Why Your Ventilator Design Needs to Consider Electromagnetic Interference (EMI)

In the last months, our normal routines have almost come to a complete stop. At the time this article was written, there are over 300 million people in the USA alone being told to stay home to reduce the growth rate of the novel coronavirus (officially named COVID-19). Experts state we may be nearing the peak in April or May, however, history has shown with the 1918 Flu Pandemic that a second wave in the fall/autumn season is possible.

COVID-19 is showing how our global medical system was unprepared for a pandemic such as this. The lack of ventilators is obvious. American hospitals currently have about 160,000 ventilators; however, estimations show 742,500 ventilators may be needed.

Although there is a push by existing professional manufacturers to step-up ventilator production , there is still a movement in the “maker” or open-source realm that want to use their skills in design (medical, electrical, mechanical, 3D printing, etc.) in order to help as much as they can.

The Google search for “homemade ventilator” increased dramatically in the month of March in the United States. While their passion to help is applauded, they still have to follow many of the same design rules. However, some of these regulations are being relaxed in order to meet the demand for more ventilators.

Is this the right decision? Will these ventilators cause more harm than good?

For starters, I am not a medical professional. I make no claims whether the medical use of these devices is a good or bad idea. In this article, we will only be considering the electromagnetic interference (EMI) these designs may potentially emit to surrounding devices.

Many medical devices contain some type of electric circuit or electronic components. Some of these are implanted into the body and other equipment can be found inside ambulances or hospitals. These include defibrillators, pacemakers, intravenous drug-dozing pumps, glucose monitors, electrocardiograph machines, automatic blood pressure cuffs, anesthesia machines, ultrasound machines, electrocauters, suction machines, ventricular assist devices, dialysis machines, patient monitoring systems, etc.

A non-compliant ventilator, over the emission limit, could cause many undesirable outcomes for other medical devices:

1. Unexpected shutdown. This is the worst-case scenario. The device shuts down when you need it to run constantly every second of the day.

2. Metastable logic. This is caused by a logic gate being stuck between 0 and 1, causing a firmware error that requires a manual restart of the machine.

3. Intermittent issues. These issues may sporadically interrupt normal operation and can be time consuming to troubleshoot, since they cannot easily be traced back to a root cause. These intermittent failures could cause some features (alarms, lights, motors, displays, LED indicators, etc.) to not function at the most critical moment.

Some documented examples of interference include, two residents in Los Angeles that were cited for using security cameras that were interfering in a nearby LTE station.

Another investigation involved a power supply on a neon sign replacement in 2016 causing disruption to electronic devices operating within 40 feet.

Although medical devices have requirements to pass conducted and radiated immunity tests, there is still a chance that a non-compliant ventilator design could interfere with a piece of life-saving equipment.

One of our goals at Würth Elektronik is to help prevent these type of EMI problems.

If you need to design your own EMC filter in order to pass conducted emissions, a good starting point is using the “ Design your Own EMC Filter” kit.

Inside this kit, there are many different common mode chokes (CMCs), X capacitors, Y capacitors, and connectors. They will enable you to assemble a working EMC filter on the spot.

If you are not sure where to start, the kit comes with a booklet that contains examples of pre-determined filters so no calculations or component selection is needed.

If the examples do not have the proper attenuation for your project, you can typically find the perfect component using our REDEXPERT online tool.

After you have located all of your components, just solder the components on the board, and connect wires from your line impedance stabilization network (LISN) to your device under test (DUT). Then while running pre-compliance measurements you can quickly determine if the filter you designed works as intended. After you prove your design, the individual components can easily be added into your design.

If you want something that works, right out of the box, then our complete WE-CLFS EMC Line Filters are a good option. These have most components you need for an EMC filter inside of a welded enclosure for perfect shielding. All of our WE-CLFS filters have at least 65dB of peak attenuation.

With 3 different types: single stage, single-stage advanced, and two-stage, there is sure to be an option that fits your needs, with options with a rated current up to 20A.

For your convenience, these filters come with UL and VDE certifications to give you peace-of-mind that safety requirements, such as creepage and clearance distances, are factored into the design.

If you are using an off-the-shelf power supply, especially from a low-quality manufacturer, you are likely to run into EMI issues. Two common ways these low-cost manufacturers often cut corners are:

1. Choosing EMC components that allow a power supply to pass EMC testing, with little to no margin. Adding any length of cable could then result in an EMC test failure.

2. Neglecting EMC components completely.

There are two easy solutions for off-the-shelf power supplies.

For conducted emissions, the easiest solution is to use the WE-CLFS filter mentioned earlier.

For radiated emissions, especially when attaching a long cable to a noisy off-the-shelf power supply, the easiest solution is to use a cable ferrite. It is very easy to find the right cable ferrite. For example, if I need 20dB of attenuation at 100 MHz, REDEXPERT will sort through all of the cable ferrites to find the best solution in about 30 seconds. It is even possible to select only parts that fit your cable!

For more help with other aspects of your design, check out our 70+ Application Notes covering topics including DC/DC converters, USB, LAN, Wireless Power, and much more.

If you need assistance at any point in your design, please reach out to your local Würth Elektronik representative.

Contact your local Würth Elektronik representative

Jared Quenzer is an "Applications Engineer - EMC and Power Specialist" at Würth Elektronik with experience designing magnetics (transformers, inductors and common mode chokes) used primarily in switched-mode power supplies. Beside having a background in design, he also has a passion for test and measurement, especially regarding electromagnetic interference.