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Implement digital power control using LLC resonant converters

Posted: 07 Sep 2012 ?? ?Print Version ?Bookmark and Share

Keywords:microcontrollers? analog systems? communications?

With the availability of new low-cost, high-performance microcontrollers (MCUs), the advantages of digital power control can be introduced to a wider range of embedded, industrial and control applications. Traditional analog systems are susceptible to factors such as drift, aging of components, variations caused by temperature and component tolerance degrading. Developers are also limited to classical control implementations. In addition, analog-based systems offer little flexibility to accommodate different environmental operating conditions or even simple changes in system requirements.

When designed using a digital approach, portions of the power system can be implemented in software, resulting in a level of flexibility that enables a single architecture to provide optimal performance across a range of applications and operating conditions. With software-based control algorithms, developers can:

???Ensure precise and predictable system behavior through configuration C both in the factory and at power up C to adjust for component tolerance issues
???Improve efficiency through the use of advanced algorithms (i.e., nonlinear, multivariable, etc.), which are not feasible to implement in analog-based systems
???Maintain performance over an extended system lifetime through dynamic recalibration
???Support multiple systems with a single controller
???Increase system reliability through self-diagnostics
???Enable intelligent management through a communications link
???Simplify system design by allowing developers to work with model tools and C rather than having to rework analog designs with every requirement change
???Reduce system cost by supporting other system functions on the same MCU
This article describes a digital power control implementation using LLC (line level control) resonant converters based on a flexible, 32bit, low-cost, high-performance microcontroller. Key elements of digital power control will be explored; including duty cycle control, dead-band adjustment in real time, frequency control, and adaptive thresholds for maintain different safe operation regions.

Tuning of the voltage compensator using coefficients during an active load will show the flexibility of the implementation, and the use of programmable soft start/stop capabilities and slew rate control will demonstrate how to avoid inrush current and reduce audible noise. Finally, developers will learn how hybrid burst mode control dramatically increases light-load and stand-by efficiency.

Digital control with MCUs
Consider the right MCU to provide all of the necessary performance and peripherals needed to control a system with a single standalone controller. MCUs with ample headroom and specialized peripherals will enable developers to implement more advanced control algorithms to further improve performance while lowering system cost.

Few microcontrollers have an architecture optimized for digital control applications with advanced architectural features to enhance high-speed signal processing. The main CPU core needs built-in DSP capabilities such as a single cycle 32 x 32bit multiply and accumulate (MAC) unit to greatly speed processing of computations. Integrated control peripherals, such as the analog-to-digital converter (ADC) and PWMs, are designed to be very flexible and easily adapt to almost any use with very little software overhead. For example, the ADC has a programmable auto-sequencer that cycles through samples in a specific order so that values are ready when the application needs them. With more intelligent control peripherals and a powerful CPU core, control loops run tighter, both improving the dynamic nature of control algorithms and resulting in better disturbance behavior.

Microcontrollers need to provide the significant PWM features needed for real-time digital control including:

???Duty cycle control for soft startup avoids inrush currents and enables various burst mode configurations to enhance light-load efficiency
???Real-time dead band adjustability guarantees ZVS at all operating points and optimizes efficiency
???Trip-zone and internal comparator options enable instantaneous PWM disabling to ensure system reliability and safety
???High-resolution frequency adjustment capabilities down to 150 ps for precise output voltage regulation

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