What is Pulse Width Modulation
Pulse width modulation is that technique by which a low-frequency signal is being generated from high-frequency pulses. The main use of pulse width modulation is to let the higher load electrical devices to take over the control of the power supplied to the system. In a simpler sense, pulse width modulation uses digital signals to control high power applications. Besides, it is easy to convert the PWM signals back to the analog form with the minimum use of hardware. The power is controlled by turning a switch on and off between the supply and load at an increased rate. The power supplied to the load is directly proportional to the duration of the on time. Pulse width modulation is also called pulse duration modulation.
The proportion of ‘on’ time to a period of time is called duty cycle. Duty cycle is expressed in percentage. Duty cycle is the inverse of the frequency of the waveform. The switching frequency of pulse width modulation must be higher than the rate which would affect the power device. This frequency can be different for different applications and devices.
By imparting pulse width modulation in an application, the power loss in the switching device can be considerably reduced. When the switch is made on, the voltage drop across the switch is practically zero. And when the switch is off, there is no current. In this case, also power loss is negligible, as the power is the product of voltage and current.
The on/off nature of pulse width modulation is another advantage because of which the PWM is widely used in various digital control applications. It is easier with a PWM to set the required duty cycle. Because of this property, pulse width modulation has found its applications in communication systems as well.
The heat produced during the operation of a digital system is lesser compared to that during the working of an analog system. Majority of the heat in a switching device is generated during the transition phase. The device is at a state between on and off at this time. This is because power is the product of voltage and current. During the working of pulse width modulation system, either current or voltage is nearly zero. And therefore the heat produced is almost zero in such systems.
A pulse width modulation signal can be generated by a sawtooth waveform as well as using a comparator.
Basic Principle
Pulse width modulation makes use of a rectangular pulse wave of which the pulse width is modulated. This modulation of pulse width results in the variation of the average value of the waveform, where the average value is dependent on the duty cycle D.
Duty cycle is the period in which the signal is on and is denoted as a percentage. Duty cycle is 50 percent when the signal is ‘on’ for half of the time period.
Consider the case of 20% duty cycle for a voltage switching between 0V and 5V, the average voltage will be 5×0.2=1V. Similarly, for 50% and 80% duty cycle it is 2.5V and 4V respectively. This property of PWM is very useful in motor speed control and LED brightness control.
PWM Generation in Analog Circuits
To generate PWM signals in analog circuits, a sawtooth triangle waveform is used. It can be easily generated using a simple oscillator. A comparator can also be included. The logic of generating PWM signals is very simple. If the value of the reference signal (sawtooth triangle signal) is more than the modulation waveform, the resultant PWM signal is a high signal, otherwise, the resultant waveform is in the low state. This method is also called as a carrier-based generation where the reference signal act as the carrier waveform.
PWM Generation in Digital Circuits
Most of the day-to-day digital circuits can produce a pulse width modulation signal. for example, microcontrollers generate PWM output signals. In microcontrollers, in order to generate a pulse width modulation signal, a counter which increments periodically is used. This counter is resent at the end of every period of the PWM. The PWM output changes state from high to low when the counter value is more than the reference value. This technique is called time proportioning or time proportioning control.
This counter is considered as the counterpart of the reference sawtooth signal in the analog circuits. The output PWM waveform is generated by comparing the current counter value and the digital reference value. The duty cycle varies in discrete steps. As the resolution of the counter increases, the performance of the counter becomes better.
Examples and Applications
One of the very popular applications of pulse width modulation is LED lighting. Usually, an LED has the tendency to produce non-linear light by applying a current. By applying the pulse width modulation, one can produce a linear light output. The brightness of the LED can be controlled by adjusting the duty cycle. In a Red-Green-Blue LED system (RGB), if the brightness of all the three LEDs is equal, then the resultant output light will be white. By varying the duty cycle of various LEDs, different colors are produced.
Another important application of pulse width modulation signal is a motor. Pulse width modulation is used in a servo motor to control the angle of the motor which is attached to a robot arm or similar things. The shaft attached to the servo motor turns to specific positions based on the control line. There are some microcontrollers with built-in PWM.
Pulse width modulation is highly useful in motor speed control applications. By incorporating pulse width modulation, it becomes quite easier for the motor to vary the speed. And this increases the efficiency of the system. It is more effective to control the motor using PWM at low RPM than linear methods. H-Bridge makes use of the pulse width modulation in an effective manner. Since the power supply can be switched across both sides of the load, the effective voltage across the load will be doubled.
In a class-D audio amplifier, the maximum output is increased by increasing the voltage. PWM is the pinnacle of such amplifiers and is used by choosing a frequency beyond human hearing. Pulse width modulation is being used by switching mode power supplies.
PWM is also used in switched mode power supplies (SMPS). By switching voltage to the load in a controlled manner with respect to duty cycle, the voltage can be maintained at the desired level. By using the feedback, the load can be monitored and power supply becomes much more efficient.