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Digital power comes of age

THE RELENTLESS MARCH OF DIGITAL TECHNIQUES INTO TERRITORY THAT WAS ONCE EXCLUSIVELY THE DOMAIN OF ANALOGUE CIRCUITRY IS NOW SET TO CONQUER THE POWERSUPPLY MARKET. DEDICATED ICs COMPETE WITH SOFTWAREBASED SOLUTIONS TO WIN YOUR BUSINESS.

After years of research, digital power-conversion techniques are making their way out of the laboratory and into everyday products. According to a recent report from Darnell Group, the portion of the power-supply market that will employ some form of digital-loop control will grow at an average annual rate of 45% for at least the next five years (Reference 1). Over the same period, Darnell estimates, the worldwide ac/dc and dc/dc power-supply market will grow by 10% annually, meaning that the digital- power segment will enjoy five times faster growth that the overall market. The company believes that by 2013, as many as 1.4 billion digital-loop converters will be in everyday use. So, what is a digital power converter, and how do under pressure designers become familiar with this approach to a traditionally analogue design problem?

The term digital power converter often means different things to different people. For instance, one interpretation is a power supply that has a digital interface such as PMBus. The MAX8688 digital power-supply controller from Maxim is a typical example of this interpretation: its chip provides a method of adding programming and monitoring to a PoL (point-of-load) converter whose inner control loop employs conventional analogue circuitry. By contrast, Darnell’s definition refers to a design that replaces the analogue control loop’s op-amps and resistor-capacitor compensation networks with ADCs, DSPs, and firmware—this is the definition that we adopt here.

The potential benefits that digital power conversion offers are compelling. A paper by Ericsson Power Modules compares a conventional analogue 18A synchronous-buck PoL converter—the company’s PMH8918L—with an equivalent design that uses Zilker Lab’s ZL2005 digital controller (Reference 2). It reports that the digital version consistently betters conversion efficiency by over 1% and that its low-load performance improvement peaks at around 3%. Also, the digital version’s noise and transient-response performance is practically indistinguishable from that of a analogue converter. Yet, in the context of a 12V-input, 1.5-to-5.5V-output PoL, the digital version increases power density by 307% while slashing component count by 58%. These statistics promise substantial reductions in cost and size, together with reliability improvements that the company estimates at 11% for this digital implementation.

Proving the digital converter’s power-density advantage, Ericsson’s engineers managed to squeeze 40A out of the original PoL’s footprint, noting, as they did so, that thermal resistance quickly became the limiting factor. Another benefit of the digital converter is the opportunity to integrate DPM (digital-power-management) functions with minimal silicon area. By contrast, analogue converters almost invariably require dedicated support ICs such as the MAX8688, which adds PMBus programmability to virtually any PoL. PMBus is a de-facto power-industry standard that combines SMBus hardware—effectively a more robust version of the I²C bus—with a command set and protocols that suit the needs of embedded power-supply systems (Reference 3).

Ericsson’s 3E evaluation kit allows you to examine the potential that digital techniques offer without having to do any hardware design. The kit consists of a two-channel base board, with each channel providing plug-in receptacles that accommodate the company’s recently released BMR453 intermediate- bus converter and up to three samples of its new ZL2005-based BMR450- series PoLs in a conventional intermediate- bus-architecture configuration. The converter is an isolated full-bridge dc/dc converter that packs almost 400W into the popular 58x37-mm quarter-brick footprint (Figure 1). The converter’s core employs a full-bridge variant of Texas Instruments’ (TI’s) UCD91xx digital-controller family, with Ericsson’s engineers working alongside the silicon vendor to tailor the UCD9125 for the BMR453. In this application, regulation improves from the +4/-9% level that is typical of competing analogue converters to ±2% while increasing efficiency to achieve around 96%—from about 3 to 33A. It’s well worth studying the layout of the BMR453 and PoL circuit boards for examples of best-practice physical layout—the layout being the “missing component” that switch-mode power-converter schematics rarely specify but that is critical to performance.

