Medical-grade isolated DC/DC converters can be used in conjunction with AC/DC power supplies or battery-powered devices, which not only reduces the cost of end products in medical applications but also meets the highest level of safety standards.
Electronic devices are increasingly being used in various medical and home healthcare environments, and the devices used must strictly comply with relevant electrical safety standards: IEC 60601-1:2005 and its accompanying documents and national standards, including the European standard EN 60601-1:2006.
When considering the AC/DC power supply required for a product, refer to the application guidelines of the latest safety standards, which mention the "Modular Protections" (MOPs) required for operators and patient environments. Specifying a power supply with the highest safety rating, i.e., dual Motion Protections (MOPPs), seems to be the wisest approach, ensuring the maximum rated supply voltage and leakage current meet the requirements for Type CF (cardiac floating) contact with patients; Type CF refers to power being connected to organs for monitoring or stimulation. Battery-powered devices may be considered unsuitable for safety standards; however, in some cases, a fully certified AC/DC is insufficient, while in others it exceeds specifications. Some applications require internal safety isolation for battery-powered devices.
For example, AC-powered equipment requires at least 2 x MOOP or MOPP isolation from the AC line to the output, depending on whether the equipment will be used in the operator's or patient's environment. However, the output connected to the patient must be isolated from ground by at least 1 x MOPP to prevent the patient from becoming a "live wire" for other faulty equipment, allowing lethal current to flow through the patient to the ground. In this case, an additional isolated DC/DC converter can power the circuit connected to the patient. If it has suitable medical-grade isolation, an AC/DC converter with only 1 x MOPP output isolation can be used.
In another scenario where the signal connection is not specified, such as the communication port of an AC or battery-powered device, there must be 2 x MOPPs between the patient connection and the signal to prevent external faults from conducting the signal. Similarly, an additional DC/DC converter can provide the necessary isolation. Figure 1 illustrates an example of a special case: a Class I AC/DC power supply with 2 x MOPPs and an unspecified signal input, plus a 2 x MOPP DC/DC converter, can provide the necessary isolation for the signal input, allowing the use of a lower-cost "operator-class" AC/DC power supply in a patient-connected environment.
DC/DC converters typically only require low power for the patient connection interface, making them relatively small and inexpensive. Due to their size, their coupling capacitance is usually very low, resulting in very low leakage current and providing a higher level of patient connectivity compared to using AC/DC converters alone.
Figure 1: High-level medical power supply solutions for patient connectivity can be achieved using affordable components.
Careful selection of DC/DC converters for medical applications is essential.
DC/DC converters have become commercial devices, available from numerous sources with varying isolation levels and certifications, both authorized and uncertified. Designers should be aware that high voltage ratings often only indicate transient immunity; without certification according to relevant standards from specific organizations, the product may not be suitable as a safety barrier for any type of application. For medical applications, the DC/DC datasheet should specify the level of protection, MOOP, or MOPP for the specified system voltage (typically 250VAC). While it's possible for a DC/DC converter to legally obtain 2 x MOPP certification, this is only possible at 30VAC, which is dangerous if the system voltage is from "normal" AC. Medical-grade DC/DC converters are uncommon because achieving high isolation levels internally is challenging. Standards require robust solid insulation or significant spacing between inputs and outputs; for example, 2 x MOPP/250VAC certification requires an 8mm creepage distance, which presents a significant challenge for internal design.
RECOM's recently released R05CT05S is an example of a DC/DC converter achieving high-grade medical-grade isolation (Figure 2). This economical 0.5W device features a nominal input of 5V and outputs of 3.3V, 5V, 3.7V, or 5.4V to power a low-dropout regulator (LDO). The converter is housed in a compact 10.3mm x 7.7mm SMD package, with a height of only 2.65mm, making it suitable for space-constrained applications. Highlights for medical applications include a 2 x MOPP/250VAC continuous rated voltage and a test voltage of 5kVAC compliant with IEC/EN 60601-1. It also features a 3.5pF coupling capacitor, resulting in negligible leakage current for 250VAC/50Hz applications. Its rated voltage is even better for non-medical applications – an enhanced isolation operating voltage of 800VAC compliant with EN 62368-1. The maximum operating temperature is 140°C with derating, and device features include enable, synchronization, regulation, and undervoltage lockout.
Figure 2: 2 x MOPP/250VAC rated isolated DC/DC converters
Advanced power packaging enables medical-grade isolation
The designers of the R05CT05S had to adopt a completely new approach to product packaging to achieve both functionality and a high level of isolation. Traditional toroidal transformers could not meet the insulation requirements, while spool-type E-cores or similar materials were too large. Therefore, the switching frequency was pushed up to 8MHz, allowing for fewer winding turns and a smaller planar transformer core, while robust inter-winding insulation was used to meet medical-grade requirements. Even at high switching frequencies, the advanced circuit design of the embedded die maintains good efficiency. The chip is wire-bonded to a DVE SOIC-16 leadframe before the entire product is encapsulated.
in conclusion
Medical-grade certified high-isolation DC/DC converters are valuable devices that provide the required isolation levels for the most sensitive applications, including CF patient connections. With careful application, using the RECOM R05CT05S in system design can minimize costs without compromising safety. The advanced circuitry and 3D power packaging technology employed in this product are essential technologies in RECOM's latest products, continuously driving the company towards smaller, more efficient, and more cost-effective DC/DC and AC/DC converters for a wide range of applications, including medical.
