Advantages of Digital Power Control in IoT Cloud Computing

The power supply is often the last design task performed in system development. Design discussions often begin with, “Okay, let’s design a power supply that takes this much input voltage, delivers this much current to multiple outputs, and has an efficiency target to keep thermals under control.” However, today’s designs are becoming more complex. Power supply designers know that a good power supply, especially one that runs into the hundreds of watts, must balance often conflicting goals. These goals include output flexibility, dynamic line/load performance, high and low load efficiency, component tolerances, thermal issues and temperature coefficients, monitoring and protection, and dynamic changes in voltage or current.

The difficulty of designing high-performance power supplies is not fully appreciated except by power experts who have long been working on developing solutions to meet the latest load requirements. This is exemplified in the challenges of designing power supplies for servers and storage networking equipment that make up the infrastructure of the Internet of Things (IoT) cloud computing and telecommunications. Performance requirements that were once relegated to high-performance CPUs now extend to medium-sized and even small loads.

Analog control topology slows down

To meet these requirements, design engineers have developed many innovative analog control loops and techniques, such as constant on-time or hysteresis control, as well as typical voltage and current mode schemes. Although these designs have many features, they are no longer able to meet user requirements. In the case of PWM controllers, functional blocks such as error amplifiers, comparators, and ramp generators are limited by their design constraints and variables. They are affected by time and phase shifts in the signal chain, as well as noise injection and large signal response issues when selecting components for the compensation network (see Figure 1).

Figure 1. Analog PWM power supply controller.

The nature of these designs—where the circuit must be tuned to the specific application needs with minimal compromise in performance—means that signal controllers cannot provide the flexibility required by the latest system requirements. This limitation has caused the performance and use of analog controllers to stagnate. Fortunately, digital power control (also known as "digital power") provides a better alternative.

What is digital power control? To some, it is still an analog control loop, but some parameters can be adjusted through a digital interface. For example, the output voltage can be changed by issuing a command through the PMBus interface. But this statement does not mention the benefits that digital control and digital power can bring to the market.

Digital power to the rescue

True digital power control based on PWM DC/DC controllers is much more than just the management of the internal analog loop. Instead, the actual closed-loop feedback path is performed entirely in the digital domain, starting with the analog/digital conversion of the feedback voltage. Once all information is in the digital domain, new advanced control techniques and digital signal processing can be applied. For example, changing states and algorithms based on the conditions of the load in order to achieve an optimized response even under changing conditions (see Figure 2).

Figure 2. Implementation of a digital power controller.

The benefits of digital control extend beyond advanced control strategies and implementation flexibility. The algorithms are able to adapt to changes, thus overcoming limitations previously seen in analog power supplies, such as component variation due to temperature rise, tolerances, and even aging. Note that some passive components, such as inductors and capacitors, can easily have initial tolerances of ±20% or more, and using components with tighter tolerances can significantly increase the cost of a design. Digital power controllers no longer need to be stretched to balance the many passive and analog components in a complex design and be held accountable for their non-ideal performance. With advanced digital control algorithms, loop stability challenges are no longer an issue.

Digital solutions help engineers succeed

While digital control may be new and too good to be true to some design engineers, the reality is quite different, as demonstrated by Intersil's ZL880x series of DC/DC step-down controllers, an innovative 4th generation mixed-signal power conversion power management IC family. Figure 3 shows the ZL8802 dual-channel, dual-phase controller, which integrates a high-performance step-down converter for a wide range of power applications. It eliminates the need for loop compensation for stability without sacrificing system bandwidth.

Figure 3. The ZL8802 block diagram shows its internal complexity, much of which is transparent to the design engineer.

As a result, the ZL880x series controllers have been adopted by multiple manufacturers in released OEM power supplies, including CUI's NDM3Z series of digital DC/DC POL modules. They are high-performance devices designed to meet the needs of the most demanding intermediate bus power systems.