Providing the USB-to-SMBus interface, the 3E kit’s base board carries a plug-in version of the same adapter that accompanies Zilker’s kits. A CD contains the 3E CMM (configurationmonitoring- management) software that communicates with the board’s converters via PMBus commands, together with system documentation. You provide a suitable power supply and loads to connect to the board’s 4-mm sockets, together with appropriate test instruments. An interesting paper contrasts the history of the development that culminated in the BMR453 by comparing the early prototype with one of the best-performing quarter-brick analogue converters, Ericsson’s PKM4304BPI (Reference 4). In particular, the paper demonstrates that going digital doesn’t automatically improve electrical performance, and that you still need considerable development effort to extract optimal performance over the converter’s operating range.

DSP DRIVES DIGITAL DC/DC

Designers who wish to implement their own hardware basically have two choices: use a dedicated chip, or take a software-driven route with a processor that’s sufficiently powerful to control the converter’s loop responses in real time. Announcing itself as “a great way to experiment and learn about digital power control,” the TMS320C200 digital-power experimenter kit from TI comprises a base board, a plug-in ControlCARD, two CDs, a universal-input wall adapter, a serial-port link cable, and an instruction card. The base board carries a pair of 10A buck-converter modules, with the first converter supplying a pair of 2Ω, 7W resistors in parallel and a software-controlled 1 Ω, 5W switched-load resistor that suits dynamic load tests. The second converter connects to a terminal block and a small light bulb that provides visual feedback. A standalone 3-½ digit DMM with a 0-to-20V range and 10 mV resolution measures the output voltages of both converters via a selection switch. Various headers and jumpers provide for connection to a PC or emulator and determine boot modes for the F2808 controller that resides on the plug-in card.

Other components on the 100-pin, DIMM-format ControlCARD include a 20-MHz crystal, a TPS70102 dual-channel regulator that derives 1.8 and 3.3V for the F2808’s core and I/O from the main board’s 5V rail, and a MAX3221 3.3-to-5V RS-232 level translator that tolerates up to ±25V on its RxD input. Unusually, the RS-232 channel’s path includes an ISO7221 dual digital isolator chip that makes it possible to use a separate 3.3V supply for the level translator. This facility helps alleviate issues with noise and ground currents when connecting to external hardware—TI intends its ControlCARD series for general use, and the company supplies hardware and software support that enables developers to use these cards as supplied or to construct new assemblies that use the same basic schematics.

In this application, it’s the interconnections between the F2808 and the pair of TI’s PTD08A010W SyncBuck modules that is of primary interest to hardware engineers. The modules are nonisolated power stages that typically convert 4.75 to 14V down to 0.7 to 3.6V under the control of a UCD9111/12 or UCD9240 digital power-control IC. These dedicated controllers embody a programmable core that interfaces with PMBus systems to support single- and dual-phase single-channel applications, as well as applications that require as many as four output channels and eight phases. Here, the F2808 implements the control and monitoring functionality for a pair of single-channel, singlephase converters with an effective control range of 0 to 4V. The 9V, 2A-rated offline adapter appears capable of supplying enough current to run channel 1 at full output voltage, which creates an indicated current draw of about 5.2A in the load resistors, which—with a combined rating of 14W and over 20W dissipation— unsurprisingly get very hot.

Each PTD08A010W module occupies a footprint of 18.92x15.75x8.0 mm and comprises a UCD7230 MOSFET driver, a pair of Vishay’s Si7108DN MOSFETs, a 0.9-µH power inductor, and various ceramic capacitors. Four power pins carry input-and outputpower and -ground connections, while a further eight pins apply power and furnish the control and feedback link. The onboard UCD7230 responds to three digital inputs—PWM (pulsewidth modulation), SRE (synchronousrectifier enable), and INH (enable/inhibit)— and returns a single fault line to supervisory logic. Only SRE is nonobvious. When its voltage is high, as it is in this application, the SRE input enables the synchronous rectifier FET and allows the module to source and sink current; when low, SRE disables the sync FET and the module can only source current. Under the control of supervisory logic, this facility allows the driver to ramp the output voltage up and down even when a pre-bias voltage is present on the output-supply rail. The PTD08A010W also provides two analogue-output pins that represent output-current and module-temperature information.

