LCL filters
Grid filters for PV, storage and bidirectional inverters. Resonance, damping and harmonic limits.
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Custom power and EMI filters
A power filter is only as good as its inductors at the real operating point: the frequency band that has to be attenuated, the DC or 50 Hz bias, the switching ripple, and the temperature and cooling conditions. We design filters for that operating point, and supply them as magnetics only or as a complete, assembled and tested filter.
Which filter do you need?
EMI filters
Common mode and differential mode filters sized from your conducted emissions scan.
Sine wave and dv/dt filters
Output filters that protect motors and cables in fast-switching drives.
Why filters fail in the converter and not on paper
Most filter designs start from ideal inductors and capacitors. The converter then shows the difference. Powder core inductors lose part of their inductance at peak current, so the resonance moves. Ferrite chokes with many turns have a self-resonant frequency above which they stop filtering. Capacitors age and their value drifts with voltage and temperature. A filter that meets the attenuation in simulation can still miss it on the bench, or cause resonances with the grid or with the converter's control loop.
We design the inductors and passive components for the conditions they will actually see, modelling their parasitic elements, checking where the resonances fall, and adding damping where it is needed. That is where most of the filter performance is decided.
What we need to start
- The converter: topology, switching frequency, modulation and power.
- The source and the load: grid or DC bus, cable, motor or the next stage.
- The target: the standard and limit to meet, a measured scan, or the attenuation at given frequencies.
- Constraints: size, cooling, ambient temperature, insulation and safety standard.
Magnetics only, or the complete filter
We supply the filter in two ways. As magnetics only, the inductors and chokes are designed for your operating point and you integrate them with your own capacitors and board. As a complete filter, we also select the capacitors and damping resistors, and assemble them on a board or in a dedicated enclosure. Both start from the same design review.
Every complete filter is measured with an impedance analyser, and you receive the measured attenuation curve against the one agreed in the specification. Insulation tests are available on request.
Straight answers
Custom filters, the usual questions.
Do you design the whole filter or only the inductors?
Both. We can supply the inductors and chokes for your own filter, or the complete filter with capacitors and damping resistors, assembled in a dedicated enclosure and tested.
How do you test a complete filter?
We measure it with an impedance analyser and send you the measured attenuation curve, so you can compare it with the specification before it goes into your converter. Insulation tests are available on request.
Which standards do you design filters for?
The ones your product must meet. For grid-connected inverters, usually the harmonic current limits of IEEE 1547, IEEE 519 or IEC 61000-3-12. For conducted emissions, CISPR 11, CISPR 32 or CISPR 25. For motor drives, the voltage stress guidance of IEC 60034-25 and NEMA MG1. The filter is designed so that your system meets those limits, which also depend on the rest of the system.
Can you start from a measured EMI scan?
Yes, and that is the best starting point. The difference between the measured spectrum and the limit, plus a margin, gives the attenuation each part of the filter has to provide.
What is the minimum order?
One unit. Prototypes are built on the same drawing that goes to production.