What Surge Protection Should an Industrial or Medical Power Supply Have?

Power supply surge protection for industrial and medical equipment comes down to three things on the datasheet: an immunity level tested to IEC 61000-4-5, the performance criterion held during that test, and how its overvoltage, overcurrent, short-circuit, and overtemperature circuits behave when they trip. Those figures are fixed when the supply is chosen, so they belong in the design stage rather than in the first field report. A surge protector at the outlet helps, but the equipment still has to ride through whatever reaches its own input.

Do power supplies have built-in surge protection?

Yes, to a degree. Most AC/DC power supplies are designed to withstand some surge at their input, but how much varies widely, and only the datasheet says. Look in the EMC immunity table for the surge entry: the standard, the test voltage in kilovolts between conductors and to earth, and a performance criterion. If a datasheet lists none of these, it has not said what the supply will survive.

What causes power surges in industrial and medical facilities?

Most surges come from lightning striking on or near power lines and from switching heavy loads such as motors, transformers, and capacitor banks. A surge lasts tens of microseconds, but it carries far more energy than the fast transients from relay switching or an electrostatic discharge from a hand on the enclosure, and the power supply’s input stage is where it has to be dealt with. Sites where drives and heavy motors share the electrical distribution see surges more often than an office does.

An input that is not rated for its environment shows up as resets, a damaged input stage, or protection components that degrade with each surge they absorb.

What does IEC 61000-4-5 surge immunity testing measure?

IEC 61000-4-5, published in Europe as EN 61000-4-5, defines how equipment is tested for surge immunity: a combination wave generator applies a defined impulse (1.2/50 µs open-circuit voltage, 8/20 µs short-circuit current) to the power input at set test levels, and the equipment’s behavior is recorded. Two figures on the datasheet matter:

• The test level, in kilovolts, applied between lines (differential mode) and to earth (common mode).
• The performance criterion. As the generic immunity standards define them, criterion A means the equipment kept working within specification during and after the test. Criterion B allows a temporary loss of function or performance that recovers by itself. Criterion C allows a loss of function that needs an operator or a reset to restore.

A supply rated at criterion B may drop out briefly when a surge hits. For equipment that has to keep running, ask for criterion A at the test level the application needs.

What surge immunity does a medical power supply need?

A medical power supply has to meet IEC 60601-1-2 for EMC, which includes surge immunity, as well as IEC 60601-1 for electrical safety. Our application note on medical USB Type-C adapters gives ±1 kV differential and ±2 kV common mode as typical surge expectations under IEC 60601-1-2 (Ed. 4.1).

The Cincon LFM300M series, one of many medical power supplies we offer, shows what a complete specification looks like. Its datasheet lists:

• Safety approval to IEC/EN/UL 60601-1 (Ed. 3.2) with 2 x MOPP (means of patient protection), and 4,250 VAC input-to-output isolation for one minute.
• EMC immunity to EN 60601-1-2 (Ed. 4.1).
• Surge immunity to EN 61000-4-5, Level 4: ±2 kV line to neutral and ±4 kV line to earth, at criterion A.
• Fast transient (EFT) immunity to EN 61000-4-4 at ±2 kV, and ESD to IEC 61000-4-2 at ±15 kV air and ±8 kV contact discharge, both at criterion A.
• Maximum leakage current of 300 µA (earth) and 100 µA (touch).

That surge rating is twice the typical figures in our USB Type-C note, held at criterion A. The series is also 1 inch high, and our medical kiosk power application note recommends it for sealed kiosks, running fanless up to 250 W through conduction cooling. For higher power, Inission Power and Cincon also offer medical-grade supplies up to 600 W and 550 W respectively.

Certification of the power supply does not carry over to the device. The finished equipment is still tested to IEC 60601-1-2, so the supply’s margin is what leaves room for the display, the computer, and the cabling around it.

What should an industrial power supply meet?

In a factory or plant, look for EN 61000-6-2, the generic immunity standard for industrial environments, which includes surge tests on AC power ports. The LFM300M datasheet lists EN 61000-6-2 alongside its medical approvals, and rates the series for -40 °C to +80 °C ambient along its derating curve and for altitudes up to 5,000 m.

Where equipment runs from a DC bus rather than the mains, transients arrive on the DC side instead, and the converter’s input protection and filtering have to handle them. Cincon DC/DC converters accept inputs up to 1,500 VDC. Autronic, part of the FORTEC Group, develops DC/DC converters from 3 W to 500 W with integrated EMI solutions, and modifies them or builds custom designs for OEMs. Its engineering is in Germany, with production there and in the Czech Republic.

Which protection features should a power supply datasheet list?

At minimum, overvoltage, overcurrent, short-circuit, and overtemperature protection, with the datasheet stating how each one recovers. Overvoltage protection keeps a fault in the supply from reaching the load. Overcurrent and short-circuit protection guard the supply and its wiring when the load fails. Overtemperature protection shuts it down before heat does damage. On the LFM300M datasheet, for example:

• Overvoltage protection latches off, and the AC input has to be cycled to reset it.
• Overcurrent protection trips between 110% and 150% of rated load and recovers automatically.
• Short-circuit protection is continuous, with automatic recovery.
• Overtemperature protection recovers automatically.

Latch-off and auto-recovery suit different equipment. A latched output stays off until someone cycles power, so the fault gets looked at; auto-recovery brings an unattended system back by itself. Either way, as our USB Type-C note puts it, protection must operate “without unsafe oscillation or repeated reset cycling.”

Why choose the power supply at the design stage?

Because the input stage sets the surge margin for everything behind it, and changing the power supply after a failed compliance test or a field problem means reworking the design. In medical and industrial equipment, the display, the embedded computer, and the power supply share that input, so we look at them together: surge and EMC ratings, grounding, EMI layout, and the thermal path out of the enclosure. Display, embedded, and power expertise sit under one roof at FORTEC, so one engineering team is accountable for the whole assembly.

The same thinking applies to the rest of the system, in how to choose an edge AI computer for a medical device and how to design a display for harsh environments.

Working with FORTEC on power

Tell us where the equipment has to work and which standards it ships to, and we will take it from there. We recommend a match from more than 13,000 standard power supplies or engineer a custom design with private labeling options, then integrate it with the display and the embedded computer. Our technical services take the project from first samples to series production. When a part receives an end-of-life notice, we tell you early and plan the next step with you through obsolescence management: a last-time buy to cover remaining builds, or qualifying a replacement before supply runs out.

Contact us with your requirements.