In the design of APC uninterruptible power supply, the DC input side of the inverter comes from the rectified output of the rectifier or the DC output of the battery, which belongs to the voltage source. Therefore, the inverter circuit in this design belongs to the voltage type inverter circuit. It generally adopts a 180 ° conduction mode, which means that each arm conducts 180 ° in one cycle. The typical control methods for voltage type inverter circuits include phase control and PWM control. Phase control refers to controlling the phase of the triggering pulse, that is, the pulse triggering time, to change the width of the output voltage pulse, thereby achieving the effect of adjusting the inverter output voltage. This control method outputs a rectangular wave voltage, which contains many harmonics and has adverse effects on the load; And the power factor is not high, and the dynamic response during adjustment is slow. Adopting PWM control can better overcome the above disadvantages. PWM, also known as pulse width modulation control, is a method of controlling the on/off of switching devices in an inverter circuit to output a series of pulses with equal amplitude but different width, which are used to replace sine waves or the desired waveform. According to certain rules, modulating the width of each pulse can change both the magnitude of the inverter output voltage and the output frequency.
Theory and practice have proven that PWM inverter circuits have strong harmonic suppression capabilities. At the same time, it also has the following characteristics:
(1) The output waveform of the inverter is close to a sine wave;
(2) Fast dynamic response;
(3) High power factor.
With the emergence and maturity of self turn off devices, P-type control technology has developed rapidly, and PWM inverter circuits have been widely used. Nowadays, PWM control technology has become a very important component of power electronics technology, playing a huge role in improving the performance of power electronic devices and promoting the development of power electronics technology.
PWM type three-phase bridge inverter circuit
Due to the rapid development and widespread application of PWM control technology, more and more APC uninterruptible power supply inverter circuits are adopting PWM control method. The basic principle of PW inverter is described in detail below. In the theory of sampling control, there is an important conclusion: narrow pulses with equal impulse but different shapes applied to inertial links have similar effects, that is, the output response waveform of the links is basically the same. This conclusion is an important theoretical basis for PWM control. Taking sinusoidal pulse width modulation SPWM as an example, the basic principle of PWM control is explained below. Divide the sine half wave into N equal parts, and consider the sine half wave as a waveform composed of N interconnected pulses. These pulses have equal widths, all equal to n/, but varying amplitudes. The top of the pulse is not a horizontal straight line, but a curve, and the amplitude of each pulse varies sinusoidally. If this pulse sequence is replaced with an equal number of rectangular pulse sequences of equal amplitude but not equal width, so that the midpoint of the rectangular pulse coincides with the midpoint of the corresponding sine equal division, and the area or impulse of the rectangular pulse and the corresponding sine equal, the pulse sequence is obtained. This is the PWM waveform. It can be seen from this that the width of PWM pulses varies sinusoidally. According to the principle of equal impulse effect, this PWM waveform is equivalent to a sine half wave. For the sine negative half cycle, the corresponding PWM waveform can be obtained using the same method. The width of this pulse varies sinusoidally and is equivalent to a sine wave in PWM waveform, becoming a SPWM waveform. In PWM waveform, to change the amplitude of the equivalent output sine wave, simply change the width of each pulse in the same proportion.