Conducted emissions are basically electromagnetic interference (EMI) or noise that originates from frequencies generated internally by an electronic or electrical device. These emissions are then propagated along interconnected cables such as signal ports, wired ports such as telecommunication ports or power conductors.
Conducted emissions are typically divided into two categories: Discontinuous and continuous disturbances.
Discontinuous disturbances are typically caused by thermostatically controlled appliances (such as cooker hot-plates, refrigerators or automatic control heaters) and automatic programme controlled machines (such as washing machines which include the switching on and off of pumps, motors and other electrically controlled appliances). To summarize an event driven or non-constant timed switching event of energy.
When an electronic device has a source of interference which continuously emits at a given frequency the interference is what is known as continuous disturbances. Continuous disturbances can be caused by a number of things but typical sources include: Switch-mode power supplies, micro-controllers, data traffic (such as Ethernet traffic) and electrical motors. To summarize any constant timed switching event of energy.
Both kinds of conducted emissions/disturbances/interference can be mitigated using similar techniques, however it is best to identify whether the emissions are common mode (CM) or differential mode (DM) in nature.
Common Mode vs. Differential Mode
Common Mode (CM): Common mode emissions occur along two lines of a closed loop simultaneously, in the same direction and in phase. The signal on each line finds a return path through a common ground. Common mode noise can be mitigated by the addition of a common mode choke, which has multiple windings on a single core. The addition of capacitor from each line to ground (known as a Y capacitor) will also aid in shunting the unwanted emissions to ground. A quick method of determining if the emission is common mode is to measure the active and neutral lines using a current probe, with both conductors passing through the current probe in the same direction. This will allow the common mode emissions to be measured while differential mode currents become cancelled out.
Differential Mode (DM): Like common mode emissions, differential mode emissions occur along two lines of a closed loop, however the current flow is in opposite directions. A series EMI inductor may be enough to suppress the unwanted interference signals, though care must be taken to select a component which has a core material that can support the current of the entire line without saturating under full load. The addition of a shunt capacitor across the lines (known as an X capacitor) may also be required. A quick method of determining if the emission is differential mode is to measure the active and neutral lines using a current probe, with the neutral line conductor passing through the current probe in the opposite direction to the active line conductor. This will allow the differential mode emissions to be measured while the common mode currents become cancelled out.
When selecting an EMI inductor/choke there are three things to consider
- Frequency – the frequency range in which emissions are a problem.
- Impedance – usually specified as a minimum value at a given frequency.
- Current – the inductor windings and core material should be physically capable of supporting the current.
Conducted disturbances will seldom be either common mode or differential mode and will probably be a combination of the both. Often several different techniques used in conjunction with each other will be required in order to achieve the desired effect with respect to suppression of emissions. Simple filter circuits can be made from resistors, capacitors and inductors though there are several line filters made by several different manufacturers on the market that utilise combinations of components in one or more stage. These line filters are designed for all types of applications, from AC mains line filtering (either single phase or three-phase) to Ethernet and even USB 3.0. In the case of high speed data or signal lines additional integrated circuitry maybe required to filter the EMI noise whilst maintaining the signal integrity. Typically these types of filters can be sourced from specialist suppliers such as Wurth Electronics etc. In many cases the use of such filters can be costly; however investigations with such filters can help designers work towards a cheaper onboard solution. The diagram below shows a typical layout of a commercially available line filter circuit.
It is best practice to consider EMI and conducted emissions along with radiated emissions at the design stage. Using pre-compliance testing at the product development stage can highlight these types of issues at an early stage. This in turn can help mitigate future issues and result in an overall better product whilst reducing the overall product development cost.