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The Benefits of Digital Power Conversion Using DSCs

Tue, 08/31/2010 - 5:05am
Magal Srinivasprasad and Mohammad Kamil, Microchip Technology
Magal SrinivisprasadMohammad KamilPower supplies have been an integral part of many telecom, consumer and industrial products for many years. The power supplies in these products must work to the peak of their performance, without compromising on extensive reliability to ensure proper end-equipment functionality. Power-supply designs have transitioned from linear to switch-mode, to improve the performance and efficiency, and decrease the cost. In order to keep up with the technological advancements while providing performance and reliability, power supplies need to be digitally intelligent. This has caused a paradigm shift to digital power conversion and power management. Therefore, from the product lifecycle perspective, digital power supplies play a vital role. Research has shown that decisions made during the design phase determine 70% of the product's costs, while decisions made during production only account for 20% of the product's costs.

This article describes the benefits of using Digital Signal Controllers (DSCs) in digital power conversion and power management, during the design phase of the product lifecycle.

Figure 1 show the typical product lifecycle. The first phase is a Requirement phase, where customer wish lists are converted to marketing requirements and then to product requirements. The second phase is a Design phase as shown in Figure 2, where circuit design, simulations, board layout, software architecture and coding are done. In the third phase, the product Prototype is built. The fourth phase involves software/hardware integration and testing of the product. In the fifth and sixth phases, the product is produced and commissioned. 

Figure 1: The Product Lifecycle of a Digital Power Supply



Figure 2: The Design Lifecycle of a Digital Power Supply

Requirements of Digital Power Supplies
Power-supply requirements are becoming more challenging every day, due to various added features, without increasing the cost and size. Energy-efficiency initiatives, in the form of government regulations, demand higher efficiency and noise-free power supplies. This is combined with the end customer’s demands for higher reliability and hassle-free operation. Manufacturers want higher yields in production by reducing the variety and number of components with design-to-cost objectives. To achieve better production yield and target costs Design For Manufacturability (DFM), Design For Testability (DFT) and Design For Serviceability (DFS) are also important.

When the above requirements get translated to design requirements, along with input/output specifications, the results include:

• Increasing power density by increasing switching efficiency and reducing component count
• Implementing in-rush current control and Power Factor Correction
• Multiple output voltages and adaptability to load changes
• Output voltage sequencing with power management
• Remote monitoring and control capability
• Paralleling of outputs for N+1 redundancy
• Reduced effect of component tolerance/drifts and End Of Life (EOL) prediction, to achieve higher reliability & performance
• Protected Intellectual property

Design and Development of the Digital Power Supply
This phase consists of Hardware and Software design and development. Hardware design involves selecting the hardware architecture, component selection and developing circuit diagrams. Hardware architecture, for power supplies, means deciding on the power-conversion topology or combination of topologies, such as half-bridge, Full-bridge, Push-Pull, etc. For example, the Phase Shift Full Bridge Zero Voltage Transition (ZVT) conversion topology is best suited to minimize switching losses and improve the efficiency in higher-wattage power supplies. After the control strategy and power architecture are finalized, simulations are carried out for the proof of concept. Results from these simulations act as the basis for selecting critical components. While selecting components, cost, switching frequency and form factor play a major role.

Selecting a Digital Signal Controller (DSC)
A compact power supply with increased power density requires the converter to switch at a higher frequency and achieve higher efficiency. This, in turn, requires innovative topological architectures, such as a variable-frequency resonant converter, to achieve zero voltage and current transitions. Different topologies require different PWM modes to be supported by the DSC. To meet these requirements, the DSC should have flexible Pulse Width Modulation (PWM) modes, with a high-speed, high-resolution PWM that can operate at a high switching frequency.

The DSC must also have a fast Analog-to-Digital Converter (ADC), with flexible data conversion modes such as simultaneous, Synchronous/Asynchronous sampling to optimize the control loop. High-speed analog comparators built-in into the DSC help to detect faults without any delay and take corrective actions quickly. A peak current-mode control algorithm requires high-speed analog comparators with a built-in Digital-to-Analog-Converter (DAC) reference. To enable efficient power-supply operation, Intelligent Power Peripherals (PWM generator, ADC and High-Speed Analog comparator) should be tied to each other without the intervention of the CPU.

Hardware Design
For better utilization of available energy sources, the power supply should have Unity Power Factor (UPF), hence input current should be in phase with the input voltage and Total Harmonic Distortion should be near to unity. A single DSC can be used to achieve UPF and secondary output voltages, thus eliminating the cost of a separate controller to achieve UPF.

Passive in-rush current control reduces overall efficiency, hence it is desirable to have active in-rush current control to maintain high efficiency and also reliability. This can be achieved by implementing peak current mode control of the input-capacitor charging current, using Intelligent Power Peripherals.

For trouble-free operation of the end system, properly sequenced multiple outputs are needed. This can be achieved using one DSC, without adding any external sequencer circuits.

Adding communication features, remote monitoring & control and thermal management to an analog power supply requires an additional microcontroller and many discrete components. On the other hand, the same DSC that is used for power conversion can be also be used to manage the above functions, saving cost and board space.

High-voltage/current tracks and control signals should be properly routed on the PCB to avoid cross talk. The digital pin remapping feature on some DSCs helps in optimizing PCB layout.

Software Design
The Software Design phase consists of developing the control loops needed for the power conversion. Here, complex compensation circuits are replaced by software loops. Self calibration, communications, power and thermal management features are done in software. To achieve optimum control-loop gain and phase margins, execution of control loops is given highest priority. Time available after the control loops is used for other tasks. Therefore, the execution speed of the DSC becomes very important, and deterministic interrupt response plays a major role in optimizing the power-supply performance. In digital power supplies, control loops are software-based; additionally, tuning is relatively easy and optimum stability points are achieved very quickly without re-spins of the PCB. Ready-made peripheral libraries and code examples from semiconductor manufacturers come in handy and save development cycle time.

Achieving Design to Cost Objectives
DFM, DFT & DFS become key elements during the Design and Development stage. DSCs help in developing value-added features at a reduced cost

Along with other DFM guidelines, it is very important to reduce the number and types of components. DSCs with Intelligent Power Peripherals reduce the usage of external components.

Software features can be written to introduce a self-testing feature, which consists of JTAG boundary-scan tests or a self-test capability. This helps to identify the location of faults and reduces the overall testing time.

Service costs can be minimized with the help of software that includes a self-calibration feature, which helps to nullify the component errors from aging by adjusting the control co-efficient. Software features can also help determine end of life. Communication features help in reporting faults and status, remotely.

Summary
In summary, low-cost DSCs with Intelligent Power Supply Peripherals have features finely tuned to the demands of the new age of intelligent power-supply needs. Digital-power control techniques enabled by DSCs provide advanced capabilities that enable high efficiencies at low cost.
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