17 Jun 2015

RF Generator Experience for Medical Applications

John Simpson, Senior Manager of Engineering, EMEA, at Plexus, looks at the need for custom magnetics and capacitors in medical device development.

When building RF generators for medical applications there are many hoops the engineers have to go through before the final design can be passed through for manufacture.

With complex regulations on wireless and waveband usage, and the continual need for an increased usable lifespan of a product, the designer has to make sure that all eventualities have been covered prior to production.

To customise or not?

One of the first considerations is whether it will be necessary to use custom magnetics and, if it is, what challenges that will bring. For some applications there’s just no way around using custom magnetics. There are off-the-shelf magnetics for run-of-the-mill applications, or pre-made DC-DC converters, and in many cases the best solution will be the drop-in one - there is no point in wasting engineering time to re-invent the wheel for something that is a good fit.

 RF generator development

However, for the critical, therapeutic parts of a medical device design where energy is crossing the patient isolation barrier, operating at a specific frequency and with tight performance constraints, there is nothing off the shelf that will come close to meeting the applications requirements.

Using custom magnetics is ideal but utilising them in RF generators introduces additional challenges such as creepage and clearance distances, EMC issues, leakage current and low capacitance requirements. Not everyone has the competency to develop individual components to meet these challenges and will instead go to a custom magnetic manufacturer to design for a specific requirement.

The problem with this approach is that the vendor then owns that IP and the OEM may well not have access to the design details. If there is a need to source parts in a different region, to go after more competitive pricing or a requirement for volumes in excess of the original supplier’s capabilities, this could prove difficult. If the OEM owns the design then that print can be taken to a custom magnetic manufacturer anywhere in the world.

Alternatively an OEM may already be working with an EMS on the project and will look to them to resolve this issue. Ideally that company will already be working closely with a custom magnetic manufacturer that understands how to achieve the manufacturability of the component. Through a good long-term working relationship the EMS will be able to achieve a fast turn-around on prototypes alongside success in building to meet requirements.

This is important because when you dig down into the requirements of a design there are performance and cost trade-offs that the OEM and partner companies will understand and the magnetic manufacturer may not. For example, the use of more exotic materials may prove better for performance but the increased cost and longer lead times may well outweigh that incremental gain. It’s very complex to relate all the information that a designer has at his fingertips to a magnetics house effectively and, even if you do, you still cannot be sure they will make the right choice for your product. The need, therefore, is to find a custom magnetic manufacturer that is the right fit for the design needs and where access to the IP is guaranteed and is available for use elsewhere.

Where one-size does not fit all

With capacitors there is also a requirement for customisation in medical applications. There are seemingly simple applications where it would appear that any compatible part should do. With blocking caps for RF generator it is possible to think that with the need for a capacitor of a certain value and voltage rating you can just drop that in.

However, without the understanding of what the design standards are for failure modes (60601-2-2 specific standard), whether you need redundancy and what can prove hazardous to the patient this will not work. It would be disastrous to the production timeline to find out the hard way at the UL or TUV stage and have the designs are sent back to the drawing board because of the wrong component selection.

When talking about patient-applied parts in these kinds of applications, there no such thing as “just a cap”. There are obviously many capacitors such as decoupling caps and ‘popcorn parts’ in a design that have nothing special about them, but the trick is knowing where the critical parts are and what’s critical about them. Working with someone who has been down that road a few times means they will know right away where to look and what needs to be taken into consideration. A closely-knit team with overlapping circles of design responsibility will be able to define, even early on in the architecture level, details such as how to achieve high-bandwidth data across a patient isolation barrier

Moving on to the PCB

Alongside selecting the bespoke magnetics and capacitors the PCB design is critical to the usability of the device. With the specifics of an RF generator, and many other medical products, the need is to apply that understanding of what the physical design of the board means for the broader restrictions of safety testing, creepage and clearance, spill tests, moisture ingress and touch temperature.

All of these are challenging and if the placing or mounting is incorrect, whether it’s one board or multiple boards, the chances are that 95% of the requirements will be met but the missed 5% will mean a redesign of the whole board.

This is another good example of where there needs to be close collaboration between the design and engineering team. The engineer understands what has to happen with the PCB, but if you let the engineer do the layout as they think best the chances are that the board will prove unmanufacturable. Conversely the PCB designer has to listen to the requirements of the engineer. If everybody brings what is most important to the design to the table then you will get the best of both worlds.

Designing for a long lifespan

Working alongside these designers are the software engineers who have to contend with recent statistics that indicated that around 80% of features in software products are never used or very lightly used. Some argue that a never used feature should not have been implemented at all or that the lightly used features are equally unnecessary.

When a customer has a grand vision of what they want a product to do, and a tight timeline, the software engineer will have to get them to prioritise the absolute must-have from the nice-to-have features.

Omitting features should never be viewed as optimal, but it does allow realistic decisions to be made. However, what is often not understood is that many extra features are actually customer defined far forward-looking requirements that not due to be utilised in a first, or even second, release.

Future proofing is essential in today’s fast paced medical device market and upgrades need to be achieved both efficiently and simply. While some upgrades are purely a software modification there are some that may require hardware changes. So the engineers will aim define at an early stage in the design process what hardware will need to be in place for these upgrades, even if it is not initially utilised, and get that in place so that it passes verification and manufacturing tests.

Then future software updates will not require hardware changes because the hardware has already undergone tests to ensure it will meet these latest requirements.

Delivering the upgrade

With USB connections all pervasive now, there is the ability to plug in a USB flash drive with a software upgrade image and have a bootloader or software upgrade management software recognise the image, verify it and then install it on the device. There is, however, a lot of complexity involved. There may be a single upgrade image but it could include UI software, FPGA and control software. These will need to be extracted and a protocol put in place to transmit the images from a processor with USB access to the other programmable devices. This then makes for a nice, clean upgrade process that is easily managed by sales staff in the field.

All this different elements have to be achieved whilst meeting the customer’s high expectations. For the most part, the customer main priority is the end product that will be presented to the patient, its therapeutic benefit and its user interface. The customer is not necessarily concerned about what’s under the hood as long as the product is reliable, meets their original requirements, comes in on time and at the right price. It is up to the EMS to manage those expectations and ensure that it has the design and engineering team in place to deliver a medical device that hits the therapeutic targets and has proven longevity.

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About the author

John is Senior Manager of Engineering, EMEA, at Plexus and responsible for recruiting, training, allocation and developing the functional teams at the Livingston Design Centre. The role also includes staff and management development, oversight of the quality system, continual improvement and the introduction of new services at Plexus.

Plexus delivers comprehensive end to end solutions for OEMs through its unique Product Realization Value Stream service model. This award winning customer-focused services model seamlessly integrates innovative product conceptualisation, design, commercialisation, manufacturing, fulfilment and sustaining services for customers in the Americas, European and Asia-Pacific regions. It is the industry leader in servicing mid-to-low volume, higher complexity customer programmes that are characterised by unique flexibility, technology, quality and regulatory requirements. Plexus works with over 140 branded product companies in the Networking/Communications, Healthcare/Life Sciences, Industrial/Commercial and Defence/Security/Aerospace market sectors.

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