Common-mode noise is one of the most difficult problems in modern PWM converters. The switching output of an inverter can generate a high-frequency common-mode voltage relative to earth. This voltage drives currents through parasitic capacitances between motor windings and frame, heatsinks and chassis, semiconductor devices and mounting structures, and cables and surrounding metallic structures.

These currents can travel through the protective-earth conductor and return through the converter chassis. In industrial installations they may interfere with encoders, PLCs, sensors, communication networks and other drives.

BLA provides common-mode chokes, CM inductors and common-mode filtering assemblies specifically intended to increase impedance to these unwanted currents. Its published product categories include Common Mode Chokes, CM Chokes and custom inductive components.

For high-current applications, BLA can engineer the magnetic component around DC bias, saturation flux density, core material, copper loss, core loss, temperature rise, leakage inductance and high-frequency impedance. This is particularly important in high-power converters where the choke cannot simply be selected according to inductance alone.

Differential-mode noise occurs between conductors and is generated by switching currents, rectifier operation, DC-link ripple and inverter commutation. In a DC converter, DM noise can appear between positive and negative rails. In an AC system, it can occur between line conductors.

BLA can suppress differential-mode components through differential-mode chokes, series inductors, DC chokes, X capacitors and LC filter networks. The filter must be designed around the converter's source and load impedance because an LC filter that is not properly damped can create resonances.

This is particularly important in modern converters using high-frequency switching because a filter designed only around the fundamental 50/60 Hz frequency will not necessarily provide useful attenuation at the switching-frequency harmonics.

A DC choke introduces controlled inductance into the DC bus and can reduce unwanted current ripple and high-frequency current propagation. In frequency converters and UPS systems, the DC choke can also influence the current waveform and the interaction between rectifier, DC bus and inverter stages.

BLA lists DC Chokes as a dedicated power-quality product category, alongside PFC chokes, CM chokes and reactors.

The magnetic design becomes particularly important in DC applications because the core operates with significant DC bias. BLA can design the choke considering DC current, ripple current, saturation margin, core losses, copper losses, thermal rise and mechanical constraints.

A line reactor is fundamentally different from a conventional EMI filter. Its purpose is to provide controlled series impedance in the power circuit, whereas an EMI filter is primarily designed to attenuate unwanted high-frequency electrical energy.

BLA's reactor portfolio includes input reactors, output reactors, line reactors and custom inductive components. Its published product information describes reactor and transformer solutions ranging from input-output line reactors to multiple transformer technologies.

In frequency converters and UPS systems, a line reactor can be used to control current ripple, reduce certain transient currents and provide additional impedance between the converter and the upstream electrical network. When combined with an EMI filter, the reactor can become part of a broader power-quality and EMC architecture.

The output side of a VFD is electrically very different from its input. The inverter produces a PWM waveform with fast voltage transitions rather than a smooth sinusoidal voltage.

When long motor cables are used, cable impedance and parasitic capacitance become significant. Reflected waves can increase motor-terminal voltage, while capacitive coupling can increase common-mode current.

BLA can provide output reactors, motor chokes, AC chokes and custom inductive filtering solutions for these applications. The objective can be to reduce high-frequency components, limit voltage transition rates and control unwanted common-mode current while maintaining the required motor voltage and frequency.

A sine-wave filter is a more advanced output filtering solution designed to convert the PWM output of an inverter into a waveform that is substantially closer to a sinusoidal waveform. This can be particularly valuable where long motor cables, sensitive motors, acoustic noise, motor insulation stress or stringent EMC requirements are involved.

BLA lists Sine Wave Filters as part of its power-quality product portfolio.

A sine-wave filter generally combines inductive and capacitive elements to attenuate the switching-frequency components while passing the fundamental motor frequency. The design must consider the converter's switching frequency, motor frequency range, rated current, voltage, load characteristics and resonance behaviour.

EMI should not be confused with low-frequency harmonic distortion. A converter can simultaneously have high-frequency conducted EMI and lower-frequency harmonic current components.

BLA offers harmonic filters as part of its power-quality product portfolio. These solutions can be incorporated where the application requires control of harmonic currents in addition to high-frequency switching noise.

For large UPS systems, industrial converters and grid-connected inverters, the complete architecture may therefore include EMI filtering + line reactors + harmonic filtering + DC-link inductance.

Modern UPS systems frequently employ active power-factor correction (PFC) to control input current and reduce distortion. However, the PFC switching stage itself becomes another source of high-frequency noise.

BLA's product range includes PFC chokes, which can be integrated into appropriate converter architectures.

The PFC choke must be designed for the switching waveform, peak current, RMS current, inductance tolerance, core losses and thermal conditions. Its parasitic capacitance and high-frequency behaviour can also influence the overall EMI performance.

The output of a UPS may supply highly sensitive loads such as servers, medical equipment, communication systems, industrial control systems and data-centre equipment. Although the UPS is designed to provide stable voltage, the switching frequency and control circuitry can still introduce high-frequency components.

BLA can provide output EMI filters, common-mode filtering, differential-mode filtering, output reactors and sine-wave filtering depending on the UPS topology.

The filter must maintain the required output voltage regulation and transient response while attenuating switching-frequency components. Excessive filtering can increase reactive current or interact with the UPS control loop, so the complete system impedance must be considered.

