LED driver using UC3842

LED driver using UC3842 current mode controller IC, In this article you will learn What is LED driver? What is a purpose of LED driver? Types of LED driver and practical example of LED driver with circuit diagram and its working. Following are the main contents of this topic :

• What are LED drivers?
• Applications
• Types of LEDs configuration
• Practical example
• circuit diagram
• circuit description
• Practical consideration

LED drivers are power electronics circuits which are used to regulate output voltage for string of LEDs which can be connected in series, parallel, matrix and independent strings. All these LED strings have different type of drivers.Output voltage also depend on number of LEDs and type LED strings used for LED driver. These drivers are usually constant current source. Because brightness of LEDs depends on constant current.

Most of the Power supplies available are not suitable for powering outdoor LED applications. Voltage output of each LED driver varies in different cases.Its depend on number of LEDs one is using per string and also depend on configuration of  LED strings. LEDs can operate at higher surrounding temperature than other lamps.

Most outdoor lamps used now a days are consist of LEDs and they are connected commonly in following four configurations. But in this article, I will discuss only series configuration.LED driver design for these four configuration also different according to current and voltage output requirement of driver to drive string of LEDs more efficiently.

1. Series configuration : All LEDs connect in series in this configuration.
2. Parallel configuration : All LEDs connect in parallel in this configuration.
3. Matrix configuration : In this configuration, String of parallel LEDs connect in series with each other.
4. Independent configuration. In this configuration series connection of LEDs strings are used and each string is drive separate constant current source LED driver.

Series configuration :

In this configuration all LEDs connect in series. Cathode of first LED connect with Anode of second LED and Cathode of second LED connect with Anode of third LED. Voltage output of driver can be calculated by multiplying forward voltage drop of LED wiht total number of LEDs used in string. For example, if you are using 5 LEDs string in series and forward voltage drop of each LED is 3 volt. Output voltage of LED driver should be 5 x 3 = 15 volt . So all LEDs are connected in series, same current will flow through each LED and a constant current source LED driver is used for series configuration. I have given a practical example of series LEDs configuration and LED driver using UC3842 current mode controller which is given below. One of the major disadvantage of series configuration is that if one LED burn due to any reason, string will become as open circuit. No current will flow. Hence complete LED lamp will remain off.

In this example, 3 LEDs having forward voltage drop of 3 volt are connected in series . Hence LED driver output voltage should be equal 9 volt. Because 3 LEDs are connected in series and each LED have a forward voltage drop of 3 volt. So voltage of LED driver is :

output voltage of LED driver = Number of LEDs x Forward voltage drop of each LED ;

output voltage required = 3 x 3 = 9 volt

For example input voltage to LED driver is between 10-25 volt and we need regulated 9 volt output of LED driver for three LEDs strings of series configuration. Circuit diagram of LED driver for Series configuration of LEDs is shown below:

Above circuit can be used to power 1.5A series LEDs .In above circuit following are the main components of LED driver:

• UC3842 current mode controller IC.
• MOSFET IRLZ34NS as a switch
• Current limiting resistor R1 .
• inductor design

1. Label 1 : It is input side of driver. As I have already mentioned, input voltage range is between 10-25 volt. But we want output voltage of driver regulated 9 volt. So we need to step down input dc voltage above 9 to regulated 9 volt. To step down dc voltage buck converter is used. Buck converter is a type of dc to dc converter which is used to step down dc voltage. It have many application in power electronics based circuits. Combination of inductor, diode and a MOSFET is used to make buck converter.
2. Label 2 : Label two in above diagram shows string of three LEDs connected in series configuration.
3. Label 3 : Label three is showing current limiting resistor. This resistor is used to set current limit of LED driver circuit. Now the question is how to select value of current limiting resistor? To select value of current limiting resistor, value of current should be known. For example above circuit, LED driver can power LEDs upto 1.5 Ampere. Current limiting resistor is basically used as a current sensing shunt resistor. R1 shunt resistor is connected with UC3842 current sensing pin. Because UC3842 is current mode PWM controller. It will use to control current upto 1.5 ampere. Reference voltage set for this pin is 1 volt. Hence we have both current and voltage values that is v = 1 volt and I = 1.5 A. Ohm law can be easily use to calculate current limiting resistor value.

V = IR

R = V / I = 1 / 1.5 = 0.67 ohm

Note resistor power rating should be more than .5 watt.

 4. Label 4 : UC3842 is used as a current mode PWM controller. Whenever current increase  or decrease from 1.5 Ampere, It adjust the on or off time of MOSFET Q1 by adjusting the duty cycle of PWM.

Note : Inductor used in above circuit diagram should be made using ferrite core and select proper wire to make inductor according to current rating of LED driver.

you don’t have a knowledge about PWM controller ICs? Don’t worry, I will write a complete article on UC3842 PWM  controller in coming days. It will give you brief idea of PWM controllers and their applications.

If you still have any issue, It would be pleasure for me to reply to your comments. Thanks  🙂