Intelligent Power with Digital Feedback Loops

Electronics Technical ArticlesSouth-East European INDUSTRIAL Мarket - issue 5/2008

DSCs are replacing analogue PWM controllers to bring new levels of flexibility and cost reductions for intelligent power supplies.

Bryan Kris,
Microchip Technology

Market factors are driving the development of intelligent power supplies that involve external control of the power supply and soft configuration in production. One way to achieve this is with digital feedback control of the power-conversion loop. Power-supply designers have employed microcontrollers (MCUs) in intelligent power supplies for communications, monitoring, control and deterministic functions, such as power sequencing, soft start and topology control. However, until recently, complete digital control of the power-conversion loop has been impractical, due to the lack of suitable cost-effective technology. With the advent of Digital Signal Controllers (DSCs) that contain specialised peripherals, complete digital control is now practical.

DSCs Enable Digital Feedback Loop
With so many inexpensive, dedicated analogue Pulse-Width-Modulation (PWM) controllers available, why should designers opt for digital feedback control using DSCs? DSCs provide the capability to implement new control methodologies and power-conversion topologies that are impractical or impossible with conventional analogue power-conversion controllers. The operating modes of voltage versus current, or continuous versus discontinuous, can be changed during operation, to meet changing input or load conditions. Digital control loops implemented in DSCs can take advantage of Flash-based technology that permit systems to be configured and calibrated for different customers using standard platform designs, which reduces inventories, time to market and NRE costs.
A synchronous buck converter SMPS control system based on a typical DSC architecture that combines fast arithmetic operations and control peripherals, such as counter-based PWM modules, with analogue comparator-based feedback and coordinated ADC sampling, is shown in Figure 1.

Digital Feedback Loop Benefits
The digital nature of a DSC eliminates the thermal drift and much of the analogue component variations in a typical design, therefore design tolerances can be minimised, reducing the size and cost of components, such as transformers and inductors. If the product requires the design to become adaptive to changing environments, parameters in DSCs can be reprogrammed during run time. This capability is impossible with analogue designs.
A single DSC can replace multiple analogue controllers, such as PFC and DC/DC, providing tight coordination among the power-conversion stages, or individual power control for multiple outputs. The ability to implement multiple independent power-control loops can provide tighter output regulation than is possible with conventional controllers.
The ability to implement multiple control loops also provides for very adaptable system designs. Consider a standard multi-phase buck converter, implemented with an analogue controller. The relationship between the phases is fixed by design, and the functionality is also fixed at a specific voltage. By implementing the same multi-phase buck converter with digital control loops in a DSC the phase relationship can change on-the-fly to meet changing load conditions, phases can be disabled as the load drops, to reduce switching losses, and the remaining phases can continue to operate in continuous-current mode. This is comparable to a modern V8 engine deactivating cylinders as the horsepower requirement drops.
Now, imagine the same digitally-controlled multi-phase buck converter. Another customer may not require 1.8 volts at 100 amps, but may instead need 4 outputs at, for example, 1.2V @ 20 amps, 1.8V@ 20 amps, 2.5V @ 15 amps, and 3.3V @ 10 amps. The digital controller is easily adaptable to treating the PWM phases as related or independent. Updating the DSC’s Flash memory with revised parameters can enable a basic power-conversion block to provide different functionality.
DSCs enable the power-supply designer to develop new power-conversion topologies and control strategies that are not possible with standard, off-the-shelf analogue PWM controllers, unless or until, an analogue device vendor designs and markets a controller specific to the design’s specific requirements.
Analogue PWM controllers require resistors and multiple pins to set options; DSCs use code, making the chip smaller. Analogue PWMs only give you a few options, whilst DSCs can be completely reconfigured. Also, analogue PWMs are typically locked into one mode on power-up, but DSCs can reconfigure dynamically in response to changing conditions.
If a product requires the design to become adaptive to changing requirements, a DSC can be reprogrammed, whilst analogue-based designs need to resort to replacing the module. Through on-chip Flash, DSCs can enable a simplified power-supply production assembly line, allowing a single hardware design to be configured for customer voltage and/or current requirements. Power-supply trimming and calibration can be performed by programming the Flash memory, thereby eliminating trim pots or laser trimming of resistors.

