Medical Power Supplies Are in a League of Their OwnNovember 18, 2020 by Jake Hertz
Power supply design is always extremely important, but no application may have more stringent design requirements than medical devices.
In any field, power supply is an important consideration when it comes to system efficacy and safety. The medical field is no exception; when it comes to healthcare devices, the stakes of power supply design are even higher.
An electronic medical device. Image used courtesy of Janco Electronics
Given the immediate risks associated with medical devices, the medical field has developed its own standard for power supply design. In this article, we’ll explore why power supply design is unique in the medical field, what these designs look like, and some news in the field.
Why Medical Power Supply Design is Different
Medical power supply designs differ from standard commercial designs for two main reasons.
First, in medical designs, the safety of the patient is a paramount concern. Exposure to even tiny amounts of leakage current can drastically impact a patient's health.
For this reason, the acceptable value of leakage currents (unintended currents that may pass through the human body) in medical equipment cannot be higher than a few hundred microamps. In industrial power supplies, on the other hand, small leakage currents can be tolerable, since a healthy operator will not be affected by it.
Vox Power’s new medical PSU, the VCCS300. Image used courtesy of Vox Power
Beyond this, electronic equipment in hospitals generally operates with very low-level signals. This kind of equipment is intrinsically more sensitive to electromagnetic interference, making EMC compliance and performance a key concern while designing medical power supplies.
The Challenges of Medical PSUs
Given these requirements, medical devices must comply with a unique power supply standard: IEC/UL 60601.
Within this standard, there are three categories of equipment:
- B-rated (body) equipment, which will have only brief contact with a patient
- BF-rated (body floating) equipment, which will have routine contact with the skin
- CF-rated (cardiac floating) equipment, which is likely to make direct internal contact with a patient
The red line shows the isolation barrier between primary (top) and secondary (bottom) components in a medical PSU. Image used courtesy of Power Systems Design
Within all three of these categories, there are a couple of main design considerations that must be taken into account for PSU engineers:
- Creepage distance and air clearance: To block stray currents that can harm patients, medical PSUs require greater isolation between the primary and secondary than do industrial or consumer PSUs.
- Leakage current: Attempting to limit current from the AC line to the patient can complicate the transformer design, especially with CF-rated supplies.
- Dielectric strength: The dielectric strength between primary and secondary is 4000 VAC for medical PSUs, compared to 3000 VAC for industrial and consumer designs.
- EMI requirements: Medical power supplies must meet UL/IEC61000-1, which poses strict EMC compliance requirements.
An example of creepage and air clearance, according to the UL60601-1 requirement. Image used courtesy of Power Systems Design
A Real-World Example: Vox Power's New PSUs
With a clearer understanding of medical PSU requirements, it may be helpful to assess a real-life example newly released in the industry. This week, Vox Power announced a power supply family tailored to the medical industry.
The company's new PSUs, the VCCS300 series, is designed for BF-rated equipment and is said to deliver 300 W of continuous output power in a 2” x 4” x 1.61” package. This gives the device a power density greater than 23W/in^3. In line with Moore's Law, increasing power density is important as medical technology is miniaturized.
Key specifications of the VCCS300M series. Image used courtesy of Vox Power
As expected from a medical power supply, the VCCS300 offers patient leakage currents of <100uA, according to the datasheet. It also complies with IEC60601 EMC standards and offers an array of safety features.
A Niche Field Growing in Prominence
Designing medical PSUs comes with a specific set of challenges. Keeping the patient safe and shrinking EMC to ensure correct device operation are two large concerns that other industries don’t encounter to the same degree.
The new device from Vox Power seems to fit the bill and does so impressively with a small form factor and high power density. While medical PSU designs may seem like a niche field, as the medical device industry continues to develop, this branch of design work will only become increasingly important.