PWM principle

PWM principle

PWM (Pulse Width Modulation) control - Pulse width modulation technology, by modulating the width of a series of pulses, to equivalently obtain the required waveform (including shape and amplitude).
PWM control technology is most widely used in inverter circuits, and most of the inverter circuits used are PWM type. It is precisely because of its application in inverter circuits that PWM control technology has determined its important position in power electronics technology. 1 Basic principles of
PWM control Theoretical basis: When narrow pulses with equal impulses but different shapes are added to a link with inertia, the effect is basically the same. Impulse refers to the area of ​​the narrow pulse. The effect is basically the same, which means that the output response waveform of the link is basically the same. The low frequency band is very close, and there is only a slight difference in the high frequency band.

Figure 1 Various narrow pulses with different shapes but the same impulse
Area equivalence principle:
The narrow voltage pulses shown in Figure 1 are added to the first-order inertia link (RL circuit) respectively, as shown in Figure 2a. The response waveforms of the output current i(t) to different narrow pulses are shown in Figure 2b. It can be seen from the waveform that in the rising section of i(t), the shape of i(t) is slightly different, but its falling section is almost exactly the same. The narrower the pulse, the smaller the difference in the response waveform of each i(t). If the above pulse is applied periodically, the response i(t) is also periodic. After decomposing it with Fourier series, it can be seen that the characteristics of each i(t) in the low frequency band will be very close, and only differ in the high frequency band.

Figure 2 Response waveforms of various narrow pulses
with the same impulse A series of pulses with equal amplitude and unequal width are used to replace a half-sine wave. The half-sine wave is divided into N equal parts, which can be regarded as N connected pulse sequences with equal width but unequal amplitude; a rectangular pulse is used instead, with equal amplitude, unequal width, overlapping midpoints, equal area (impulse), and width changes according to the sine law.
SPWM waveform - a PWM waveform equivalent to a sine wave whose pulse width changes according to the sine law.

Figure 3 Use PWM wave to replace half sine wave
To change the amplitude of equivalent output sine wave, change the width of each pulse in the same proportion.
PWM current wave: The current type inverter circuit performs PWM control, and the obtained current wave is PWM current wave.
PWM waveform can be equivalent to various waveforms:
DC chopper circuit: equivalent DC waveform
SPWM wave: equivalent sine waveform, and can also be equivalent to other required waveforms, such as equivalent required non-sinusoidal AC waveform, etc. Its basic principle is the same as SPWM control, and is also based on the equivalent area principle.