What Test and Measurement Challenges do you face when sourcing Power Supplies Used in Medical Equipment?

The healthcare industry is changing. The increasing age profile of patients, an increase in lifestyle-related conditions, as well as the demand for higher levels of healthcare in the developing world has resulted in changes in diagnostics and treatments.

What Test and Measurement Challenges do you face when sourcing Power Supplies Used in Medical Equipment? Source: ShutterStock
What Test and Measurement Challenges do you face when sourcing Power Supplies Used in Medical Equipment? Source: ShutterStock

The treatment environment is also changing, with increased levels of outpatient care for many illnesses and treatments including more home care and clinic-based activities. There have been major changes in the safety standards that govern medical equipment, with the full adoption of the harmonized standard IEC60601-1 3rd edition, from which national standards are derived (UL, EN, CSA, for example).

In this article we explore:

  • Isolation, insulation, MOPPS and MOOPS – Isolating patients and operators from the risk of electric shock
  • Leakage current – Factors affecting leakage current and application variations
  • EMI – Emissions, immunity and improving EMI performance
  • Reliability testing – Highly Accelerated Life Test (HALT) and Highly Accelerated Stress Screening (HASS)
  • Environmental ROHS and REACH – Impact of new environmental regulations on commercialization

Content Summary

Safety First
Isolation, Insulation, MOPPs, and MOOPs
Leakage Current
Electromagnetic Interference (EMI)
Reliability Testing
Environmental ROHS and REACH
Conclusion

Applications are wide-ranging, including imaging (MRI, CT, PET, X-ray, and ultrasound), diagnostic equipment, dialysis, surgical robotics, lasers, and patient monitoring to name but a few. Like other industries, there is the challenge to develop medical equipment that is smaller, more reliable, offers greater functionality, higher efficiency, and is competitively priced.

However, the one area that has not changed is that the safety of the patient and of the operator is of paramount importance. The nature of medical devices leaves no room for compromise in terms of reliability, quality, and above all, safety. Power supplies are a critical component in medical equipment. Connected directly to the AC power grid, they are often in the first line of defense to ensure patient and operator safety.

Safety First

The first steps to safety take place months and often years in advance of a product being placed on the market. Understanding and adherence to the required safety and EMI standards need to be part of the power supply design process and must be adopted from first principles of product design.

IEC60601-1 (3rd edition), identifies the primary objective as the reliable and effective isolation between the AC input to the power supply, the internal working voltages, and the DC output. Adequate spacing between conductors and components is critical to achieving effective isolation. Reliable insulation design also contributes to effective isolation, ensuring that when a power supply is stressed to a voltage higher than its normal operating range, it does not fail or break down in a manner that could harm the patient or the operator.

Electromagnetic Interference (EMI) must also be considered as part of the power supply design. For example, imaging equipment, clinical chemistry, and patient monitors have complex detection and signaling components that are very sensitive to EMI. The following sections of this paper identify the critical criteria of the standards, how they apply to switch-mode power supplies, and their verification.

Isolation, Insulation, MOPPs, and MOOPs

The 3rd edition of IEC60601-1 introduced the concept of MOOP (Means of Operator Protection) and MOPP (Means of Patient Protection). The standard now recognizes that the potential risks to the operator are very different from that of the patient. An operator is typically less sensitive to shocks, generally is in good health, and has been trained in the use of the equipment. In order to address the differences, IEC60601-1 has defined different requirements for protection of the operator (MOOP) and more stringent protection of the patient (MOPP).

This distinction between patient and operator can result in quite different safety insulation and isolation requirements for circuits that operators and patients interact with. Specifically, anything that falls within the remit of operator protection only must meet the creepages and clearance requirements of IEC60950 (safety for information technology equipment). By contrast, circuitry that falls within the realm of patient protection must meet the more stringent requirements of IEC60601.

Medical equipment must incorporate one or more Means of Protection (MOP) to isolate patients and operators from the risk of electric shock. This ensures that in the event of one MOP failing, there is always another MOP to protect the patient and operator. Any insulation, creepage distances, air clearances, or protective impedance will contribute to MOP. These measures must be designed into the power supply. PCB layout, isolation barriers as well as component selection, must take all of these factors into account in the product design stage. MOOPs and MOPPs will become the common phrases when describing creepages, clearance levels, and isolation.

The table below shows the requirements and the insulation level of each MOP and the creepages, clearance, and test voltage required to attain the insulation level.

Requirements and the insulation level of each MOP and the creepages, clearance, and test voltage required to attain the insulation level.
Requirements and the insulation level of each MOP and the creepages, clearance, and test voltage required to attain the insulation level.

Insulation Testing Power Supplies

The Dielectric Withstand Test is used to determine the ability of equipment to protect against electrical shock. The test is often referred to as a Hi-Pot (High Potential) test. The testing applies high voltage between the points being tested and measures the resultant current to ensure the isolation barrier does not get breached.

