Electromagnetic Interference (EMI) compliance testing naturally comes at the end of a product development cycle. Not passing EMI is the system engineer’s worst fear. It means a major setback to the product shipping schedule and could also mean a costly total power re-design. Fortunately, designing a power solution that is EMI compliant doesn’t have to be a hit or miss effort. A well-planned design using a proper filter, low EMI components, low EMI power regulator ICs, and/or low EMI power modules, along with good PCB layout and shielding techniques, will assure a high chance of first-pass success.
What is EMI Noise? Why Should You Care?
EMI is the disruption of operation of an electronic device when it is either connected to, sharing the same power source with, or near another electronic device that generates the EMI. EMI can be either conducted or radiated. EMI problems can prevent adjacent pieces of electronics equipment from working alongside one another.
Common examples of EMI in our everyday life that you may have encountered are:
- Disturbance in the audio/video signals on the radio or TV due to an aircraft flying at a low altitude
- Transmitters that prevent local TV stations from displaying their pictures. In the worst case, the whole picture could disappear, or there may be some patterning of the picture.
- Interference caused by a cell phone’s handshake with the communication tower to process a call (this is why airliners ask passengers to switch off their cell phones during flights)
- Interference from a microwave oven that affects the nearby WiFi signal
With the vast growth in the usage of electronic equipment, the issue of electromagnetic compatibility (EMC) has become an important topic. Hence, standards bodies have been established to ensure proper performance of electronic equipment even with EMI. Now, with modern electronic equipment, it is possible to operate mobile phones and other wireless devices near almost any electronics equipment with little or no effect. This has come about by ensuring that equipment does not radiate unwanted emissions, and also by making equipment less vulnerable to radio frequency radiation.
EMI Design Requirements
The CISPR 22 (often referred to as EN55022 in Europe) EMI specification divides equipment, devices, and apparatus into two classes:
Class B: Equipment, device, and apparatus that are intended to be used in the domestic environment and meet CISPR22 Class B emission requirements
Class A: Equipment, device, and apparatus that do not meet the CISPR22 Class B emission requirements, but comply with the less stringent CISPR22 Class A emission requirements. Class A equipment shall have the following warning: “This is a Class A product. In a domestic environment this product may cause radio interference in which case the user may be required to take adequate measures.”
EMI testing comprises two parts: conducted and radiated. Conducted emission testing is done in the frequency range of 150kHz-30MHz. This is the AC current conducted into the line source and is measured using two methods: quasi-peak and average, each with their own limits. Radiated emission testing is done in the higher radio frequency range of 30MHz-1GHz. This is the radiated magnetic field from the device under test (DUT). The testing upper range, 1GHz, applies to the DUT that has an internal oscillator frequency up to 108MHz. This upper range extends to 2GHz with an internal oscillator up to 500MHz, 5GHz with an internal oscillator up to 1GHz, and 6GHz with an internal oscillator higher than 1GHz.
Below are graphical illustrations of the CISPR 22 specification: The y-axis is the EMI magnitude measured in dBuV. The x-axis is the frequency measured in Hz.
EMI Noise Sources in Switching Power Supplies
Switching power supplies can generate electromagnetic energy and noise as well as be impacted by electromagnetic noise from external aggressors. Noise generated by a switching power supply can be categorized as both conduction and radiation. Conducted emissions can take the form of voltage or current, and each of these can be further categorized as common-mode or differential-mode. Also, the finite impedance of connecting wires enables voltage conduction to cause current conduction and vice versa, and differential-mode conduction to cause common-mode conduction and vice versa.
Let’s examine the noise sources in a switching power supply. Here is a simplified buck regulator schematic and its operational circuit waveforms: