How are voltage and duty cycle related in pulse width modulation (PWM)?

Relationship Between Voltage and Duty Cycle in Pulse Width Modulation (PWM)

Pulse Width Modulation (PWM) is a technique that regulates the average output voltage by controlling the duty cycle of a switching signal. PWM is widely used in applications such as motor control, power management, and LED dimming. Understanding the relationship between voltage and duty cycle in PWM is crucial for correctly using and designing PWM systems.

1. Basic Principle of PWM

• PWM Signal: A PWM signal is a periodic square wave with a fixed frequency but a variable proportion of high (on) and low (off) levels within each cycle. This proportion is called the duty cycle.

• Duty Cycle: The duty cycle is the ratio of the time the signal is high (on) to the total period of the PWM cycle. It is typically expressed as a percentage or as a fraction between 0 and 1. For example, a 50% duty cycle means the signal is high for half of the cycle and low for the other half; a 100% duty cycle means the signal is always high; and a 0% duty cycle means the signal is always low.

• PWM Frequency: The frequency of the PWM signal determines the duration of each cycle. Higher frequencies result in shorter cycles, and the PWM signal changes more rapidly.

2. Relationship Between Voltage and Duty Cycle in PWM

• Average Voltage: In PWM, the average output voltage is proportional to the duty cycle. If the peak voltage of the PWM signal is ${\mathrm{<br>}}_{}$Vmax, the average output voltage ${\mathrm{<span\; style="font-family:\; arial,\; helvetica,\; sans-serif;\; font-size:\; 16px;">Vavg\; can\; be\; calculated\; using\; the\; following\; formula:</span>}}_{}$

Vavg=D×Vmax

Where:

• Vavg is the average output voltage.

• D is the duty cycle (0 ≤ D ≤ 1).

• Vmax is the peak voltage of the PWM signal (usually the supply voltage).

• Effect of Duty Cycle on Average Voltage:

• When the duty cycle is 0%, the PWM signal is always low, and the average output voltage is 0.

• When the duty cycle is 100%, the PWM signal is always high, and the average output voltage equals the peak voltage Vmax.

• When the duty cycle is between 0% and 100%, the average output voltage is a proportion of the peak voltage. For example, a 50% duty cycle results in an average output voltage that is half of the peak voltage.

3. Application Examples of PWM

a. Motor Control

• In motor control, PWM is used to regulate the speed or torque of a motor. By changing the duty cycle of the PWM signal, the average voltage applied to the motor can be controlled, thereby adjusting the motor's output power. For instance, reducing the duty cycle decreases the average voltage, slowing down the motor, while increasing the duty cycle increases the average voltage, speeding up the motor.

b. LED Dimming

• In LED dimming applications, PWM is used to adjust the brightness of an LED. By changing the duty cycle of the PWM signal, the average current through the LED can be controlled, thus adjusting its brightness. For example, a 50% duty cycle results in an LED brightness that is half of its maximum, while a 100% duty cycle makes the LED fully bright.

c. DC-DC Converters

• In DC-DC converters (such as buck converters or boost converters), PWM is used to regulate the output voltage. By adjusting the duty cycle of the PWM signal, the on-time and off-time of the switching device can be controlled, which in turn adjusts the output voltage. For example, in a buck converter, increasing the duty cycle raises the output voltage, while decreasing the duty cycle lowers it.

4. Advantages of PWM

• High Efficiency: PWM controls voltage through switching operations rather than linear regulation (e.g., using resistive voltage dividers), resulting in lower energy losses and higher efficiency.

• Precise Control: By precisely adjusting the duty cycle, PWM allows for fine control over the output voltage or current.

• Flexibility: PWM can easily adapt to various applications, such as motor control, LED dimming, and power management.

5. Limitations of PWM

• Electromagnetic Interference (EMI): Since PWM signals are high-frequency switching signals, they can generate electromagnetic interference, especially at higher frequencies. Proper filtering and shielding techniques should be employed in PWM system design.

• Noise: In some applications, PWM signals may introduce audible noise, particularly in audio equipment or motor drives. This issue can be mitigated by selecting an appropriate PWM frequency.

Summary

In Pulse Width Modulation (PWM), the average output voltage is directly proportional to the duty cycle. The duty cycle determines the proportion of time the signal is high within a PWM cycle, which in turn affects the average output voltage. By adjusting the duty cycle, the output voltage or current can be flexibly regulated without changing the supply voltage. PWM technology is widely used in motor control, LED dimming, power management, and other applications, offering high efficiency and precise control.