The Role of Digital Control in Power Supplies
Supplying power to a device, board or system is, by definition, an analog function that is characterized with parameters such as voltage, current and efficiency. However, the increased performance of digital control, combined with the lower-cost point of low pin-count, easy-to-use microcontrollers, is making digital control a viable option for various power-supply functions. But which functions are better suited for digital and where is it better to stay with the proven analog methods?
Although there's a push for digital control in the main power-control loop, digital control has a broader function in power supplies. There are basically four levels of digital integration in power supplies.
Level I adds simple functions that are difficult to perform with analog components. For example, a 6-pin microcontroller provides a PWM waveform that ramps from 0% to 100% duty cycle to provide a soft-start function to the switcher circuitry.
Level II provides a secondary management function around the more traditional analog circuit. In this case, the digital controller monitors the output parameters and uses existing external controls to enhance the functionality of the power supply, but the power-control loop is still completely analog. This is done using standard microcontrollers with an integrated A/D converter for measuring the various parameters.
Level III is a higher level of integration, with the switcher circuitry integrated on the microcontroller. The microcontroller also controls switching and gain, although implementation of the feedback loop is still primarily analog. This requires a more specialized digital controller with switching circuitry and various analog functions integrated onto the device.
Level IV is complete digital control, with all parameters digitized and analyzed by the controller to provide the appropriate outputs. This typically requires a DSP with high-speed A/D converters and PWM outputs.
The appropriate level of digital integration depends on the requirements of the design. Most of these digital-control functions center on achieving deterministic behavior in what are sometimes complex interrelated designs.
Digital controllers do well at making decisions and handling “what if” conditions. When statements such as “if X voltage is higher than Y, then adjust Z” describe the circuit, it's best to have a microcontroller perform that function. This secondary monitoring of power-supply parameters and reacting to any exceptions accounts for most of the practical uses of microcontrollers in power supplies today.
Digital controllers are also good at sequencing events or any timing-related functions. Microcontrollers run from a clock, which makes time measurement and event execution at specified times fairly straightforward. For example, changing the over-current protection level during startup and then throttling it back after 20 ms can be done with a Level II design that uses a low pin-count microcontroller with an internal oscillator. Again, the digital-control portion fits as a complimentary peripheral function to the main power-supply control loop.
Once in the power supply, using digital controllers offers several other advantages. Monitoring and diagnostics of the supply now can be communicated to the rest of the system. Because many newer microcontrollers come with data EEPROM, event counters and logs can be stored and extracted at a later stage to provide valuable information about failures and power-supply usage. Also, the EEPROM is valuable in storing calibration values that can be used to linearize a temperature coefficient or reduce the cost of a voltage reference.
The most overlooked benefit of using digital control in power supplies is during the design and production of the power supply. The ability to change behaviors simply by reprogramming the controller accelerates the rate of change and thus time-to-market. One design can be used for different applications by changing just the firmware. This results in lower inventory costs and fewer manufacturing setup changes. It also enables electronic (rather than manual) calibration and delayed customization of a design, providing substantial cost savings beyond the bill of materials.
Fanie Duvenhage manages Microchip Technology's portfolio of low pin-count PIC microcontrollers. He holds a BSEE degree from the University of Pretoria, South Africa, and MBA and MSD degrees from Arizona State University.