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The Methods and Benefits of PWM EMI Reduction in PCBA

Author Cadence System Analysis

Key Takeaways

●     Learn about PWM and EMI reduction techniques.

●     Gain a greater understanding of the mechanisms that produce EMI and how you can mitigate it.

●     Learn more about EMI standards.

 

 Graphic of the mean output signal of a pulse width modulation signal

The mean output signal of a pulse width modulation signal.

The Class D amplifier continues to be one of the most efficient amplifier classes since its introduction in 1958. In the field of electronics, the method or technique of pulse width modulation (PWM) is synchronous with both the Class D amplifier and controllers.

However, what is also synchronous with devices that utilize PWM is electromagnetic interference (EMI), which incidentally degrades performance, functionality, and device lifecycle.

Pulse Width Modulation (PWM)

The method of generating an analog signal by utilizing a digital source is called PWM or (Pulse Width Modulation). A typical PWM signal incorporates two primary components that characterize its overall behavior:

  • Duty cycle

  • Frequency

We call the amount of time the signal is in an on or high state, as compared to the total time it takes to complete one cycle, the duty cycle. In other words, the term duty cycle references the duration of the on state against the entire period where the signal completes a single cycle. For example, a signal or waveform will have a 50% duty cycle if its high state occupies half of the total duration.

Furthermore, the frequency dictates how fast the PWM will achieve a complete cycle. For example, 500 Hz equates to 500 cycles per second. This also means it controls how quickly the PWM signal switches between its on and off states. In summary, cycling a digital signal on and off at a quick enough rate, and with a specific duty cycle, will cause the output to appear characteristically like a continuous-voltage analog signal while supplying power to a device.

Electromagnetic Interference (EMI)

EMI, or electromagnetic interference, is a type of electromagnetic emission that can disrupt another component or electrical device. In general, there are two possible causes of EMI: One likely reason involves direct physical contact with a conductor, which we call conducted EMI. The second probable cause of EMI is through induction, which does not require direct physical contact, and we call this type radiated EMI.

When referring to the radio frequency spectrum, we call EMI, RFI, or radio frequency interference. This particular type of disruption is generated by external sources that affect electrical circuits through conduction, electrostatic coupling, or electromagnetic induction. As you might imagine, these types of disturbances can prevent functionality or degrade circuit performance.

When referring to data paths, disturbances such as these can effectively increase error rates and even cause the complete loss of data. Overall, whether naturally occurring or human-made, both source types can produce changing electrical currents and voltages that attribute to EMI.

Categories of Electromagnetic Interference

These are the two categories of EMI we’ll be focusing on today:

  • Narrowband

  • Broadband

Whether it is narrowband EMI or RFI, they both typically originate from intended transmissions such as TV stations, radio stations, or smartphones. However, broadband EMI or RFI, generally, has unintentional radiation sources such as electric power transmission lines.

Keep in mind that conducted EMI requires physical contact with a conductor, whereas radiated EMI is through induction, thus not requiring direct contact with the conductor. Also, EM disruptions in the electromagnetic field of conductors will cease to be restricted to the conductor's surface and will radiate outwardly. These characteristics exist in all conductors, and the shared inductance between two radiated EM fields will result in EMI.

The Significance of EMI in PWM Techniques

In the area of electronics, both high-frequency and high-current PWM signals are synonymous with producing and radiating EMI and RFI. Therefore, when utilizing PWM currents to control power devices, be sure to make connections that mitigate or limit the radiated noise. In summary, the use of shielded twisted-pair cables and single-point grounding is vital to reducing PWM noise and, therefore, the prevention of EMI.

The overall design of power electronics contends with three significant challenges:

  • EMI

  • Power losses

  • Harmonics

Furthermore, the use of power electronics (PE) also negatively affects the electrical grid by way of EMI. For this reason, PE designs must comply with the standards set by CISPR 22 and the FCC. For example, static converters, which can bring about signal disturbances in the EM environment in both near and far-fields. The leading cause of these disruptions is, of course, high-frequency interference from the semiconductor switching components inside power electronics.

Note: EN 55022 / CISPR 22 provides the information (Europe) (FCC Part 15 in the US) for IT equipment (ITE) for the radio disturbance characteristics for EM compatibility compliance. To ensure the utilization of either standard to certify digital electronic equipment, CISPR 22 and FCC Part 15 are comparatively congenial, although there are a few differences.

EMI Reduction through PWM Techniques

Typically, the challenge of mitigating EMI utilizing shielding and filtering methods is not cost-effective. Pursuant to those demands and a desire for greater efficiency, promotes the use of switching mode power supplies to achieve increased efficiency. As I am sure you are aware, PEs like power inverters generate CMV (common-mode voltage) and CMC (common-mode current). These two by-products create high-frequency EMI noise and leakage currents in both electrical drive applications and grid-connected systems, which lower system efficiency.

However, we can mitigate this CMV by designing proper EMI filters and investigating the effects of numerous modulation strategies. For example, there are examinations of the impact of various modulation methods on CMC and CMV in two-level and three-level power inverters. These results indicate that the modified third harmonic injection method reduces the CMC and CMV in a system by 60%. In this case, they utilize this modified PWM method in conjunction with EMI chokes to reduce system distortion.

Overall, there are passive techniques such as shielding, filtering, and accurate PCB layout designs of the power components that focus on mitigating the EMI levels. There are also active techniques that rely on the modification of the PWM control signal's shape to spread the signal's energy over a wide frequency range, or reduce EMI.

EMI Reduction Checklist

Limiting EMI radiation is paramount to any electrical design involving PWM signals. Therefore, the following checklist will provide a guideline to help eliminate PWM noise generation or diagnose EMI issues:

  • Utilize field supplies that incorporate a slew-rate-limited output that will limit output surge current.

  • Regardless of field supply location, connect its negative return, which is usually its ground wire, to a docking panel or board ground. This PS ground reference wire connection is vital.

  • Utilize shielded twisted-pair cables for your connections, which includes power and your device connections (internal and external). Also, ground the shield to the enclosure at both the input and output.

  • Utilize a ground connection from the board assembly directly to your enclosure's ground.

  • When using AC for main power into the device or an enclosure, connect the AC ground line directly to the chassis or enclosure. Also, use an additional wire (failsafe) from the AC ground line connector to the board ground.

EMI mitigation is a vital and necessary design decision that dictates a device's functionality, performance, and lifecycle. Furthermore, the use of PWM is critical to many power electronic devices, and the side effect, of course, is EMI radiation. Therefore, designing PCBAs that address the reduction of EMI is paramount because EMI negatively affects all devices within its range.

Twisted pair cable with shield structure to mitigate EMI

Twisted-pair cable with a shield structure to mitigate EMI.

Work towards implementing EMI mitigation techniques for all of your company's design and manufacturing needs. Whether you are utilizing a single-sided board or a multi-layer design, create designs from verified component models and analyze all aspects of their functionality. 

You can rest easy with better attention and stronger tools for your EMI performance. 

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