When an instantaneous large current is drawn from a power supply, the voltage output dips slightly. A significant ripple in the current drawn will induce a voltage ripple on the power line. Conducted Emission (CE) test measures this ripple (in frequency domain) introduced by the Device under Test on to the Input Power Line.
Voltage ripples can be seen on the AC power line or a DC power rail. Any ripple from the DC voltage level can be tracked backwards to the AC power line if proper filters are not present on the path.
One of the sources of DC ripples is DC-DC switching converters. DC-DC Converters are used for converting one voltage to another & will be present in most of the systems. Due to inherent switching architecture, the load on the input power of DC-DC converter will continuously vary in accordance with the switching frequency. This can result in CE at the switching frequency & harmonics of switching frequency. Ripples can also be introduced on DC power lines due to the loads which draw intermittent high currents such as Speakers, Vibrators etc.
There are multiple approaches to resolve the CE issues depending on the architecture. If the system takes in the AC power directly, then the CE issues can be eliminated by designing the filters to attenuate the emissions at the particular frequency on the AC side directly. Improving the load regulation of the first stage AC-DC converter can also help to reduce the emission levels.
If the system takes DC input from a standard off the shelf SMPS (Switch mode Power Supply) / AC – DC Adapter, then the issue needs to be resolved at the DC side only. One important aspect is that a certified adapter doesn’t guarantee successful CE testing. Even though the adapters will have internal filtering, the frequency response of the filter may not match with the emission frequencies of the board, resulting in emissions crossing the allowed limits.
One of the approaches to reduce emissions on the DC input lines is to design a filter (typically LC filter) to attenuate the voltage ripple. The frequency response of the filter should designed to match the emission frequency. If the emissions are caused by any specific chipsets, then these chipsets can be powered on by a separate power rail derived from the input supply. This will make sure that the voltage ripple will be seen at the output of the intermediate power rail & can be handled more easily.
Selection of the appropriate power architecture along with the power chipsets, analysis & design of required filters on the power lines can make sure that the conducted emission tests are passed without multiple design iteration.