One of the opening technical sessions on Thursday, September 11, at Darnell’s Energy Summit focused on issues related to the digital control of ac-dc power supplies and dc-dc converters. Among the presenters were representatives from Cirasys, Inc., Analog Devices, Microchip Technology, Inc. and Ericsson Power Modules. “Short-Cutting the Digital Converter Design Process with Advanced Control Methods,†was addressed by Paul Gregory, President, Cirasys, Inc.
“As a power controls company, Cirasys is commercializing modern, nonlinear methods designed to solve many of the issues inherent in nonlinear converters. For example, our Input Output Linearization (IOL) method delivers dramatic improvements in the design cycle and in the performance of buck, boost and buck-boost converters. But the greatest impact is on boost and buck-boost, bringing them up to the performance level and design simplicity of bucks,†Gregory stated.
“Since IOL incorporates feedback and an exact model of the converter plant, no special compensation design is required, and all operating points are available with the same control design and with identical performance. Control design is in fact somewhat simpler than current buck designs. Other attributes of IOL control include: Greatly improved stability and very wide operating range, Very high bandwidth, not limited by averaged model/small signal, and Fast transient response. As a result, IOL controlled boost and buck-boost converters are as simple to design as linear converters and deliver comparable performance,†he continued.
Modern controls like IOL have the ability to level the topology playing field and give options back to the design and system engineer. Rather than dealing with right half plane zeroes and complex poles and making compromises on stability, performance, and cost, they can take advantage of a purpose-built control method designed to take these effects into account and require only the input of operating parameters like desired voltage and current. Not only is the design process greatly simplified and shortened, but performance is also improved without sacrificing other features – a welcome advance in a time of increasing customer pressure.
Finally, with intelligent modern control, the converter itself can evolve into a much more capable device, one with abilities never seen before. For instance, since IOL uses an exact model, not a linear approximation, it means that any and every possible operating point supported by the physics of the plant can be selected at any time. This allows “on-the-fly†voltage switching on either the input or output, and effectively lets the converter perform the additional functionality of a regulator or output tracker – all done by the control firmware.
“This opens the door to a whole new class of converter products that can operate in variable voltage environments and offer inherent stability. An example of this is realizing a decades-old concept of affordable and accurate envelope tracking of power - the ability to deliver only the power precisely needed at any particular moment. Motor control, radio and audio amplifiers, and even batteries can benefit from a converter that can quickly track fluctuating loads and deliver precise levels of voltage,†Gregory concluded.
During the same session, Kevin Huang, a Power Applications Engineer with Analog Devices presented a “ High Performance Isolated DC/DC Converter Implementation Using Digital Control .†Huang reviewed the benefits of using the company’s ADP1051, an advanced digital controller with a PMBus interface targeting high density, high efficiency dc-to-dc power conversion. This controller implements voltage mode control with high speed, input line feed-forward for enhanced transient and improved noise performance. The ADP1051 has six programmable pulse-width modulation outputs capable of controlling most high efficiency power supply topologies, with added control of synchronous rectification. The device includes adaptive dead time compensation to improve efficiency over the load range, and programmable light load mode operation, combined with low power consumption, to reduce system standby power losses.
Huang reviewed several features that can be implemented with the ADP1051 to enable a robust system of parallel and redundant operation for customers that require high availability or parallel connection. The device provides synchronization, reverse current protection, pre-bias startup, accurate current sharing between power supplies, and conditional overvoltage techniques to identify and safely shut down an erroneous power supply in parallel operation mode. This controller is based on flexible state machine architecture and is programmed using an intuitive GUI. The easy to use interface reduces design cycle time and results in a robust, hardware coded system loaded into the built-in EEPROM. According to Huang, the small size (4 mm × 4 mm) LFCSP package makes the ADP1051 suited for ultracompact, isolated dc-to-dc power module or embedded power designs.
Turning to ac-dc power supply designs, Andreas Reiter with MicroChip discussed, “Implementing Adaptive Algorithms for Enhanced Efficiency in Fully Digital-Controlled AC/DC Power Supplies.†During his talk, he reviewed the flexibility and power of SMPS dsPIC® Digital Signal Controllers in switch-mode power supplies. The design that was the focus of the presentation has a peak efficiency of 94.1% and achieves the ENERGY STAR CSCI Platinum Level. It features a 2-phase interleaved power factor correction boost converter followed by a 2-phase interleaved two-switch forward converter with synchronous rectification.
Specifications of this digital-controlled ac-dc power supply include: Input of 90-264Vac, 47-63 Hz; Output Voltage of 11.94 – 12.1 Vdc; Output Current of 60A; Output Power Rating of 720W; Line Regulation of +/- 0.5% (max); and Load Regulation of +/- 1% (max). This design is implemented using two dsPIC33F “GS†digital-power DSCs from Microchip that provide the full digital control of the power conversion as well as all system management functions. As shown during the paper presentation, the dsPIC33F ‘GS’ devices enable designers to easily and cost effectively develop products using advanced adaptive control algorithms that help improve efficiency at light loads.
Key features of the resulting ac-dc power supply include: Full digital control; Standard 1U form factor; 20 ms minimum hold-up time to compensate drop-outs during UPS step-in; Parallel operation, including load/current sharing capabilities; Hot-plug capability for easy maintenance during operation; EMI/EMC, which satisfies EN55022, Class B; Under/Over voltage lock-out; Sustained short-circuit protection; Over-temperature shutdown; Power de-rating; and Adaptive control.
Closing this session, Andreas Larsson, R&D Head Designer with Ericsson Power Modules gave a preview of the company’s new, “ Automated Optimal Phase Spreading for POL Arrays,, that will be formally announced at the end of November. The new on-line design tool will help system designers to synchronize multiple POL converters and provide a controlled phase offset for each POL. Larsson pointed out that finding the optimal phase spread requires; first, finding phase offsets for Lowest ICin,RMS (this enables the minimization of EMI, Cin size & stress, power dissipation) and, second, find the phase offsets for Lowest dVinpp (thereby improving regulation). This turns out to be a complex problem. Evaluating all possible offset combinations is not generally possible as N (number of Rails) increases. Ericsson has developed a proprietary algorithm that employs uses smart sorting and evaluation methods to identify optimal solutions.
