Benefits of Digital Feedback Loops in Power Conversion Applications

Market factors are driving the development of intelligent power supplies, including external control of the power supply and software configuration in production. One way to achieve this is to use digital feedback control in the power conversion loop. Power supply designers have already adopted microcontrollers ( MCUs) in intelligent power supplies.

to implement communication, monitoring, control and deterministic functions such as power sequencing, soft start and topology control. However, until recently, full digital control of the power conversion loop was not possible due to the lack of suitable and cost-effective technology. The advent of digital signal controllers (DSCs) with dedicated peripherals has made full digital control possible.

DSC Enables Digital Feedback Loops

Designers often ask why they would choose to use DSCs for digital feedback control when there are so many inexpensive dedicated analog pulse width modulation (PWM) controllers? DSCs provide new control methods and power conversion topologies that are not possible or even impossible to implement using traditional analog power conversion controllers. The operating mode (voltage or current) can be changed continuously or discontinuously during operation based on changes in input or load conditions. The digital control loop implemented in the DSC takes advantage of flash memory technology, allowing the system to be configured and calibrated for different users using a standard platform design, thereby reducing inventory and NRE costs and shortening time to market.

The functional block diagram shown in Figure 1 depicts a synchronous buck converter SMPS control system based on a typical DSC architecture that combines control peripherals such as fast arithmetic operations and counter-based PWM modules with feedback from analog comparators and ADC sampling working in conjunction with them.

Advantages of Digital Feedback Loops

The digital nature of the DSC eliminates the temperature drift and numerous analog component variations in typical designs, thereby minimizing system errors. This in turn reduces the size and cost of components such as transformers and inductors. If the product requires the design to adapt to changing conditions, the parameters of the DSC must be reconfigurable at run time. This is not possible with analog designs.

A DSC can replace multiple analog controllers (such as PFC and DC/DC) if there is tight coordination between the power conversion stages or independent power control of multiple outputs. The ability to execute multiple independent power control loops provides more precise output regulation than traditional controllers.

The ability to execute multiple control loops is also required for highly adaptable system designs. Take a standard multiphase buck converter implemented with an analog controller. The relationship between the phases is fixed at design time and the function is fixed at a specific voltage. When the same multiphase buck converter is implemented with a DSC using a digital control loop, the relationship between the phases can be changed on the fly to meet changing load conditions. When the load decreases, one phase can be disabled to reduce switching losses while the remaining phases continue to operate in continuous current mode. This is very similar to a modern V8 engine stopping cylinders when horsepower demand decreases.

Now, taking this multiphase buck converter with digital control as an example, let’s say that the customer may not need 1.8V output at 100 amps, but may need 4 phases (for example: 1.2V @ 20A, 1.8V @ 20A, 2.5V @ 15A, and 3.3V @ 10A). The digital controller can be easily adjusted to make the PWMs dependent or independent of each other. Updating the DSC flash memory with modified parameters allows the basic power conversion circuit to provide different functions.

DSCs allow power supply designers to develop new power conversion topologies and control strategies that are impossible to achieve with standard off-the-shelf analog PWM controllers unless they are willing to wait for analog device vendors to design and sell controllers that meet the needs.

Analog PWM controllers require resistors and multiple pins to set options; DSCs use code to make the chip smaller. Analog PWMs only provide a few options, while DSCs can be completely reconfigured. Additionally, analog PWMs are typically locked into one operating mode at power-up, whereas DSCs can be dynamically reconfigured to respond to changing conditions.

If a product requires a design that can adapt to changing requirements, the DSC needs to be reprogrammed. In analog designs, the old module needs to be discarded and replaced with a new one. With on-chip flash memory, DSCs can simplify the production assembly line of power supplies—a hardware design can be configured to the customer's voltage and/or current requirements. Adjustments and calibrations of the power supply can be performed by programming the flash memory, eliminating the need to adjust potentiometers or laser trim resistor values.

Other Advantages of Digital Control

Implementing a digital power controller with a DSC can add many additional features to the system without increasing the cost of the system.

If a product requires coordination of multiple output voltages during system startup and shutdown, a DSC can provide this functionality without increasing the cost. In contrast, implementing analog power sequencing and tracking devices in the system would be very expensive. Many products require coordination of multiple output voltages under fault conditions. This is because if one output voltage fails, the other output voltages must usually be reduced or shut down to prevent the load circuit (such as the motherboard) from entering a "latch-up" state.

