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.
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.
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:
|Frequency (MHz)||Modulation||Test Level (V/m)|
|385||18 Hz pulse, 50%||27|
|450||FM +/- 5kHz dev.|
|710, 745, 780||217 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:
- 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
Check out my video presentation on PCB grounding and HF current flow.
- 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,