Discrete-component buck converter drives HB LEDs

HB (high-brightness) LEDs require a large amount of current to operate. When driving HB LEDs from a voltage source, you can set the required current with a suitable series resistor. If the voltage source is a battery, then, as the battery drains, the LED’s intensity decreases. Also, a series resistance has the disadvantage of power loss through the resistor. A better option is to use a suitable dc/dc converter. If the LED’s turn-on voltage is lower than the battery voltage, as would be the case with a 6V sealed-lead-acid battery, then you can use a buck converter (reference 1 and reference 2). You can build a simple buck converter using only discrete components. It requires two bipolar transistors, a P-channel MOSFET, an inductor, a Schottky diode, and a few resistors (Figure 1).

When you switch on the battery voltage, the voltage across R1, the resistor in series with the HB LED, is 0V. Thus, transistor Q2 is off, and Q1 is in saturation. The saturated state of Q1 switches on the MOSFET, thereby applying the battery voltage to the LED through the inductor. As the current through resistor R1 increases, it turns on Q2, which turns off Q1 and thus turns off the MOSFET. During the MOSFET’s off state, the inductor continues to supply current to the LED through Schottky diode D2. The HB LED is a 1W, white Lumiled LED. Resistor R1 helps control the LED’s intensity. Using a larger value for R1 reduces the intensity.

The SwitchCAD-III software, which is available as a free download from Linear Technology, simulated the circuit; the simulated MOSFET was an International Rectifier IRF9Z24S instead of an IRF9540 because the model for IRF9540 is not available in SwitchCAD-III. Figure 2 plots the MOSFET-drain waveform and the voltage at Q1’s base. The circuit was wired on a prototyping board and tested for various supply voltages. Figure 3 shows oscilloscope screenshots for the MOSFET-drain voltage and the voltage at the base of Q1. They fairly well match the simulated waveform. Conversion efficiencies were 60 percent to 95 percent for supply voltages of 6V to 10V.


References
1. Saab, Alfredo H, and Steve Logan, “High-power LED drivers require no external switches,” EDN, July 19, 2007, pg 78.
2. Gadre, Dhananjay V, “Buck regulator controls white LED with optical feedback,” EDN, Oct 25, 2007, pg 72.

Caption
Figure 1: A buck converter provides current sufficient to drive a high-brightness LED.
Figure 2: In a SwitchCAD simulation, the upper trace is the MOSFET-drain voltage, and the lower trace is the base voltage of Q1.
Figure 3: In an oscilloscope screenshot, the upper trace is the MOSFET-drain voltage; the lower trace is Q1’s base voltage.

Click here for the illustrations:

Figure 1, Figure 2, Figure 3