Why does PWM behave like EMI?

Several factors contribute to PWM's ability to generate EMI. These include:

Rapid switching

PWM generates EMI due to high clocking and continuous switching action of internal digital circuitry. Rapid switching introduces high-frequency components in nearby electronic devices. More action of PWM noise is seen in power devices with high current ratings. PWM operation in the RF domain with high switching frequency contains multiple harmonics and ripples. Simply put, PWM functions as an unintentional antenna to all nearby devices.

Fast edge rate

The edge rate is a measure of time taken by a signal to transition between low (0) and high (1) states. A faster edge rate corresponds to the fast rise and fall times in a PWM signal. PWM pulses transitioning abruptly between high and low values produce sharper edges that are trapezoidal in shape rather than square waves, increasing the likelihood of EMI generation.

Large duty cycle

A large duty cycle indicates that the device remains on for a longer period of time. In high-current rated and RF devices, continuous operation of the PWM generator circuit introduces pulses directly into the environment. As the duty cycle is larger, pulses are radiated in the environment for a long time. This kind of continuous PWM noise synchronizes nearby drivers and motors to hinder their operation.

Can EMI disturb the PWM signal itself?

EMI, mainly, distorts the amplitude of any pulse-modulated wave. In such cases, PAM (Pulse Amplitude Modulation) signals are adversely affected. EMI can also change the position of the pulse in PPM (Pulse Position Modulation) and quantized levels in PCM (Pulse Code Modulation).

PWM is quite immune to EMI and noises compared to other pulse modulation techniques. Information is encoded in the pulse width of a PWM signal. EMI cannot affect the width of the pulse, leaving the information signal unharmed. However, strong EMI can alter the PWM duty cycles.

PULSE MODULATION

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Six ways to minimize PWM noise

Fortunately, there are several design techniques that can effectively mitigate PWM-induced EMI. Let's explore these methods:

Follow EMC constraints

Smart design that follows EMC (Electromagnetic Compatibility) constraints can easily avoid chances of EMI generation due to PWM signals. The term “smart design” here incorporates an effective hardware and PCB design. Mindful selection of components, meeting EMC constraints, and running simulations enable designers to theoretically minimize the effects of EMI in PWM-based systems.

Avoid high slew rate

Converters and high-speed drives in motors have a high slew rate. A high slew rate means how fast an output changes. A rapid change in output gives rise to surge currents. Hence, the design should aim to minimize the slew rate. As mentioned above, high-duty cycles can generate PWM noise. However, duty cycle value is the subject of application. The key is to choose pre-determined on and off times.

Selecting an appropriate switching frequency

In SMPS, soft switching converters, and motors, PWM is used to synchronize and control parameters like frequency and current. Loop functions as an antenna to radiate EMI. The size of the loop affects its radiating efficiency. Reducing the size of the loop to a quarter wavelength of the switching frequency is a method to reduce the chances of EMI generation.

Place filters

Placing EMI filters (or low pass filters) can block noise or localize it. A filter ensures that high-frequency components are not radiated. Filters should be placed near the sources of high-frequency generating components, i.e. PWM generating circuit. Commonly used filters in converters and motors is a single-stage or two-stage LC circuit.

Implement single-point grounding

Ground loops, voltage drops, and improper routing can cause EMI to simultaneously appear on the signal and return lines in a circuit. In simple words, EMI appears in phase at signal and grounding paths. Single-point grounding is a way to eliminate unnecessary grounding paths and make all the grounding connections to a single reference point.

Use shielding

Just as the name suggests, shielding is a protective “gear” that protects components from EMI. Conductive shielding is commonly used in electrical and electronic equipment as a measure to protect against noise. In the case of EMI generation due to PWM, shielded twisted pair cables can be used. Additionally, metal sheets, foils, films, and coatings also function as shielding.

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