EMC and the CISPR 32 / EN 55032 Marks

Nearly every power supply datasheet quotes EN 55032 (the European form of CISPR 32), and nearly every one adds a letter after it. That letter matters more than the number.

What the standard covers

EN 55032 governs emissions — the electrical noise a product pushes back down the mains lead and radiates into the air. Switching supplies are noisy by nature, so they are usually the component that decides whether the finished product passes. It replaced the older EN 55022 that many long-running datasheets still quote.

Class A and Class B

• Class A — for equipment used in industrial and commercial environments. The limits are looser, on the reasoning that a factory is already electrically hostile and the neighbours are not watching television.
• Class B — for equipment that may be used in residential settings. The limits are tighter, typically by around 10 dB.

So Class B is the harder test, and a supply certified only to Class A cannot be dropped into a product destined for someone’s home or office without re-testing. This is the single most common reason a design passes on the bench and fails at the test house.

Two things the letter does not tell you

• Emissions are not immunity. How well the equipment tolerates interference arriving from outside is a separate standard, and a separate test.
• The class is measured on the supply, not on your product. Your enclosure, cable routing and load all change the result. Treat the figure as a starting point, not a pass.

Tell us which class you need to meet and we will point you at supplies that give you headroom rather than a borderline pass.