If the design requires that the power conversion process be synchronized with external events or other devices, DSCs can also provide this function without increasing the cost. Standard analog PWM controllers with this function are much more expensive than ordinary analog PWM controllers. In addition, multiple DSCs can be daisy-chained to provide more coordinated resources.

In certain applications, it is necessary to compare and monitor information from sensors, such as those used in fan control and fault detection and temperature monitoring. DSCs can use processor resources not used by the digital control loop to perform these additional tasks.

In telecommunications and other critical applications, system power is often implemented by multiple independent power modules, which provide a total power that exceeds the system requirements. This ensures that if one power module fails, the remaining power modules can continue to power the system. The power modules are connected together with wires, which forces each power module to output an equal share of the total power required. (This is called "load balancing.") DSCs can achieve load balancing without increasing the cost. In fact, analog load balancing interface devices are more expensive than many analog PWM controllers.

A function related to "load balancing" is called "hot swapping." When a power module fails in a load-sharing application, users often want a service technician to replace the bad power module with a new one while the system is still running. Hot swapping requires that the power module can disable and enable itself in a controlled manner and control its behavior so as not to interfere with the operation of other power modules. Implementing "hot swapping" functions with analog devices can be very expensive.

If the system already requires a microprocessor to perform other tasks, the DSC can perform these tasks while controlling the power supply, thereby reducing the number of components and reducing costs.

If the system requires error logging or communication functions, the DSC can provide such functions, which are impossible to provide with analog controllers. DSC allows power supply designers to measure complex physical quantities such as power and efficiency. And, the DSC can use this information to help designers adapt its response characteristics to any changing load conditions.

Another potential feature of digital loop control is that since revisions can be completed through software, there is no need to use expensive multi-layer circuit boards during the development of new products, thus saving time and cost. Product development costs can be further reduced by loading easy-to-use test software for circuit board testing or customizing multiple products on a single hardware.

Considerations for Digital Loop Control

Learning about topics such as embedded system design and MCU programming can be a learning curve, especially for designers who are accustomed to discrete analog power solutions. Fortunately, software tools, reference designs, and libraries are available to simplify the learning curve. Selecting the right DSC for a particular application is critical to the success of the design, and careful consideration of the required DSC features is an important step. The key consideration in selecting a DSC for an application is to ensure that the on-chip PWM module provides sufficient resolution for the power design. The resolution and speed of the DSC's on-chip ADC provide the system with status information (feedback) that is input into the control loop, so the ADC should also have sufficient resolution. Therefore, the selected DSC should have on-chip PWM and ADC that are suitable for power applications.

The selected DSC should also have analog comparators. This is because the ADC can continuously monitor the signal and sample it at a rate not exceeding its maximum sampling rate in megasamples per second (MSPS). However, if the monitored signal only needs to be compared to a fixed limit value, using a DSC with an ADC alone is a waste of processing resources. On-chip analog comparators free up the processor and ADC to perform other more valuable tasks while still allowing the selected DSC to support fast power fault handling and current limiting functions. The

right choice of a controller-centric DSC (such as the example in Figure 2) simplifies the task of implementing the control algorithm. Typically, the digital control loop is implemented as a proportional-integral-derivative (PID) algorithm. However, complex DSP programming techniques are not required to handle the DSP functions of a controller-centric DSC.

Modern DSCs also offer monitoring features such as a watchdog timer (WDT) that can reset the system software and hardware in the event of a software or hardware failure. WDTs enhance system reliability by allowing digital control systems to reset or at least enter a safe state.

Conclusion

This article discussed the benefits that digital feedback loops offer to power supply designers and their designs. Analog PWM controllers are essentially reactive devices (some controllers have some limited feed-forward functionality). This means that they are unlikely to provide good performance under widely varying conditions. In contrast, implementing a digital feedback loop in a power supply allows the power conversion process and load changes to be simulated. This provides many benefits, such as increased reliability, reduced component count, better transient response, and the ability to dynamically change topologies and switch from single-phase to multi-phase outputs. It also allows the design to be customized before production is complete through software rather than hardware.