Specifying Inductors for EMC Noise Filtering in Power Converters

Most modern power converters, and certainly all isolated DC/DC converters, are of the ‘switched-mode’ type, where external DC voltages are ‘chopped’ at high frequency to produce AC for the internal isolation transformer. The transformer AC output is rectified back to DC, with regulation achieved by duty cycle control, all with high efficiency and low losses. A disadvantage is that the switching process generates high-frequency ripple on the input and output, along with conducted and radiated noise spikes that can disrupt other equipment. There is a trend for power converters to operate at ever higher frequencies with faster slew rates to increase efficiency, but the resulting noise spectrum becomes much broader.

Any commercial power converter will include minimal internal filtering to reduce ripple and noise to a typical peak-to-peak value of about 1% of the DC output. This is acceptable in most cases, but if lower levels are required for a sensitive application, a simple solution is to add an external LC filter (Figure 1).


The inductor impedance is theoretically zero at DC, and the capacitor impedance is infinite, so the desired DC is unaffected. As frequency increases, however, inductor impedance ZL increases and capacitor impedance ZC decreases, producing an increasing ‘voltage divider’ effect. The filter corner frequency is chosen to reduce ripple at the converter switching frequency, but predicting the attenuation of noise spikes, which comprise a spectrum of frequencies up to tens of MHz, is more challenging. At certain frequencies, when ZL and ZC become equal, the LC network ‘resonates,’ and noise can be amplified rather than attenuated, although this effect is damped by the load resistor.

Above resonance, there is still some noise attenuation, but other parasitic effects begin to occur. For example, the self-capacitance of the inductor produces another resonance at a much higher frequency. This capacitance also allows noise to partially bypass the inductor. At higher frequencies, core losses in the inductor increase and AC resistance of the inductor wire rises due to the ‘skin effect.’ The capacitor also begins to act as a resistor as its impedance becomes small compared with its Equivalent Series Resistance (ESR). Capacitor Equivalent Series Inductance (ESL) introduces additional high-frequency effects. When these parasitic elements are included, the equivalent circuit of an LC filter resembles Figure 2.

LLOSS 1 and 2, along with RLOSS 1 and 2, are a simplified way to include the effect of frequency-dependent core losses in the circuit. Different values of LLOSS give different impedances, which allow different resistive elements RLOSS 1 and 2 to influence the circuit at varying frequencies. More LLOSS/RLOSS networks can be added to increase model accuracy, but component values are difficult to calculate from inductor datasheet information.

For a complete model of a particular inductor and core, values are typically determined empirically. Figure 3 shows a simulation plot of the attenuation of a filter with and without LLOSS/RLOSS networks included, using assumed values for L and C and their parasitic components, demonstrating that core loss can strongly affect high-frequency noise attenuation – a difference of 20 dB at around 10MHz in this case. Unfortunately, core loss does not appear in typical inductor datasheets and can vary widely.

When choosing a commercial inductor for the EMC filter on the input of a DC/DC converter (Figure 4), the inductor manufacturer’s datasheet usually provides little more than inductance, DC resistance, and sometimes resonant frequency. While this may allow the reflected input ripple to be attenuated by a known amount, the attenuation of noise spikes and their spectrum is difficult to predict without data on parasitic components.

As seen with the output filter analysis, high-frequency effects such as core loss again strongly influence noise attenuation. It is understandable that inductor manufacturers do not provide this information, as many variables affect performance. Core loss, for example, depends on the amplitude and shape of the AC component of the waveform, frequency, DC current bias, and temperature.


Choosing an optimum inductor is therefore challenging and can lead to conducted and radiated noise levels exceeding operational or statutory limits. This may only be discovered during independent EMC testing of the end product, at which point modifications are costly.

If appropriate test equipment is available, potentially including antennas and an EMC chamber, samples of inductors with the same headline ratings from different suppliers can be tested in-circuit to evaluate real-world results. A large inductance value may seem beneficial, but the resonant frequency decreases and a physically small component is likely to have high DC resistance, causing voltage drop depending on converter loading and dissipating some power. Large inductors also exhibit high self-capacitance, reducing high-frequency attenuation.

A smaller inductance combined with a larger capacitor is an alternative, but if an electrolytic type is used for cost and size reasons, high-frequency performance may be poor. Ceramic capacitors perform well at high frequency but are expensive and large for high capacitance values.

The optimal combination of L and C is a compromise influenced by cost, size, and performance. Once an inductance is chosen, there is a confusing array of types available on the market. Ferrite and iron powder cores are common, with some exotic options such as polycrystalline cores, while drum, ring, and ‘E’ core shapes are also considerations, along with through-hole or SMD mounting, which affect performance. Buyers may also see wide price variations for parts with similar nominal specifications of inductance and current rating.

Each inductor type suits particular applications. Ferrite cores have the lowest losses but are more expensive than iron powder, which tolerates over-current better and maintains inductance more effectively. Ring or toroid cores have low magnetic field leakage but are harder to wind and terminate than drum or ‘bobbin’ cores. Design, production, EMC, purchasing, and process engineers must all collaborate to select the optimum solution.

RECOM, a manufacturer of AC/DC and DC/DC converters, understands the complexity of selecting the right inductor for effective noise suppression. To address this, RECOM offers a range of cost-effective inductors and carefully matched capacitors designed for compatibility with most of their power converter models. These filter components are validated through conducted and radiated noise testing in RECOM’s in-house EMC chamber. This ensures customers receive a proven, ready-to-implement solution that simplifies design, reduces development time and cost, and accelerates time to market.