In the world of DIY electronics, many enthusiasts find satisfaction in building practical projects that challenge their understanding of power electronics and circuit design. If you're an advanced hobbyist or a professional looking to create a high-power, efficient DC-AC inverter, this project might be the perfect challenge. In this article, we'll walk through a specific project that involves using the 7MBP50RA120-55/50A1200V power module, a device that can handle significant current and voltage, making it suitable for creating a robust inverter capable of supplying power to AC loads.

The 7MBP50RA120-55/50A1200V is an IGBT (Insulated-Gate Bipolar Transistor) power module designed for high-voltage and high-current applications. It is commonly used in industrial applications, such as motor drives and power converters. For this project, we will design and build a DC-AC inverter circuit using this module, which will convert a 48V DC input into a 120V AC output, suitable for powering household appliances or smaller tools.

Materials and Components

Before we dive into the design, let’s take a quick look at the components we’ll be using:

● 7MBP50RA120-55/50A1200V Power Module: This module integrates multiple IGBTs and diodes, providing high efficiency and robustness for high-power applications.

● Power Supply (48V DC): A battery bank or a regulated power supply that can provide a stable 48V DC output.

● Transformer (1:1 step-up): A transformer capable of stepping up the voltage from the inverter’s output to the required AC voltage (120V in this case).

● Gate Driver Circuit: A circuit that is used to properly drive the gates of the IGBTs, ensuring that they turn on and off as needed.

● Filter Capacitors: To smooth out the DC voltage and reduce ripple.

● Heat Sink: For cooling the IGBT module, as it will dissipate heat during operation.

● Inductors and Capacitors: For the output filter to smooth the AC waveform.

● Rectifier Diodes: To ensure proper current flow in the correct direction.

Design Overview

The inverter we are going to build will be a full-bridge inverter design. A full-bridge inverter consists of four switching devices arranged in an H-bridge configuration. The 7MBP50RA120-55/50A1200V power module includes four IGBTs that will serve as the primary switching devices. These IGBTs will alternately switch on and off to generate a pulse-width modulated (PWM) AC waveform, which will then be fed into the transformer for voltage step-up.

The system will be powered by a 48V DC source. We will use a gate driver circuit to control the switching of the IGBTs, ensuring that the transistors switch at the correct timing to generate an alternating current. The high-frequency AC signal from the inverter will then pass through a low-pass filter to smooth out the waveform, followed by the transformer to step up the voltage.

Step 1: Power Stage and IGBT Configuration

The 7MBP50RA120-55/50A1200V power module is the heart of the inverter’s power stage. The module features four IGBTs, and it’s designed to handle high-voltage and high-current switching applications. Each of these IGBTs can switch large amounts of current with minimal power loss. To set up the power stage, the key idea is to configure the IGBTs in a full-bridge arrangement.

Here’s how the full-bridge configuration works:

● Two IGBTs are connected in series to handle the positive voltage half-cycle of the AC waveform.

● The other two IGBTs are connected in series to handle the negative voltage half-cycle of the AC waveform.

● The center point of the IGBT series connections serves as the AC output.

The IGBT module also includes internal diodes that will allow current to flow back to the source when the IGBT is off, ensuring that energy stored in the inductive load is dissipated safely.

Step 2: Gate Driver Circuit

The gate driver circuit is essential for ensuring that the IGBTs switch in the correct sequence and at the proper time. The IGBT gate requires a voltage between 12V and 20V to turn on, and the gate driver must deliver this voltage to each of the four IGBT gates. Additionally, the gate driver should be able to handle the high-speed switching characteristics of the IGBTs to produce a clean PWM signal.

For a simple driver, you could use a dedicated IGBT driver IC like the IR2110 or TC4420. These ICs will amplify the low-power PWM signals from a microcontroller (or external controller) to the higher voltages needed for the gates of the IGBTs.

The gate driver should be designed to handle both high and low-side switching. The low-side IGBTs can be driven directly from the driver IC, but the high-side IGBTs require a floating driver that can generate a voltage higher than the supply voltage.

The gate driver should also include a dead-time feature to prevent both top and bottom IGBTs in the bridge from being turned on simultaneously. This is crucial because turning on both IGBTs in the same half-bridge at the same time would result in a short circuit, damaging the power components.

Step 3: PWM Control

The PWM (Pulse Width Modulation) signal is what controls the IGBTs to switch on and off in a precise timing pattern. The PWM signal determines the frequency and the duty cycle of the AC output. In our case, we will be generating a 60Hz AC signal with a duty cycle of around 50%. This will give us a square wave AC output, which will be the basis for generating the desired 120V AC output.

The frequency of the PWM signal can be controlled by a simple microcontroller or a dedicated PWM controller IC. In this project, we are focusing on the hardware setup, so we won’t dive into microcontroller programming. However, a microcontroller such as an Arduino or PIC microcontroller can be used to generate the PWM signal that will control the gate driver.

Step 4: Transformer

After the IGBTs have created the PWM signal, the next step is to step up the voltage from the low DC side to the required AC voltage. This is where the transformer comes into play. In this project, we will use a 1:1 step-up transformer to boost the voltage from 48V DC to approximately 120V AC.

While the frequency of the AC signal from the inverter will be 60Hz, the waveform will initially be a square wave due to the nature of the PWM signal. However, the transformer will help smooth out the waveform to some extent, and additional filtering components will further improve the quality of the AC output.

Step 5: Filtering and Output Smoothing

One of the challenges of building an inverter is producing a clean, usable AC output. The PWM signal, even after passing through the transformer, will still contain high-frequency harmonics and noise. To smooth the output and make it more suitable for powering AC devices, you’ll need to use a combination of capacitors and inductors.

Start by placing capacitors at the output to filter high-frequency components. You can use large electrolytic capacitors in parallel with smaller ceramic capacitors to cover a broad range of frequencies. The capacitors will help to smooth the voltage ripple caused by the switching.

You may also want to include an inductor in series with the output to further filter high-frequency noise and smooth out the waveform. The inductor will help reduce the rate of voltage change (di/dt), making the output more sinusoidal.

Step 6: Heat Dissipation and Cooling

The 7MBP50RA120-55/50A1200V power module is capable of handling large currents, but it will still generate significant heat during operation. To ensure the longevity of the components and prevent thermal damage, you will need to attach an appropriate heat sink to the power module. The heat sink will absorb and dissipate the heat generated by the IGBTs during operation.

In addition to the heat sink, you may also want to consider active cooling methods, such as fans or forced air cooling, depending on the power levels you intend to handle.

Step 7: Final Assembly and Testing

Once all components are connected, it’s time to assemble the inverter in a safe enclosure. Make sure to mount the power module securely to the heat sink and ensure that all connections are solid and insulated. You should also include fuses or circuit breakers to protect the inverter from short circuits or overcurrent situations.

Before powering the inverter, double-check all the wiring, particularly the IGBT gate connections and the transformer wiring. Once the setup is complete, you can connect the inverter to the DC power supply and test the output voltage. Use an oscilloscope to observe the waveform and ensure that the inverter is producing a clean AC signal.

Conclusion

This DIY high-power DC-AC inverter project demonstrates how you can use the 7MBP50RA120-55/50A1200V power module in a practical application. By carefully designing the power stage, gate drivers, and filtering stages, you can create an inverter capable of converting a 48V DC input to a 120V AC output.

Although the project requires some advanced knowledge of power electronics, it offers a rewarding challenge for anyone interested in building a real-world application for high-power electronics. With a good understanding of IGBT switching, gate drivers, and PWM control, you'll have the foundation for building a versatile and reliable DC-AC inverter.