It’s not news that a power supply’s performance is critical to an efficient and reliable system. Even so, the supply often doesn’t get the appreciation and respect it deserves. Very often, whether the product is a low-power, portable device operating from a battery or a larger one using the AC line, designers move on to other issues after a few words about efficiency and quality. The thinking is “just get some power-supply box with AC input, sufficient DC-output current at the right voltage, and high-enough efficiency to keep dissipation low” — what more is there to worry about, right?
The reality is much more complex, of course. Power-supply designers and OEMs know that a good supply – especially one for mid-to-higher power levels of hundreds of watts and above – must juggle and then balance often conflicting goals, including output flexibility, dynamic line/load performance, high- and low-load efficiency, component tolerances, thermal issues and temperature coefficients, monitoring and protection, and changes in voltage or current requirements.
The difficulties of designing a high-performance supply are not appreciated, except by supply experts, who have managed to develop many clever and effective topologies. The greatest challenge is in supplies for servers, data centers, and similar higher-power applications that demand exceptional performance across an array of parameters. Further, performance demands are now extending down to medium and even smaller-size supplies.
Digital supplies offer dramatically better solution
To meet these requirements, designers have developed many innovative, all-analog topologies and techniques, including multistage regulation, POL (point-of-load) conversion, SEPIC (single-ended primary-inductance converter), constant-on-time control, continuous conduction mode, and discontinuous conduction mode, to cite just a few examples of a sometimes bewildering array. Despite the features that these designs offer, they are no longer capable of meeting the user demands. {Figure 1, part a: analog-control block diagram}
Their complexity of these very clever designs — they implement the electronic equivalent of wheels within wheels within wheels — and their sensitivity to unavoidable component tolerances indicates that they have reached a performance plateau. As a further limitation, once a supply is designed and built using one of these approaches, its operation is fixed in hardware. Fortunately, there is a superior alternative to which OEMs are transitioning: digital power control (aka “digital power”).
What is digital power control? There’s some ambiguity in the phrase. To some, it means analog closed-loop feedback, but with some parameters adjustable via digital control. For example, the output voltage could be changed when the controller finds that system conditions have changed. As with the all-analog approach, the control algorithm itself is still fixed in hardware. {Figure 1, part b: true digital-control block diagram }
True digital-power control, however, is much more than just digital supervision of an inner analog loop. Instead, the actual closed-loop feedback path is executed entirely via dedicated digital circuitry, beginning with A/D conversion of key signals, digital signal processing either in hardware or software, and output control via D/A converters. Since the control strategy is implemented by algorithms which the device executes, these can be both sophisticated and even change on-the-fly to meet changing situations.
The benefits of digital control go beyond advanced control tactics and flexibility of implementation. The algorithm can adapt to changes in component values due to temperature rise, tolerance variations, and even aging. (Note that the initial tolerance of some passives, such as inductors and capacitors, can easily be ±20% and more, and tighter-tolerance parts are extremely costly or not available.) The power controller no longer needs to precariously balance a long list of passive and analog components, all arranged in a complicated design, which must also account for tolerance, drift, and other realities. Even more noteworthy is the right approach provides a unique solution to the challenging problem of loop compensation, see sidebar, “Digital control leads to compensation freedom.”
Some designers assume the word “digital” means a microcontroller or microprocessor running an application block, and think that a successful digital power controller can be realized by a standard processor and firmware. However, this is an unwise solution in practice, see sidebar, “Go micro or go dedicated?”
Intersil’s approach helps OEMs succeed
While digital control may seem too new and too good to be true, or perhaps “just around the corner,” the reality is quite different as proven by Intersil’s ZL8800, an innovative 4th-generation, mixed-signal power-conversion and power-management IC. This dual-channel, dual-phase controller integrates a high-performance step-down converter for a wide variety of power-supply applications. It eliminates the need to compensate the loop for stability without compromising system bandwidth. {Figure 2: block diagram of ZL8800}
As a result, it is already designed into released OEM supplies from CUI. CUI’s NDM3Z series of digital DC/DC POL modules, which are high performance devices designed to meet the needs of the most demanding intermediate bus power systems. {Figures 3: photo of CUI supplies}
Mark Adams, VP of Advanced Power at CUI noted, “we selected Intersil’s ZL8800 family controller for our NDM3ZS-60 digital POL module because we felt that it provided the most advanced feature set on the market. Simply converting one voltage to another voltage is no longer an option for our customers. Now, perfect voltage conversion under all conditions, all of the time, is required. Intersil’s digital IC technology helps us achieve this.”
