We are all familiar with inverters, which are also called DC-AC converters, which are devices that convert DC voltage to AC voltage. The inverter converts the battery voltage (such as 12V DC or 24V DC) into 110V AC or 220V AC. The power sockets we use in our lives are 220V/110V. We have no way to store AC power, but we can store DC power in batteries. Therefore, in order to save the electrical energy of household sockets, the development of AC-DC converters (rectifiers) has appeared.
These DC batteries can provide DC current, and our equipment is designed to use AC power. Therefore, the development of inverters came into being.
Full bridge inverter
This inverter is constructed with four switches connected by a bridge. By properly opening and closing two switches at a time, a 2-level AC (pulsating DC) output can be obtained.
According to the different states of the switch, the output level will fluctuate between +VDC and -VDC
S1 and S2 closed output = +VDC
S3 and S4 closed output = -VDO
S1 and S3 closed output = 0
S2 and S4 closed output = 0
A real switch requires switching transition time, which is called blanking time.
Square wave inverter
This is the simplest inverter. The power supply voltage is Vs. Consider the RL load. When Q1 and Q2 conduct, the output is connected to +Vs, and when Q3 and Q4 conduct, the output is connected to -Vs. The AC output voltage is not exactly the same as the sine wave, but it has some AC characteristics that may be useful in some applications.
The current flowing through the switch must be bidirectional, but true switches, such as insulated gate bipolar transistors (IGBTs), can only flow in one direction. So we added anti-parallel feedback diodes to each switch. During the interval when the switching current should be negative, the diode will conduct current. In a positive cycle, the diode will generate a reverse bias.
So far, we have seen single-supply 3-level (+VDC, -VDC, 0) square wave inverters. This concept can now be extended to multilevel inverters with multiple DC power supplies and multiple output DC levels.
Suppose the system has two square wave inverters, and each square wave inverter has a battery. There are two voltage sources in total. So the output level is +2VDC, +VDC, o,VDC, -2VDC
The output of the first square wave inverter is connected in series with the next inverter to form a multi-level or multi-level inverter. The output shape of these types of inverters is very similar to sine waves, and they have a wide range of applications, including speed-regulating motor drives and connecting renewable energy sources such as photovoltaics to the grid.
In a dual-source inverter, one voltage source and H-bridge must output longer pulses than the other voltage source and H-bridge to obtain a sine wave output. In this way, one DC power supply (battery) will discharge faster than the other. A technique called mode switching is used for each DC power battery for the same amount of time on average. The first source must perform a longer period of time in the first half of the cycle, and the other source must perform a longer period of time in the second half of the cycle. In this way, the behavior of the two sources is equal during a complete cycle.
The diode clamped multi-level inverter has a multi-level inverter circuit with multiple DC voltage source batteries. We can use multiple capacitors and only one DC power battery. The capacitors will be connected in series with each other and use a single DC battery as the power source. The output voltage level of the dual-capacitor single-cell multi-level inverter is (VDC, VDC12,0, -VDC12, -VDC)