Power Electronics

Digital Power for the Analog Engineer

With system designers increasingly requiring more from their power solutions, digital solutions such as hybrid controllers provide key features like dynamic output voltages, adaptable sequencing, margin testing and in-circuit power system monitoring.

CONVENTIONAL ANALOG controllers require significant external support to meet these system needs. Voltage supervisors, analog-to-digital converters (ADCs) and current monitors add cost, board space and complexity to power supply design. Digital controllers integrate these desirable features, but the powerful, high-speed processor cores and flash memory are larger and more expensive than their analog counter parts. So, is there a compromise to meet the digital needs of system designers, without the expense of digital processors or the need to understand digital programming?

The newest development in digital power solutions are hybrid controllers - analog pulse-width modulation (PWM) controllers integrated with digital programming, monitoring and communications into a single integrated circuit (IC). These hybrid controllers provide power designers with the ability to meet the digital demands of the next-generation systems, while still leveraging their experience designing proven analog power solutions.

So, what can hybrid controllers offer analog power and digital systems designers? A few key benefits include digitally programmable sequencing, active voltage adjustment, dynamic programmable protection features, and the system level reporting and monitoring of the power supply status.

Fig. 1 shows the functional block diagram of a hybrid controller. A conventional analog PWM core uses the outputs of a series of digital-to-analog converters (DACs) to set key functions of the controller, such as its error amplifier reference voltage, switching frequency or current limits, providing the programmability of a digital power solution with a conventional analog PWM. Meanwhile, ADCs monitor key power supply parameters such as output voltage, temperature and load current, and store them in internal registers, where as a digital communications core allows a host controller to retrieve data from the registers or dynamically adjust the values stored in programming registers. This combination provides the designer with the confidence of an analog power solution and the flexibility of digital communications.

Fig. 2 demonstrates four point-of-load (POL) converters based on conventional analog controllers plus an external monitor and supervisor to provide programmable sequencing and reporting of output voltage and individual load current. This requires significant routing, four current sense amplifiers, and a four-channel monitor with sequencing, communications and eight ADCs for just four power rails.

Fig. 3 shows the same system based around a hybrid controller. Integrating the monitoring and sequencing controls of each channel of the supervisor dramatically eases routing, saving precious printed circuit board (PCB) space and reducing the over-all cost of achieving the system requirements. Adding programmability provides a vast array of new possibilities previously available only to digital designers building power around high-performance digital processors, or using complex interfaces between the digital controller and the analog converter.

Let's look at a few benefits a hybrid controller brings to the analog designer.

SEQUENCING

While most PWM converters today include enable and power good functions that allow simple sequenced start-up of various POL converters. This basic scheme requires dedicated wiring, connecting the power good indicator of one converter to the enable of the next in the start-up sequence. While this is a very effective solution when powering well-defined systems, designers are often forced to develop power solutions in parallel with the complex digital circuits, processors and systems. When system power sequencing requirements are unknown or change during the development process, these hardwired sequencing solutions can drive rework wires, PCB changes, and even major layout rework efforts. Such efforts delay the development process, as designers learn more about the actual performance and sensitivities of overall solution.

By giving analog designers the ability to enable and disable individual POL converters via a digital communications bus through a host controller, hybrid controllers allow designers to forgo hardwiring sequencing choices during initial PCB development. This moves sequencing control to firmware programming and facilitates rapid adjustment of sequencing without PCB changes or rework wiring. Now, designers are able to both test a wide range of sequencing solutions and quickly adapt to evolving system requirements.

Routing hardwired enables can be difficult, especially on today's high-density multi-layer boards with hundreds to thousands of individual components. For example, a hardwired sequencing solution for a system with 12 separate rails could require 11 dedicated supply-to-supply routing lines crisscrossing the entire PCB, or 24 dedicated lines between a common supervisor and the 12 separate POL converters. Using a digital bus with an enable command reduces this tangle of routing to a common two- to four-wire bus, allowing routing, branching and serial connections as demanded by the system layout.

VOLTAGE ADJUSTMENT

In a conventional PWM controller, the output voltage is often programmed via a resistor divider, with the voltage at the common node of the divider being compared to a precision reference voltage and the controller adjusting its operation to drive the two voltages to be equal. This allows designers to select low tolerance, precision resistors to set the very precise voltage levels demanded by high-performance processors, logic, memory, sensors and other circuits. However, this precision resistor divider solution imposes a sometimes undesirable restriction - a fixed voltage set by this divider ratio and the precision reference.

Like sequencing requirements, this solution is often preferred when working with well-defined, mature systems, processors and logic circuits designs to run on standard supply voltages. However, just like with sequencing requirements, when developing new systems, the exact voltages required for higher performance or lowest power consumption are not always well known. Digital adjustment of the precision reference allows system designers a huge breadth of flexible choices during development, qualification and protection phases of the product life cycle.