Adaptive algorithms within the IC automatically change the operating state to increase efficiency and overall performance with no need for user interaction. The device’s fully digital loop achieves precise control of the entire power conversion process with no software required, resulting in a very flexible device that is also very easy to use. The control algorithm is implemented to respond to output current changes within a single PWM switching cycle, achieving a smaller total output voltage variation with less output capacitance than traditional PWM controllers.
The device provides best-in-class transient response for digital POL converters, saving on output capacitors and board space. It enables designers to fully control and monitor, via the PMBus interface, every power rail to maximize reliability. Intersil’s proprietary single-wire DDC (Digital DC) serial bus lets multiple ZL8800s communicate with other Intersil digital power ICs for inter-IC phase-current balancing, sequencing and fault spreading, thus eliminating additional complicated power-supply manager designs requiring external discrete components.
The ZL8800 includes circuit protection features that continuously safeguard the device and load from damage due to unexpected system faults. It can continuously monitor input voltage and current, output voltage and current, including cycle-by-cycle output-overcurrent protection, internal temperature, and the temperature of two external thermal diodes. Monitoring parameters can also be pre-configured to provide alerts for specific conditions.
The key element of the ZL8800 is the integration of the proprietary digital-modulation technology. An advanced digital controller needs to meet three key requirements: it should support sufficiently high bandwidth; ideally, it should be compensation-free; and it should support fixed-frequency switching. The ZL8800’s digital voltage-mode control provides the ability to achieve high bandwidth using its patented ChargeMode control technology.
Traditionally, voltage- or current-mode hysteretic controllers have offered the best loop response, but these come with the drawback of switching with variable frequencies. Modern telecommunication equipment that uses digital power controllers requires fixed frequency operation, allowing tight control of the noise spectrum in end-user applications. The ZL8800 achieves all of these with its unique digital modulator and compensation technique.
Its digital modulation technology allows the controller to react to voltage deviation in a single PWM switching cycle. The ZL8800 samples the error and computes the modulation signal multiple times during a switching period, which significantly reduces group delay and therefore supports very-high-bandwidth operation. (Note that one switching cycle represents the smallest quantum of control in a PWM loop, and thus represents the upper limit in terms of speed in responding to a transient.) {Possible figure 4: some key waveform from ZL8800 operation}
Ingeniously, the ZL8800 does not need to know the actual output-capacitor value; instead it relies on digital algorithms to make the correct adjustment, even for stability. The result is a reduction in the amount of capacitance needed to support a specific application, while providing a compensation free design. The controller’s response ensures that any transient conditions are met while preserving stability and minimizing any ringing or over-shoot. The system-level benefits of this approach are that designers are now no longer limited in their power-component choices. Furthermore, the controller eliminates the effects of component aging or environmental variations since the digital loop is constantly monitoring and accounting for the change.
Setup via GUI an additional OEM benefit
Another advantage of the all-digital approach to closed-loop control is that it is compatible with a user-friendly and powerful graphical user interface (GUI). Intersil’s PowerNavigator™ software allows simple configuration and monitoring of multiple devices using just a PC with a USB interface. It makes it easy to change all features and functions of the digital power supply design using a simple GUI. Using this tool, engineers can set up the power architecture, with defining voltage rails as well as current-sharing operation, establishing event- or time-based sequencing, monitoring hardware and faults and, of course, saving project and configuration files. {Figure 5: a GUI screen of PowerNavigator}
Conclusion
As Mark Adams at CUI noted, “power density, transient response, and efficiency are all of chief concern to our customers as they are tasked with powering today’s advanced ICs. The ZL8800’s superior transient response performance and compensation-free design which allows our customers the ability to autonomously balance the trade-offs between dynamic performance and system stability on a continuous basis is ideal for our new family of high current digital modules.”