The CDs that accompany the kit comprise a free, 32-kbyte-limited version of TI’s Code Composer Studio for theF28x and a 60-day trial copy of the VisSim Embedded Controls from Visual Solutions, a code generator for use by developers. But unless you have an emulator—which TI does not include in this kit—you can only use the TestDrive GUI that is available from TI’s Web site. You first download and install the baseline-software package for the F2808. This step builds two directories for system hardware and software components such as support files, examples, and documentation; installing the power kit material adds further files to these directory trees, including the TestDrive-GUI. This software requires an RS-232 connection that uses just the RxD and TxD lines at 57,600 baud, and it can appear only marginally stable. Killing all other applications and unnecessary processes on the host Win2K PC resolved this problem.

The first of the GUI’s four tabs accesses the main panel that sets and reads back the output voltages along with current and temperature values. A separate button allows you to turn the outputs on and off using the soft-start/ sequencing facility that the second tab sets. It is also possible to set the PC-to-board update rate, which a blinking LED on the control card affirms. The third transient-response-tuning tab lets you experiment with different values for the PID (proportional integral- derivative) constants that set the feedback- loop’s response. A graph displays a scopelike representation of the system’s dynamic response that the 1-kHz-switched dynamic load stimulates (Figure 2). It is interesting to compare this graph with a real scope trace, which mirrors the graph’s response and also reveals switching breakthrough due to the load-switching IRFR024N’s gate-source capacitance. This exercise provides a feel for the effect of each PID control and its interdependencies, but it would be helpful to include an explanation of the theory that creates the responses that you observe. The fourth tab sets calibration constants to match the individual board’s output levels with a monitor, such as the integral DMM.

Further exploration requires you to install the Code Composer Studio (CCS) software and attach a compatible emulator, such as the XDS510LC JTAG emulator from Spectrum Digital ($249) or Blackhawk’s broadly similar USB510 ($299)—which the company’s UK distributor, Direct Insight, kindly loaned for our evaluation. With the exception of a mysterious message that suggested that the environment requires you to install ActivePERL 5.8 for scripting support that is unnecessary for our purposes, installing the free edition of CCS that accompanied the kit ran flawlessly. Install the emulator’s drivers, set up CCS for a F2808 target and your emulator, then read the Two Channel Buck CCS User Guide that describes how to explore the Code Composer project that appears in the system-software directory. If you’re new to CCS, download and familiarise yourself with its “getting started” document that appears on its Web page.

The first lab exercise builds a program that runs open-loop, returning the input-busbar voltage and the converters’ output voltages, currents, and temperatures to a watch window. In open-loop operation, you have direct access to the PWM modulator and can adjust the PWM constants while observing the output voltage changing— a concept that’s unfamiliar to analogue designers, who are unable to break this converter’s feedback loop. As in other exercises, assemblylanguage files that execute in-line code handle time-critical tasks such as controlling the PWM duty cycle, while C-language routines control and supervise non-time-critical tasks. The guide succinctly describes the program structure, which follows a round-robin model with three CPU timers sequentially triggering processes.

This exercise works correctly providing that you ensure that the project configuration is set to RAM and insert both boot jumpers—and that you have correctly configured CCS. Failure to observe these points creates some creatively unhelpful error messages, such as this one, which repeatedly appeared when you try to run the Go Main command:

Trouble Setting Breakpoint with the Action “Terminate GEL_Go()” at 0x8718: Error 0x00000008/-1076 Error during: Break Point, Cannot set/verify breakpoint at 0x00008718 Breakpoint Manager: Retrying with a Legacy Hardware breakpoint.

The program would then not run. Much trial-and-error revealed that this message stems from a simple failure to include a file called f2808.gel in the set-up procedure. It would be helpful if the instructions that accompany the kit included explanations how to set up CCS correctly for these exercises, or if the exercises included all the necessary dependencies.

