Lately, designers have been raving about the benefits offered by constant current sources as a means for driving LED lighting fixtures. Many older designs relied on simple resistors to limit LED drive current. The primary advantage with the constant current source is the ability to maintain a steady forward current through the LED as the voltage drop across the LED junction varies. The constant current source also allows for variations in the power source of the LED circuit, without affecting LED forward drive current. As a result the LED lights will provide a continuous luminous output during operation.
As the forward voltage across the LED light varies, the voltage drop across the LED constant current source will also vary. Designing an LED circuit around a fixed voltage value can lead to potential complications, even when including a constant current source. Light emitting diode manufactures commonly bin their LEDs according to forward voltage drop. All LED lamps within the same lot (binned for voltage) should contain similar forward voltage drops, while operating at a specified drive current. However, upon ordering, many manufactures do not guarantee the same LED voltage bin. In addition, the manufacture may not even offer the customer with the convenience of selecting a preferred voltage bin. It is important to consider these factors during the design process of an LED circuit utilizing a constant current source. Otherwise, the LED lighting fixtures may face potential failures.
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 disipation 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.
The most effective way to gain control over the power disipation 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.
Another useful method for controlling power disipation from the LED driver circuits involves the copper area located on the LED circuit board itself. Technically, this method does not limit power disipation, but rather the internal die temperature of the LED driver itself. Many manufactures publish data for LED driver power disipation 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 didoes causes increased power disipation 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.
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 disipation 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 constnat 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 disipation 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 disipation 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.