[Leapfrog: First Look]
ICs Boost Next-Generation Power-Supply Features, Flexibility
A 30-MIPS processor, ultra-fast PWM, and an ADC make digital (feedback) power supplies economically viable.
Hailed as the next generation of digitally controlled power-supply ICs, Microchip Technology's Digital Signal Controller (DSC) ICs give power supplies internal digital control. While they aren't the first ICs intended for this purpose, their price tag starts at $2.99 each in lots of 10,000, which is far below any of their predecessors.
These devices are configured within the supply s feedback loop, so speed is critical. That's because the analog-to-digital converter (ADC) and processor must sample the output and react as quickly as possible to any changes. The Digital Signal Controller ICs meet all these requirements.
First, consider the basic internal digital control IC configuration for a switch-mode power supply (Fig. 1). Compared with analog supplies, Digital Signal Controller ICs provide extensive fault monitoring, better transient response, and lower-cost redundancy options.
Also, the digital approach eliminates a power supply's drift and need for temperature compensation. This digital technique can program operational settings, which eliminates the manual tweaking of power-supply adjustments.
External digital control used with the PMBus and Power-One's Z-One system also can set output voltages and monitor power-supply performance. But they require an external power manager or processor and the appropriate power-supply interface. In contrast, Digital Signal Controller ICs can perform the same task primarily using firmware and in some cases sensors.
Digital signal control requires fewer hardware platforms because firmware can control power-supply performance while using the same internal hardware. Plus, this type of control can change topologies and configurations on-the-fly: from buck to boost or boost to buck, as well as from continuous to discontinuous.
About the only necessary change might be to use different external power output stages. External digital control can change a power supply's operating parameters, but it cannot change topologies on-the-fly.
A unique aspect of these DSCs is that their analog comparators can terminate the pulse-width modulation (PWM) signal early. This allows cycle-by-cycle current limiting, which is required for current-mode power supplies.
Additionally, digital signal control enables the implementation of power factor correction (PFC) by adding appropriate firmware and some external hardware. An analog-based supply would require considerably more hardware. And, external digital control cannot provide this capability.
Digital signal control opens the door to extensive creativity, limited only by the available features, the designer s imagination, and the amount of available program memory. For example, this controller can monitor the performance of an individual component within the power supply.
Furthermore, the same basic circuit could be used for uninterruptible power sources (UPSs), power inverters, and digital lighting. This degree of flexibility isn't available with an analog-based supply or external digital control supply.
ECONOMIC VIABILITY To be economically feasible, the DSC IC must cost-effectively provide the necessary high-speed power-supply functions. Besides the IC s price, there are other cost-related issues, such as learning the digital control design philosophy and developing the necessary firmware. But once this learning curve is mastered, a similar design approach can be used for all supplies.
Designers should ask themselves two necessary questions. First, what applications fit DSC-based power supplies? The obvious answer is ac-dc and dc-dc converters. The second question is what power levels make the most economic sense. It all depends on the price-performance characteristics of the DSC IC.
Today, the cost of this controller IC could become a small percentage of the overall power-supply cost at 100 W and above. This could include front-end power supplies, high-power dc-dc converters, and bus converters. In the future, economies of scale could permit use of the DSC IC for power supplies below 100 W.
It now appears that Microchip Technology has the solution with a DSC family that meets the requirements for speed, price, and features. This family includes the dsPIC30F1010, dsPIC30F-2020, and dsPIC30F2023. Virtually, their only external components are in the power output stage (see Fig. 2 and 3 and the table).
These devices are able to coordinate the analog-to-digital sampling point relative to the PWM cycle. Configurable control also allows the analog comparators to react to random events and prevent their effect on power-supply performance.
On-chip multiple PWM circuits enable these devices to function independently without adding to processor overhead. Also, multiple PWM circuits permit their parallel use for different functions, such as voltage regulation and PFC control.
The devices analog comparator can be used to terminate the PWM pulse early, enabling cycle-by-cycle current limiting. The multiple-channel ADC permits the parallel sampling of voltages and current throughout a power supply.
The MPLAB integrated development environment supports these ICs to meet the need for simplified firmware development. This tool includes an assembler, compilers, a visual device initializer, and graphical user interface plug-in options. An evaluation board familiarizes designers with the hardware and firmware in a typical buck converter.
AVAILABILITY Selected devices are available for early adopter sampling. General sampling is expected in July at http://sample.microchip.com. Volume production is expected in August.
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