Additional Digital-Control Benefits
With a DSC implementation of a digital power controller, many additional features can be added to a system without incurring additional cost.
If a design requires the coordination of multiple output voltages during start-up and shut-down, a DSC can provide this functionality at no added cost. By contrast, deploying analogue supply sequencing and tracking devices can be very expensive. During fault conditions, many products require the coordination of multiple output voltages. This is because if one output voltage experiences a fault, the other output voltages usually must be reduced or turned off to prevent the load circuitry (such as a mother board) from experiencing a latch-up condition.
If the power-conversion process needs to be synchronised to external events or other devices, DSCs can provide this capability at no additional cost. Standard analogue PWM controllers which integrate this capability are significantly more expensive than ‘jelly bean’ analogue PWM controllers. DSCs can also be daisy-chained together to provide additional coordinated resources.
In certain applications, such as those in fan control and failure detection, and temperature monitoring. sensor information needs to be collated and monitored. A DSC can handle these additional tasks with processor resources not utilised by the digital control loops.
In telecommunications and other critical applications, the system power supply is typically implemented with multiple independent power modules that provide more total power capability than the system requires. This is done so that, if a particular power module fails, the remaining modules can continue to power the system. These collections of power modules are physically wired together, so it is imperative that each power module equally provides its fair share of the total power required. This is called load sharing. A DSC can implement load sharing at no added cost. Analogue load-sharing interface devices can cost more than many analogue PWM controllers.
Associated with load sharing is a function called hot-swap. When a failure occurs in a power module within a load-sharing application, it is often desirable to enable a service technician to replace the defective power module with a new module while the system is operating. Hot-swap requires that the power modules are able to disable and enable themselves in an orderly manner, and control their behaviour as not to interfere with the operation of the other power modules. With analogue parts, implementing hot-swap capability can become very expensive.

If a system already requires a microprocessor for other tasks, a DSC may be capable of performing that task as well as controlling the power supply, thereby reducing component count and costs.
If a system requires error logging or communications capability, a DSC can provide this capability, which is not possible with an analogue controller. DSCs give power-supply designers the ability to measure complex metrics, such as power and efficiency. DSCs can also use this information to help them adapt their response characteristics to any changing load conditions.
Another feature of digital-loop control is the potential to save time and money during the development of new products by eliminating the need for an expensive multi-layer board spin, because the fix can be made in software. The ability to load test-friendly software for board test, or the ability to make multiple custom products based on a single piece of hardware, can further reduce product development costs.

Digital-Loop-Control Considerations
There may be a learning curve for topics such as embedded systems design and MCU programming, especially for designers used to stand-alone analogue power solutions. Fortunately, there are software tools, reference designs and software libraries that can ease this transition. Choosing the correct DSC for the particular application is critical for the success of the design and careful consideration of the required DSC features is an important step. A key factor to consider in DSC selection is to ensure that the on-board PWM module provides adequate resolution for the power-supply design. The resolution and speed of the ADC on-board a DSC provides the system with status, or feedback, to the control loop and should also have adequate resolution. Therefore, it is necessary to choose DSCs with on-board PWMs and ADCs suitable for power-supply applications.
Also, DSCs that offer analogue comparators, or ADCs, can continuously monitor signals and process the samples up to their mega-samples-per-second (MSPS) rating. However, this is a waste of processing power if the signal being monitored is just being compared to a fixed limit. On-board analogue comparators free the processor and ADC to perform other more valuable tasks, while still allowing the DSC to support fast power-supply fault- and current-limiting functions.
The correct choice of a controller-centric DSC such as that shown in Figure 2) can ease the task of implementing the control algorithm. Often, digital control loops are implemented with a Proportional, Integral and Derivative (PID) algorithm. However, complex DSP programming skills are not needed to handle the DSP features of controller-centric DSCs.
Modern DSCs also provide supervisory functions, such as Watch-Dog Timers (WDTs), which can reset the system software and hardware in the event of a failure. WDTs enhance system reliability by enabling the digital-control system to recover, or at least enter a safe state.

This article has discussed the advantages that a digital feedback loop brings to power-supply designers and their designs. Analogue PWM controllers are basically reactive devices, although a few have some limited feed-forward capability. This means that they can never provide optimal behavior under widely changing conditions. In contrast, implementing a digital feedback loop in power supplies enables the designer to model the power-conversion process as well as the load process. This offers advantages, such as increased reliability, reduced component count, better transient response, the ability to change topologies and from single- to multi-phase on-the-fly. It also allows design customisation toward the end of production through software rather than hardware.