Power Electronic Riddle No.12 – LED Driver

An LED driver is a self-contained power supply that has outputs matched to the electrical characteristics of your LED or array of LEDs. There are currently no industry standards, so understanding the electrical characteristics of your LED or array is critical in selecting or designing a driver circuit. Drivers should be current-regulated (deliver a consistent current over a range of load voltages). Drivers may also offer dimming by means of pulse width modulation (PWM) circuits. Drivers may have more than one channel for separate control of different LEDs or arrays. LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in two ways. The first method is to use the LED V-I curve to determine what voltage needs to be applied to the LED to generate the desired forward current. This is typically accomplished by applying a voltage source and using a ballast resistor as shown in Figure below. However, this method has several drawbacks. Any change in LED forward voltage creates a change in LED current. With a nominal forward voltage of 3.6 V, the LED in Figure below has 20 mA of current. If this voltage changes to 4.0 V, which is within the specified voltage tolerance due to temperature or manufacturing changes, the forward current drops to 14 mA. This 11% change in forward voltage causes a much larger 30% change in forward current. Also, depending upon the available input voltage, the voltage drop and power dissipation across the ballast resistor waste power and reduce battery life. The second, preferred method of regulating LED current is to drive the LED with a constant-current source. The constant-current source eliminates changes in current due to variations in forward voltage, which translates into a constant LED brightness. Generating a constant-current source is fairly simple. Rather than regulating the output voltage, the input power supply regulates the voltage across a current-sense resistor. Figure below shows this implementation. The power-supply reference voltage and the value of the current-sense resistor determine the LED current. Multiple LEDs should be connected in a series configuration to keep an identical current flowing in each LED. Driving LEDs in parallel requires a ballast resistor in each LED string, which leads to lower efficiency and uneven current matching. Many of today