During development, designers can rapidly test the performance of their system under a wide range of supply voltages, determining the best power solution for their manufactured systems, even testing a vast array combinations of voltages in multi-rail systems to determine the best over-all solution for their application. During the test and qualification phase, the final supply voltages can be margined up or down to ensure design integrity over the expected range of mass production variables. This assures system and quality designers that their design is effective and tolerant to normal variations in the production environment.

During the production phase, firmware can be updated without hardware changes to adapt to changing requirements of processors or even adapted to trim out errors introduced by less expensive, lower precision components or the imperfect selection forced by discrete component selection values.

As designs of digital circuitry become ever more sophisticated and the optimum performance point of each manufactured device changes slightly, digitally programmed output voltages enable system designers to allow the powered device itself to drive the voltage selection. Each powered device is able to make small changes in the voltage provided, allowing these high performance processors, controllers, field-programmable gate arrays (FPGAs) and application-specific integrated circuits (ASICs) to operate at peak performance.

Finally, digital adjustments to the output voltage allow designers the ability to select the specific voltage a single device needs to operate best, as well as actively scale that voltage to its current operating state. This enables the power solution to adapt to the needs of its load dynamically, providing the system designer with the capability of meeting the ideal balance of efficiency and performance - even as that balance changes drastically during a systems operational cycle.

PROTECTION FEATURES

Like many features in standard analog controllers, system protection features such as soft-start, over-current, over-voltage and under-voltage protection are often hardwired, programmed by external resistors and capacitors placed around the controller itself. During development, designers often make their best guesses at what these values should be. Or they set them artificially high to prevent unintentional triggering of these protection features from interfering with initial product development when the exact characteristics of the final system are not well known.

Providing digital access to these features, and even the converters response to the faults, provides designers with a unique ability to ensure proper functioning during product development while testing the target level of these key features. By adding warning levels, non-stopping fault conditions, and digital monitoring of key parameters, designers can access real-time feedback about their design choices and actual system requirements, without the risk of interrupting operational tests due to a poorly set threshold or overlooked fault condition.

With digital power, you can test assembled systems outside their normal fault limits to determine operational design margin, improving system design confidence and reliability.

MONITORING AND REPORTING

Telemetry, or recording key operational states of an entire system, is becoming an increasingly important part of product development and production failure analysis. Solutions exist to monitor, record and report the state of power supplies. They are commonly integrated into digital power solutions or included in external voltage supervisors. However, for economic reasons, supervisors are rarely built to monitor a single voltage rail. Instead, integrating several voltage and current monitors with enable drivers to provide sequencing for two to eight POL converters. Power designers may need to route sensitive, low-voltage analog signals for monitoring voltage and current long distances, or include many more supervisors as system power solutions become increasingly complex. By contrast, hybrid controllers incorporate these monitoring circuits directly into the control IC itself. This drastically limits the routing of sensitive signals and incorporating the analog-to-digital conversion at each converter. The result is improved accuracy and eased routing demands.

ANALOG CORE

So far, we have been focused on the advantages hybrid controllers present to the power designer versus their conventional analog counter-parts, but what advantages do they offer versus a purely digital solution?

Digital power has long heralded the benefits of adaptable, non-linear control algorithms to improve transient response, stability and even adapt to changing or aging power components. However, they have been slow to realize. Developing non-linear digital control loops requires an unusual combination of power expertise and digital programming capability not available in most design groups, and trusting a function as critical as power to unproven or poorly understood control methodologies is risky at best. As a result, most digital power designs emulate conventional analog control methods.

The processing speed and power required to make cycle-by-cycle control decisions often drives designers to lower bandwidths than with analog control. This eliminates much of the promise of digital power for all but the most sophisticated users. Another promise of digital power is one of its greatest weaknesses. Just as digital power allows the control loop to be dynamically adjusted through a simple firmware update, this can easily be a double-edged sword since each firmware update poses the risk of changing the control loop, stability and transient response characteristics of the power converter. This is one of the analog core's greatest strength. Since the error amplifier's loop compensation is fixed once the PCB is populated, the converter's loop characteristics, stability and loop response are well known. Being protected from changes or errors in the software gives the power designer confidence that a software change won't result in an unstable converter and damage to the sensitive devices it powers.

Analog controllers with integrated digital monitoring and communications, or hybrid controllers are not bringing new functionality to the power designer. Instead, hybrid controllers are providing analog power designers the in-circuit adaptability, monitoring and reporting demanded by system designers. This includes the trusted performance of a well-known analog control core, meeting the needs of the next generation of system requirements, without the risk or expense of fully digital power solutions.

REFERENCES

For more information about digital power, visit: www.ti.com/digitalpower-ca.

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