In-depth analysis of DC-AC converter inverter

    A device that converts DC voltage to AC voltage is called a DC–AC converter or "inverter". The inverter converts the battery voltage (such as 12V DC or 24V DC) to 110V AC or 220V AC. The power sockets we use in our lives have 220V/110V. We have no way to store AC power, but we can store DC power in batteries. Therefore, in order to save power in household sockets, the development of AC-DC converters (rectifiers) has emerged.

    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 using four switches connected in a bridge. By appropriately opening and closing two switches at a time, a 2-level AC (pulsating DC) output can be obtained.

    Depending on the state of the switch, the output level will fluctuate between +VDC and –VDC

    S1 and S2 are closed, output = +VDC

    S3 and S4 closed output = -VDC

    S1 and S3 are closed, output = 0

    S2 and S4 are closed, output = 0

    Real switching requires switching transition time, which is called blanking time.

    Square wave inverter

    This is the simplest inverter. The supply voltage is Vs. Consider an RL load. When Q1 and Q2 are conducting, the output is connected to +Vs. When Q3 and Q4 are conducting, the output is connected to –Vs. This AC output voltage is not exactly like a sine wave, but has some AC characteristics that can be useful in some applications.

    The current flowing through the switch must be bidirectional, but real switches such as insulated gate bipolar transistors (IGBTs) can only flow in one direction. So we add anti-parallel feedback diodes to each switch. During the intervals when the switch current should be negative, the diode will conduct current. During a positive cycle, the diode will be reverse biased.

    Multilevel Inverter

    So far we have seen a single supply 3 level (+VDC, -VDC, 0) square wave inverter. Now this concept can be extended to a multi-level inverter with multiple DC sources and multiple output DC levels.

    Assume the system has two square wave inverters, each with a battery. There are two voltage sources. So the output levels are +2VDC, +VDC, 0, -VDC, -2VDC

    The output of the first square wave inverter is connected in series with the next inverter to form a multistage or multilevel inverter. The output shape of these types of inverters is very similar to a sine wave, and their applications range from variable speed motor drives to 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 get a sinusoidal output. This way, one DC source (battery) will discharge faster than the other. A technique called mode commutation is used to make each DC source battery perform the same amount of time on average. The first source must perform for a longer period of time in the first half cycle, while the other source must perform for a longer period of time in the second half cycle. In this way, over a complete cycle, both sources behave equally.

    Diode clamped multilevel inverter

    Multi-level inverter circuit with multiple DC voltage source batteries. We can use multiple capacitors and only one DC source battery. The capacitors will be connected in series with each other and use a single DC battery as the power source. The output voltage levels of the dual capacitor single battery multi-level inverter are (VDC, VDC/2, 0, -VDC/2, -VDC)