LEDs have gone a very long way since their firs appearance and nowadays they come in all sorts of types and applications, posing new design and manufacturing challenges. High-power LEDs have emerged as decent replacements for bulbs due to their high energy-conversion efficiency. However, they are not as simple to use as the old and common 3mm and 5mm LED types, which need as little as a simple microcontroller pin to be controlled. The high-power LEDs require special drivers, capable of delivering high constant currents across a wide range of input voltage and ambient temperature.
The concept of a high-power LED driver is relatively simple as shown in figure 1.
Figure 1: General layout of a high-power LED driver
It is critical that a constant current driver is used instead of a constant voltage source. If it were possible to use a constant voltage source, then any off-the shelf voltage regulator would have been appropriate to use in such an application. Unfortunately, high-power LEDs have a voltage-current characteristic which is highly dependant on temperature. If, we consider the fact that a 1W or 3W power LED dissipates a substantial amount of heat, a constant voltage source will not be good enough. When first turn on the LED, it is “cold”, therefore the constant voltage would determine a specific current through the LED. As the seconds pass by, the LED gets hotter and for the same applied voltage it will draw a different amount of current. As a result its brightness will change. And that is only under the effect of its own dissipated heat, without even taking into account the variation of ambient temperature! These well known problems have triggered the need for high constant current sources, and there are many semiconductor manufacturers on the market which now manufacture dedicated ICs for LED control.
2. LED Control
The main drawback of these products however is their cost. Of course, it does not matter when the manufacturing quantities of an end product are small, or when you deal with a hobby project. But LEDs have made their way into a wide range of industrial and even automotive applications. As a result, the cost of such a constant current driver has come to dictate the criteria of design for such applications. As example, we consider NCP3063 switching power supply IC provided by OnSemi which was initially meant to be used as a voltage regulator. Fortunately, it also has a constant current mode, which allows it to be used as a driver for an LED. It also has a small package, allowing it to be incorporated in miniature applications and it is automotive rated, which brings all the known benefits: -40 to +85 Celsius operation, high input voltage, low failure rate. The schematic (Figure 2) is pretty simple, with the current through the LED being easily adjustable through a simple resistor (R1).
Figure 2: Schematic for driving a high power LED
As a short description:
- D1 – diode to protect against reverse supply voltage (“fool’s diode”)
- C1, C2 – energy reserve and filtering capacitors
- R1 – current adjusting resistor
- L1, D2, C4 – components required for operation as buck regulator
- C3 - timing capacitor
- C5 - filtering capacitor
- D3 - LED
- U1 - constant current driver
The schematic makes much more sense when it is looked at after knowing the block diagram of the NCP3063 (Figure 3).
|Figure 3: Block diagram of the NCP3063|
3. Theory of operation
The IC integrates a lot of features but not all of them are used here. The most important one, which we take advantage of, is the current limiting comparator inside the NCP3063. As it may be seen from the block diagram in figure 3, the comparator has the inverting input connected to pin 7, and the non-inverting input connected to VCC-0.2V (this 0.2V voltage is generated internally). The purpose of the comparator is to turn off the high power switch inside the IC in case the voltage difference between its two internal pins gets higher than these 0.2V. The way the schematic is built, this voltage difference is dependant upon two parameters:
- The current flowing through the internal switch
- The R1 resistor connected between pins 6 and 7 of the IC.
As such, this comparator gives us the ability of establishing the maximum current that is flowing through the LED by simply choosing the appropriate value for R1. The current that flows through R1, goes into the NCP3063 through pin 1 and it flows then out through pin 2, eventually ending up through the LED. When this current increases, the voltage drop across R1 is also increasing, and when this voltage drop reaches 0.2V, the internal current sense comparator is triggered and its shuts down the switch between pins 1 and 2, thus cutting of the current through the LED. The switch is turned back on by the internal oscillator of the NCP3063, at the end of each oscillating period. The internal oscillating frequency is given by the value of the C3 capacitor, which is called timing capacitor. The value of 2.2nF gives an oscillating frequency of about 150kHz. The provided schematic will work across a decent range of high power LEDs. It will provide about 200mA through the LED, which ensures brightness dependent on the LED (Figure 4).
Figure 4: Current voltage characteristic of Rebel Luxeon LED used in the project
The current seems pretty small for this type of LED, however, be warned: the higher the current through the LED, the higher the dissipated heat. With more power, several options can be considered: heat sink or metal core PCB, but both these methods are expensive and in a design where the final price is the driving force.
Based on text written by brumbarchris