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Industry:

Telecommunication, Computer, Industrial, Automotive

Networks Application Note
Industry:
Telecommunication, Computer, Industrial, Automotive
Application:
DC-DC Converters in Power Supplies
Statement of the Problem:
Designers of electronic systems are challenged to provide circuits with improved efficiency and the ability to handle greater currents and higher voltages while reducing current leakage and decreasing the board space required for these circuits. At the same time, purchasing groups strive to reduce the list of suppliers on the approved vendor list and to ally with strategic suppliers focused on meeting their global requirements. Bourns offers a range of passive components for the designer of linear and switched mode power supplies. Chip resistors, chip diodes, and inductors are just three of these product lines. Further, Bourns is a global supplier of components working with companies throughout the world. The Design Challenge: The DC-DC converter is an integral element of power supplies. Low-value current-sense resistors, diodes and inductors are three associated components whose correct selection will optimize performance.
Input DC Voltage
The principle of operation of a DC-DC converter is shown in Figure 1. In its most basic form, the circuit uses a capacitor and an inductor as energy storage elements to transfer energy from input to output in discrete packets. Feedback circuitry regulates the energy transfer to maintain a constant voltage within the load limits of the circuit.
Reference
Oscillator Input
Bourns® Model SDR0805 Inductor Bourns® Model CD1005-B0034 Schottky
Regulated Output
Rsense Bourns® Model CRL2010-FU-R051E
In Figure 1, the control technique is a peak inductor current limit. As soon as the output voltage goes out of regulation, Figure 1: Schematic of a DC-DC Converter the switch (transistor) turns on until the current reaches the programmed current limit, set with a low ohmic resistor (Bourns type CRL) in the inductor (Bourns type SDR) current path. The Schottky diode (Bourns type CD) prevents the capacitor from discharging through the switch to ground. When the inductor current reaches the programmed limit, the switch turns off for a time constant. At the end of the feedback constant, the feedback circuit compares Vout to the regulation voltage and either turns the switch on again (if Vout remains out of regulation) or holds the switch off until Vout falls out of regulation.
Solution:
Supplier Vishay KOA Yageo
Part Number(s) CRCW2010 SR732H RL2010
Bourns® Compatible CRL2010 CRL2010 CRL2010
The Solution:
Chip Diode During the "off time" of the transistor, the diode is forward biased and charges the capacitor C1. The inductor supplies the current. To improve the efficiency of the DC-DC converter, the power loss through the diode should be reduced by using a device with a minimum forward voltage (Vf). The power loss will be If x Vf. Schottky Barrier diodes, such as Bourns® CD1005 series, have lower forward voltages than PN junction diodes. For example, CD1005-B00340 has a Vf of only 0.37 V. During the "on time" of the transistor, the diode is reverse biased and prevents the charged capacitor from discharging to ground through the transistor. The output voltage of the DC-DC converter is provided by the capacitor. Bourns supplies a wide range of switching, Schottky, Schottky rectifier and TVS diodes for telecom, industrial, computing and automotive applications. Among the distinguishing features of Schottky barrier diodes for power supply applications are low forward voltages, flat leadless packages and lead free terminations. Furthermore, the 1005 size (1.0 mm x 0.5 mm) package can be a drop-in replacement for industry standard SOD-323 and Micro-melf packages. The following table provides cross reference information between the Bourns® Schottky barrier diode CD1005-B00340 and other models on the 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
The Solution:
Inductors When the switch closes, the input voltage is impressed across the inductor. Because the input voltage is DC, current through the inductor rises linearly with time at a rate that is linearly proportional to the input voltage divided by the inductance. Assuming a fixed inductor peak current, the inductor selection is straightforward size the inductor core to meet the fixed limit. Assuming an output average current of 2 A, there are several versions of the SDR0805 SMT power chokes which offer saturation currents of 6-7 A. The saturation current begins the area of non-linear performance. The power choke that is chosen should have a saturation current rating much greater than the current limit of the regulation circuit. The inductor and capacitor also act as a low pass filter removing any harmonics introduced by high frequency switching and attenuating of the voltage ripple at the output. Generally, switching frequencies are in the kilohertz range depending on the capability of the power transistor to turn on and off at high speeds. Furthermore, high frequencies enable the use of a smaller inductor and capacitor. The following table shows a cross reference for the SDR0805 power choke.
Supplier JW Miller Sumida Central Tech Dale & Vishay API Delevan
Part Number(s) PM75 CD75 CTGS75 IDCP-3020 PD75
Bourns® Compatible SDR0805 SDR0805 SDR0805 SDR0805 SDR0805
Bourns supplies a wide range of precision low ohmic resistors, magnetic and chip diodes for power supply applications. Bourns® products include surface mount chip inductors, power chokes, chip beads, chip bead arrays, as well as radial-leaded and axial-leaded inductors. These components are used in computer, communication, instrumentation, industrial and medical applications.
Please contact your local Bourns Application Engineer or Bourns Sales Representative for additional information.
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