideas to mitigate this interference include

  1. Keep traces/connection as short as possible between sensors and ADC
  2. If you can mount them all on the same circuit board then do so
  3. This circuit board will have one layer dedicated to a solid ground plane fill over the entire plane. All ground pins

    Check out my video presentation on PCB grounding and HF current flow.
  4. Cables = antennas that are good at receiving the interference. Minimise use of cables where possible.
  5. Figure out what your minimum bandwith requirements for airflow are and filter the signal appropriately. You probably won’t need to sample the airflow faster than 10kHz so put a low pass filter right next to the ADC input. Something like a 4k7 and a 1nF will give you a 3dB of 34kHz. This will reduce the risk of RF noise being demodulated by the ADC input.
  6. Decouple the supply lines to the pressure sensor well
  7. Add a small filter to the pressure sensor input, perhaps another RC filter as shown above. This will help prevent the pressure sensor from being affected by the test.
  8. It is possible that the pressure sensor will be directly affected by the radiated noise picked up by the sensor body itself and not by the traces. It would be prudent to provide a PCB footprint for a shielding can near the sensor. I have seen this effect on gas sensors in the past.

Risk Analysis

Assuming that the advice above is followed, the risk to the EUT is manageable.

One of the interesting features of Radiated RF Immunity testing is that of the Problem Band where most issues occur.

radiated rf immunity susceptibility characteristics

Most of the time, the problem band is in the 100MHz to 300MHz area (I’ll cover this in more detail in a future article). Cables tend to be the best antennae at these frequencies and, hopefully, our ventilator only has one cable of interest – the AC power cable. This has plenty of filtering for conducted emissions reduction which should handle this noise.

Probably the two biggest problem frequencies from the spot frequencies above are going to be 385 MHz and 450 MHz.

Then we are into the realms of direct pickup on internal signal cables and PCB traces at higher frequencies. If we’ve laid out our PCB well as highlighted above (short analogue traces, filtering, good ground plane, shielding provision) then this will help mitigate our risks.

ESD

Overview

The levels of ESD testing are almost twice that of the regular EMC standards with a requirement for 8kV contact and 15kV air discharges.

ESD is very good at upsetting digital systems and it has a particular fondness for edge triggered pins e.g. reset lines and interrupts.

Design Recommendations

If the reset line for the CPU controlling the RVM is shared with other digital circuit blocks or supervisory controllers then an RC low pass filter at the input to the CPU is highly recommended. This helps prevents unwanted resets.

Checking can be implemented in the Interrupt Service Routine to ensure that an interrupt condition actually exists, effectively de-bouncing the input.

Thankfully the Ingress Protection requirements for the RVM of IP22 and the requirement to provide flat, easily cleanable surfaces will probably dictate the use of some kind sealed membrane keyswitch panel. These have good ESD immunity as no direct contact discharge can take place on an switch where the plastic covering remains in place.

Whatever user interface technology the RVM employs, this will be a key risk area for ESD. If this is on a separate PCB to the main controller, all interfaces will need some kind of filtering. A small capacitor to ground on each of the lines that goes to the keypad would be a good idea. 0603, 100pF usually works well here.

Lastly on the mechanical design, keeping the electronics well away from the enclosure seams will also reduce the risks of creepage of any discharge into the circuit board.

Summary Test Plan

Emissions

  • Radiated Emissions, Class A, 30MHz to 1GHz (EN 55011)
  • Mains Conducted Emissions, Class A, 150kHz to 30MHz (EN 55011)

Immunity

Text in bold is highlighted as a risk item.