The second lab exercise implements closed-loop control using three assembly-language routines that an interrupt-service routine executes at the default 300-kHz PWM rate, with additional library modules furnishing soft-start and output-sequencing functions. Again, a watch window provides user access to programmable parameters and returns results. This exercise also demonstrates one of the graphing capabilities within CCS, with a window opening to display the converter’s output readings in scope-like format. But the major components of interest to programmers are the three core control modules that take an ADC reading every PWM cycle, calculate the PID block’s corrections, and adjust the duty cycle.

Tuning the constants that program the PID block are the subject of the third lab exercise, which uses the same code base as its predecessor. The watch window allows you to enter new constants while CCS graphs the output response as it switches the active load—functionally equivalent to the earlier exercise that used the RS-232 connection and slider controls that Figure 2 shows. Because tuning the loop response properly requires mathematical analysis in a tool such as MathCAD, the software simplifies coefficient selection by reducing its range from five degrees of freedom to three. This results in watch-window entries that allow you to independently change the P, I, and D gains that control an IIR (infinite impulse- response) filter with two poles and two zeroes. The graph display in Figure 3 shows the response using the default low-gain settings. After tuning, which you carry out by iteratively adjusting the constants, should allow you to achieve a response similar to that of Figure 2. Just as in a real application, excessively large increments can stimulate total oscillation that is unhealthy for hardware! Concluding the lab exercises, you verify that the loop remains stable by observing the response while ramping the output voltage on and off at different rates.

Clearly, this software-driven approach offers an unprecedented level of control at the expense of development ease. To use it properly, you need to be an expert C- and F28x-assemblylanguage programmer, be thoroughly familiar with Code Composer Studio —an extremely powerful and hence complex environment—and be able to model PID-loop responses to ensure adequate stability and transientresponse performance, typically in a third-party math tool that provides Bode-plot analysis.

PRAGMAS OR PRAGMATISM?

If the software-coding approach is an interesting intellectual exercise, many hardware engineers will find it easier to apply dedicated ICs such as TI’s UCD9111/12 or UCD9240 that you can program at a higher level via PMBus commands. Supporting such development, TI offers evaluation modules and the free Fusion Digital Power Designer software, which includes features such as gain and phase plots and simulation capabilities for single- and multi-phase buck converters (Figure 4). The software has been evolving, and while there’s still evidence of beta features such as Auto Tune, the latest version—1.5.59—appears stable. Usefully, you can run the program in offline mode to explore the effects of its various controls, or connect to evaluation modules using the USB-interface adapter that accompanies the currently available $49 UCD9112EVM. Also, TI has just announced the UCD9240EVM in a four-phase configuration for $149.

Ease-of-use is a prime objective for Zilker Labs’ mixed-signal chips such as its ZL2006, which implements a voltagemode synchronous-buck converter that you can program using nothing more than pin straps and resistors, but that also offers full PMBus control. Suiting input voltages from 3 to 14V and output levels of 0.54 to 5.5V with 1% accuracy, the 36-pin QFN device integrates MOSFET drivers capable of peak currents as high as 3A alongside a powermanagement subsystem that supports a range of soft-start/stop, sequencing, and voltage-tracking features. Other features include adaptive light-loadefficiency optimisation, current sharing and phase interleaving to support multi-phase configurations, secure nonvolatile memory for storing user-configuration data, and output voltage and current monitoring and protection.

The key to the ZL2006’s flexibility is its PWM loop, which combines analogue and digital blocks that can autonomously and efficiently control the power-conversion process. In this standalone scenario, multimode input pins—which you leave open, tie high or low, or connect to ground via a programming resistor—set parameters such as output voltage, current-sensing method and limits, soft-start-delay and ramp values, sequencing configuration, undervoltage-lockout threshold, and the converter’s switching frequency. Alternatively, the device offers full programmability, including the ability to modify its PID constants using the I2C/SMBus-compatible hardware interface and PMBus protocols. A USBto- SMBus adapter—for which a reference design is available—allows you to control evaluation cards or your own prototype hardware using Zilker’s freely available PC-compatible software.