The transition from conventional silicon IGBTs toward SiC MOSFETs and other wide-bandgap technologies is changing the EMI problem. Faster switching allows designers to reduce switching losses and increase switching frequency, but it also produces steeper voltage transitions.

The result can be increased common-mode displacement current and greater sensitivity to parasitic inductance. A few nanohenries of stray inductance can generate significant voltage during very high di/dt events.

BLA's capability in custom inductors, common-mode chokes, reactors and EMI filters allows the filtering network to be designed around these high-frequency characteristics rather than simply scaling an older silicon-based design. BLA describes its engineering capability as including sophisticated EMI/EMC filters, inductive components, transformers and reactors, supported by R&D engineering and custom development.

A modern high-performance converter may require more than one filter stage. A first stage may address lower-frequency conducted emissions, while a second stage provides additional attenuation at higher frequencies.

BLA can develop multi-stage EMI filters combining common-mode inductance, differential-mode inductance, X/Y capacitors, ferrites and damping networks. The BL4233, for example, is described as a dual-stage three-phase filter.

Multi-stage architecture can be particularly useful when a converter must achieve high attenuation across a wide frequency range rather than only at a narrow switching-frequency band.

One of the more advanced problems in power-conversion EMC is the interaction between the converter input impedance and the EMI filter output impedance. An EMI filter may have excellent standalone insertion loss but can destabilize the converter if its impedance characteristics interact unfavourably with the converter's control loop.

This is particularly relevant in UPS systems, active-front-end converters, grid-connected inverters and high-power DC-DC converters.

BLA's custom engineering approach allows the filter to be considered as part of the complete converter rather than as an isolated component. Inductance, capacitance, damping, parasitic resistance, switching frequency and source/load impedance can all be considered when developing the filter architecture.

For very high-frequency noise, ferrite cores, ferrite rings, ferrite beads and cable ferrites can provide additional impedance without significantly affecting the low-frequency power path.

BLA's broader filter category includes ferrite-related and common-mode technologies alongside custom EMI solutions. These components can be used around DC cables, gate-drive wiring, control cables, communication lines and power conductors where high-frequency current requires additional attenuation.

Ferrite selection must be based on complex impedance versus frequency rather than simply the nominal inductance value.

A converter can fail an EMC test even when its internal filter is correctly designed if the filter installation creates a poor high-frequency return path. Long wires between the filter and chassis can introduce parasitic inductance, reducing attenuation at higher frequencies.

BLA provides feedthrough filters, IEC inlet filters, DIN-rail filters and chassis-mounted EMI solutions within its product range. These solutions allow filtering to be positioned directly at the enclosure boundary.

For high-frequency applications, the mechanical bond between the filter and metal chassis becomes part of the electrical design.

BLA's published range gives OEMs the ability to source multiple parts of the EMC and power-quality architecture from one engineering partner. Its EMI filter portfolio includes BL1220 DC EMI Filters, BL2010/2020/2030/2060/2070/2080/2090 single-phase EMI filters, BL330 and BL358 three-phase EMI filters and BL328H three-phase-neutral filters.

The broader portfolio includes common-mode chokes, CM chokes, DC chokes, PFC chokes, reactors, sine-wave filters, harmonic filters, feedthrough filters, IEC inlet filters, inductors, transformers and custom EMI filters. This breadth is important because frequency converters, UPS systems and inverters rarely have a single noise mechanism.

BLA can also engineer products beyond standard catalogue ratings. Its BL1220 documentation, for example, lists ratings extending to 2300 A and notes that higher-current filters for large central inverters up to the MW range are available on request.

This opens the possibility of designing custom high-current DC filters, high-current three-phase EMI filters, large common-mode chokes, high-current reactors and custom power-conversion filter assemblies for large industrial converters, UPS systems, renewable-energy inverters and grid-interface equipment.

The ability to customize the filter around the OEM's mechanical envelope is equally important. BLA states that it provides custom filters, OEM/ODM services and application-specific engineering, with rapid prototype and production capabilities.

The advantage of using BLA across frequency-converter, UPS and inverter applications is that the customer does not have to treat the EMI filter, reactor, choke and power-quality problem as separate engineering issues. BLA's portfolio spans EMI/EMC filters, magnetic components, reactors, transformers and power-quality filters, allowing the complete noise-control architecture to be considered together.

For example, a VFD application may require a three-phase EMI input filter + AC line reactor + common-mode choke + output reactor. A solar inverter may require a high-current DC EMI filter + DC choke + three-phase AC EMI filter + common-mode filtering. A UPS may require a three-phase input EMI filter + PFC choke + DC-link choke + inverter output filter + harmonic filter. The appropriate combination depends on the converter topology and measured noise.

The strongest solution to a converter EMI problem starts with understanding the frequency-domain behaviour of the equipment. BLA can develop the filtering architecture around parameters such as conducted-emission spectrum, common-mode current, differential-mode current, switching frequency, harmonic content, source impedance, load impedance, insertion loss, leakage current, rated current, thermal rise and mechanical constraints.

The result is a filter designed for the actual equipment rather than a generic filter selected solely from the voltage and current rating.