Feature: Power electronics
Ideally, the filter should be simulated using appropriate tools, and validated using measurement equipment such as a VNA. When filters are outsourced, it becomes essential for design engineers to carefully review the datasheet, understand the stated insertion loss and follow the manufacturer’s installation guidance. In practice, however, from the author’s experience, these steps are often not carried out thoroughly, frequently leading to problems. A failed EMC test can result in multiple visits to the test
laboratory, followed by repeated cycles of re-design and re- testing. In some cases the product may continue to fail EMC tests, at which point specialist expertise is sought. This is often when consultants like me are brought in to assist. This article is therefore aimed at engineers involved in
product and power electronics design. The intention is that after reading it you will be able to avoid common pitfalls and design or install EMI filters correctly, helping to reduce costly delays before bringing a product to market.
Understanding the insertion loss of filters in power electronics applications By Min Zhang, Founder and Principal
EMC Consultant, Mach One Design E
ngineers who design power electronics products like EV chargers, flywheel energy storage systems or variable speed drives are often involved in specifying input and/or output filters for power converters. Some of these filters may be designed in-house, but in
many cases they are sourced as off-the-shelf solutions from third-party filter manufacturers – particularly for high-power products installed in metal enclosures. When filters are designed in-house, the quality of the design naturally depends on the company’s engineering resources.
20 March 2026
www.electronicsworld.co.uk
Insertion loss basics To understand the insertion loss of a filter, it is essential to recognise that its performance depends strongly on both the noise source impedance and the load impedance. This applies to differential-mode and common-mode noise paths; see Figure 1. The simulation in Figure 2 shows how variations in source
and load impedance can significantly affect the measured insertion loss of a filter. Four different source-load scenarios are simulated, with each resulting in different insertion loss. This highlights an important point: accurately simulating real filter performance is not trivial. Even in a relatively simple simulation setup, factors like coupling within the common- mode choke, parasitic elements of inductors and capacitors, and any damping components must be considered to obtain meaningful results. The example shown here focuses only on the differential-
mode performance of the filter. It also shows how insertion loss is calculated in practice. A fixed-amplitude noise source is applied (in this case, 1Vpeak-to-peak
), and a frequency sweep
is performed whilst keeping the source amplitude constant. The resulting voltage across the load is then measured. The insertion loss is calculated as: Insertion Loss = 20 log (V2/V1), where V2 is the voltage
measured across the load with the filter in place, and V1 is the voltage measured across the load without the filter.
Source and load impedances for power electronics applications This naturally leads to the next question: How do we know the source and load impedances? The honest answer is that, in most cases, we don’t – at least
not precisely. Different circuits and products can exhibit very different impedance characteristics. That said, there are some useful rules of thumb, particularly if working with variable
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