The diagram below shows a typical power supply isolation diagram. This diagram identifies the isolation levels required at various points in the product. These include Primary to Secondary, Input to Ground, etc.

Figure 1. Example isolation diagram showing various insulation points on a power supply.
Figure 1. Example isolation diagram showing various insulation points on a power supply.

When testing primary to secondary circuits, it is possible to overstress the basic insulation. As many secondary low voltage circuits are connected to ground, by applying a 4000 VAC from input to output, the Hi-Pot voltage is unavoidably applied from the primary circuits to ground and may damage the power supply. Testing protocols can be implemented to mitigate against this while ensuring safety.

Type testing is carried out by the safety test agency to ensure compliance and is outlined above; however, as stated, this is not a guarantee of compliance in production.

Product testing by the power supply vendor can be carried out during the manufacturing process. Reinforced insulation testing can be performed separately at the isolation barriers. This can be applied to transformers, opto-couplers, and PCBs independently prior to insertion to ensure their compliance. 100% testing of these components is considered the best practice, thus ensuring the appropriate levels of insulation are present when the power supply is assembled.

Leakage Current

Leakage current is specified to ensure that in the event of direct contact with the medical equipment, the operator or patient is unlikely to experience electric shock. Patients may be in a weak condition, and exposure to small leakage currents may have detrimental effects on them. The allowable leakage current from medical equipment can vary depending on application from a few μA to 500 μA. Leakage tests are designed to simulate a human body encountering different parts of the equipment and these measurements must be below certain limits, as defined in the IEC60601-1 standards. For worldwide approvals, the maximum permissible Earth leakage current is 300 μA. This applies to the equipment, and not just to the power supply. There may be other leakage components in the system that contribute to Earth leakage and these must be considered for system testing.

There are several factors that affect leakage current:

  • The parasitic capacitance of the transformer and other components.
  • Input line voltage. Earth leakage is directly proportional to the input line voltage. Therefore, the same power supply tested at 110 VAC will have lower leakage current than when tested at 230 VAC.
  • The line to ground capacitance value of the “Y” capacitors. The lower the Y-capacitance, the lower the leakage current. However, as we will see later, there is a trade-off in EMI performance of the power supply and the leakage current that must be managed.
  • Input line frequency. Earth leakage is directly proportional to line frequency.

As part of the product design, capacitor tolerances must be considered to ensure that even in the worst case (capacitor values +20%), the leakage current will still be below the maximum allowable limits. This is verified as part of the Safety Agency testing of the power supply to IEC60601-1.

Production testing should be carried out on 100% of power supplies. Leakage current can be measured as part of the final testing of a power supply using dedicated testers that have been set up to simulate the worst-case conditions of operation, i.e. input voltage set to the highest rated input voltage.

Electromagnetic Interference (EMI)

Component power supplies are not standalone parts and as such, are not required to meet EMI standards from a regulatory perspective. However, switch-mode power supplies are active parts in medical equipment and their EMI characteristics can have a direct impact on the equipment’s EMI performance. As mentioned in the previous section, there is a fine line between designing and manufacturing power supplies to meet low leakage requirements while ensuring compliance with the international standards for EMI. Leakage current is safety critical, whereas EMI can have detrimental effects on the equipment in the vicinity of the system.

Emissions: Conducted and Radiated

Power supplies by their very nature create noise. Any source of changing voltage or current with respect to time will result in ringing. Any switch-mode power supply will have a number of these events during every switching cycle.

High di/dt loops on a forward converter
High di/dt loops on a forward converter
High dv/dt areas on a forward converter
High dv/dt areas on a forward converter

EMI must be factored into the product design from the initial stages and monitored throughout design. Pay attention to the PCB layout by:

  • Minimizing ground loops.
  • Leaving no floating parts (making sure all loops are brought back to ground).
  • Keeping signal and power connections separate to improve EMI performance.

Appropriate design consideration to this and on-board filtering will help power supply emission performance. While it is not viable to complete conducted and radiated emissions tests on 100% of production parts, designing the power supply and verifying that it meets limit lines with some margin (3 dB, for example) will most likely ensure all parts meet the required limits of EN55022/ CISPR22/FCC part 15.

Emissions: Conducted and Radiated

Often described as power factor, harmonic current emissions are concerned with the effect that a piece of equipment has on the public AC line network. Limits have been defined to ensure that the system does not induce harmonics which can damage the quality of this network.

Once again, a power supply design must take this into account to ensure that all harmonics are within the defined limits of EN61000-3-2, by appropriate filtering and line conditioning.

As part of the power supply test, power factor should be measured during the final production procedure. Power factor is inversely proportional to input line voltage, i.e. it is worse at 230 VAC than at 110 VAC. In order to ensure performance, testing should be carried out at the worst case, with input voltage set at the highest rated input voltage. High quality power supplies should have a power factor of >0.95 over all load and line conditions.

