Bridge Rectifiers and EMI Filters: What's the Best Order?

Cadence System Analysis

Key Takeaways

EMI in AC-DC conversion begins with conducted currents into the input.
Conducted EMI can also appear as common-mode or differential mode noise at various stages during power conversion.
Different components are used at each stage in power conversion to reduce conducted EMI.

Ferrites come in all shapes and sizes, and they’ll be used to remove EMI on input cables in your power system

When we talk about EMI, it is usually in the context of high-speed/high-frequency PCB layout, where routing mistakes and poor stackup design produce excess crosstalk during operation. However, EMI also interferes with power integrity and reliability, particularly during power conversion. Any device that pulls its power from the grid can receive noise during operation, both from the input power lines and due to unintended feedback that couples noise back to the input.

Enter EMI filters, which can be used to clean up noise throughout a power conversion system. However, when using EMI filters, there are some things designers should consider. In particular, a common question is where to place the bridge rectifier and EMI filter stages to provide maximum noise reduction. By taking advantage of multiple EMI filter circuits and components, a designer can properly remove noise from power conversion.

Where Should Bridge Rectifiers and EMI Filters Be Placed?

Some designers will argue about whether to place an EMI filter before or after a bridge rectifier in a power converter. In general, there are some specific points in a power converter where EMI filtration occurs, either with intentionally placed filters or due to the inherent function of each conversion stage.

The block diagram below shows the general layout of a production power converter system and the locations of some standard EMI filter components. EMI can result from unintended coupling anywhere along the signal path in this block diagram.

Simple block diagram showing the typical locations of standard EMI filter components

Here, there are several instances where EMI filters are placed around a bridge rectifier to help reduce or eliminate conducted EMI. Typically, the coupling mechanism between different sections of this converter block diagram will determine the type of noise that propagates to the output, and the different filter blocks in the above image will address specific noise sources along the signal chain. Note that radiated EMI is not specifically addressed in the above block diagram because, practically, it could be received in any stage of power conversion.

Also omitted from the above block diagram is common-mode choke placement, which could be placed at the input or output, depending on the strength of common-mode noise in the system. Generally, ferrite beads only act as a differential band-stop filter, not as a common-mode choke, so this noise source would still need to be addressed in three-phase AC systems as well as in any system with strong parasitic coupling involving chassis ground.

The table below summarizes the role of each of the EMI filter components in the above block diagram.

EMI filter component or circuit	Problems it solves	Problems it does not solve
Ferrite choke	Acts like a band-stop filter, only removes mid-band frequencies on power line inputs.	High-frequency harmonic content on the input power lines.
Common-mode choke	Suppresses broadband common-mode conducted currents, which helps reduce radiated common-mode noise produced via parasitic feedback.	Contrary to misconceptions, a common-mode choke is not a low-pass filter. It does not produce a DC differential output.
Pi filter	Provides low-pass filtering to help eliminate switching noise (~1 MHz differential-mode noise) and suppress ripple from the switching converter.	Does not remove common-mode noise that propagates through the rectifier bridge, can pickup noise from a ground plane if not placed properly.
Rectifier output capacitor	Dampens leftover ripple from the rectification stage.	Does not remove common-mode noise that propagates through the rectifier bridge.

What EMI Filters Can’t Solve

While EMI filters can greatly reduce noise from particular sources, there are some problems that some EMI filters can’t fix:

Common-mode noise: The pi filter used on the output only targets differential-mode noise, not common-mode noise. A common-mode choke is normally used on the output to provide broadband filtering of common-mode noise.
Radiated EMI: While EMI filters don’t directly target and suppress radiated EMI, they can reduce conducted EMI that originates as radiated EMI.
Power system harmonics: The problem with harmonics on the output from a power converter cannot be totally solved with an EMI filter. This is because these harmonics are prominent up to dozens of orders (40th harmonic or higher is commonly seen in EMC tests), so they will still be present after low-pass/band-pass filtering. Instead, a PFC circuit is used between the rectifier and switching regulator to remove harmonic content.
Power ripple rail: When high-speed components switch states and draw transient current, they will excite an underdamped transient oscillation on the power rail, even when the rail is supplied with a perfect DC source. EMI filters cannot do anything to address this noise source, which might be seen around ~100 MHz. Instead, it can only be reduced with decoupling.

Since EMI filters won’t eliminate all noise sources in a power system, some other steps should be taken to help clean up noise on the power rail. Design tools with an integrated SPICE engine and electromagnetic field solver can help you pinpoint noise sources and experiment with methods to eliminate them.

When you’re designing advanced power systems that use bridge rectifiers and EMI filters, make Cadence’s PCB design and analysis software your go-to application for every task in electronics design. You’ll have access to a range of simulation features you can use for pre-layout design evaluation to ensure stable power delivery, and you can seamlessly move into physical layout with schematic capture features and a complete set of CAD tools.

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