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Pulse Width Modulation Characteristics and the Effects of Frequency and Duty Cycle

Key Takeaways

●     Learn about pulse width modulation (PWM).

●     Gain a greater understanding of PWM as a controlling method.

●     Get a better understanding of the effects of duty cycle and frequency in PWM.

 Graph of the mean output signal of a pulse width modulation (PWM) signal at the input

The mean output signal of a pulse width modulation signal at the input.

In electronics, modulation is the application of a controlling or altering influence on something. We also refer to it as a variation in the pitch, strength, or tone of a frequency, like in the human voice.

However, in terms of applications, we typically encounter modulation techniques in use for control of devices like DC motors or LEDs. In cases such as these, the technique is called pulse width modulation (PWM).

Pulse Width Modulation Techniques

As stated previously, modulation refers to the ability to exert control over a device or system. Therefore, methods such as this exist in a myriad of applications within the field of electronics. One of the more common uses for modulation as a control method is PWM.

We encounter the extensive use of PWM due to its adaptive nature. PWM is a technique that mitigates the average amount of deliverable power of an applied electrical signal. Moreover, the process is achieved by effectively chopping up the signal into distinct parts. In terms of functional operation, PWM achieves this control by controlling the average current and voltage it delivers to the load. This method is accomplished by rapidly turning the switch between the load and the source, on and off.

However, if we compare on and off periods of the switch, an increase in on-time versus the off-time increases the total power supplied to the load. In general, this method of control has many beneficial applications. For example, PWM paired with maximum power point tracking (MPPT) is one of the principal methods for reducing a solar panel's output to facilitate its use by a battery.

Pulse Width Modulation Frequency

Overall, PWM is principally suited for running inertial devices like motors, which are not as quickly affected by this distinct switching. This is also equally true for LEDs with PWM because of the linear fashion in which their input voltage affects their functionality. However, the PWM switching frequency needs to be high enough not to affect the load, yet the resulting waveform that the load perceives should also be smooth.

Typically, the frequency in which the power supply must switch will vary extensively depending on the device and its application. For example, the switching has to be done several times a minute in an electric stove and well into the tens or hundreds of kHz for PC power supplies and audio amplifiers. One of the significant advantages of using PWM is that power loss in the switching devices is substantially low. In fact, during the off phase of a switch, there is virtually no current. Also, during the on phase of a switch, there is practically no drop in voltage across the switch while transferring power to its load.

Since power loss is a consequence of both voltage and current, this translates into virtually zero loss in power for PWM. Moreover, PWM is perfectly suited for digital controls, due to the nature of digital technology (i.e., 1's and 0's, or ON and OFF states). In general, the intrinsic nature of digital technology lends itself effortlessly to PWM's functionality, and thus, it is easy to set the necessary duty cycle.

PWM Characteristics

A PWM signal is a method for creating digital pulses to control analog circuits. There are two primary components that define a PWM signal's behavior:

  • Duty cycle: A duty cycle is the fraction of one period when a system or signal is active. We typically express a duty cycle as a ratio or percentage. A period is the time it takes for a signal to conclude a full ON-OFF cycle.

  • Frequency: The rate at which something repeats or occurs over a particular period. In other words, the rate at which a vibration happens that creates a wave, e.g., sound, radio, or light waves, typically calculated per second.

About the duty cycle, while the signal is high, we refer to it as ON, and the duty cycle describes the amount of time a signal is in its ON-state. We measure or quantify a duty cycle as a percentage. This percentage represents the specific time a digital signal is ON during a period (interval), and this interval is the inverse of the waveform frequency.

For example, a digital signal that spends half of the time in an ON-state and half the time in an OFF-state will have a duty cycle of 50%, i.e., an ideal square wave. A digital signal that spends three-quarters of the time in an ON-state and one-quarter of the time in an OFF-state will have a duty cycle of 75%.

PWM Characteristics Continued

We discussed the vast array of applications that ideally suit PWM's functionality, including LEDs and motors (servo). Since frequency is a primary component of the PWM technique, it is understandable that frequency affects PWM's ability to exert control within an application. Therefore, the square wave frequency does need to be sufficiently high enough if controlling LEDs, for example, to get the proper dimming effect.

As an example, a duty cycle of 20% at 1 Hz will be noticeable to the human eye that an LED is turning OFF and ON. Whereas, a duty cycle of 20% at 100 Hz or higher will merely exhibit a slightly less dim light output.

As I am sure you are aware, we can utilize PWM to control motors (servo). We can also use it to control a servo motor's angle. In terms of applications, this is beneficial when we attach it to a mechanical device like a robotic arm in an assembly or manufacturing environment. This is ideal because a servo utilizes a shaft which turns to a specific position depending on its control line.

PWM Frequency

A frequency or period is specific to controlling a particular servo. Typically, a servo motor anticipates an update every 20 ms with a pulse between 1 ms and 2 ms. This equates to a duty cycle of 5% to 10% at 50 Hz. Now, if the pulse is at 1.5 ms, the servo motor will be at 90-degrees, at 1 ms, 0-degrees, and at 2 ms, 180 degrees. In summary, by updating the servo with a value between 1 ms and 2ms, we can obtain a full range of motion.

PWM is also currently in specific communication systems, and its duty cycle is in use to convey information over communications channels. Overall, PWM is a methodology or technique to generate low-frequency output signals from high-frequency pulses.

By quickly switching the output voltage of an inverter leg between the upper and lower voltages (DC rail), the low-frequency output basically becomes the average voltage over the switching period.

PWM as a controlling technique is ideally suited to a vast array of applications. Along with its duty cycle, the PWM frequency is the foundation of its functionality as a controlling method.

Sine wave PWM graph

Sine wave with a PWM.

Designing functional circuits that use pulse width modulation requires having a suitable PCB Design and Analysis software that can help you get it done right the first time. OrCAD, by Cadence, is one such software with a suite of robust tools to help with all of your PCB designs. 

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