DSCs Ease Migration to Digital Loop Control

Nov 1, 2006 12:00 PM
By Bryan Kris, Staff Architect, Architecture & Applications, Digital Signal Controller Division, Mic

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Application flexibility and cost concerns are creating a demand for intelligent power-supply designs that support soft configuration in production and external control. These advanced power supplies require a digital power-conversion feedback loop. Recent advances in digital signal controllers (DSCs) with high-performance on-chip peripherals targeting power conversion, coupled with their ease of use and affordability, enable many more power-conversion products to migrate to digital loop control.

To implement digital loop control in switch-mode power supply (SMPS) designs using DSCs, designers need to understand the DSC architecture and the factors that influence the selection of the DSC in a particular design. A DSC-based synchronous buck converter will serve as a tutorial example to introduce the design principles associated with developing digital loop control for SMPSs.

Enabling SMPS Designs

When a DSC is used to implement digital loop control in a synchronous buck converter, we arrive at Fig. 1, which describes a typical DSC architecture. DSCs provide the fast-math operations of a DSP, with practical control peripherals and the look and feel of a microcontroller. Control peripherals include counter-based pulse-width-modulation (PWM) modules, analog comparator-based feedback and coordinated analog-to-digital converter (ADC) sampling.

This efficient combination of features and functionality is excellent for power-supply applications, as DSCs provide the high execution rates needed for control-loop software. Even better, complex DSP programming skills are not needed to design using DSCs. Instead, using familiar analog components and microcontroller-like software, designers can use DSCs to quickly and cost-effectively build advanced power supplies. Nevertheless, in this synchronous buck converter SMPS control system example, delays are associated with each function block.

Choosing the correct DSC depends on the particular needs of each application. To choose the right DSC, designers must first select the topology: stepup or stepdown (boost or buck), or isolated (forward, half-bridge or full-bridge). Next, designers must adopt the appropriate switching technique such as hard- or soft-switching and then select a control methodology based on voltage or current mode.

The choice of DSC is determined largely by how well the DSC's peripherals match the desired application. For example, if a soft-switching technique such as zero-voltage transition (ZVT) is desired, then the PWM module onboard the DSC must support dynamic phase shifting. If current-mode control is to be implemented, then analog comparators with reference digital-to-analog converters (DACs) and an ADC with asynchronous-sampling capabilities simplify this task. If the SMPS application is operating at high PWM frequencies to reduce the size and cost of the pc-board components, a high-resolution PWM is needed to reduce voltage and current ripple.

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