  • ESD, (EN 61000-4-2), 8kV contact, 2/4/8/15kV air. Test to connectors as well.
  • Radiated RF Immunity (EN 61000-4-3)
    • 80MHz to 2.7GHz @ 3V/m
    • Various spot frequencies at up to 27V/m
  • EFT (EN 61000-4-4), AC Mains Port, 2kV
  • Surge (EN 61000-4-5), AC Mains Port, 1kV line-to-line, 2kV line-to-ground
  • Conducted RF Immunity (EN 61000-4-6), AC Mains Port, 3V/m (6V/m in ISM bands)
  • Dips and Interrupts (EN 61000-4-11), AC Mains Port, various

Conclusions

Not only has this article identified key EMC risks to Rapidly Manufactured Ventilators but also provided some design guidelines to dealing with the problems that might arise.

Some of the guidelines within might be useful to anyone designing a Medical Device. We haven’t covered the requirements for Patient Coupled Ports or SIP/SOP ports from an EMC perspective as they aren’t of too critical a concern for an RVM.

We can see how looking at the standard and pulling out the required tests can help us understand the risks involved in the design.

Experience of knowing how the tests will typically affect the EUT is the key to unlocking good design practices. In my case, this comes from having worked on many designs with problems and the learning that comes from fixing the issues that crop up.

Remember EMC test success comes from good EMC design. For a time critical RVM there is one chance to get it right – no do-overs!

I hope you found this article useful. See you when all this has calmed down.
All the best,
James.

When ESD Protection Gets Bypassed

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

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

 

The EUT

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

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

system under test showing front panel, esd protection and cables

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

 

EUT + ESD = ???

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

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

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

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

Analysis

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

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

 

Lessons

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

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

 

 

 

stainless steel camera system

TWITL – Underwater Camera System Industrial EMC Testing

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

stainless steel camera system

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

A Simple EMC Fix

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

The fault finding process was relatively straightforward this time.

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

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

 

 

Simple RF Current Transformer for EMC / EMI Investigation

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.

crude differential mode surge spice circuit

Surge Testing, MOV Position and Fuse Current

I’ve been working on a power supply product for a customer with a very tight limit on the AC mains fuse rating. One of the problems this causes is during differential mode surge testing.

When the metal oxide varistor (MOV) connected line-to-line fired, the resulting current was enough to blow the fuse after a couple of surges at the specified 1kV surge (1.2/50us, 2 ohm). Clearly there wasn’t enough headroom for the product to pass the test. A different MOV with a higher clamping voltage would have reduced the peak current but at the cost of higher voltage stress elsewhere in the circuit.

I decided to look at if the position of the varistor within the circuit made a difference to the surge current in the fuse. It started off in the middle of the mains filter (PCB routing convenience I suspect) but perhaps mounting it before the filters would help? What about at the end of the filter chain, then the X2 capacitors can go to work on the surge pulse first.

The easiest way to try these scenarios was to stick it into SPICE (I like SiMetrix) and have a look at the variables. I crudely modelled the input stage of the power supply as shown below. I guessed at many of the series impedances for the fuse and the capacitor. However the leakage inductances and DCR for the inductors I measured using my excellent Peak Electronics LCR45 component meter. The MOV was simply a 1N4004 diode with a 400V reverse breakdown and the surge was only applied in the +ve direction.

crude differential mode surge spice circuit

I varied the position of the “MOV” between positions A, B and C to see if there was a difference in the surge current through the fuse (R15). Interestingly enough, there was.

surge test spice output

Red = A, Green = B, Blue = C

So the further down the filter chain that the MOV is placed, the less the peak surge current (56% lower) and the RMS current (23% lower) through the fuse.

The results were positive too. The power supply went from failing on the 5th strike at 1kV to passing 10 strikes at 1.75kV. A marked improvement resulting in a more robust product.

 

conducted rf immunity calibration impedance and measurement voltages

When is a Test Level Not a Test Level?

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.

conducted rf immunity cdn calibration impedances

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 LevelUo (Vrms)Umr (Vrms)
110.167
230.5
3101.67

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.

conducted rf immunity calibration impedance and measurement voltages

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 LevelUo (Vrms)Umr (Vrms)Umr (Vpk-pk)
110.1670.467
230.51.4
3101.674.67

 

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.