How to modify MedTech power supplies

This is efficient and competitive and reduces procurement and storage costs - but it requires a large amount of flexibility and performance potential of the power supply chosen, as the authors will prove from the example of respiratory machines for home use.

Designers of a new medical device don’t want to replace their peripheral components just because they have to add some convenience features and new functions to the higher or modernised device class. Neither do they want to replace their chosen and proven suppliers. But additional functions are extremely energy-hungry. Just think of a device that is available in the simple version with buttons, but in the next category with touch display operation.

In consequence the performance potential of a power supply is of big importance. To give an example, one of our customers from a German Medical Device Company, Loewenstein, producing respiratory systems, just realised, that they ten years ago used the 150W power supply, or even 200W. 

Now they got aware, they needed a 350W peak power supply for the next device generation. The reason: Faster motor acceleration, higher pressures or flows, as well as comfort features such as hose heating and humidifiers constantly drove the power requirements for the further development of ventilators and high-end device variants upwards; and much faster than this can even be approximately compensated for by efficiency increases in the power consumption of the components used.

Fortunately, this often is a problem which can be coped with by using a stronger AC/DC Converter, as the size of new, well-designed power supplies – using the latest parts in their design - is shrinking, either. While the VLT150 which had initially been used for the respiratory devices had been considered to be ‘small’ on a 5.0x3.04x1.07” (9.22 W/inch³) form factor eight years ago, the ‘smallest of its class’ MWLP, EOS has been launching three years ago, already offered 225 Watts on a 4.0*2.0*1.0 inches form factor. That makes a performance jump to maximum 28 Watts peak/inch³. The ultraflat MULP presented in the year following reached 225 Watts on a 5*3*0.75 inches platform – max. 37,5 Watts per cubic inch. 

Provided, the supplier of the power supplies offers the higher device class in comparable, and medically approved version. So it is hardly surprising, that same time manufacturers of medical power supplies are broadening their range of pre-certified medical supplies. During the last two years, the EOS Power design-team has also deliberately chosen this path and developed its first 600 to 1000 Watts power supply with medical approval - the MVPS600/1000. 

Typical applications in this power class are dialysis machines, lasers, clinical diagnostic equipment such as CRTs and X-ray machines, imaging applications, laboratory analysis and metrology. But today they also address the named smaller, more and more complex and therefore more energy-hungry applications, which are attempted to run without noise, dirt and failure-prone external cooling. So today also clients in the now 350 Watt range are happy to know they are available pre-approved and by standard.

On size, the EOS VPS 600 W AC/DC converters, which have been presented for the first time at Electronica 2018, are based on a standard 5*8.5*1.6 inch (1U) profile. In line with all EOS Power supply series released in recent years, they can be modified in many cases to customer specifications and are available in versions for industrial, ITC and medical use up to Class II / BF. 

Versatility of the ‘Versatile Power Supply’ (VPS) series is provided by the wide power spectrum of the series from 600 Watts to 1000 Watts output. This is achieved by different cooling modes - convection-cooled only, convection-cooled with internal cooling, or convection-cooled plus additional fan. Without external cooling or fans, convection cooled 600W in the U-frame, 420W in the punched cover and 360 W in the closed cover maximum power is available. 

The 350W mentioned at the beginning of this article are close at hand now, but using this supply the unit can be operated without external cooling. Provided, the design makes it considerably more reasonable to not use the smaller device (5*3*1‘’ in case of the EOS MWLP350) PLUS external fan and provide some reserve performance potential as mentioned initially.

With efficiencies are up to 94%, an MTBF of over 1.28 million hours, as well as error signalling and easy modification of anchors, cable connections and many other features ensure satisfied users. Double fuse and thermal cut-out ensure safety. If more power is required, it is also possible to connect up to three VPS devices in parallel. 

In addition, the standard supplies have to offer the latest medical approvals: IEC 60601 2nd/3rd, IEC 60601-1-2 fourth edition (EMC). Of similar large importance is the leakage current - in the case of the VPS of less than 100µA and the EMC rating according to class B. And even BF as standard approval in a power supply series can turn out to be useful, requiring the double mains fuses and the insulation for direct patient contact according to Body Floating (BF) standard with double MOPP protection.

BF requirements
Reason is, that approval requirements can be the next issue to hold surprises for medical power supply designers and the design-in of their open frame power supplies. This came into effect in the respiratory project mentioned before. Because of a changed medical classification system and because of sensors which were built into the respiratory masks of the latest generation of the devices, the connection between patient and suddenly has been regarded as conductive. 

This meant, the otherwise insulated section, suddenly had to meet BF requirements, 2xMOPP and protection class II. This way, the fact that all EOS series are without exception approved BF and according to the latest EN60601-1 and the relevant EMC standards, suddenly became the decisive next generation success factor. 

Success factor modified standard
As much important for the success of the open frame customer project in many medical technology projects is the ability to modify the power supply without changing the technical values. The main topics often are connectors, EMC and ultimately also the load profile. 

This already had been exceptional when designing the Loewenstein predecessor models prisma VENT30/40 and the ventilator VENTIlogic LS, both of which were equipped with the older EOS power supply MWLT 150. The Gretchen question of ‘make or buy’ had already been close to ending up in ‘make’ then. This could have been prevented, but just because EOS had already made prior efforts concerning the approvals. The MWLT version of the EOS WLT150 had already been one of the few power supplies in this area with Class II approval on the market at that time - since 3rd edition of the EN60601 all standard versions also feature 2xY2 caps for 2xMOPP as a Spec retrofit. 

