Resistors, Diodes, Chokes For Power Supplies Converters

Resistors, Diodes, Chokes For Power Supplies Converters
Designer’s Guide to Chip Resistors, Chip Diodes & Power Chokes
For Power Supplies & DC/DC Converters
APPLICATION NOTE
INTRODUCTION
CRL2010
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 their approved vendor list and to ally with strategic suppliers focused on meeting
their global requirements. Bourns offers a range of passive components for designers 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
CD1005
SDR0805
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. 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.
In figure 1, the control
technique is a peak
inductor current limit.
As soon as the output
voltage goes out of
regulation, 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.
Reference
Oscillator Input Input DC Voltage
S
R
U1
Bourns® Model SDR0805 Inductor
Bourns® Model CD1005-B0034 Schottky Barrier
Q
Q
Regulated
Output
Rsense
Bourns® Model CRL2010-FU-R015E
Figure 1.
Schematic of a DC/DC Converter
The Schottky barrier 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.
11/14 • e/N1416
Designer’s Guide to Chip Resistors, Chip Diodes & Power Chokes
For Power Supplies & DC/DC Converters
LOW OHMIC CHIP RESISTOR SOLUTION
The sensing voltage, (Vsense), is used to determine the inductor current, given by the following equation:
CRL2010
Vsense = Io x Rsense
Typically, Vsense will be set to 100 mV to save power and at the same time keep the level high enough
above the noise. To sense a 2 A average output current, for example, Rsense must be
100 mV/2 A = 50 mW.
The amount of ripple voltage allowed at the output-regulated voltage will determine the allowable
error of the resistance value required. Therefore, as the precision of the voltage increases, the allowable
error of the resistance value decreases.
CD1005
The two most significant contributors to error of a resistor’s value are initial tolerance and Temperature
Coefficient of Resistance (TCR). Initial tolerance is the accuracy of the resistance value as supplied by
the manufacturer. Commonly available values have an initial tolerance of 1 % or 5 %.
SDR0805
TCR causes the value of the resistance to vary with change in temperature. The change in temperature
of the resistor can occur from two causes: 1) the change in ambient temperature and 2) self-heating
due to power dissipation of the resistor. The ambient temperature can vary due to causes such as the
heating of a PC board by components near the resistor or the increase or decrease of the temperature
of the air around the resistor.
Self-heating occurs as power is dissipated by the resistor in response to current flowing through the
resistor. Resistors have derating curves from which their self-heating may be determined. For example, the
Bourns® Model CRL 2010 device has a power rating of 0.5 watt at 70 ˚C, derating to zero watts at
+125 ˚C. The self-heating may be determined by the equation:
Rate of self-heating =
(125 °C - 70 ˚C)
(Power rating at +70 ˚C) - (Power rating at +125 ˚C)
For the Model CRL2010, the self-heating rate is (55 ˚C / 0.50 watt) = 110 ˚C per watt.
Two amps flowing through a 51 mΩ resistance creates 0.2 watts. In the case of Model CRL2010, the
application of 0.2 watts will create a self-heating of 22 ˚C (0.2 watts x 110 ˚C/watt).
11/14 • e/N1416
Designer’s Guide to Chip Resistors, Chip Diodes & Power Chokes
For Power Supplies & DC/DC Converters
LOW OHMIC CHIP RESISTOR SOLUTION (Continued)
CRL2010
If Model CRL2010-FW-R015E (51 mΩ, 1 %, TCR = ±200 PPM/˚C) is used in an application in which
the maximum error can be ±5 % and the operating temperature is 0 ˚C to +70 ˚C, will the
CRL2010-FW-R015E remain within the allowable error? Again, the three sources of error are: Initial
tolerance, TCR error due to change in ambient temperature and TCR error due to self-heating.
A. Initial tolerance = 1.0 %
B. TCR Error due to ambient temperature: (70 °C –25 ˚C) x 200 PPM/˚C = 11000 PPM = 1.1 %
C. TCR Error due to self-heating: 110 ˚C/watt x 0.2 watt x 200 PPM/˚C = 4400 PPM = 0.44 %
CD1005
Maximum error = 1.0 % + 1.1 % + 0.44 % = 2.54 %
Therefore, the Model CRL2010-FW-R015E would produce a worst-case error of almost half of the
desired amount.
As you can see, it is straightforward to calculate the error budget of a resistor when TCR is given and
self-heating has been calculated.
SDR0805
The CRL series from Bourns are thick-film chip resistors ranging from 0603 (0.06 x 0.03 inch) to 2512
(0.25 x 0.12 inch) in size, and from 0.01 watts to 9.10 watts at 1 % and 5 % tolerances. The maximum
power rating is 1 watt at 70 ˚C in the 2512 size package.
The table below shows a cross reference of competitor models:
Supplier
Vishay
KOA
Yageo
Table 1.
11/14 • e/N1416
Part Number
CRCW2010
SR732H
RL2010
Bourns® Alternative
CRL2010
CRL2010
CRL2010
Bourns® Chip Resistor Cross Reference
Designer’s Guide to Chip Resistors, Chip Diodes & Power Chokes
For Power Supplies & DC/DC Converters
CHIP DIODE SOLUTION
CRL2010
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 the Bourns® Model CD1005 series, have lower
forward voltages than PN junction diodes.
For example, the Model 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.
CD1005
Bourns supplies a wide range of switching, Schottky barrier, 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® Model CD1005-B00340
Schottky barrier diode and other models on the market.
SDR0805
Supplier
Rohm
Panasonic
Panasonic
Philips
ON-Semi
GS/Vishay
Table 2.
11/14 • e/N1416
Part Number
RB751V-40
MA2J728/MA728
MA2J732/MA732
1PS76SB40
RB751V40
SD104XWS
Bourns® Alternative
CD1005-B00340
CD1005-B00340
CD1005-B00340
CD1005-B00340
CD1005-B00340
CD1005-B00340
Bourns® Schottky Barrier Diode Cross Reference
Designer’s Guide to Chip Resistors, Chip Diodes & Power Chokes
For Power Supplies & DC/DC Converters
INDUCTOR SOLUTION
CRL2010
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 Model SDR0805 SMT
power choke offering 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.
CD1005
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 Model SDR0805 power choke.
Supplier
Sumida
Central Tech
Dale & Vishay
API Delevan
SDR0805
Table 3.
Part Number
CD75
CTGS75
IDCP-3020
PD75
Bourns® Alternative
SDR0805
SDR0805
SDR0805
SDR0805
Bourns® Inductor Cross Reference
SUMMARY
Bourns supplies a wide range of precision low ohmic resistors, inductors and chip diodes for power supply
applications. Bourns® inductive components include surface mount chip inductors, power chokes, as well as
radial-leaded and axial-leaded inductors are used in computer, communication, instrumentation, industrial
and medical applications.
ADDITIONAL RESOURCES
Please contact your local Bourns Application Engineer or Bourns Sales Representative for additional
information. Visit Bourns online at:
www.bourns.com
COPYRIGHT© 2014 • BOURNS, INC. • 11/14 • e/N1416
“Bourns” is a registered trademark of Bourns, Inc. in the U.S. and other countries.
Americas: Tel +1-951 781-5500
11/14 • e/N1416
Fax +1-951 781-5700
EMEA:
Tel +36 88 520 390
Fax +36 88 520 211
Asia-Pacific: Tel +886-2 256 241 17
Fax +886-2 256 241 16
5
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising