Fig: PWM signal

Pulse width modulation (PWM) is a widely used energy-efficient technique to control electrical loads on demand and conserve energy. In the PWM technique, a PWM controller generates the PWM signal (rectangular pulse wave). The PWM signal is used to switch ON and OFF semiconductor switches (e.g., MOSFETs, IGBTs, GaN transistors) used in power converters (e.g., AC-DC converter, DC-DC converter & DC-AC converter); thereby controlling the average power delivered to the load. The applications of PWM-based power converters include variable-speed computer fan controllers, light dimmers, personal electronics, UPS, Electric vehicles, car heaters, electric stoves, soft-blinking LED indicators, DC motor controls, ultrasonic cleaning, and many more applications.

Duty cycle D = T_on/(T_on+T_off)

The PWM signal switches (ON & OFF) the semiconductor switches at a high switching frequency, typically in the range of a few kilohertz or even higher. Here, the pulse width (ON time) or duty cycle of the PWM signal is adjusted or modulated to control the average voltage (and current) received by the load (e.g., motor), resulting in the average power delivered to the load being controlled. 

Fig: Concept of duty cycle variation

The average output delivered to the load depends on the duty cycle or Ton time of the PWM signal. For example, a 50 % duty cycle means that the load (e.g., motor) will receive an average voltage of 50 % of the supply voltage and run at half speed. Similarly, a 100 % duty cycle means that the load (motor) will receive an average voltage of 100 % of the supply voltage and run at full speed. Thereby achieving efficient control of power consumption.  Note that in the PWM method of control, due to the high switching frequency with a specific duty cycle, the output appears characteristically like a continuous-voltage analog signal while delivering power to a load/device. PWM is a popular approach due to its effectiveness in minimizing energy waste while maintaining optimal load performance.

EMC aspect of Pulse Width Modulation:

EMC (Electromagnetic Compatibility) is a device's, equipment's, or system's ability/capability to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic or radio frequency disturbances (called EMI) to anything in that environment.

High switching frequency in PWM converters causes a higher voltage (dv/dt) and (di/dt) current rise, which produces electromagnetic interference (EMI) from the power converters. The EMI (i.e., undesired electromagnetic disturbance) may be conducted EMI and/or radiated EMI, leading to interference with neighboring electronic devices and systems. EMI is proportional to the switching frequency (increases with switching frequency). 

Conducted EMI is the unwanted electromagnetic emissions generated by devices during operation, which can travel through power cords or power lines and potentially impact other connected devices on the same line. Radiated EMI is the unwanted electromagnetic emissions generated by devices during operation, which travel through the air and potentially impact other nearby devices/systems in the same environment.

The high switching frequency also causes switching losses, switching stress on power devices, and reduces the system's efficiency. Hence, the choice of switching frequency in power electronic design is decided/limited by several factors. The factors are:  switching stresses on the power devices (during the turn-on and turn-off transients), switching losses, and severe di/dt and dv/dt which produce electromagnetic interference (EMI). 

EMI reduction technique in Power electronic designs involving PWM signals: 

The conducted & radiated EMI in Power electronic design (e.g., in power electronic converters) involving PWM signals can be reduced through various techniques as mentioned below:

The EMI reduction techniques include the use of EMI filters, shielding, use of shielded twisted pair cables, proper grounding, proper PCB layout or design, and choosing components with low EMI characteristics. 

Frequency converter having input, output filters, shielding to suppress EMI

Both solutions (EMI filters & shielding) have limitations and increase the cost and weight of a power converter. There is another technique called random pulse width modulation (RPWM) to reduce EMI. In the RPWM technique, one of the switching parameters of the PWM signal, such as switching frequency, duty cycle, and pulse position is randomly varied in order to spread the energy of the PWM signal. This randomization helps to distribute the spectral energy of the switching waveform, reducing the concentration of energy at specific frequencies and thereby reducing electromagnetic interference (EMI).  

EMC standards for pulse width modulation (PWM) based electronic devices: 

Electrical and electronic products/devices can be brought to market if they meet the requirements of EMC (Electromagnetic Compatibility) standards for emissions and immunity. The EMC emission & immunity standards have been developed to ensure that the product does not emit more than a specified level of EMI and can withstand EMI from nearby devices & & operates satisfactorily in its intended electromagnetic environment.

There are several EMC standards applicable to PWM (Pulse Width Modulation) signal-based electronic devices. Some common standards are listed below:

CISPR 22/EN55022: This standard sets the limits and measurement techniques for electromagnetic emissions from information technology equipment (ITE), both Class A & Class B equipment.

CISPR 11: This standard sets the limits and measurement techniques for electromagnetic emissions from Industrial, scientific and medical equipment. 

FCC Part 15: This standard is for electronic devices sold or used in the United States. The FCC part 15 limits the amount of electromagnetic interference/emission from electronic devices, including intentional & unintentional radiators.  

IEC 61800-3:  In this standard, EMC requirements for adjustable speed power drive systems (PDSs) and machine tools (MTs) are specified.

EN / IEC 61000-6-1: It is the generic EMC immunity standard for residential, commercial, and light-industrial environments. 

EN / IEC 61000-6-2:  It is the generic EMC immunity standard for electrical & electronic equipment used in industrial environments.

EN / IEC 61000-6-3:  It is the generic EMC emission standard for electrical & electronic equipment used in residential environments.

EN / IEC 61000-6-4:  It is the generic EMC emission standard for electrical & electronic equipment used in industrial environments.