To ensure a high-power LEDs operates at its maximum brightness and has an extended working life. It must be driven at the specified optimal current. It is crucial to avoid allowing the current to exceed the permissible value, as this would significantly shorten the device’s lifespan. Using a power supply or a battery with a small current-limiting resistor is not an ideal solution. This is because not only does this approach waste energy by heating the resistor. But also, if a small resistor value is chosen to minimize wastage, even minor fluctuations in the applied voltage can result in significant variations in the flowing current. LEDs are known to have a small dynamic resistance around their optimal operating point. Therefore, meeting the requirements demands a more sophisticated electronic solution than a simple series resistor.
Efficient Current Regulation with Power MOSFET
Achieving a stable current amid minor fluctuations in supply voltage is traditionally accomplished through a regulated current source, albeit at the cost of energy wasted in the series transistor. The introduction of a power MOSFET as the series component mitigates this inefficiency, limiting power loss to the current sense resistor and the minimal ‘on’ resistance of the switching transistor.
Application: Driving Luxeon LED
The proposed circuit drives a commercial Luxeon LED using a BUZ71. For a 5 W LED drawing 0.7 A, a voltage drop of 0.175 V occurs across R9, resulting in a power dissipation of 122 mW. With an ‘on’ state resistance of 85 mΩ for T1, approximately 60 mV is dropped across it, leading to a dissipation of at least 42 mW. To accommodate these drops and ensure stability, a supply voltage of around 7.2 V is recommended, allowing some margin for T1 and R9.
Power Supply and Operational Mechanism
A serendipitous discovery reveals that six NiCd or NiMH cells offer an ideal supply voltage of approximately 7.2 V under load. An unregulated mains power supply with a 6 V transformer, bridge rectifier, and 2200 μF/16 V electrolytic smoothing capacitor provides a similar output. IC1b, in conjunction with T1, establishes a current source adjustable between 360 mA and 750 mA using P2. IC1a functions as an under-voltage cutout switch, preventing deep discharge of the connected battery. The circuit incorporates hysteresis for stability, with IC1b shutting off the LED if the current through R9 is deemed too high.
Component Considerations and Efficient Heat Dissipation
It’s crucial to employ opamps with input stages constructed using PNP transistors for this circuit. Considering an average voltage of 7.4 V during discharge with six cells, approximately 0.55 V must be converted into heat. T1 dissipates about 0.4 W, eliminating the need for additional cooling. The circuit exhibits impressive efficiency, exceeding 90%, ensuring optimal utilization of power resources.