The uses of DC-DC converters span across a wide range of industries, from aerospace and military applications to commercial and industrial spaces. Regardless of the circuit topology of the DC-DC converter employed, there are basic parameters, certifications, and a level of ruggedization that designers must meet in order to qualify for the most stringent medical requirements.
Certifications and testing around medical applications for power supplies, or converters, are defined primarily by their isolation and leakage current. Both these parameters relate to the level of protection a patient will experience when in electrical contact with the power supply. There are however, also a myriad of other parameters to consider when choosing a DC-DC converter in order to ensure optimal performance over the lifetime of the device. This article dives into the certifications for medical-grade DC-DC converters and the basic parameters to look at when choosing these devices.
Power considerations in medical devices
The range of power solutions for electronic medical devices is as varied as the applications themselves. Between the power source (e.g., battery, outlet, generator, etc.), the power delivery network (PDN), and the connected circuitry (e.g, sensors, actuators, processors, etc.), it is critical that every component function nominally in the event of a power loss, voltage fluctuations, or potential interference. For a ventilator, this means that an energy storage device such as a battery is required to rapidly switch to backup power in the event that a power loss has occurred. This avoids the catastrophic failure mode of the ventilator shutting down before the longer-term backup power such as a diesel generator is engaged.
Medical devices should be able to tolerate a degree of voltage fluctuations around their nominal values. This very same reasoning applies to all critical medical equipment such as blood pressure monitors, incubators, defibrillators, and dialysis machines. On the system level, redundant backup power and energy storage devices might be employed. However, on the device level, the power supply and/or AC-DC and DC-DC conversion must ensure both electromagnetic compliance (EMC) as well as overcurrent, overvoltage, overload, short circuit, and over-temperature protections in order to minimize unwanted emissions and protect the component and the load from undesired conditions.
The safety of the device for both the operator and the patient is also critical in medical applications. Most safety standards present parameters and testing guidelines in order to ensure this level of safety. However, the quality of the device is dependent on some basic electrical and mechanical specifications.
Basic parameters to look at in a DC-DC converter
DC-DC converters are found in virtually any electronic medical device in order to take an input DC signal and provide a tightly regulated output DC signal. Regardless of the DC-DC converter’s topology, there are specific electrical, thermal, and mechanical variables that are critical to optimize for the particular end application. The most basic parameters are the required input voltage and output voltages as well as output current/power for the use case, and they are readily defined by the system requirements. DC-DC converters begin to differentiate themselves with factors such as efficiency, power density, reliability, and certifications/standards compliance. A high-power density will allow the designer to save on space by fitting a DC-DC converter with larger power ratings in a smaller form factor. For a medical application, medical certification is the major qualifying factor. Outside of this, reliability is key, so circuit protections are necessary to buffer the component from unpredictable voltage fluctuations and surge events.
Depending upon the environment in which the DC-DC converter will be deployed, additional component ruggedization can be critical. In outdoor environments, it would be necessary to ensure the converter will not meet any failure modes from cycling between hot and cold temperatures or from a high humidity environment. The MIL-STD-810 specification is the most common standard referred to in electronic component datasheets for environmental engineering considerations.
This is due to the exhaustive guidelines and test conditions listed out within the standard for potential environmental stressors from high and low-temperature conditions to fungus, explosive atmosphere, and ballistic shock. While most medical components will likely not undergo mechanical strain from gunfire, parameters such as thermal shock, relative humidity, vibration, and even altitude can be tested in order to represent the mechanical robustness of the device. This illustrates the thermal stability of the DC-DC converter as well as its ability to operate in both shock and vibrations (e.g., mobile emergency/ first aid equipment) and high altitudes (e.g., mountains, aircraft, etc.).
While many of these parameters ensure the device does not meet premature failure due to undesired electrical, mechanical, and environmental conditions, the mean time between failures (MTBF) is a critical parameter that represents the long-term wear out of while the device is operating in steady-state conditions.
Power supply requirements for medical applications
The default standard for medical devices is the IEC 60601-1 standard that defines the “general requirements for basic safety and essential performance.” For power supplies (i.e., AC-DC and DC-DC converters) within medical equipment, the standard is essential to make certain the medical equipment functions within safety guidelines. These guidelines are defined by the vicinity of the patient to the power supply. The classifications are as follows:
- Type B (body): no contact with the patient
- Type BF (body floating): physical contact with the patient
- Type CF (cardiac floating): physical contact with the heart
Devices without patient contact might include x-rays, MRI scanners, medical lasers, and hospital beds, while devices with limited contact include blood pressure monitors, incubators, and ultrasound equipment. The most stringent CF classification of devices is surgical equipment, dialysis machines, defibrillators, and heart-lung machines. The power supplies themselves do not qualify as medical devices and therefore are not subject to these classifications. However, it is still immensely helpful to the medical equipment designer to understand whether or not the power supply meets the isolation, creepage, insulation, and leakage current requirements for the various categories (Table 1).
