PCB Design

LED PCB Design: Essential Techniques for Efficient Lighting Solutions

LED PCB Design

LED PCBs are very common and these types of PCBs are found almost everywhere, hence the demand for such PCBs is higher than ever. LED PCBs require a carefully planned design for designing schematic and PCB  as there are many variables to maintain while designing such boards. LED PCBs are hence very susceptible to errors and the chance of error increases even more when designing high-density LED boards using low-pitch LEDs and big LED boards. Things get even more complicated in the LED PCB circuit design of multi-colour or RGB LEDs.

What is LED PCB Design?

LED PCB design involves creating a circuit board designed specifically for mounting and operating LED (Light Emitting Diode) components. Before LED PCB design, we must understand the characteristics of various LEDs (light-emitting diodes) and their usage methods for reasonable use. LED PCB design also involves the service life of LED PCBs and their usage efficiency, so we must understand LED before the LED PCB design.

Working with an LED in LED PCB design

Characteristics of LED

Fig1. A Basic Representation of LED’s Ports

Fig1. A Basic Representation of LED’s Ports

LED or Light-emitting diode in LED PCB board design is a special type of semiconductor device that emits light after passing a current through it after overcoming a threshold voltage. It has two ports like any other diode which are cathode and anode.

Working Principles of LED

Basic Working Principle: The LEDs work on the concept of overcoming the band gap of doped semiconducting materials to produce light. Hence all the LEDs have threshold voltage which a voltage applied across the anode has to overcome.

Explanation of LED at the Physical Level: The LEDs require a potential difference between the anode and cathode which must be greater than the threshold voltage of the LED or the forward voltage, which is the minimum potential difference required between the cathode and anode during the flow of current. The voltage at the cathode will be the difference between the voltage applied at the anode and the forward voltage as there is a voltage generated against the direction of potential applied on the anode. Forward current which is the maximum current a LED can handle comfortably without damage. If a current is passed which is over the forward current of the LED then the diode of the LED burns out due to excess current flowing through the diode.

Therefore when using an LED in LED PCB design, one must use a current limiting element such as a resistor or a constant current IC over a cathode or anode to improve the life of the LED.

How to Use an LED in  LED PCB design

Fig2. Using a Resistor to Limit the Current Flowing through the LED

Fig2. Using a Resistor to Limit the Current Flowing through the LED

In Fig2. we can see how a resistor is used to limit the current through an LED in an LED PCB circuit design. Now putting a resistor of the right resistance and wattage is also important.

By Ohm’s law, we know that R=V/I, and using the formula for power in a conductor we can say that P=V*I. Where R is the resistance V is the Voltage applied through the conductor (in this case the resistor) and I=current passed through a conductor.

For example in Fig 2 let’s take the Forward current value as 60 mA and the applied voltage as 5V and we want the LED to glow under 20mA of current so how do we find the wattage and resistance of the resistor?

  • We first find the wattage by multiplying the applied voltage by the expected current i.e. 5V*20mA=100mW
  • Then we find the resistance of the resistor by V/I which in this case is 5V/20mA= 250Ω

So we will use a resistor of at least 100mW and a resistance of 250Ω to achieve 20mA through the LED.

Working with More than One LED in LED PCB design

What happens when we consider a group of LEDs connected in LED circuit board design, There are generally two ways to connect more than one LED which is either in series or in parallel connection in LED circuit board design.

A Group of LED in Series Connection in LED PCB Circuit Design

 

A Group of LED in Series Connection in LED PCB Circuit Design

Fig3. A Group of LED in Series Connection in LED PCB Circuit Design

In a series connection, the LEDs have uniform brightness and hence the current regulation becomes much easier but the LEDs in this configuration share two critical flaws, i.e. even if one LED is damaged or stops working then all the LEDs stop working as the connection between ground and supply is lost.

Secondly, the voltage at the anode of the second LED is the output voltage from the first LED’s cathode so the voltage drops for the second LED due to the forward voltage of the first LED. The same goes for the following LEDs after the second LED. Hence, the voltage keeps on dropping after subsequent LED that, at a point, there might be a possibility that there is not enough voltage to overcome one of the last LED’s forward voltage, again the connection between the ground and supply will break. The circuit won’t be completed so none of the LEDs will glow.