For our tests, Zilker supplied two ZL2006EV1 evaluation boards and one ZL2106EV1 for the ZL2106—a chip that’s similar to the ZL2006 but integrates the power MOSFETs to suit applications that require up to 6A— together with its USB/SMBus adapter and a resources CD containing software and documentation. Available from the company’s Web site, the PowerNavigator GUI’s group-monitor screen allows you to set output voltages and margins, to sort start/stop delays and slew rates, and to configure tracking and sequencing. The screen returns input and output voltage, output current, and temperature values in analogue format while continuously reporting the duty cycle of each converter. Clicking the “device config” softbutton shows three tabs, initially displaying the “configure device” screen for the device address that you select. This screen echoes the group-monitor screen’s configuration abilities but adds settings for under- and overcurrent fault limits, switching frequency, and global fault responses. The second tab—file I/O—lets you load and store a configuration file that consists of PMBus commands.

But the true extent of the software’s configuration abilities becomes apparent when you click on the “PMBus commands” tab that controls every PMBus command that the device implements. Six sub-tabs access entry screens for basic commands, fault-limit settings, monitor-configuration and status data, advanced settings, settings for devices that implement the DDC (display-data-channel) bus, and PMBus-command storage. These interfaces support complex set-up capabilities— for instance, the “basic settings” page accesses basic output parameters, power-management parameters, output- current calibration, and operational modes (Figure 5). Here, you can set output-voltage parameters, including margins, independently of one another, whereas the “group monitor” and “configure device” screens automatically adjust margins around the voltage that you set. Other general settings include switching frequency, maximum duty cycle, and soft-start/stop parameters. This screen also allows you to enter new PID constants in numeric or analogue-like formats—see sidebar “RC goes PID”. You can override the algorithm that dynamically determines the deadtime between the control and sync MOSFETs switching by setting your own static values in 4-nsec increments. This, potentially, can improve efficiency by over 1% compared to an analogue converter’s static setting. As Ericsson’s engineers report for the broadly similar ZL2005 (Reference 2), it is possible to use these controls to match the converter’s performance to alternative MOSFET types in order to gain clear efficiency advantages.

MORE OPTIONS

In May 2008, Zilker and Fairchild announced a joint partnership agreement to manufacture and market digital- power-converter products. Since that time, Fairchild has launched its first digital PoL products, all of which employ Zilker’s proprietary Digital-DC power-conversion architecture and use the same development tools. As a result, Fairchild provides a useful second source for the FD1505, FD2004, FD2004-1, FD2006, and FD2106, which mirror Zilker’s product numbering system. Infineon, too, is getting involved in the digital-power market, in this case by purchasing fabless design house Primarion in April 2008. Primarion has been in existence since 2000 and offers families of parts that target general-purpose PoL, PC-VRM (voltage-regulatormodules), graphics-processor, and memory- system power supplies.

Other companies that offer digitalconverter silicon include CHiL Semiconductor. Formed as recently as 2006, CHiL is a fabless concern that specialises in designing mixed-signal digital-power products that currently include the CHL8100, a four-phase buck converter, and the companion CHL8500 driver, which targets the PC market. Interestingly, the company also holds several patents for ac/dc- and dc/ac-conversion techniques that focus on resonantconverter topologies that typically find use in high-power offline converters and some inverters.

A more familiar but late entrant into this marketplace, Analog Devices’ first digital-converter IC is the ADP1043. The preliminary datasheet shows that this device is a secondary-side controller that targets isolated ac/dc and dc/dc power supplies, and that are appropriate for use in offline and intermediate-busconverter applications. The chip operates under microcontroller supervision via an I2C port that supports all programming and monitoring functions. Crucially—and unlike most dedicated chips—the ADP1043 supports multiple topologies, including single-ended, active- clamp, and interleaved forward converters; half- and full-bridge designs; and two-stage configurations that cascade a buck-converter pre-regulator ahead of the full bridge to gain a possible 1% efficiency increase (Figure 5). Intriguingly, the company’s presentation promises a graphical development environment that requires no programming expertise. The ADP1043FB100- EvalZ evaluation kit, which implements an isolated 100W full-bridge converter, was not available by press time, but should be—for $300—by the time that you read this. The similarly priced ADP1043IF300-EVALZ, which demonstrates a 300W offline converter, is also available since October 2008.