Know LEDs & Drivers

Why Constant Current:

Refer to the current/voltage characteristics. In case of LEDs current varies exponentially as voltage. That is a small change in voltage can cause a large change in current. This damages LEDs. So the typical solution is to use constant current power supplies, or driving the LED's at a voltage much below its maximum rating. Since most common power sources (batteries, mains) are not constant current sources, LED fixtures must include a constant current converter

Why MELCON LED drivers:

All Melcon drivers meet the maximum current ratings provided by the manufacturer and therefore, guarantee a long life & optimum performance of LEDs. To ensure a constant current, LEDs on a circuit must be connected in series but not beyond SELV limits. One can think of series-parallel combinations as discussed below.

Why Multi channel Drivers:

As per SELV standard, (safety handling DC voltage) considering average 3.5V/led, maximum 15 LEDs can be connected in series. Multi-channel LED drivers are with supervisory micro controller, which monitors current flow through each LED channel and is protected from over-drive. PWM bus control prevents the excess current resulting the optimum performance of the LEDs.

Manufacturing – 100% in-house:

In order to design, develop and manufacture the most advanced range of LED drivers & Electronic ballasts, we choose to maintain control of all the production process. This ensures quality, consistency, reliability and ultimate value to our customers. From Electronic circuitry to injection molding, our infrastructure is designed to produce consistent quality.

Pitfalls of Parallel LED Arrays

Whenever LEDs are placed in parallel the potential exists for a mismatch in the current that flows through the different branches. The forward voltage, VF, of each LED varies with process, so unless each LED is binned or selected to match VF, the LED or LED string with the lowest total forward voltage will draw the most current. (Figure 1)This problem is compounded by the negative temperature coefficient of LEDs (and all PN junction diodes). The LEDs that draw the most current suffer the greatest increase in die temperature, and as their die temperature increases their VF decreases, creating a positive feedback loop. Elevated die temperature both reduces the light output and decreases the lifetime of the LEDs. Mismatched LEDs in Parallel.The system of Figure 1 also illustrates a potential over-current condition if one of the LEDs fails as an open circuit. Without some protection scheme, the entire drive current IO will flow through the remaining LED(s), likely causing thermal overstress. Likewise, if one of the LEDs fails as a short circuit, the total forward voltage of that string will drop significantly, causing higher current to flow through the affected branch. To maintain safety and reliability in a parallel LED system, forward voltage should be binned or matched. Fault monitoring should detect LEDs that fail as either short or open circuits. Finally, the entire array should have evenly distributed heat sinking, to ensure that VF change with respect to die temperature occurs uniformly over all the LEDs. Melcon Drivers ensure safety for LEDs Load.

Dynamic Resistance

In a standard power supply that regulates output voltage the load resistance has a simple calculation: RO = VO / IO LEDs are PN junction diodes with a dynamic resistance that shifts as their forward current changes. When the load is an LED or string of LEDs, the load resistance is replaced with the dynamic resistance, rD.Simply dividing the LED forward voltage by forward current yields a value that is 5 to 10 times higher than the true dynamic resistance. LED dynamic resistance is provided by some manufacturers, but in most cases must be calculated using I-V curves. (All LED manufacturers will provide at least one I-V curve.) To determine rD at a certain forward current, draw a line tangent to the I-V slope as shown in Figure 2. Extend the line to the edges of the plot and record the change in forward voltage and forward current. Dividing 1VF by 1IF provides the rD value at that point. Figure 2 also shows a plot of several rD values plotted against forward current to demonstrate how much rD shifts as the forward current changes. One amp is a typical driving current for 3W LEDs, and the calculation below shows how the dynamic resistance of a 3W white InGaN was determined at 1A:
1VF = 4.0V – 3.45V
1IF = 1.35A – 0A
rD = 1VF / 1IF = 0.55 / 1.35 = 0.47

Dynamic resistances combine in series and parallel like linear resistors, hence for a string of ‘n’ series-connected LEDs the total dynamic resistance would be:

rD-TOTAL = n x rD + RSNS A curve-tracer capable of the 1A+ currents used by high power LEDs can be used to draw the I-V characteristic of an LED. If the curve tracer is capable of high current and high voltage, it can also be used to draw the complete I-V curve of the entire LED array. Total rD can determined using the tangent-line method from that plot. In the absence of a high power curve tracer, a laboratory bench-top power supply can be substituted by driving the LED or LED array at several forward currents and measuring the resulting forward voltages. A plot is created from the measured points, and again the tangent line method is used to find rD.
Output Voltage Changes when LED Current Changes With a standard forward voltage, VF, and forward current,& select a different LED current, and if they do the forward voltage will change in the VLED The change in voltage comes from LEDs’ V-I curve. Figure 2 shows a typical curve from a 5W white (InGaN) LED. The cross-hairs intersect at the standard, or typical VF and IF values of 3.5V and 0.35A, respectively. LED manufacturers provide these curves in their datasheets, and they can also be generated using curve tracers. V-I Curve with typical VF and IF Once the VF of the LEDs has been determined from the V-I curve, the LED driver’s output voltage is calculated using the following formula: VO = n x VF + VSNSIn this equation ‘n’ is the number of LEDs connected in series, and ‘VSNS’ is the voltage drop across the current sense resistor.
In practice VO changes with both process and with the LED die temperature. The more LEDs in series, the larger the potential difference between VO-MIN and VO-MAX.
An LED driver must therefore be able to vary output voltage as needed to maintain a constant current. IF is the controlled parameter, but output voltage must be predicted in order to select the proper regulator topology, IC, and passive components

                       
 
 

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