Introduction to inverter
An inverter converts the DC voltage to an AC voltage. In most cases, the input DC voltage is usually lower while the output AC is equal to the grid supply voltage of either 120 volts, or 240 Volts depending on the country.
The inverter may be built as standalone equipment for applications such as solar power, or to work as a backup power supply from batteries which are charged separately.
The other configuration is when it is a part of a bigger circuit such as a power supply unit, or a UPS. In this case, the inverter input DC is from the rectified mains AC in the PSU, while from either the rectified AC in the in the UPS when there is power, and from the batteries whenever there is a power failure.
There are different types of inverters based on the shape of the switching waveform. These have varying circuit configurations, efficiencies, advantages and disadvantages
An inverter provides an ac voltage from dc power sources and is useful in powering electronics and electrical equipment rated at the ac mains voltage. In addition they are widely used in the switched mode power supplies inverting stages. The circuits are classified according the switching technology and switch type, the waveform, the frequency and output waveform.
Full bridge inverter
The above figure is a full bridge inverter circuit consist of 4 power switches. Each of the switches will operate alternately according to the switching pattern. Different from the half bridge inverter, the output from full bridge inverter will equal to the supplied voltage. This inverter also will have have both positive and negative square wave signal from its output.
Half bridge inverter
Referring to the figure above, this is a half bridge circuit consist of two power switches and two similar voltage sources. The output voltage of this inverter will be half from the supplied voltage. Besides, it only have a positive square wave signal from its output.
Cascaded multilevel inverter
The above figure shows the cascaded multilevel inverter circuit. It consist of several full bridge inverter circuit cascaded together to form a new inverter circuit with multiple stages of output voltage. The most attractive applications of this technology are in the small voltage to high voltage ranges. Advantages of this multilevel approach include good power quality, good electromagnetic compatibility, low switching losses and high voltage capability.
If all dc voltage sources in the circuit are equal to Vdc, the inverter is known as symmetric multilevel inverter. The effective number of output voltage steps (Nstep) in symmetric multilevel inverter may be related to the number of full bridges (n) by:
Nstep = 2n + 1
and the maximum output voltage (Vo,max) of this n cascaded multilevel inverter is:
Vo,max = n x Vdc
Besides, if the voltage sources supplied to the circuit are different, this circuit is known as asymmetric multilevel inverter. It advantage is it can increased the number of output steps without increasing the number of inverters. The dc voltages sources are proposed to be chosen according to a geometric progression with a factor of two or three. For n cascaded multilevel inverters, the number of voltages steps are given as follow:
Nstep = 2n + 1 - 1 if Vj = 2j - 1 Vdc for j = 1,2,..., n
Nstep = 3n if Vj = 3j - 1 Vdc for j = 1,2,..., n
The maximum output voltages of these n cascaded multilevel inverters are:
Vo,max = (2n - 1) Vdc if Vj = 2j - 1 Vdc for j = 1,2,..., n
Vo,max = ((3n - 1)/ 2) Vdc if Vj = 3j - 1 Vdc for j = 1,2,..., n