Medical-Grade Power Supplies: Ten Important Criteria
The term "medical grade" is applied to power supplies in a variety of package styles and, at a glance, these supplies may look no different than their commercial-grade counterparts.
However, medical power supplies have been specially designed to meet and be approved to the IEC60601-1 medical equipment safety standard, which influences the internal power supply design. However, safety is not the only factor that distinguishes medical-grade power supplies from commercial models. Medical equipment typically requires longer design cycles than other types of equipment and medical equipment designers often require more design support from the power supply vendor. In addition, medical equipment usually has greater life expectancy than other equipment. Therefore, power supplies designated as medical-grade generally need to be supported by the power supply vendor for many years.
When selecting a medical-grade power supply consider the following criteria as they relate to your application requirements:
Output Voltage (V) and Current (A):
Application requirements for supply voltage vary widely. To accommodate these needs, medical power supplies offer either single or multiple output voltages, typically in the range of 3.3 to 48Vdc. When determining the supply-voltage levels required in an application, not the current levels that will be required at each voltage.
Power Rating (W):
If only a single voltage is needed, the power rating required of the supply will simply be the supply voltage times the maximum current required in the application. If multiple outputs are needed, add the voltage-times-current products for all of the supply voltages to determine the power rating. Good engineering practice dictates that you select a power supply model with a rating that provides some design margin.
Form Factor and Mounting:
Medical grade supplies are offered in a variety of package styles including open-frame, enclosed, encapsulated, and external adapter with a range of mounting options such as PCB mount and chassis mount. The selection of a particular package style depends on a number of application considerations.
The options for power supply input and output connections (I/Os) are related to the choice of mounting options. With pc-board mount packaging, I/O pins on the bottom of the supply insert into plated-through holes or sockets on an application pc board. With chassis-mount packaging, the supply may include Molex headers or screw terminals that enable interconnects to be made using wire harness assemblies.
The ambient temperature range expected in the application together with power supply thermal specifications will determine whether convection cooling is sufficient or is forced-air cooling may be required. Note that the packaging style will affect thermal performance and cooling requirements.
For applications where the power supply may be exposed to vibration or contaminants, power supply reliability can be enhanced by the selection of an encapsulated power supply.
Medical-grade power supplies must meet the IEC60601-1 safety standard for medical electrical equipment in regional versions such as UL60601-1 and EN60601-1. These standards impose additional safety requirements (relating to creep-age and clearance spacing, isolation voltage and leakage current) beyond the requirements of the general safety standards of UL/EN60950-1. Medical-grade supplies also must meet these general standards. With regards to leakage, UL60601-1 specifies a limit of 0.3 mA of leakage current from input to output for Class I equipment. Within a power supply, most of the leakage is caused by the EMC filter on the input. However, equipment designers should be aware that other components in the system can contribute additional leakage.
Patient Connect vs. Patient Vicinity:
According to UL60601-1, electrical equipment is classified as "patient vicinity" if It is intended to be located within 6 feet of a patient. If electrical equipment is intended to come into direct contact with a patient, it is classified as "patient connect" and requires an additional level of isolation. Medical-grade power supplies are typically certified under the patient vicinity requirements since they do not come into direct contact with patients.
Additional Horizontal Standards:
In addition to meeting the IEC60601-1 standard, applications may also demand compliance with the collateral medical equipment standards such as IEC60601-1-2 which defines requirements for electromagnetic compatibility. There are numerous other collateral standards relating to other hazards and specific medical applications. Determine which standards apply in your application and whether they affect power supply selection.
Dual Line Fusing:
UL/EN60601-1 requires fusing on both the line and neutral wires in equipment with a protective earth ground. This fusing can be accomplished in the power supply or externally in the system.