The Time for Modern Control Methods is Now
Enabled by the availability of faster and more computationally capable microcontrollers, an increasing number of power systems are being implemented with digital (software) control. Yet the promise of digital control – tremendous gains in performance, flexibility, and configurability – is often only weakly delivered in applications. This is largely due to the lack of modern control methods that can take full advantage of digital architectures. The greatest success in implementing digital control has been realized with linear plants (e.g. buck) because of their design simplicity.
Enabled by the availability of faster and more computationally capable microcontrollers, an increasing number of power systems are being implemented with digital (software) control. Yet the promise of digital control – tremendous gains in performance, flexibility, and configurability – is often only weakly delivered in applications. This is largely due to the lack of modern control methods that can take full advantage of digital architectures. The greatest success in implementing digital control has been realized with linear plants (e.g. buck) because of their design simplicity.
As with analog controls, the designer of a digital nonlinear converter (e.g. boost or buck-boost) today still uses averaged models and linearizes around a specific operating point, carrying with the design all of the limitations and compromises of small signal control methods: complex, iterative design cycles, low bandwidth, narrow operating ranges, limited stability, and slow transient response to name a few. Like the old adage regarding bringing a knife to a gunfight, the designer is fighting a nonlinear battle with linear methods. As a result, boost and buck-boost converters are often bypassed by engineers in favor of simpler bucks, even if the application would be better served with these nonlinear topologies.
Other industries have faced similar challenges in migrating to digital control. As high-performance military aircraft evolved, implementations of “fly-by-wire” computer-controlled flight were only marginally successful until nonlinear control methods such as Feedback Linearization were developed. These methods were designed specifically to solve the nonlinearities of flight control and were the key element that moved fly-by-wire into the mainstream, enabling outstanding stability, safety, and jaw-dropping maneuverability. Today, fly–by-wire implementations are an essential part of military and commercial aircraft.
Modern, nonlinear control methods can have a similar, dramatic impact on both the design cycle and performance of boost and buck-boost converters. These methods deal with the actual nonlinear system without having to do small signal linearization or requiring a new control design at each operating point. Computationally intensive, these methods have only become practical for broader application with the rise in microcontroller performance.
As a power controls company, Cirasys is commercializing modern, nonlinear methods designed to solve many of the issues inherent in nonlinear converters. For example, our Input Output Linearization (IOL) method delivers dramatic improvements in the design cycle and in the performance of buck, boost and buck-boost converters. But the greatest impact is on boost and buck-boost, bringing them up to the performance level and design simplicity of bucks. Since IOL incorporates feedback and an exact model of the converter plant, no special compensation design is required, and all operating points are available with the same control design and with identical performance. Control design is in fact somewhat simpler than current buck designs. Other attributes of IOL control include:
- Greatly improved stability and very wide operating range
- Very high bandwidth, not limited by averaged model/small signal
- Fast transient response
As a result, IOL controlled boost and buck-boost converters are as simple to design as linear converters and deliver comparable performance.
Modern controls like IOL have the ability to level the topology playing field and give options back to the design and system engineer. Rather than dealing with right half plane zeroes and complex poles and making compromises on stability, performance, and cost, they can take advantage of a purpose-built control method designed to take these effects into account and require only the input of operating parameters like desired voltage and current. Not only is the design process greatly simplified and shortened, but performance is also improved without sacrificing other features – a welcome advance in a time of increasing customer pressure. Finally, with intelligent modern control, the converter itself can evolve into a much more capable device, one with abilities never seen before. For instance, since IOL uses an exact model, not a linear approximation, it means that any and every possible operating point supported by the physics of the plant can be selected at any time. This allows “on-the-fly” voltage switching on either the input or output, and effectively lets the converter perform the additional functionality of a regulator or output tracker – all done by the control firmware.
This opens the door to a whole new class of converter products that can operate in variable voltage environments and offer inherent stability. An example of this is realizing a decades-old concept of affordable and accurate envelope tracking of power – the ability to deliver only the power precisely needed at any particular moment. Motor control, radio and audio amplifiers, and even batteries can benefit from a converter that can quickly track fluctuating loads and deliver precise levels of voltage.
After the microcontroller, the plant design, and the choice of components, the fourth piece of the digital power puzzle is control. As the power industry pushes more applications into digital solutions, and digital solutions become more complex, control will take on a more prominent role. Companies will develop and showcase their modern control methods and begin marketing them as their defining competitive advantage. Additional techniques and methods will emerge, be evaluated, and either be discarded or improved. The question that will face power solution and system providers is do they want to risk falling behind their competitors, or will they want to work with suppliers who understand modern control and incorporate these new control techniques into their products. They can realize practical and complete solutions to their existing problems, and then begin the much more exciting and intriguing prospect of exploring the new field of applications opened up by modern control.
