Application Note

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Light Emitting Diode

Electrical properties ofGaN LEDs & Parallel connections

Table of contents
1. Overview ................................................................... 2
2. Parallel Connection of GaN-basis LED ........... 2
3. Summary................................................................... 5

The part number NSCW215 in this document is the part number ofNichia’s product, and does not have any relevance or similarity toother companies' products that may have trademark rights.

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1. Overview
For reference on current and forward voltage, please consult Nichia’s LED Product Guide 2004. In thisexample, Table 1 provides the forward voltage of NSCW215 as 3.6V (Typ.), 4.0V (Max.) at IF=20mA. This
indicates for a group of LEDs, the forward voltage - forward current characteristic is not a single fixedvalue. In a group of LEDs operating at 20mA the voltage will be an average of 3.6 volts with a maximumof 4.0 volts. But for any given LED in the group the VF may be any value between 2.8 and 4.0 volts.
Therefore, when constructing a system where GaN-basis LEDs are connected in parallel, please note thefollowing conditions.
･Even if you energize the same voltage to the same product, the current can be different.
･If the current value is different, the luminous intensity is also different.
･If the current value is different, the color can also be different.

Light Emitting Diode

Table 1. Forward Voltage of NSCW215 (IF=20mA)
Forward Voltage VF (V)
Typ.

Max.

3.6

4.0

2. Parallel Connection of GaN-basis LED
2.1. Example 1 (A circuit with 2 resistors)
Designing following circuit with LEDs under different forward voltage characteristics.

I1I2

LED1
VF=3.2V@20mA

LED2
VF=3.8V@20mA

I1 + I2

5V

R190Ω

VD1

VR1R290Ω

VD2VR2

Figure.1: Circuit Diagram
Supposition
Suppose as follows:
Power-supply voltage: 5V
LED1 side: 3.2V at 20mA with 90Ω
LED2 side: 3.8V at 20mA with 90Ω

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The current value of I1 side is calculated as follows.
Postulation#1
I1 = 10mA
VR1 = 10mA × 90 Ω = 0.9 V
VD1 = 5 V - 0.9 V = 4.1 V
Refer to Figure 2 which shows: VF = 3.06V at 10mA
Therefore, VD1 should be 3.06V.

Light Emitting DiodeAs the result, 10mA of Postulation#1 is not correct.

Figure 2. Forward Voltage - Forward Current
Postulation#2
I1 = 15 mA
VR1 = 15 mA × 90 Ω = 1.35 V
VD1 = 5 V – 1.35 V = 3.65 V
Refer to Figure 2 which shows: VF = 3.14V at 15mA
Therefore, VD1 should be 3.14 V
As the result, Postulation#2 is not correct but the “15mA“ is more accurate than “10mA”.
Postulation#3
I1 = 20 mA
VR1 = 15 mA × 90 Ω = 1.8 V
VD1 = 5 V – 1.8 V = 3.2 V
Postulation#3 equal to the “Supposition”
The same way of calculation will result as I2=15mA.

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The circuit with one resistance that is cost-effective.
I1 + I2

Light Emitting Diode5V

I1

I2

LED1
VF=3.2V@20mA

LED2
VF=3.8V@20mA

VD1

VD2

Figure 3. Circuit Diagram
Supposition
Suppose as follows:
Power-supply voltage: 5V
LED1 side: 3.2V at 20mA
LED2 side: 3.8V at 20mA
The current value of I1 side is calculated as follows.
Postulation#1
I1 = 20 mA
VD1 = 3.2 V
VD1 = VD2
Therefore, VD2 = 3.2 V
The current value of I2 is 2mA which is derived from Figure 2
Therefore, I1:I2 =10：1
Postulation#2
I2 = 20 mA
VD2 = 3.8 V
VD2 = VD1 = 3.8 V
The current, I1, exceeds the scope of the graph of current vs. voltage shown in Figure 2. An LED used inthis manner may also exceed Nichia's defined absolute maximum current. The luminous intensity ofLED1 compared to LED2 will be noticeably different.
Please refer to Nichia's product specifications for reference on the maximum allowable forward current.
Please do not exceed the absolute maximum current when using Nichia LEDs.
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3. Summary
When two LEDs of similar luminous intensity are driven at the same current, they will appear identical.
This can be achieved using a parallel connection where both LEDs are at the same forward voltage.
However, if the voltage changes the luminous intensity will change according to the forward current
change of each LED.
As shown in Figure 1, adjusting the voltage with separate resistors will reduce variation in forward current
resulting giving similar luminous intensity values. For the example of Figure 1, both resistors were thesame thus the luminous intensity would not be the same. If the resistors were matched to the VF of theLED, the same luminous intensity could be achieved.
Theoretically one could consider very narrow VF binning as a solution. Practically this is not an answer.
When the range of available product becomes small, the ability to ship decreases and the cost drasticallyincreases. The best answer in this case is one which considers VF variation as a function of the product

Light Emitting Diodeand strives to limit its effect on the system.

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Light Emitting DiodeDisclaimer
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