LED Driver Circuit Design with Constant Current Source

By Sanjay Chawla On Nov 12, 2008 Site: Electronics Design Engineering Lounge (Public) Type: Blog - Tags: Electronics - # of views: 22588

Many of the times designers get frenzied of designing LED Drivers by constant current sources. They know that the advantage with constant current source design is primarily to maintain steady forward current through the LED as the voltage drop across the LED junction varies and also the constant current source allows variations in the power source of the LED circuit without affecting LED forward drive current, but they get out of control of these benefits just for not taking the following considerations into their designs:

Voltage Drops in the LED Circuit

Designing an LED circuit based on the typical forward voltage drop stated in the manufacture data sheets can lead to overheating of the LED driver. This occurs when the forward voltage drop across the light emitting diodes decreases to a value significantly less than the typical stated value. During such an event, the increased voltage across the LED driver naturally results in a higher total power dissipation from the driver package, which may lead to decreased LED driver life or reliability issues. This problem becomes especially hazardous when LED manufactures publish a conservatively high typical forward voltage rating. In reality, many LED lights feature a lower measured voltage drop than that stated within the data sheets. So why not simply design the LED circuit based on values slightly lower than published? Because deviations in the manufacturing process can cause the forward voltage to vary, this allows for the possibility of future LED shipments or lots to contain significantly higher forward voltage values.

Controlling LED Driver Power Dissipation

The most effective way to gain control over the power dissipation from the LED driver is to adjust the source voltage, when possible. In many LED lighting applications utilizing an AC to DC power supply, a trimmer potentiometer located on the power supply can be adjusted to vary the DC voltage output. The trimmer should be calibrated to allow for a minimal overhead voltage drop across the LED driver. This value can be located within the data sheets corresponding with the driver device. It is important to perform the trimmer adjustment based on measurements taken just after power has been applied to the LED circuits. Otherwise, a circuit that has been active for an extended period will result in lower measured values across the LED lights. Adjusting the power supply trimmer according to these values may result in a circuit that does not operate properly when engaged, due to insufficient voltage across the LED driver.

Increasing Copper Area for LED Drivers

Another useful method for controlling power dissipation from the LED driver circuits involves the copper area located on the LED circuit board itself. Technically, this method does not limit power dissipation, but rather the internal die temperature of the LED driver itself. Many manufactures publish data for LED driver power dissipation capabilities based on the total copper surface area, to an extent. When space permits, designing a board with excessive 2 ounce copper pours in proximity to the pin on the LED driver components, will offer an additional means of cooling for the device. When a lower than expected forward voltage across the light emitting diodes causes increased power dissipation from the LED driver, excessive thermal energy can escape and dissipate outward into the copper planes surrounding. This simple but effective method permits additional flexibly to the LED circuit designer, and promotes a healthy LED driver circuit.

Load-Bearing Series Resistors in LED Circuit

Consider an automotive LED lighting design with very limited board space. The automotive power source does not feature an adjustable output. The circuit board containing the LED drivers does not offer additional surface area for copper due to space restrictions. This situation creates somewhat of a challenge for the circuit designer considering the numerous possibilities associated with LED voltage drops. Once again, a problem with lower than expected LED voltages can result in excessive power dissipation from the LED driver circuits. However, a single resistor placed in series with the LEDs can provide an additional method for controlling and limiting power dissipation from the drivers. Since the constant current source circuit always delivers a continuous constant current, a series resistor will always feature a constant voltage drop. This voltage drop across the resistor can provide relief for the LED driver by dissipating excess heat that would otherwise result in increased heat dissipation from the driver. The resistor value must be selected prior to final assembly, based on the unique forward voltage measured from each specific LED lot or shipment. To determine the value for the series resistor, simply divide your desired forward voltage drop by the LED circuit drive current. Do not forget to calculate for power dissipation from the resistor. When choosing the most desirable voltage drop for the relief resistor, simply refer to the minimal overhead voltage required for the LED driver, as published on the driver data sheets.