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The principle of PWM pulse width modulation
- May 25, 2018 -


PWM pulse width modulation is based on changing the pulse width to control the output voltage and controlling the output frequency by changing the cycle. The change of output frequency can be achieved by changing the modulation period of this pulse. In this way, the two functions of voltage regulation and frequency modulation are consistent, and are independent of the intermediate DC link, thus speeding up the regulation speed and improving the dynamic performance. Because the output constant amplitude pulse only needs constant DC power supply, the rectifier can be replaced by an uncontrolled rectifier, so that the power factor of the grid side can be greatly improved. The use of PWM inverter can suppress or eliminate low order harmonics. With the use of self closing devices, the switching frequency is greatly improved, and the output waveform can be very close to the sine wave.

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The PWM frequency conversion circuit has the following characteristics:

1. the output voltage that is quite close to the sine wave can be obtained.

2. rectifier circuit using diode, can get close to 1 of the power factor.

3. circuit structure is simple

4. by controlling the output pulse width, the output voltage can be changed, and the dynamic response of the frequency conversion process will be accelerated.


Now the general inverter basically uses PWM control mode, so we introduce the principle of PWM control.


The basic principle of PWM


Pulse width modulation (PWM). The control mode is to control the switching of the inverter circuit switch device, so that the output end gets a series of pulse with equal amplitude, and uses these pulses to replace the sine wave or the required waveform. It is to produce multiple pulses in the half cycle of the output waveform, so that the equivalent voltage of each pulse is sinusoidal, and the output is smooth and the low order oblique wave is less harmonic. By adjusting the width of each pulse according to certain rules, the output voltage of the inverter circuit can be changed, and the output frequency can also be changed.


In the theory of sampled data control, there is an important conclusion that the impulse is equal and the narrow pulse with different shape is added to the inertial link. The effect is basically the same. The impulse refers to the area of the narrow pulse. The effect is basically the same here. The output response waveform of this link is basically the same. If the output waveforms are analyzed by Fourier transform, their low-frequency characteristics are very close, only slightly different in the high frequency range.


According to the above theory, we can replace sine waves with rectangular waves of different widths, and simulate sinusoidal waves with different frequencies through the control of rectangular waves.


For example, if the sine half wave waveform is divided into N equal parts, the sinusoidal half wave can be regarded as a waveform composed of N connected pulses. The width of these pulses is equal to N, but the amplitude is not equal, and the top of the pulse is not a horizontal line, but a curve. The amplitude of each pulse varies according to the law of sine. If the above pulse sequence is replaced by the same number of equal amplitude and unequal rectangular pulse sequences, the middle point of the rectangular pulse and the middle point of the corresponding sinusoidal equals are reclose, and the rectangular pulse and the corresponding sinusoidal area (i.e. impulse) are equal, and a set of pulse sequences is obtained, this is the PWM waveform. It can be seen that the pulse width varies according to the sine rule.  According to the same impulse equivalent effect, the PWM waveform and the sine half wave are equivalent. For sine negative half cycle, PWM waveform can also be obtained in the same way.


In the PWM waveform, the amplitude of each pulse is equal. In order to change the amplitude of the equivalent output sine wave, as long as the width of each pulse is changed according to the same ratio coefficient, in the AC direct AC converter, the rectifier circuit uses an uncontrollable diode circuit, and the output pulse voltage of the PWM inverse transformer is DC. The amplitude of the side voltage.


According to the above principle, the width and interval of each pulse in the PWM waveform can be accurately calculated after the number of the sine wave frequency, the amplitude and the half period of the pulse number. According to the result of calculation, the PWM waveform can be obtained by controlling the on-off of all switching devices in the circuit.



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