PWM

Control the speed of a DC motor by varying the width of drive pulses. The SG3525 controller chip relies entirely on this technique for power supplies.

Core Concept

Pulse width modulation keeps the pulse amplitude constant but changes the duration. A circuit compares the continuous message signal against a high frequency sawtooth wave. Higher message voltages intersect the ramp later and produce wider pulses.

This method wastes almost zero power in linear operation. Switching transistors remain either fully on or fully off. Class D amplifiers utilize this exact property to drive heavy speakers with high efficiency.

Since the pulse amplitude remains completely fixed clipping limiters can strip away additive channel noise. The true information sits strictly in the timing of the rising and falling edges.

Key Equations

The duty cycle indicates the percentage of time the pulse is high. D = (ton / Ts) * 100 expresses this ratio.

The average DC voltage extracted from a PWM wave is directly proportional. Vavg = D * Vmax gives the filtered output.

Given:\nPeak voltage (Vmax) = 12 V\nDuty cycle (D) = 40 percent (0.4)\n\nWhy this formula:\nThe average voltage is simply the peak voltage multiplied by the duty cycle ratio.\n\nFormula:\nVavg = D * Vmax\n\nSubstitution:\nVavg = 0.4 * 12\n\nCalculation:\nVavg = 4.8\n\nFinal Answer:\n4.8 V

Exam Tip: PWM has much better noise immunity than PAM. The information sits securely in the time transitions rather than the fragile amplitude peaks.

Key Properties

• Output transistors operate purely in cut-off and saturation regions for maximum efficiency.
• Transmitted average power varies with the message signal amplitude.
• PWM signals require significant transmission bandwidth to preserve sharp vertical edges.
• A simple low pass LC filter successfully recovers the original baseband signal.
• It remains highly susceptible to phase and timing jitter.

Quick Revision

• Amplitude remains completely constant.
• Pulse width varies directly with signal amplitude.
• Noise clipping improves signal integrity.
• Efficiency is exceptionally high for power circuits.
• Bandwidth requirements are very large.
• Exam trap: Forgetting that PWM signals naturally contain the original message baseband. You can recover the signal using just a passive low pass filter.