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2. Relation between Flicker, LED Current and LED Voltage Ripple
In order to determine the relation between light flicker, LED current ripple and the LED driver output voltage ripple, the LED string characteristics need to be examined.
Figure 2 shows the relation between LED current and relative luminous flux of a Cree XLAMP MX-6 LED high brightness LED.
Figure 2. Extrapolating LED current ripple to luminous flux variation for a Cree high brightness LED
A sine shaped LED current ripple is drawn in the graph, and the resulting luminous flux variation is extrapolated. So changing LED current has an immediate effect on the light output, but it can be seen that the curve is not exactly linear. The relation between LED current ripple and resulting flicker % is therefore not linear either, and for most LEDs, the % light flicker is lower than % current variation.
In most offline LED drivers, the circuit parameters control the output (LED) voltage ripple, and the LED current ripple is a result of the output voltage ripple. Therefore it is important to know the relation between the voltage ripple across the LED string and the current ripple through the LED. This relation can be found from the LED I/V curve in Figure 3. (The same Cree XLAMP MX-6 LED)
Figure 3. LED I/V curve with dynamic resistance measurement
The dynamic resistance of the LED at a certain operation point will determine the relation between the voltage ripple on the LED and the resulting current ripple thought the LED. This dynamic resistance is quite small, which means that a very small voltage ripple can already result in large current ripple. Since the slope of the I/V curve changes at different operating points, the dynamic resistance needs to be determined around the average LED current operating point.
Most LED lamps use multiple LEDs. When placing LEDs in series, the dynamic resistance needs to be multiplied by the number of LEDs. When paralleling LEDs, the dynamic resistance needs to be divided by the number of LEDs in parallel.
2. Relation between Flicker, LED Current and LED Voltage Ripple
In order to determine the relation between light flicker, LED current ripple and the LED driver output voltage ripple, the LED string characteristics need to be examined.
Figure 2 shows the relation between LED current and relative luminous flux of a Cree XLAMP MX-6 LED high brightness LED.
Figure 2. Extrapolating LED current ripple to luminous flux variation for a Cree high brightness LED
A sine shaped LED current ripple is drawn in the graph, and the resulting luminous flux variation is extrapolated. So changing LED current has an immediate effect on the light output, but it can be seen that the curve is not exactly linear. The relation between LED current ripple and resulting flicker % is therefore not linear either, and for most LEDs, the % light flicker is lower than % current variation.
In most offline LED drivers, the circuit parameters control the output (LED) voltage ripple, and the LED current ripple is a result of the output voltage ripple. Therefore it is important to know the relation between the voltage ripple across the LED string and the current ripple through the LED. This relation can be found from the LED I/V curve in Figure 3. (The same Cree XLAMP MX-6 LED)
Figure 3. LED I/V curve with dynamic resistance measurement
The dynamic resistance of the LED at a certain operation point will determine the relation between the voltage ripple on the LED and the resulting current ripple thought the LED. This dynamic resistance is quite small, which means that a very small voltage ripple can already result in large current ripple. Since the slope of the I/V curve changes at different operating points, the dynamic resistance needs to be determined around the average LED current operating point.
Most LED lamps use multiple LEDs. When placing LEDs in series, the dynamic resistance needs to be multiplied by the number of LEDs. When paralleling LEDs, the dynamic resistance needs to be divided by the number of LEDs in parallel.