A Group of LEDs in Parallel Connection in LED PCB Circuit Design

A Group of LEDs in Parallel Connection in LED PCB Circuit Design

Fig4. A Group of LEDs in Parallel Connection in LED PCB Circuit Design

These problems in LED PCB board design are solved by parallel connection, here the current adds up as usual but the voltage across each LED’s anode is the same, and even if one LED is dead it will only make the other LEDs brighter as more net current flows through each LED. Sometimes this net current may exceed the forward current of other LEDS and it might cause a burning up of the LED hence making current regulation a challenging task for a parallel connection.

Techniques Used for Increasing the Efficiency of LED Lighting PCB Boards

Techniques Used for Increasing the Efficiency of LED Lighting PCB Boards

Techniques Used for Increasing the Efficiency of LED Lighting PCB Boards

The quality of an LED PCB board depends on the average life of an LED used on the PCB, decreasing the power consumption of the LED PCB Board, decreasing power loss and power attenuation, maintaining desired brightness, and less need for repairs during the lifecycle of the PCB. Some of the methods in the LED PCB board design to improve the quality of an LED PCB board are

Multiplexing and Matrixing

Multiplexing and matrixing in a PCB LED board refers to a technique that is commonly used for controlling multiple LEDs as a matrix, this way of LED PCB board design the circuit becomes less complicated and is very effective for creating various images for many lighting solutions.

Example of 2x2 LED Matrix

Fig5. Example of 2×2 LED Matrix

Now in this picture to toggle D1, we have to apply low (GND) at column 1 and high (VCC) at  Row 1 similarly by dividing the LED solution as a matrix one can very easily control the whole LED solution as a matrix. In this LED PCB board design, by adding a PWM signal in every row we can control the brightness for the whole matrix.

This technique in LED PCB board design is widely used for text-generating LED PCB boards. One can generate a clear text image on the display board by generating a bitmap of each character. In LED PCB board design applying this technique to a PCB board mostly requires an LED driver IC which controls the column and row.

Row Scanning

Row scanning is a popular method of LED PCB design adapted from the concept of Cathode Ray Tube displays. Row scanning basically divides the display into groups of equal parts of a row. So say there are 16 rows and 32 columns matrix display. So we divide 16 rows into 4 groups of rows in LED circuit board design. Now we lit all the first rows of all the groups in LED circuit board design, then after 10-15ms we lit the second row of all the groups and after we completed the 4th row, again we lit all the first rows of each group in LED circuit board design.

Since the switching is so fast, to the human eye it seems as if all LEDs are on as usual but if you look in a camera you will be able to see the scanning of rows according to the shutter speed of the camera. For this method of LED circuit board design, we use driver ICs and MOSFETs for fast switching. This method of LED PCB design is most widely used in Big LED boards to reduce power consumption and also improves the average lifespan of each LED used in a PCB board.

Thermal Management

Thermal management of an LED PCB board in an LED PCB design is very important for a number of reasons. The first reason is that all LEDs emit heat when turned on. In an enclosed space, a PCB board with around 400 bright LEDs can very easily raise the temperature inside the enclosed space by at least 5 degrees Celsius, if we assume that 1 LED consumes 20mA of current that is 8 Ampere of load on the PCB, so PCB must have enough copper( minimum 2oz) to sustain such high current load.

Also, the components used such as constant current drivers, display drivers, and MOSFETs must be able to take a load of such high current loads. As mentioned earlier all the current regulating components must be chosen carefully or else they might generate heat, thereby reducing the efficiency of nearby components.

Better Quality Materials

The use of better quality materials in LED light circuit board design is by far the simplest yet most overlooked factor determining the Efficiency of an LED PCB solution. The use of high current and temperature tolerance components in LED light circuit board design is important for any LED PCB board. The use of LEDs from reputed make reduces the chance of a faltering LED though they are generally more expensive, they mostly deliver a better price-to-performance ratio.

 

LED board designers widely use these techniques in LED light circuit board design, but there might come situations where you might have to decide between two techniques each having its own tradeoffs for example choosing between a series and a parallel or some LED light circuit board designs also utilise a combination of series placed LEDs with parallel connection. It all ultimately falls upon the requirements of the board and clever choices of selection and placement of components in LED light circuit board design.

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