Medical Power Supplies Information
Medical power supplies are power supplies which produce conditioned outputs for medical devices, instruments, and equipment. They are built to satisfy the IEC601(EN60601)& UL60601 safety standard which covers electrical, medical, and dental equipment that is intended for professional use. This standard ensures the device's reliability, which is especially important in the medical field. By satisfying the standard, power supplies are defined as "medical-grade" devices. An important feature of these power supplies is their ability to isolate the patient from any electrical signals.
Medical power supplies also typically require longer design cycles than commercial devices, and designers often require more design support from the power supply vendor. They also tend to have a longer life expectancy, meaning they typically need to be supported by the vendor for many years.
Power supplies are categorized by the mechanism used to convert and transfer the input power to the output power. For medical power supplies, there are two main categories:
Linear power supplies accept AC inputs and provide one or more DC outputs for a wide variety of computer and industrial applications. They use an active element (normally a power transistor) operating in its linear region to generate a desired voltage. The output voltage is regulated by dropping excess input power in ohmic losses (heat) in a series dissipative component (resistor) or a transistor. Linear power supplies provide excellent regulation, very small ripple, and very little output noise.
Switching power supplies use a switching element or regulator (normally a power transistor) to generate the desired voltage. They are also called switch-mode products or switching mode power supplies (SMPSs). These power supplies incorporate electronic components that continuously switch ON and OFF at a very high frequency. This switching action connects and disconnects energy-storing devices (inductors or capacitors) to and from the input source voltage or the output load. A SMPS design results in high power density (smaller size for the same power output) and reduced power consumption (higher efficiency) in comparison to linear power supplies.
Form factor designates the packaging of the power supply design. There are several available for medical power supplies, the most common of which are listed below:
- Open frame - components are fitting together on an open board with no outer packaging or enclosure.
Image Credit: PSUI
- Enclosed frame - components are contained inside an enclosure to protect users and equipment.
Image Credit: PSUI
- Modular - device is design to be fitted or installed in sections or separate parts.
Image Credit: Ametek Programmable Power, Inc.
- Encapsulated - device is tightly enclosed in a (typically plastic) capsule or casing.
Image Credit: Sunpower, Ltd.
There are many parameters needed to fully characterize a power supply; however for most power supply types there are a set of parameters that are common. These include input and output voltage (specified in volts [V]), the output current (in amps [A]), the rated output power (in watts [W]), the input signal frequency (in Hertz [Hz], kilohertz [kHz], or megahertz [MHz]), and the regulation.
- Input voltage is the magnitude and type of the voltage applied to the power supply.
- Input frequency is the frequency of the input signal.
- Output voltage is the magnitude of the voltage at the output of the device. Because application requirements vary widely, medical power supplies offer either single or multiple output voltages, typically in the range of 3.3 to 48 VDC.
Selection tip: When determining the supply-voltage levels required in an application, note the current levels that will be required at each voltage.
- Output current is the current associated with the output voltage.
- Output power is the power (in watts) delivered to the load.
- Regulation indicates the stability of the output voltage.
- Line regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in input line voltage.
- Load regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in load.
Mounting specifications are often less important but should be considered as needed to properly fit the power supply to the application or system. Mounting options include:
- Board mount
- Circuit mount
- Wall mount
- DIN rail mount
- Rack mount
Features for medical power supplies add functionality such as circuit protection and cooling which may be important for certain applications.
Several factors can affect the performance and/or the physical integrity of DC power supplies. Circuits to protect the power supplies are normally included in the design and construction of the device. Some of these are:
- Short circuit protection
- Overload protection
- Over current protection
- Over voltage protection
- Under voltage protection
- Over temperature protection
Several cooling methods are used to protect medical power supplies:
- Fan cooling
- Water cooling
Medical power supplies can also incorporate a number of other features:
- Battery backup
- Power factor correction
- Temperature compensation
Medical power supplies are used in several medical applications. Each one is designed taking into consideration the required power, isolation, voltage, and current needed for the application. Normally these power supplies are not interchangeable. Some of these applications include:
- Dental - power supplies are used to power dental devices.
- Ophthalmic - power supplies are designed to be used for ophthalmic procedures.
- Ultrasound - power supplies are used to power ultrasound devices.
- Surgical - power supplies are used in surgical procedures inside operating rooms.
- Ventilator - power supplies are used to power medical ventilators for patients.
For a more detailed overview of power supplies selection, visit the Power Supplies Specification Guide on Engineering360.