This project explores the use of the GreenPAK™ SLG46621 ICs in power electronics applications and will demonstrate the implementation of a single-phase inverter using various control methodologies. Different parameters are used to determine the quality of the single-phase inverter. An important parameter is Total Harmonic Distortion (THD). THD is a measurement of the harmonic distortion in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
Below we described steps needed to understand how the project was programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.
1.1. Single-Phase Inverter
A power inverter, or inverter, is an electronic device or circuitry that changes direct current (DC) into alternating current (AC). Depending upon the number of phases of the AC output, there are several types of inverters.
- Single-phase inverters
- Three-phase inverters
DC is the unidirectional flow of electric charge. If a constant voltage is applied across a purely resistive circuit, it results in a constant current. Comparatively, with AC, the flow of electric current periodically reverses polarity. The most typical AC waveform is a sine wave, but it can also be a triangular or square wave. In order to transfer electrical power with different current profiles, special devices are required. Devices that convert AC into DC are known as rectifiers and devices that convert DC into AC are known as inverters.
1.2. Topologies of Single-Phase Inverter
There are two main topologies of single-phase inverters; half-bridge and full-bridge topologies. This project focusses on the full-bridge topology, since it provides double the output voltage compared to the half-bridge topology.
1.2.1. Full-Bridge Topology
In a full-bridge topology 4 switches are needed, since the alternating output voltage is obtained by the difference between two branches of switching cells. The output voltage is obtained by intelligently switching the transistors on and off at particular time instants. There are four different states depending upon which switches are closed. The table below summarizes the states and output voltage based on which switches are closed.
Table 1: Switching States and Output Voltage
|1||S1 & S2||+Vdc|
|2||S3 & S4||-Vdc|
|3||S1 & S3||0|
|4||S2 & S4||0|
To maximize the output voltage, the fundamental component of the input voltage on each branch must be 180º out of phase. The semiconductors of each branch are complementary in performance, which is to say when one is conducting the other is cut-off and vice versa. This topology is the most widely used for inverters. The diagram in Fig. 1 shows the circuit of a full-bridge topology for a single-phase inverter.
Figure 1: Full-bridge Single-phase Inverter Topology
1.3. Insulated Gate Bipolar Transistor
The Insulated Gate Bipolar Transistor (IGBT) is like a MOSFET with the addition of a third PN-junction. This allows voltage-based control, like a MOSFET, but with output characteristics like a BJT regarding high loads and low saturation voltage.
Four main regions can be observed on its static behavior.
- Avalanche Region
- Saturation Region
- Cut Area
- Active Region
The avalanche region is the area when a voltage below breakdown voltage is applied, resulting in the destruction of the IGBT. The cut area includes values from breakdown voltage up to threshold voltage, wherein the IGBT doesn’t conduct. In the saturation region, the IGBT behaves as a dependent voltage source and a series resistance. With low variations of voltage, high amplification of current can be achieved. This area is the most desirable for operation. If the voltage...Read more »