Industry: Telecommunication, Computer, Industrial, Automotive

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Designer's Guide Chip Resistors, Chip Diodes, Power Chokes Power Supplies DC-DC Converters
Industry:
Telecommunication, Computer, Industrial, Automotive
Application:
DC-DC Converters Power Supplies
Statement Problem:
Designers electronic systems challenged provide circuits with improved efficiency ability handle greater currents higher voltages while reducing current leakage decreasing board space required these circuits. same time, purchasing groups strive reduce list suppliers approved vendor list ally with strategic suppliers focused meeting their global requirements. Bourns offers range passive components designer linear switched mode power supplies. Chip resistors, chip diodes, inductors just three these product lines. Further, Bourns global supplier components working with companies throughout world. Design Challenge: DC-DC converter integral element power supplies. Low-value current-sense resistors, diodes inductors three associated components whose correct selection will optimize performance.
Input Voltage
principle operation DC-DC converter shown Figure most basic form, circuit uses capacitor inductor energy storage elements transfer energy from input output discrete packets. Feedback circuitry regulates energy transfer maintain constant voltage within load limits circuit.
Reference
Oscillator Input
Bourns® Model SDR0805 Inductor Bourns® Model CD1005-B0034 Schottky
Regulated Output
Rsense Bourns® Model CRL2010-FU-R051E
Figure control technique peak inductor current limit. soon output voltage goes regulation, Figure Schematic DC-DC Converter switch (transistor) turns until current reaches programmed current limit, with ohmic resistor (Bourns type CRL) inductor (Bourns type SDR) current path. Schottky diode (Bourns type prevents capacitor from discharging through switch ground. When inductor current reaches programmed limit, switch turns time constant. feedback constant, feedback circuit compares Vout regulation voltage either turns switch again Vout remains regulation) holds switch until Vout falls regulation.
Solution:
Ohmic Chip Resistors sensing voltage, (Vsense), used determine inductor current, given following equation: Vsense R-sense Typically Vsense will save power same time keep level high enough above noise. sense average output current, example, R-sense must mV/2 amount ripple voltage allowed output-regulated voltage will determine allowable error resistance value required. Therefore, precision voltage increases, allowable error resistance value decreases. most significant contributors error resistor's value initial tolerance temperature coefficient resistance (TCR). Initial tolerance accuracy resistance value supplied manufacturer. Commonly available values have initial tolerance causes value resistance vary with change temperature. change temperature resistor occur from causes: change ambient temperature self-heating power dissipation resistor. ambient temperature vary causes such heating board components near resistor increase decrease temperature around resistor. Self-heating occurs power dissipated resistor response current flowing through resistor. Resistors have Derating Curves from which their self-heating determined. example, Bourns® 2010 device power rating watt 70°C, derating zero watts +125 self-heating determined equation: (125 (Power rating (Power rating +125
Rate self-heating
CRL2010, self-heating rate 0.50 watt) watt. amps flowing through resistance creates watts. case CRL2010, application watts will create self-heating (0.2 watts °C/watt).
CRL2010-FW-R051E ±200 PPM/°C) used application which maximum error operating temperature will CRL2010-FW-R051E remain within allowable error? Again, three sources error are: Initial tolerance, error change ambient temperature error self-heating. Initial tolerance Error ambient temperature: PPM/°C 11000 Error self-heating: °C/watt watt PPM/°C 4400 0.44 Maximum error 0.44 2.54 Therefore, CRL2010-FW-R051E would produce worst-case error almost half desired amount. see, straightforward calculate error budget resistor when given self-heating been calculated. series from Bourns thick-film chip resistors ranging from 0603 (0.06 0.03 inch) 2512 (0.25 0.12 inch) size, from 0.01 9.10 tolerances. maximum power rating watt 2512 size package. table below shows cross reference competitors' models:
Supplier Vishay Yageo
Part Number(s) CRCW2010 SR732H RL2010
Bourns® Compatible CRL2010 CRL2010 CRL2010
Solution:
Chip Diode During "off time" transistor, diode forward biased charges capacitor inductor supplies current. improve efficiency DC-DC converter, power loss through diode should reduced using device with minimum forward voltage (Vf). power loss will Schottky Barrier diodes, such Bourns® CD1005 series, have lower forward voltages than junction diodes. example, CD1005-B00340 only 0.37 During time" transistor, diode reverse biased prevents charged capacitor from discharging ground through transistor. output voltage DC-DC converter provided capacitor. Bourns supplies wide range switching, Schottky, Schottky rectifier diodes telecom, industrial, computing automotive applications. Among distinguishing features Schottky barrier diodes power supply applications forward voltages, flat leadless packages lead free terminations. Furthermore, 1005 size (1.0 package drop-in replacement industry standard SOD-323 Micro-melf packages. following table provides cross reference information between Bourns® Schottky barrier diode CD1005-B00340 other models market.
Supplier Rohm Panasonic Panasonic Philips ON-Semi GS/Vishay
Part Number(s) RB751V-40 MA2J728/MA728 MA2J732/MA732 1PS76SB40 RB751V40 SD104XWS
Bourns® Compatible CD1005-B00340 CD1005-B00340 CD1005-B00340 CD1005-B00340 CD1005-B00340 CD1005-B00340
Solution:
Inductors When switch closes, input voltage impressed across inductor. Because input voltage current through inductor rises linearly with time rate that linearly proportional input voltage divided inductance. Assuming fixed inductor peak current, inductor selection straightforward; size inductor core meet fixed limit. Assuming output average current there several versions SDR0805 power chokes which offer saturation currents saturation current begins area non-linear performance. power choke that chosen should have saturation current rating much greater than current limit regulation circuit. inductor capacitor also pass filter removing harmonics introduced high frequency switching attenuating voltage ripple output. Generally, switching frequencies kilohertz range depending capability power transistor turn high speeds. Furthermore, high frequencies enable smaller inductor capacitor. following table shows cross reference SDR0805 power choke.
Supplier Miller Sumida Central Tech Dale Vishay Delevan
Part Number(s) PM75 CD75 CTGS75 IDCP-3020 PD75
Bourns® Compatible SDR0805 SDR0805 SDR0805 SDR0805 SDR0805
Bourns supplies wide range precision ohmic resistors, magnetic chip diodes power supply applications. Bourns® products include surface mount chip inductors, power chokes, chip beads, chip bead arrays, well radial-leaded axial-leaded inductors. These components used computer, communication, instrumentation, industrial medical applications.
Please contact your local Bourns Application Engineer Bourns Sales Representative additional information.
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"Bourns" registered trademark Bourns, Inc. U.S. other countries. COPYRIGHT© 2004, MADE USA, BOURNS, INC. 05/04 e/N0406

Industry: Telecommunication, Computer, Industrial, Automotive

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