Electromagnetic interference (EMI) and electromagnetic compatibility (EMC) are two sides of the same coin: circuit designers don’t want systems to influence other circuits or receive interference from them unintentionally. EMI prevention has become more troublesome within the last few decades for numerous reasons, including gains in switch performance and denser assemblies. Faster speeds and expanding features are inconsequential when the circuit function is erratic or unpredictable, so compliance with neighboring electronics is not guaranteed.
EMI issues broadly reflect PCB best practices for power and ground layout, and designers can supplement power lines with an EMI filter to ensure minimal noise on the system power input. These filters perform the crucial role of cleaning and conditioning signals before distortion can be introduced to the network, preventing poor performance and even damage to components.
The Basics of EMI Filter Layout and Theory
The need for EMI filtering arises from the increased efficiency of switched-mode power supplies. As switching frequencies have risen, so too have instantaneous di/dt values that can induce currents in nearby lines without proper shielding. An EMI filter aims to provide insertion loss at frequencies outside its intended operating range with a high input impedance and ideal impedance matching at the targeted frequency. Circuit designers create the appropriate insertion loss with a filter network built from inductors that serve as low-pass signal pathways (i.e., blocking high frequencies). At the same time, capacitors shunt high frequencies away from sensitive circuitry.
It’s worth noting that EMI filters benefit tremendously from proper layout practices to minimize the effects of parasitics. If an EMI filter is drastically underperforming calculations/simulations, ensure these guidelines are in place:
- Separate the input and output lines to prevent coupling.
- Keep the trace and lead length short for a low impedance.
- Space filter components from successive sections apart.
- Observe clearance between the filter and sensitive circuitry nearby.
- Ensure low impedance pathway from power wiring to filter input and balanced impedance to enhance common-mode signal rejection.
The layout is only one portion of the EMI filter puzzle, however. The impact of these two noise modes is critical to understanding these filters’ performance:
- Differential mode noise manifests as a voltage difference between the hot and neutral/return lines due to load switching, transients, and surges that develop relative to the ground potential. The use of pulse width modulation (PWM) – where entirely on/off cycles control the power delivery – introduces a significant ripple current at the DC link between the battery output and the pre-load power inverter.
- Common mode noise forms between the ground and two or more lines. This noise includes hot and neutral/return, as the ground is common to both these lines and the other ground-referenced lines in the system. Common mode transients are often higher in frequency than differential mode as common mode noise generates from capacitive coupling of differential mode signals across conductors, the rate of which positively correlates to increasing frequency.
Common mode noise is more challenging to define than differential mode noise. Consider the transformer in a power delivery system: due to the wound nature of its coils, significant capacitance exists between the adjacent surfaces of the coils. During the initial moments of power conduction, current wants to flow through the transformer to fill the empty “capacitor” due to the large, instantaneous dv/dt developed. The behavior of a capacitor seeks to maintain the voltage difference between its terminals and will sink current to do so. In other words, the current will flow back to the source when the power to the transformer switches off, closing the current loop. However, the path of this current simply follows the path of least impedance, and it has many different circuit pathways it can take (individually or collectively) to do so. This winding capacitance – inherent to all inductors – also extends between the primary and secondary windings of the transformer.
AC and DC EMI Filters: What’s the Difference? |
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EMI filters are essential components in electronic circuits to mitigate electromagnetic interference. Choosing the right filter topology is crucial to ensure optimal performance and compatibility with specific applications. In this table, we explore common EMI filter topologies, their suitability for different scenarios, and factors that make them unsuitable for certain applications. Understanding these topologies will help engineers make informed decisions when selecting EMI filters for their PCB power designs. Let’s dive into the details of each topology and their respective advantages and limitations.
Common EMI Filter Topologies |
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| Description | Suitable for | Unsuitable for | Explanation | |
| Π (pi) filter | Two capacitors are in parallel from a series inductor to ground. The inductor should be twice the value of either cap. | 220A, 50Ω test | MIL-STD-461, naval, and current injection probe tests. | The line-side capacitor does not function as intended at low frequencies. |
| T filter | Two inductors are in series with the load with a bypass cap at the inductor junction. The capacitor should be twice the value of either inductor. | Low-impedance lines and high current lines (provided the value of L is reasonably small). | DC systems with PWM-switched loads. | The inductive impedance of the filter adds to low line impedance, which provides capacitor-suitable input impedance. |
| L filter | An inductor in series with the load, bypass cap at load. | Load–switched DC and high-power applications. | Doubled configuration. | Peak-to-peak voltage drop can match the switcher frequency; capacitor reactance must be below the input impedance. |
Your CM Keeps Your Power Lines Running Quiet and Smoothly
EMI filters are essential to the optimal performance of today’s fast-switching power supplies. By offering a high impedance to signals that could detrimentally affect the system circuitry, these filters form the first line of defense against noise. With system speeds climbing and EMC testing a prohibitively expensive failure, design teams have little room for error in NPIs.
Fortunately, our engineering team here at VSE is committed to building electronics for our customers: we can lean on over forty years of experience in the industry of delivering life-saving and life-changing electronics. With our valued manufacturing partners, we strive to meet industry standards and our internal expectations of high reliability in printed circuit board assembly.
