External PCB Trace Max Current

Tools介紹
Calculate the maximum current of a trace
PCB Trace Current Calculator

This tool, based on the formulas and graphs contained in the standard document , calculates the maximum allowable current that can flow through a copper printed circuit board external trace (also called as microstrip), keeping the temperature increase of the trace itself below the specified input value. As shown in the picture, the microstrip has width W and thickness T and is separated by an insulating dielectric from a large conductive ground plane.

By providing additional input parameters (ambient temperature and trace length), it is possible to calculate the trace total temperature, resistance, voltage drop and power dissipation (power loss).

### Max Current calculation

First, calculate the area according to the following formula:

A = (T · W · 1.378 [mils/oz/ft2])                 (I)

Then, calculate the maximum current:

IMAX = (k · TRISEb· Ac                                       (II)

Where:

A is the cross-section area [mils2] T is the trace thickness [oz/ft2] W is the trace width [mils] IMAX is the maximum current [A] TRISE is the maximum desired temperature rise [°C] k, b and c are constants. According to IPC-2221A Par. 6.2 (“Conductive Material Requirements”), their values for inner layers are as follows: k = 0.048 b = 0.44 c = 0.725

Equation (II) is based on a curve fit to the charts provided in  (par. 6.2, Figure B and Figure C).

### Trace temperature calculation

The overall trace temperature can be calculated as follows

TTEMP = TRISE + TAMB

Where:

TTEMP is the trace temperature [°C] TRISE is the maximum desired temperature rise [°C] TAMB is the ambient temperature [°C]

### Resistance calculation

First, convert the cross-section area from [mils2] to [cm2]:

A’ = A * 2.54 * 2.54 * 10-6

Then, calculate the resistance:

R = (ρ * L / A’) * (1 + α * (TTEMP – 25 °C))

Where:

T is the trace thickness [oz/ft2] W is the trace width [mils] R is the resistance [Ω] ρ is the resistivity parameter, whose value for copper is 1.7E-6 [Ω · cm] L is the trace length [cm] α is the resistivity temperature coefficient, whose value for copper is 3.9E-3 [1/°C] TTEMP is the trace temperature [°C]

### Voltage drop calculation

Voltage drop can be calculated as follows:

VDROP = I * R

Where:

VDROP is the voltage drop [V] I is the maximum current [A] R is the resistance [Ω]

### Power dissipation calculation

Power dissipation, or power loss, can be calculated according to the following formula:

PLOSS = R * I2

Where:

PLOSS is the power loss [W] R is the resistance [Ω] I is the maximum current [A] </span

Example 1

Inputs W = 12 mil T = 5 mil TRISE = 30 °C TAMB = 25 °C L = 12 inch

Output Cross-section Area = 60.00 mils2 IMAX = 4.17 A

Additional output Trace Temperature = 55 °C Resistance = 0.150 Ω Voltage Drop = 0.626 V Power Dissipation = 2.608 W

Example 2

Inputs W = 10 mil T = 3 oz/ft2 TRISE = 20 °C TAMB = 18 °C L = 25 cm

Output Cross-section Area = 41.34 mils2 IMAX = 2.66 A

Additional output Trace Temperature = 38 °C Resistance = 0.167 Ω Voltage Drop = 0.444 V Power Dissipation = 1.182 W

### Reference

 IPC-2221A “Generic Standard on Printed Board Design” 