Pulse Width Modulation (PWM) : An Introduction and Applications

What is PWM?

Pulse width modulation (PWM) is a method used in electronics to control the power output of a circuit by modulating the width of a pulse signal. In PWM, a fixed frequency electrical signal, often a square wave, is turned on and off at a rapid rate, and the width of the "on" part of the duty cycle is varied to achieve the desired output.

Duty Cycle of PWM :

The duty cycle of a PWM signal is a measure of the proportion of time during each cycle of the signal that the signal is in its "on" state, which is typically a high voltage state. Essentially, the duty cycle is the fraction of time that the signal is in its "on" state relative to the total time of the signal cycle, expressed as a percentage. For instance, if the duty cycle is 50%, the signal is in its "on" state for half of the signal cycle duration.

                                            Turn on Time

Duty Cycle    =    ---------------------------------------

                              Turn On Time + Turn Off Time

Process for generating the pulse width modulation :

To generate a pulse width modulation (PWM) signal, a comparator is used. The comparator has two inputs - one is the modulating signal that represents the information to be transmitted (such as the desired power level for a motor), while the other input is a non-sinusoidal or Sawtooth wave that serves as a reference signal. The comparator compares these two signals and generates a PWM signal as its output waveform.

If the saw tooth wave is greater than the modulating signal, the comparator output will be in a "High" state. The magnitude of the difference between the two signals determines the comparator output, which defines the width of the pulse generated at the output. This process is repeated for each cycle of the saw tooth wave, resulting in a train of pulses with varying widths that encode the modulating signal.

Types of Pulse Width Modulation Technique :

There are three conventional types of pulse width modulation technique and they are named as follows:

• Trail Edge Modulation:- Trail Edge Modulation is a technique used in pulse width modulation (PWM) to control the duty cycle of the output signal. In this technique, the rising edge of the signal is kept fixed, while the falling edge is modulated. This results in a constant frequency output waveform, but with varying duty cycles. Trail Edge Modulation is used in applications where a fixed frequency is required, but with varying power or intensity levels. This technique is commonly used in lighting applications, such as LED lighting, where it is used to control the brightness of the lights while maintaining a constant flicker-free output.

• Lead Edge Modulation:- Leading edge pulse width modulation (PWM), is a technique used to control the power delivered to a load. In leading edge modulation, the leading edge of the pulse or waveform is modulated, while the trailing edge remains fixed. By varying the width of the pulse's leading edge, the average power delivered to the load can be controlled. This technique is commonly used in applications such as LED lighting, motor control, and power supplies. Leading edge modulation is also used in high-frequency switching power supplies to improve efficiency and reduce electromagnetic interference.

• Pulse Centre Two Edge Modulation:- Pulse Center or Dual Edge Modulation is a technique used in Pulse Width Modulation (PWM) that modifies the timing of both the leading and trailing edges of the PWM signal. This technique involves the generation of two consecutive pulses of the same polarity but with different pulse widths. The center point of the two pulses is the reference point or center of the waveform. By adjusting the width of both pulses, the effective duty cycle of the PWM signal can be changed without altering the frequency. Pulse Center Modulation is commonly used in power electronics applications such as motor control, power inverters, and voltage regulators.

Applications of Pulse Width Modulation (PWM) :

• PWM is used in robotics to control the movement of servo motors and to control the speed of DC motors.
• PWM is used in Class D audio amplifiers to generate high-quality audio signals.
• PWM is commonly used in LED lighting systems to control the brightness of the LEDs.
• PWM is used in solar charge controllers to regulate the charging of batteries from solar panels.

Advantages of PWM :

• PWM used in telecommunication for encoding digital signals onto an analog carrier wave. It works by varying the width of pulses in a train of periodic pulses to represent information. This technique is used in applications such as remote control systems, audio and video transmission, and motor control.
• PWM is used to prevent overheating in electronic circuits. By controlling the amount of time that a signal is "on" versus "off," the average power delivered to a load can be regulated. This allows the circuit to operate more efficiently and reduces the amount of heat generated.
• PWM is used in motor control applications to generate variable speed outputs. By varying the duty cycle of a PWM signal, the speed of a motor can be controlled. This is often used in applications such as robotics, electric vehicles, and industrial machinery.

Practical use case of PWM in real life application :

PWM finds practical use in controlling the speed of DC motors by generating a PWM signal from a microcontroller to regulate the voltage applied to the motor. By adjusting the duty cycle of the PWM signal, the average voltage supplied to the motor can be varied, thus controlling the speed of the motor.

An excellent illustration of this is seen in electric scooters where the controller generates a PWM signal with a duty cycle corresponding to the desired speed, triggered by the user twisting the throttle. This technique offers precise speed control, cost-effectiveness, and durability since it reduces mechanical stress and overheating, ensuring a smooth and safe ride. Additionally, it eliminates the need for complex analog circuits and expensive components.