The switching converter (power supply, motor drive or energy converter) is a serious cause of interference emissions in the low to medium frequency range since it is essentially a high-power trapezoidal wave oscillator. Depending on the type of circuit and the power levels involved, high di/dt loops or high dV/dt nodes will be present in the circuit and will couple to the external environment via several routes, often involving parasitic components. The trend to higher power efficiencies means faster switching speeds, which increase di/dt and dV/dt further for a given power level, and can extend the upper frequency emissions into the VHF range. In the semiconductor industry, wide band gap devices such as Silicon Carbide (SiC) and Gallium Nitride (GaN) transistors have been gaining attention due to their small size, fast speed, and better thermal performance. The introduction of these new semiconductors into the consumer market came after a series of military and other commercial applications of the technology in everything from electric vehicles to radar systems.

GaN and SiC devices have enabled a much better form factor for product design than their silicon counterparts. As they become cheaper and more available, it is expected that we will see them widely adopted in power-switching modules worldwide.Technically speaking, GaN semiconductors are high-electron-mobility transistors (HEMTs), meaning they do not have the doped region in a PN junction like MOSFETs. This enables faster electron flow, hence higher switching speed.

Converter design becomes a trade-off between high volumetric and power efficiency on the one hand, and low emissions on the other.

Layout of switching converters is critical to maintaining their EMC. Loop areas can be minimized by running signal/power tracks immediately next to and parallel to their return tracks. Sensitive tracks or wires should not be run near to inductors or transformers, especially their magnetic flux leakage points; if necessary, orientate such tracks for minimum pickup. Unfiltered or “clean” circuits must be kept separate from noise sources, and large area sources of high dv/dt (particularly live heatsinks or transformer windings) should be screened or separated from other components and circuits. All ground and power rails should be carefully checked to ensure they do not offer common impedance routes within or outside the unit.

Emission characteristics

The major component of switching noise emission is due to the switching frequency and its harmonics. Asymmetry of the switching waveform normally ensures that both odd and even harmonics are present. If the fundamental frequency is stable and well defined then a spectrum of narrowband emissions is produced which can extend beyond 30MHz when the waveform transition times are fast. A measurement bandwidth of 9kHz means that individual harmonics can be distinguished if the fundamental frequency is greater than about 20kHz (see (a)). Designs in which the frequency is not stable will typically show both frequency and amplitude modulation due to input ripple or output load changes which has the effect of broadening individual harmonic lines so that an emission “envelope” is measured (b).

A further cause of broadband noise may be due to reverse recovery switching of the output rectifier diodes, This phenomenon is typically observed in MOSFET/IGBT-based switching converters, but less frequently in WBG-based converters. This is because HEMTs do not have the PN structure, they also do not have a body diode. This can have a great impact in applications such as motor drives, where we can now switch on the HEMT for freewheeling rather than relying on the body diode.

The emissions equivalent circuit

The measurement for conducted emissions uses a LISN with the test ground plane as reference, with the noise signal being picked off:

mains Live and Neutral, for a single phase mains supply;

each phase (L1,2,3 & N), if it's a three-phase supply;

DC + and DC - for a DC supply.

A switching converter's noise sources can be decomposed into three types, although they are all related to the same switching process. V_N1 appears differentially across live and neutral; part of this voltage will be measured on the live wire, part on the neutral, their relative amplitudes depending on the relative impedances to earth.

V_N2 appears in common mode between the power lines and the enclosure of the EUT. A metallic enclosure grounded via a safety earth lead is referred back to the ground reference plane at the LISN. If the enclosure is non-metallic and/or there is no safety earth, the common-mode coupling is dominated by stray capacitance C_S between the EUT and the ground plane.

As well as the noise directly coupled back to the power input, noise voltages V_N3 can drive current through any output lines - note that V_N3 is not the intentional differential-mode DC output voltage, but the common-mode noise that rides on top of this voltage with respect to the external ground. Although the output line emissions may not be measured directly, such current can flow through the impedances which are common to the power line measurement circuit (C_S and/or the safety earth) and therefore contribute to emissions measured on the power lines. This is why any power supply should have a representative common-mode load impedance as well as its differential power load, when it is under test.

This section includes the following parts:

For each of these types of emission, the coupling paths are considered in more detail

A special case of switching converter is the variable speed drive

Part of the EMC requirements for power converters is control of AC supply harmonics