Pulse width modulation—PWM is an abbreviation of English word, which means pulse width modulation. It is a series of signals with different pulse widths. It uses the digital output of the microprocessor to control the analog circuit. A very effective technology, widely used in many fields from measurement and communication to power control and conversion.
Pulse width modulation is an analog control method, which modulates the base bias of the transistor according to the change of the corresponding load to realize the change of the transistor on-time, so as to realize the change of the output of the switching regulated power supply. This method can keep the output voltage of the power supply constant when the working conditions change, and it is a very effective technology to control the analog circuit with the digital signal of the microprocessor.
PWM control technology has become the most widely used control method of power electronic technology due to its advantages of simple control, flexibility and good dynamic response. Pulse width modulation is a method of digitally encoding analog signal levels. Through the use of high-resolution counters, the duty cycle of the square wave is modulated to encode the level of a specific analog signal. The PWM signal is still digital, because at any given moment, the full-scale DC power supply is either completely (ON) or completely absent (OFF). The voltage or current source is applied to the analog load in a repetitive pulse sequence of on (ON) or off (OFF). Even when the DC power supply is added to the load when it is on, when it is off, the power supply is disconnected. As long as the bandwidth is sufficient, any analog value can be encoded using PWM. Most loads (whether inductive or capacitive) require a modulation frequency higher than 10 Hz, and the modulation frequency is usually between 1 kHz and 2 kHz.
The modulation frequency is the reciprocal of the period. Before performing the PWM operation, this kind of microprocessor requires the following tasks to be completed in the software: 1. Set the period of the on-chip timer/counter that provides the modulated square wave 2. Set the on-time in the PWM control register 3. Set the PWM output Direction, this output is a general-purpose I/O pin 4, start timer 5, enable PWM controller
One of the advantages of PWM is that the signals from the processor to the controlled system are in digital form, and there is no need to perform digital-to-analog conversion to keep the signal in digital form to minimize the effect of noise. Noise can only affect digital signals when it is strong enough to change logic 0 or change logic 0 to logic 1.
The enhancement of noise resistance is another advantage of PWM over analog control, and this is also the main reason why PWM is used for communication at certain times. Switching from analog signal to PWM can greatly extend the communication distance. At the receiving end, a suitable RC or LC network can filter out the modulated high-frequency square wave and restore the signal to an analog form.
In short, PWM is economical, space-saving and has strong anti-noise performance.
There is an important conclusion in the use of control theory: when narrow pulses with the same impulse and different shapes are applied to the link with inertia, the effect is basically the same. The PWM control technology is based on this conclusion to control the turn-on and turn-off of the semiconductor switching device, so that the output terminal can obtain a series of pulses with equal amplitude but unequal width. These pulses are used to replace sine waves or other pulses. The required waveform is modulated by the width of each pulse according to certain rules, which can change the output voltage and output frequency of the inverter circuit.
The basic idea of current control PWM is to use the desired output current waveform as the command signal and the actual current waveform as the feedback signal. The on-off of each switching device is determined by the comparison of the two instantaneous values, so that the actual output changes with the command signal. And change. The duty cycle is the ratio of the on-time to the cycle; the duty cycle refers to the ratio of the pulse wave high potential time to the entire cycle. The larger the duty cycle, the longer the circuit turn-on time and the higher the overall performance.