Creating RF generators for medical apps

Add in the continual need for the prolonged usable lifetime of a product and the designer will have to ensure that all eventualities are covered before moving into production. This article should give anyone involved some idea of the key hurdles and some ways to overcome them.

Customisation of magnetics
An initial decision is whether custom magnetics are needed and, if so, what will be required to meet the challenge their usage may bring. Some applications will use custom magnetics because there are no alternatives and there are off-the-shelf magnetics for run-of-the-mill applications, or pre-made DC-DC converters. The drop-in ones may prove best in some cases and will save engineering time for more complex issues with critical therapeutic parts of a medical device design.

However, where energy is crossing the patient isolation barrier, operating at a specific frequency and with tight performance constraints, no off the shelf product will get near to meeting the applications requirements.

Custom magnetics are ideal but in RF generators they can introduce extra challenges such as creepage and clearance distances, EMC issues, leakage current and low capacitance requirements. Without the competency to develop individual components to meet these challenges it becomes necessary for an OEM to get a custom magnetic manufacturer to design for a specific requirement. The problem is, the vendor then owns that IP and the OEM may well not get access to the full design information. When the need arises to source parts in a different region, to achieve more competitive pricing or an increase in volume above the original supplier's capabilities, it could be a problem. Ownership of the design gives the ability to take that part to a custom magnetic manufacturer in any location.

Working with an EMS on the project means the product developer should have a manufacturer with an understanding on how to achieve true manufacturability of the custom magnetic component. There are will be performance and cost trade-offs to make during the design process so a good working partnership is essential. For example, using a more exotic material may be better for performance but increased cost and longer lead times may well counteract that incremental gain.

Relating all the information that a designer has to a magnetics house effectively can be difficult too and, if that is achieved, it is still in their hands to make the right choice for your product. It is essential, therefore, to only work with a custom magnetic manufacturer that is the right fit for the design and that gives guaranteed access to the IP for global use.

Medical process capability
When developing RF/piezoelectric surgical generators, which are classified as FDA Major Level of Concern and IEC62304 Class C. it is essential that the medical regulatory process to be followed is understood, including IEC60601-1, IEC62304 and ISO14971. When the CE mark is required for the medical device and the medical device's software plays a role in the patient or operator safety of the device, IEC62304 is applicable and the IEC62304 TRF (Test Report Form) must be filled in to demonstrate compliance.

With the release of IEC60601-1 3rd edition, the definition of essential performance was defined as performance of a clinical function where loss or degradation beyond the limits specified by the manufacturer results in an unacceptable risk. Furthermore, essential performance must be maintained through single fault conditions. A surgical generator design team must therefore understand the implications of this requirement on the architecture of the system to allow essential performance to be met against this strict definition. Utilisation of redundancy (sensing circuitry, microprocessor controls & supervision, etc.) is almost certainly required to ensure this compliance.

Helpful DSP controls
RF and Piezoelectric surgical generators typically require ability to sense output voltage, current, impedance magnitude, impedance phase and real power. In order to limit the usage of feature specific ICs that can create parts availability issues in the coming years, and to provide flexibility in the capabilities being provided, it is often beneficial to architect surgical generator controls and monitoring using Digital Signal Processors (DSPs). A DSP with on-chip high speed ADC inputs and PWM outputs is capable of providing the monitoring and control needed for these applications.

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