Constant current technology for multi-channel LED output

There are many different seemingly plausible opinions on this issue. Some people say: In the volt-ampere characteristics of LEDs, the voltage is fixed and the current is also fixed. So the effect of constant voltage and constant current is the same. Some people say that when LEDs are connected in parallel, constant voltage power supply should be used, and when LEDs are connected in series, constant current power supply should be used; some people say that because LEDs are constant current devices, constant current sources should be used; some people say that when powered by AC power, constant voltage power supply should be used, and when powered by batteries, constant current power supply should be used.

As for why this is required, it seems that no one can explain it clearly. So, should constant voltage power supply or constant current power supply be used? First, let's take a look at what kind of device LED is. Because the brightness of LED is proportional to its forward current, and the structure of some LEDs determines its heat dissipation, that is, power consumption. So most LEDs will give rated currents, such as 20mA for Φ5, 350mA for 1W, etc., but this does not mean that LEDs can only work at these rated currents, nor does it mean that LEDs are constant current devices. For example, Cree's 1W LED and 3W LED are the same model. When the current increases from 350mA to 700mA, the power increases from 1W to 3W, so this LED can work at any value between 350-700mA.

To understand this issue in depth, we must first know the volt-ampere characteristics of LED.

1. The volt-ampere characteristics of

LED The Chinese name of LED is light-emitting diode, so it is a diode itself. Its volt-ampere characteristics are very similar to the volt-ampere characteristics of general diodes. However, the curve is usually very steep. For example, the volt-ampere characteristics of a 20mA straw hat LED are shown in Figure 1.

Figure 1. Volt-ampere characteristics of low-power LEDs

If it is powered by dry cells or batteries, then due to the nonlinearity of the LED volt-ampere characteristics, a small voltage change will cause a large current change. In the figure above, the LED with a forward current of 20mA when the power supply voltage is 3.3V, if it is powered by 3 dry cells, the voltage of the new battery exceeds 1.5V, and 3 cells is 4.5V, the current of the LED will exceed 100mA and it will soon burn out. The same is true for 1W high-power LEDs. Figure 2 shows the volt-ampere characteristics of a company's 1W LED. The voltage of a 12V battery can drop from 14.5V to 10.5V when it is fully charged to almost fully discharged. The difference is nearly 20%. From the volt-ampere characteristics, it can be seen that a 10% change in the power supply voltage (3.4V-3.1V) will cause a 3.5-fold change in the forward current (from 350mA to 100mA).

Figure 2.1W high-power LED volt-ampere characteristics

2. Temperature coefficient of volt-ampere characteristics

Until now, many people still think that if the LED voltage is fixed, the current is also fixed, so using constant voltage and constant current is the same. In fact, the volt-ampere characteristics of LEDs are not fixed, but change with temperature. Therefore, if the voltage is fixed, the current is not fixed, but changes with temperature. This is because LED is a diode, and its volt-ampere characteristics have the characteristics of a negative temperature coefficient.

Figure 4. Series resistance can only reduce the influence of temperature, but cannot eliminate its influence

4. Can a constant voltage power supply be used when several LEDs are connected in parallel?

Due to the discreteness of the volt-ampere characteristics of LEDs, not only are the volt-ampere characteristics of LEDs of the same wattage produced by different manufacturers different, but even the volt-ampere characteristics of LEDs of the same model produced by the same manufacturer are different.

Figure 5. Discreteness of the volt-ampere characteristics of LEDs produced by different manufacturers and the same manufacturer

Obviously, if a constant voltage power supply of 3.4V is used for power supply, the current flowing through each LED is obviously different, and the brightness of each LED is also different. Therefore, a constant voltage power supply cannot be used

for power supply. 5. After multiple LEDs are connected in parallel and powered by a constant voltage power supply, can different series resistors be used to balance the current?

It is possible at room temperature, but it cannot be maintained after the temperature rises. Figure 6 shows this problem. The volt-ampere characteristics of LEDs at room temperature are represented by a solid line. The volt-ampere characteristics of the two LEDs are slightly different in slope. When powered by a constant voltage power supply Vo, different resistors can be used to obtain the same forward current Io. However, when the temperature rises, its volt-ampere characteristics shift to the left, as shown by the dotted line. Because it is still the original constant voltage and the original resistance, the current at this time has become I1 and I2. It is not equal to the original Io.

Figure 6. The series resistor can keep its current unchanged at room temperature, but it cannot keep the current balanced after the temperature rises.

6. After N LEDs are connected in series, if a constant voltage power supply is used, its temperature effect (the increase in current caused by temperature rise) will be expanded N times. This is because after all LEDs are connected in series, it is equivalent to the volt-ampere characteristics of each LED being connected in series along the voltage axis. Figure 6. Multiple LEDs are connected in series, which is equivalent to multiple volt-ampere characteristics stacked at the constant current point. After power is turned on, the temperature rises and all volt-ampere characteristics shift to the left.

After the temperature rises, the N volt-ampere characteristics all shift to the left, which increases the current by N times. If a constant current power supply is used, then after the temperature rises, the current can still be kept constant at Io.