Embedded Systems Design Europe - August/September 2008 - (Page 25)

psu design Commercial converter characteristics. Manufacturer Model TRACO POWER XP Power RECOM C&D Technologies BOURNS RECOM Table 1 Vi (V) 12 12 12 12 12 12 Vo (V) 3.3 3.3 3.3 5.0 3.3 3.3 Io (A) 1.2 1.2 0.7 0.4 3.0 1.0 Ii (mA) Io=0. 20 38 21 40 11 7 η (%) 77 83 65 60 93 81 Isolation Yes Yes Yes Yes No No TEN 5-1210 JCA0412S03 RW-123.3S HL02R12S05 MX3A-12SA R-78A3.3-1 supply circuits operate in a hostile environment that requires high inputto-output isolation and low quiescent current in standby mode. That combination presents design difﬁculties, because the power consumption in operating mode is much greater than in idle mode. Few if any modules are commercially available for this purpose, due to the necessary trade-offs between isolation and low power consumption. To address such issues, described here is an isolated, switched-mode power supply intended for wireless devices. It accepts a nominal 12-V input and delivers an isolated 3.6-V output, and its quiescent current draw is among the lowest available. Designed as a power supply for EGSM (Extended GSM), WiFi, and ZigBee communication modules, it also provides remote control for electromechanical actuators and electronic sensors in harsh environments. INCREASING AUTONOMY Because the main characteristic of a power supply for wireless devices is battery life, the main design goal was to reduce power consumption while maintaining performance in the radiofrequency system. Thus, care was taken that the autonomy of the wireless-communication device increased when these conditions are present: depends on the transmission system. The second can be obtained with a switched-mode power supply, and the third depends directly on power consumption in the DC-DC converter itself. You must take extra care, therefore, to minimize the current draw under no-load conditions. As a result, we stress this third point in recommending design techniques for optimizing the system. Discontinuous transmission and reception Because transmitters and receivers consume the most power in a wireless device, many such devices implement discontinuous transmission/reception to optimize the resources of the air interface and the efﬁciency of the communications link. Discontinuous operation also helps to reduce power consumption, because the radio’s active elements are not continuously on. On the other hand, discontinuous transmission introduces voltage ripple and current peaks in the power supply. The level of bias voltage affects transceiver performance directly, and a drop in supply voltage degrades radio performance, mainly at extremes of the voltage range. This degradation can make it difﬁcult to meet the applicable speciﬁcations for certifying a wireless device. Battery life cycle and discharge characteristics are also sensitive to current peaks in a load, if the system is powered by a secondary cell. 4 niques described in a paper by Jose Ignacio Garate and others. The voltage is regulated with a DC-DC converter operating in linear or switched mode. Regulation is not only necessary to reduce voltage ripple, but also to reduce EMC problems and maintain performance of the radio. 5 Topology of the power supply for high efﬁciency Power-supply efﬁciency is important, so a switching topology offers the best power-supply option. However, the DC-DC converter modules listed in Table 1, which are typical of those commercially available, don’t meet the requirement we’re looking for: ultralow power consumption under noload conditions. Even the nonisolated converters draw a relatively high current with no load. Taking this data into account, we therefore set a goal of 12 mA maximum for the DC-DC converter’s noload current. To achieve that goal, we distinguish between standby current and quiescent current as follows: • • • • • Quiescent current is the supply current necessary to maintain a regulated supply voltage under no-load conditions. Standby current is the supply current drawn when the system is not producing a regulated output voltage. Discontinuous transmission and reception. Filtering or regulation of the supply voltage. A high-efﬁciency power-supply topology. The ﬁrst of these characteristics Filtering or regulation of the power-supply voltage The supply voltage can be ﬁltered with a high-valued capacitor or other tech- Finally, you may need to add galvanic isolation to ensure efﬁcient protection for devices designed to operate in hostile environments. DESIGN GOALS AND ISSUES In the design of a power supply for 25 www.embedded.com/europe | embedded systems design europe | AUGUST – SEPTEMBER 2008 024-025-026-027-028-029_ESDE.indd 25 28/08/08 11:58:34 http://www.embedded.com/europe

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Embedded Systems Design Europe - August/September 2008 Contents TI Overhauls DSP Lineup, Adds 15 Processors QNX Publishes Source Code for File System Congatec to Take on Proprietary Market Swiss Multicore Project Wins Microsoft Grant OpenCores Bundles Development Tool ARM Compiler Boosts Freescale i.MX31 LabVIEW Updated for Multicore and Wireless Cover Feature: Interactive Tool Supports Multiprocessor SoC Design Wanted: Benchmaking for Embedded VMM Hypervisors Graphical Design Empowers Spider Robots Building a Power Supply for Discontinuous Transmission Wireless Networks RTOS Selection & Best Practices Achieving Cache Coherence in a MIPS32 Multicore Design New Products Advertising Contacts

Embedded Systems Design Europe - August/September 2008

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