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Pure sine wave inverters are suitable when operating sensitive electronic devices, including communications equipment, that require high-quality waveform with little harmonic distortion.

The benefits and advantages of digital signal processing

In the quest for increased performance, flexibility, configurability, communications, and remote monitoring and control, the power electronics industry is increasingly moving from analog to digital power converters, particularly when high-density power output is required.

Many mission-critical operations in aerospace and defense, as well as some industrial applications, require high output power in the multi-kilowatt range. Given the mission-critical nature of these applications and the extremely rugged conditions in which they must operate, these power converters must also withstand stringent vibration, shock, EMI, humidity, and other environmental settings without breaking down.

Meeting all of these requirements often requires a custom solution from a power converter designer proficient in digital signal processing (DSP) techniques as well as military and aerospace specifications and high-reliability (hi-rel) standards.

“You generally don’t just go to a catalog and pick out something that’s going to provide you with 50,000 watts of power. Almost everything is custom made in that arena,” said Kamran Kazem, Vice President and Chief Technology Officer at Magnetic Design Labs (MDL), which designs and manufactures analog and digital switching and linear power supplies, dc-ac inverters, and dc-dc converters.

According to Kazem, digital power devices are capable of operating over a very wide operating range, require few external components, are easy to communicate with, and introduce a degree of flexibility in control not previously available with analog techniques.

“The main reason to go with digital over analog is that almost anything can be customized fairly easily with just a little bit of code,” says Kazem. “Whereas with an analog type of converter, once it is designed it is rather difficult to change or get any additional information from it and there is no real-time communications.”

Digital signal processing (DSP) is also far more precise than analog. Settings do not drift with time and temperature changes since they are controlled only by the DSP clock and the software, not cap or resistor values that change over temperature and time.

“Perhaps the most important feature of a digital converter is its flexibility,” said Kazem. Digital power converters offer an array of programmable parameters including output voltage settings, output current, current limit trip point, power sequencing routines, voltage margining, and multiple thresholds for warning and fault conditions for overcurrent, overtemperature, and under- and overvoltage. Fault conditions and power usage can also be stored in non-volatile flash memory for later recall.

Designers can program any of these parameters at any point during the product’s life cycle. These, and other function or feature changes, often simply require updating the flash memory, and can even be updated remotely over the internet. With analog, similar parameter or function changes require part (hardware) changes and often also require a new PC board.

Real-time communications for monitoring and diagnostics is another major benefit. Digital power conversion devices can be tied into existing networked systems as well as communication processors and the information used to monitor and control the output.

Despite these and other advantages of DSP technology, for most engineers the major downside of digital power conversion is the learning curve that the technology demands.

“It takes a very high skill level and advanced education to successfully design for digital control of analog signals using a DSP processor,” said Kazem.

Kazem adds that despite excellent programming and debug tools from the DSP chip vendors, these tools can be very difficult for the inexperienced designer.

“Designers that are used to working in the analog domain can get into a sophisticated DSP application and find that it takes too long or becomes too difficult to complete the work,” said Kazem.

Hi-reliability applications

In addition to DSP, many mission-critical electronics used in military and aerospace applications must be designed to satisfy stringent ruggedization and hi-rel requirements.

Hi-rel is defined as the probability of failure-free performance under stated conditions, usually over a specified interval of time. These electronic systems, down to component-level technology, must be able to operate for many years without failure, often without the opportunity for repair, and operate in temperatures that can range from -55 to +85°C.

Bob Seidenberg, former Senor Quality Assurance Manager with BAE Systems, describes the importance of high-reliability, ruggedized digital inverters that were installed in the M1068, a command communications variant of the U.S. Army’s M113 family of armored personnel carriers (APCs). BAE Systems designs and manufactures electronic systems and subsystems for commercial and military applications.

The original M113 APC revolutionized mobile military operations when first fielded in Vietnam. To date, an estimated 80,000 M113s, including a long list of variants, have been produced and used by over 50 countries worldwide, making it one of the most widely used armored fighting vehicles of all time.

During this time, the M113 has been continuously updated over the years to meet the ever-increasing demands of the modern battlefield. Since then the M113 family of vehicles are being upgraded, reconfigured, and introduced as entirely new systems, including the M1068.

The M1068 variant is used as a tactical operations center capable of long range communications and includes 4.2-kW auxiliary power unit (APU) mounted on the right front of the vehicle to provide 24-V power.

As part of this project, BAE Systems required two hi-rel, ruggedized 2500-W pure sine wave inverters per vehicle to convert 24-V dc power generated by vehicle-mounted APU into usable multi-kilowatt levels of ac for powering communications devices, lighting, computers, and other electronic devices.

Although more expensive, pure sine wave inverters provide cleaner, utility grade power than quasi sine wave models. Pure sine wave inverters are ideal when operating sensitive electronic devices, including communications equipment, that require high quality waveform with little harmonic distortion.

In addition, pure sine wave models have a high surge capacity which means they are able to exceed their rated wattage for a limited time. This enables vehicle motors to start easily, which can draw many times their rated wattage during startup.

Seidenberg explained that the 2500-W inverters also had to meet some shock and vibration requirements that could only be met by hi-rel inverters.

“The shock requirement for the inverters installed on the M1068 was close to 30 g, in three directions (vertical, horizontal, and transverse) and the vibration spectrum was also very demanding, but MDL was able to meet those requirements with an innovative pure sine wave inverter design that was basically non-destructive,” said Seidenberg.

According to MDL’s Kazem, despite the 30 g requirement for the M1068, the military specified that the inverters had to withstand 100 g. This was well beyond the amount of shock the vehicle would ever realistically experience, based on military tests conducted in the roughest terrain that maxed out at 15 g. Still, MDL was able to deliver 2500-W, pure sine inverters that met the requirement as verified by an independent testing lab.

“The military has a number of requirements that we had to follow,” said Seidenberg. “So, naturally, our supplier had to pay a lot of attention to those requirements and ensure that they were met in a way that was correct for the application.”

Kazem believes that high-reliability electronic systems and components are no longer the exclusive domain of aerospace and defense. Today, medical, transportation, communications, infrastructure, and industrial all have applications where the price of failure is high.

“These power converters might not have to withstand the same extreme conditions as the military, but vibration, shock, humidity, and other inherent environmental problems are still factors, so the need for hi-rel and rugged power converters certainly applies to those markets as well.”

DSP modules

DSP is also a key element in a new generation of modular, stackable inverter options designed to provide a range of 1 to 20 kW of dc-ac power via a single, customizable unit.

This type of system, available from custom power converter designers like MDL, consists of rack-mounted inverter modules that can be stacked in a parallel configuration, enabling the user to add as many inverters as needed to meet the power requirements.

Each unit connects to a communications controller that is responsible for synchronization, load sharing, and any external communications. The individual inverters are hot swappable, enabling the addition or replacement of modules on the fly.

“This type of modular design provides project managers with a system that fits their power requirements without having to develop a new unit just for their specific project,” said Kazem. “This eliminates the need for many application-specific designs, and could also enable faster delivery of the power converter at a much more economical cost.”

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