Using Copper Etch Value Resistance Larry Spaziani INTRODUCTION Cu

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

Using Copper Etch Value Resistance Larry Spaziani INTRODUCTION Cu

Datasheet Thumbnail

Top Searches for this datasheet

DN-71
Using Copper Etch Value Resistance Larry Spaziani
INTRODUCTION Current sensing power supplies motor controls demand very value resistor. Each application varies need resistance value, power rating, size, form factor, inductance, temperature coefficient accuracy. meet some these applications, copper printed circuit board (PCB) utilized, some distinct limitations. This design note provides design equations recommendations designing resistors with copper. DESIGN COPPER RESISTOR resistance, function temperature, piece metal, given equation: R(T) These constants apply standard commercially annealed copper used technology. resistivity copper, function temperature, therefore defined
S(T) 1.7241 10-6 0.0039 20)] where copper temperature
Definition which also pertinent resistor design copper defined ounce copper deposited over square foot surface area. This results copper clad with typical copper thickness 0.0014 inches ±0.0002 inches. copper simply twice thick. Using these parameters calculate resistance given length, width, thickness copper etch results formula:
S(T)
with units Resistance, S(T) Resistivity,
R(T)
Length, Area,
S(T)[ Length[cm] Width[cm] Thickness[cm] 1000mils Length(mils) S(T)[ 2.54cm Width mils) Thicknessmils
characteristics copper which pertinent design copper resistor Electrical Resistivity: 1.7241 10-6 20°C Temperature Coefficient Resistivity: +0.0039 Composition: 99.5% Pure Copper, typically
value sense resistor implies that current that resistor quite high. copper etch will self heat power dissipated resistor. MIL-STD-275E provides design guidelines relating copper etch current temperature rise etch dimensions. Figure recreates sections MILSTD-275E curves. temperature rise, above ambient temperature, found knowing current area copper etch.
1/97
Width 150mils
Current Density Curve Outer Layer Copper Etch
DN-71
Allowed Current
Step Find length resistor, insuring that designed maximum 60°C ambient +30°C rise (copper temperature 90°C). From equations S(90) 1.7241 10-6 0.0039 20)]
Cross Sectional Area (Width [mils] Thickness [mils]
Current Density Curve Inner Layer Copper Etch
Cross Sectional Area (Width [mils] Thickness [mils]
1000mils Length[mils] S(90) 2.54cm R(90) Width[mils] Thickness[mils]
Allowed Current
Solving R(90°C) 10m, using150 mils Width mils Thickness results Length 2.43 inches. Final Dimensions (inches): 2.43 0.150 0.0014
Resistance
Copper Temperature
Figure MIL-STD-275E Current Density Temperature Rise Curve Outer Inner Copper Layers Example: Calculate length width MAXIMUM resistor using outer layer PCB. resistor must carry maximum while maintaining more than 30°C temperature rise above ambient. Ambient temperature normal operation 10°C 60°C. Step Find cross sectional area carry with 30°C rise, solve minimum width resistor. From Figure mils2 required carry Amperes. copper 1.4mils thick, resulting minimum width resistor 146mils.
Figure Resistance Copper Temperature Example Design
final resistance, function copper temperature, shown Figure Table provides required dimensions copper resistor given maximum current desired voltage drop. Table assumes maximum operating ambient temperature 60°C, with width specified 30°C temperature rise. required resistance equal calculated copper temperature 90°C.
Desired Voltage Drop 10mV 25mV 50mV Etch Length [in] 0.162 0.405 0.810 0.243 0.608 1.215 0.405 1.013 2.025 0.648 1.620 3.240 0.891 2.228 4.456 1.053 2.633 5.266 1.377 3.443 6.886 1.701 4.253 8.506 2.025 5.063 10.126 2.430 6.076 12.152 2.754 6.886 13.772 3.078 7.696 15.392
DN-71
Width [in] 0.010 0.015 0.025 0.040 0.055 0.065 0.085 0.105 0.125 0.150 0.170 0.190
Vias through resistor will effect resistance. Curved serpentine resistor patterns utilized long overall width length, including curves, understood. copper resistor connected larger copper planes will have lower temperature rise than predicted MIL-STD-275E curves heatsinking those copper planes.
AMPS
CONCLUSIONS design copper resistor straightforward once operating parameters such voltage drop, operating current operating ambient temperature known. Tolerances technology will effect accuracy resistor should considered. length-to-width ratio resistor quite large resistivity copper, when area available PCB, this resistor essentially free. Applications which require current limiting ideal with resistor, current limiting quite accurately maximum operating temperature where current limiting most critical. Average current mode control applications require precise voltage drops independent temperature current levels. resistors therefore recommended Average Current Mode control sense resistor. REFERENCES Reference Data Radio Engineers, Howard Sams Inc., Sixth Edition 1977. MIL-STD-275E, NOTICE July 1986, Military Standard Printed Wiring Electronic Equipment.
Table1. Dimension Solver Given Current Desired Voltage Drop 90°C Maximum Copper Temperaturre
Some physical layout characteristics should considered when designing resistor. thickness tolerance copper vary from supplier supplier relative location PCB. typical tolerance ±0.2mils/oz. Etchback will reduce copper area, thus increasing resistance unit length. Etchback larger impact with narrow runs, where width/thickness ratio lower. recommended that widths 0.025 inches used. copper resistor, outer layer, should solder masked over entire resistive area. Solder copper will reduce resistance.
UNITRODE CORPORATION CONTINENTAL BLVD. MERRIMACK, 03054 TEL. (603) 424-2410 (603) 424-3460

Using Copper Etch Value Resistance Larry Spaziani INTRODUCTION Cu

Top Searches for this datasheet

Other recent searches