Embedded Systems Design Europe - August/September 2008 - (Page 26)

psu design portable or wireless devices, three key technical requirements should be taken into account: very low no-load power consumption, isolation, efﬁciency, and size. Issues that arise in meeting these requirements involve the correct isolation conﬁguration, the control method, and the topology of the feedback loop Isolation Isolation between input and output is achieved with a transformer, and for inverting or ﬂyback conﬁgurations, the energy is stored in the transformer’s inductance. The problem, then, is how to provide feedback from the secondary to the primary. The majority of systems use an extra auxiliary winding or optocoupler for that purpose. An auxiliary winding, however, increases the complexity while not providing enough outputvoltage precision for low outputs and variable loads. When the system is in regulation, an optocoupler needs constant current through the primary-side LED. To optimize the system, this current has been minimized as much as possible, as Figure 1 shows. The minimum limit is set by a reduction of the optocoupler’s current transfer ratio (CTR) at low current (63% at 10 mA and 22% at 1 mA), and a reduction of speed (2 µs at 20 mA and 6.6 µs at 5 mA). We must add yet another limitation, which is the minimum current (I = 100 µA) that must ﬂow from the error comparator through the precision shunt regulator TLV431. For the resistive divider connected to the shunt regulator’s output (R131 and R137), high-value resistors have been selected to minimize the current draw. You must take care to compensate for possible delays caused by the input current (I = 0.5 µA) and the input capacitance (this problem can be solved using capacitive dividers). The output ﬁltering capacitor (C47) is large, so you may need to choose a low-ESR electrolytic type kmin ref (such as tantalum, Os-Con, and organic aluminum). If so it must also be a low-leakage type, because current leakage can be signiﬁcant, especially at higher temperatures. (For a 16-V Kemet T495 100 µF capacitor, I equals 16 µA at 25°C and 160 µA at 85°C.) L 26 AUGUST – SEPTEMBER 2008 | embedded systems design europe | www.embedded.com/europe 024-025-026-027-028-029_ESDE.indd 26 28/08/08 11:58:55 http://www.embedded.com/europe

Table of Contents Feed for the Digital Edition of Embedded Systems Design Europe - August/September 2008

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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