Therefore the EOS MWLT had been recommended to Loewenstein (then still named Weinmann) by the Austrian distributor CODICO, the Hamburg based medical device specialist had already been working with successfully for years in the field of displays and components. Together, the responsible persons had decided to try to modify the WLT150 - if possible without touching the parameters relevant for approval. 

"This was followed by several phases in which we and the European sales managers at EOS could also have opened our office at Löwenstein, then still Weinmann, in Hamburg," said Manfred Schulz, FSE Active Components at CODICO, summing up the cooperation between the three medium-sized companies. 

The challenge of device modification in medical technology
The look back at this project shows what challenges lie ahead for the pre-certified power supplies competing for being used in Medical Devices.

Topic one, the connections. EOS Power often supplies its customers with special connectors from its factory in Mumbai. Users can choose between screw terminals and plug-in terminals at the output, depending on their requirements. For the respiratory system’s power module, for example, a special output connector was selected. This enables the device manufacturer to work with the same wiring harness in all his devices. 

In addition, the position holes as well as the cross-section of the fastening and the circuit board for the ventilator were individually adapted in many small steps. In addition to the connections, even in an early phase of the cooperation, several other minor details had also been changed according to the customer's specifications - including the shrink tubing for fire protection reasons.

Challenge two are the incoming goods, workflow and traceability. Since 2016, Loewenstein has been working with special barcode stickers in the areas of incoming goods, production, quality assurance and batch control, which are required in three different versions on the components in production in Hamburg. These labels, including barcodes and peel-off stickers, are already applied and registered by the manufacturer in Mumbai. 

In India, precautions have also been taken to comply with customer-specific ESD packaging regulations with the support of CODICO. In the unlikely event of a necessary recall, for example in the event of an increased failure rate, the product could be traced seamlessly from the end customer to the upstream supplier, including all component suppliers used.

More than just new plugs
Thinking about modifications, many device producers just consider topics like new plugs or cables. But the usual standard modifications from plug to output voltage very often come out to be insufficient. More profound adjustments have to be coped with. So, when once in another design process it got obvious that the everyday voltage and load profiles of the respiratory devices did not yet correspond exactly to the standard characteristics of the EOS power supplies, several even different component parts were added and modified. 

Afterwards intense test series were run jointly in Hamburg and Mumbai. What had been happening? - Not only the VENTIlogic LS received a customer-specific NTC for temperature monitoring and fan control due to ventilation phases with very high outputs; the Ventilogic LS even used other MOSFETS in the PFC stage, which tolerate particularly higher voltages. Later, also the 2xY2 capacitors were adapted to improve the load profiles and EMC performance. 

This close development cooperation only worked, as a prototype of each modified ventilator was sent to EOS Power in India for measurements. The new EMC measurements (including surge and burst) were also carried out in parallel in Germany and India.

The reason had been, that eventually critical components and safety-relevant barriers were changed. This led to additional certification requirements for the modified MWLP225. As If the separation between patient and current flow become different, or optocouplers and control parts that monitor the control system are changed, subsequent certification becomes mandatory. 

But also the project impact of those occurrances depends on how they are dealt with. According to the Loweenstein team, the re-certification, happened to´be run quickly and problem-free. At around 5,000 dollars, the costs were significantly lower than the costs for recertification, which are usually about twice as high.

Process integration in the background
Another success factor for modifications is the integration of processes in the background. In addition to the modifications made by the manufacturer, the distributor CODICO still carries out complete serial number tracking and customer-specific packaging labelling. The producers in India are taking care of additional DoC tests. These are also documented accordingly. 

Suppliers can also support customers with documentation requirements. Since an essential part of the risk analysis, in the sense of EN60601, is the handling of critical components, assembly variants and second-, third- and fourth-source components often drive welding beads on the forehead of medical technology manufacturers. Without an open approach to the parts list, more in-depth customer-specific adaptations would therefore be virtually impossible. 

EOS has provided almost 40 data sheets on capacitors and other components. Also circuit diagrams. Concerning Wolfram Heinrich, project leader at Loewenstein, this is an unusual service. He sees the reason for this in the similar corporate culture and customer orientation of the three medium-sized partners. Only in this way can the cooperation work that seamlessly "remotely".

Optimal conditions for high mix low volume
And there is another condition that makes such development partnerships in medical technology possible at all: All EOS power supplies are produced centrally at EOS Power in Mumbai. The production systems are deliberately kept flexible and adjusted to high-mix and various modifications. Even quantities of only a few thousand identical components per year can be cost-effectively produced there. 

The EOS high-mix-low-volume ‘pick and place’ systems can handle different types of printed circuit boards and components. It also can easily be handled, when the designs have different connectors to match the compact design of the power supplies; the equipment ensures reliable placement of larger components as well as the placement of resistors and very small capacitors - as well as the accurate placement of a wide range of SMT components through to finepitch components such as ICs, QFN packages and BGAs.

This manufacturing facility has received numerous awards for quality and manufacturing processes from renowned bodies such as Dun & Bradstreet, Frost & Sullivan and Elcina. Many customers audited the EOS production in India yet. And last but not least, according to Wolfram Heinrich from Loewenstein, it of course is also a plus that the price - also of the more powerful EOS Open Frame power supplies - was ‘very reasonable’.