“We selected the ZL8800 controller for our NDM3ZS-60 digital POL module because we feel it offers the most advanced feature set on the market,” said Mark Adams, senior vice president at CUI. “Simply converting one voltage to another no longer works for our customers. The requirement now is for perfect voltage conversion at all times and under all conditions. Digital IC technology helps us achieve this goal.”

The IC's internal adaptive algorithm automatically changes operating states to improve efficiency and overall performance without user intervention. The device's fully digital loop enables precise control of the entire power conversion process without the need for software, making it a very flexible and easy-to-use device. The control algorithm is used to respond to output current changes within a single PWM switching cycle, thereby achieving a smaller total output voltage change with a smaller output capacitance than traditional PWM controllers.

The device allows designers to fully control and monitor each power rail through the PMBus interface to maximize reliability. Its proprietary single-wire DDC (digital DC) serial bus allows multiple ZL880x devices to communicate with other Intersil digital power ICs for phase current balancing, sequencing and fault diffusion between ICs, eliminating the need for additional complex power manager designs using external discrete components.

The ZL8802 includes circuit protection features that continuously prevent damage to devices and loads due to unexpected system faults. It continuously monitors input voltage and current, output voltage and current, including cycle-by-cycle output overcurrent protection, internal temperature, and the temperature of two peripheral thermal diodes. Monitoring parameters can also be preconfigured to provide alerts for specific conditions.

Digital modulation technology

The key element of the ZL8802 is the integration of a proprietary digital modulation technique. An advanced digital controller needs to meet three key requirements: it should support sufficiently high bandwidth; it should not require compensation under ideal conditions; and it should support fixed frequency switching operation. The ZL8802's digital voltage mode control is able to achieve high bandwidth using its patented ChargeMode control technique.

Traditionally, voltage or current mode hysteresis controllers have provided the best loop response, but with the disadvantage of switching at a variable frequency. The latest telecommunications equipment using digital power controllers requires a fixed operating frequency to achieve tight control of the noise spectrum in the end-user application. The ZL8802 utilizes its unique digital modulator and compensation techniques to achieve all of these goals.

Its digital modulation technique allows the controller to react to voltage changes in a single PWM switching cycle. The ZL8802 can sample the error multiple times in one switching cycle and calculate the modulation signal, which significantly reduces group delay and thus supports very high bandwidth operation.

The cool thing is that the ZL8802 does not need to know the actual output capacitor value. Instead, it relies on digital algorithms to make the correct adjustments and ensure stability. The result is a reduction in the capacitance required to support a specific application while providing a compensation-free design. The controller's response ensures that any transient conditions are met while maintaining stability and minimizing ringing or overshoot. The system-level benefit of this approach is that it no longer limits the designer's choice of power supply components. At the same time, the controller also eliminates the effects of component aging or environmental changes because the digital loop can continuously monitor and be responsible for changes.

Configuration, verification and monitoring

Another benefit of the all-digital closed-loop control solution is that it is compatible with an easy-to-use and powerful graphical user interface (GUI). PowerNavigator software can configure, verify and monitor multiple devices using a PC with a USB interface. This makes it easy to use a simple GUI to change all the features and functions of the digital power design. Using this tool, engineers can set up the power architecture, define voltage rails and current sharing operations, establish event-based or time-based timing control, monitor hardware and faults, and save projects and configuration files.

in conclusion

When designing power supplies for advanced FPGAs, processors, and ASICs used in IoT cloud computing servers and telecommunications equipment, engineers are most concerned about power density, transient response, and efficiency. Digital PWM DC/DC controllers such as the ZL8802 provide excellent transient response performance and uncompensated design, helping power design engineers independently strike a continuous balance between dynamic performance and system stability.

About the Author

Chance Dunlap is the Director of the Infrastructure Power Business Unit at Intersil. During his 17-year career in the power electronics industry, he has worked in business development and marketing, supporting the development of a range of power products from digital power supplies to isolated controllers. He holds 6 patents, has published many technical papers, and has spoken and presented at many symposiums and industry conferences.

Chance holds a BSEE from Purdue University and an MBA from the University of Arizona.