Input/Output isolation and leakage current
The level of insulation protects the operators and patients from electrical shock and the insulation barrier can be air or a prespecified material for additional isolation. Clearance is the distance through air between two conductors, the goal is to keep the electrodes separated far enough away from each other to prevent arcing. Creepage distance is the distance along the surface between two conductors, and this distance ensures that there is no short-circuiting from one conductor to another in case the insulating surface becomes contaminated and begins conducting.
The level of leakage current from the power supply to the patient must be extremely low to qualify as medical grade. For instance, CF-rated supplies the leakage current limit is 10μA in normal conditions to 50μA for single fault conditions, whereas a leakage current below 500μA is satisfactory for most non-medical-grade supplies.
Means of protection: MOOP and MOPP
The IEC 60601-1 specifies both the means of operator protection (MOOP) and means of patient protection (MOPP) in order to differentiate the potential hazards that each of these users might encounter with a medical device. For operator protection (1xMOOP) one layer of insulation with a creepage distance of 2.5 mm and 1500 VAC of isolation is satisfactory. Two MOOP (2xMOOP) implies a double layer of insulation with double the creepage distance and isolation. Basic patient protection (1xMOPP) requires a single layer of insulation with 1500 VAC of isolation and a 4mm creepage distance. Two MOPP (2xMOPP) is the most stringent protection with two layers of insulation, 4000 VAC of isolation, and 8 mm creepage distance. Type CF equipment requires both an IEC60601-1 qualified supply as well as an isolation barrier between the supply and the part of the equipment that is in contact with the patient. This isolation can be accomplished with a DC-DC converter that has a 2xMOPP classification.
Product qualification and manufacturing quality –ISO 14971 and ISO 13485
The 3rd and 4th editions of the IEC 60601-1 require manufacturers to prepare and maintain risk management documentation according to ISO 14971 in order to:
- Estimate the associated risks of medical devices
- Control the risks
- Monitor the effectiveness of these controls
All of these factors must be evaluated by a certified body/test lab (e.g., UL) at each step in the process, causing compliance with the later editions of IEC 60601-1 to be much more stringent than previous iterations. This process is put in place to eliminate any unacceptable failures or degradation in device performance.
An additional production quality standard for medical devices is ISO 13485. This standard specifies requirements for a quality management system of an organization.
In order to qualify, an organization must demonstrate its ability to provide medical devices that “consistently meet customer and applicable regulatory requirements”. A manufacturing process that is ISO 13485 compliant will guarantee a production process with a high level of reliability for every component fabricated.
The MPQ60W medical-grade 60W DC-DC converter
P-DUKE’s MPQ60W medical-grade DC-DC converter is rated for the most stringent CF-type medical devices with less than 4.5 μA of leakage current, 5000 VAC of isolation, reinforced 2xMOPP insulation, and 8 mm of air clearance and creepage distance. The ISO 14971 risk management file supports designers to compile exhaustive technical documentation on their device while the ISO 13485 compliance ensures a quality, high-reliability DC-DC converter. Outside of meeting these standards, the MPQ60W features an industry-leading power density for a medical type CF DC-DC converter with its tiny quarter brick form factor. The DC-DC converter includes all the necessary circuit protections (i.e., OCP, OTP, OVP, SCP, UVP) and has been tested for thermal shock and vibration according to MIL-STD-810. The MPQ60W can function nominally at altitudes up to 5000 m and has a wide operating ambient temperature range between -40°C to 105°C.
While power supplies used in medical equipment are not considered medical devices and are therefore not directly addressed in the IEC 60601-1, these devices can still be qualified under them in order to ease the design and production process for engineers. This simplifies the quality control and time-to-market of any medical-grade device. The small MPQ60W is certified in accordance with CF-type devices with the strictest isolation requirements. This, combined with its ruggedization and reliability in compliance with ISO 13485, enables end-users to readily implement these DC-DC converters in their design. P-DUKE offers a wide range of high-grade DC-DC converters from1W to 60W and high-grade AC-DC power supplies from 15W to 450W suitable for 2 x MOPP applications.