SEPIC LED controlled using UC3842 IC


The new 350 mA light−emitting diodes (LEDs) present some new challenges in the area of the power converters that drive them. They must be driven from a “current source ” rather than a voltage source because, as with previous LEDs, their forward voltage varies from part to part, and with temperature. It makes sense that in the interest of stable operation the source should remain a constant current. A second consideration is that the original source of power may have a voltage that varies above and below the voltage of the LEDs that are to be driven. For example, the input voltage may be between 10 Vdc and 15 Vdc, while a series string of LEDs may have a voltage of nominally 12 Vdc, such as would occur with four 3−V LEDs. Given that no galvanic isolation is required between the input and output, what’s needed is a non-isolated dc−dc converter that can handle an input that is below or above the output. The single−ended primary inductor converter (SEPIC) meets this requirement. It has an inductor input, providing smooth input current, requires only one switching transistor, and can operate over a wide range of input, both above and below the output voltage.

The objective of this project is to make an LED based indicator to vary with its brightness as the input current is being altered of any changes. With this, we can know if there is a sudden rise or fall in the input current the intensity of the LED will vary. This circuit may also find applications in optimization for OFF-LINE DC to DC converters.



In this circuit a UC3842 IC is used which is a DC-DC conversion device.  Above figure shows the complete circuit for driving up to 10 high−intensity LEDs at 0.35 A, from a wide range input such as the rectified output of a low−voltage transformer. The controller is the popular UC3843 current−mode PWM device, running at 100 kHz. Current−mode control is achieved via the current−sensing resistor, R5 that senses the current in switching transistor Q2. For stable operation when the input voltage is less than the output, ramp compensation is required, as the duty ratio is greater than 50%. This is accomplished by feeding some of the ramp signal into the current sense input via Q1 and R2. The oscillator frequency is set by the timing resistor and timing capacitor, R1 and C2, respectively. Resistor R4 and capacitor C4 provide filtering to reduce the usual spike at the leading edge of the pulse across R5 due to capacitance of the FET, Q2. Resistor R3 provides level shifting to compensate for the voltage offset of the ramp signal injected by resistor R2. Capacitor C5 provides dominant−pole compensation of the main feedback loop. The LED current is regulated by sensing the voltage across resistors R7 and R8, which are in series with the load. The output current is regulated to 2.5 V (the reference input voltage of U1) divided by the resistance of R7 and R8, which sum to 7.2. This produces an output current of 0.35 A. Resistor R6 provides the input impedance for the loop compensation pole formed with capacitor C5.


Project done By:

Ajay kumar 

Nikhil kumar



Who's new

  • ravirajpatil871...
  • shubhambajoria
  • yassir
  • demiholyman890954
  • scottgillum51169040

Get Notified



We are Social


Subscribe to Syndicate