Immunity

There are a number of immunity specifications that medical equipment must comply with. These include: Electrostatic Discharge, Radiated Electromagnetic Field, Fast Transients, Input Surge Immunity, and Conducted Immunity.

It is not feasible to test these parameters on 100% of production parts. However, once again, appropriate consideration and testing as part of the product design process will ensure good and consistent characteristics of the power supply. Common best practice for the power supply manufacturer is to submit the power supply to an accredited test laboratory that will test the power supply against the following standards:

Common best practice for the power supply manufacturer is to submit the power supply to an accredited test laboratory that will test the power supply against the following standards.
Common best practice for the power supply manufacturer is to submit the power supply to an accredited test laboratory that will test the power supply against the following standards.

Reliability Testing

Reliability is one of the cornerstones of medical equipment. The critical nature of the equipment demands that it works “first time, every time.” Suppliers of individual components often offer very impressive life and reliability data; however, the effect on overall reliability when a large number of components are used is more difficult to predict. In conjunction with careful component selection, the manufacturing process is the factor that has greatest impact on power supply quality and reliability.

Highly Accelerated Life Test (HALT) is a stress testing methodology for assessing product reliability during the product development process. It is performed to identify design and process weaknesses. By subjecting the power supply to progressively higher stress levels brought on by thermal dwells, vibrations, rapid temperature transitions, and combined environments, the power supply is exposed to limits well beyond operating conditions. These fundamental technology limits are the data that drives the improvements on design robustness and development time. The end-customers’ benefits of HALT are demonstrated quality, reduced field failures, and increased reliability.

Highly Accelerated Stress Screening (HASS) or “Burn-In” is the process of eliminating the infant mortalities. These take place in the first portion of the reliability “bath-tub” curve shown below.

Highly Accelerated Stress Screening (HASS) or “Burn-In” is the process of eliminating the infant mortalities. These take place in the first portion of the reliability “bath-tub” curve.
Highly Accelerated Stress Screening (HASS) or “Burn-In” is the process of eliminating the infant mortalities. These take place in the first portion of the reliability “bath-tub” curve.

Infant mortalities are usually attributed to component faults, process, workmanship, or handling. To ensure that these early failures do not occur while in the end equipment, it is best practice to employ 100% burn in of all power supplies. This can be carried out at elevated temperatures and under varying input line and output loading conditions to simulate various modes of operation.

Environmental ROHS and REACH

New environmental regulations now affect medical device manufacturers, in particular with regard to the restrictions of hazardous materials used in their products. The EU directive for the Reduction of Hazardous Substances (ROHS), which covers electronic and electrical equipment, has been in effect for medical devices since July 2014. Medical product manufacturers will now need to anticipate the impact that even small changes in component selection can have on meeting the environmental regulations. In addition, the EU has also implemented the REACH directive (Regulation, Evaluation, Authorization, and Restriction of Chemicals). This directive identifies a number of hazardous materials and substances and defines acceptable limits for the content of these products.

REACH relates to product placed on the European market, and in the case of global medical equipment companies, this has major impacts. Furthermore, the scope of these directives is changing, with additional materials and substances coming under the umbrella of the overall directives. Selection of components that meet these directives, and will continue to meet them, is a challenge for the product manufacturer and, by direct relationship, the component supplier.

Power supply manufacturers should analyze the bill of materials to the single component level and verify conformity with each supplier based on the current lists of restricted substances. Ongoing maintenance for component changes, alternative suppliers, and newly restricted substances should be undertaken. Best practice involves the use of a third-party company to objectively analyze the current bill of materials and suppliers to ensure compliance. No new components should be approved by the power supply manufacturer without supporting documentation from the component supplier assuring compliance. This is an ongoing activity and requires regular updating of databases and compliance statements from the power supply manufacturer.

Conclusion

Compliance with safety standards for the medical industry takes precedence over all other factors. It is compulsory for the equipment supplier to protect patients and operators to the highest level. In order to ensure this, appropriate component selection is paramount. Power supplies, and in particular AC/DC power supplies, are safety critical components in any system. Medical equipment demands power supplies that are efficient, cost effective, compliant with safety and environmental standards, but above all safe and reliable. OEMs of medical equipment should take care in selection of the power supply to ensure they meet the existing (and possibly future iterations) of the IEC, EN UL and CSA safety standards. This greatly simplifies the system approval from the safety agencies and the FDA (Food and Drug Administration).

It is the challenge and the responsibility of the power supply manufacturer to conduct, record and monitor thorough pre-production (component inspection), in-production (process and test inspection), and final test (burn-in and test) of their power supplies. A combination of testing, characterization, and continuous monitoring of changes in regulations and legislation continually challenges power supply manufacturers to define, modify, and add to their testing protocols. But they must do so in order to ensure the power supplies manufactured comply to the relevant standards and deliver the expected performance and reliability to the customer.

Source: Advanced Energy