Does LED drive power need to be a constant current source?

01 Introduction

There are many different specious opinions about this now. Some people say: In terms of volt-ampere characteristics of LEDs, if voltage is fixed, current is also fixed. Therefore, effect of using DC voltage and DC current is same. Some say that when LEDs are connected in parallel, they must be powered by a constant voltage source, and when LEDs are connected in series, they must be powered by a constant current source; When powered by constant voltage power supply, it must be powered by DC power when it is powered by battery. Why this is necessary, no one can really explain.

So, should we use a DC power supply or a DC power supply?

First of all, let's see what kind of device an LED is. Because brightness of an LED is proportional to its forward current, and structure of some LEDs determines its heat dissipation, that is, power consumption. Therefore, most LEDs will give a nominal current, such as 20mA for F5, 350mA for 1W, etc., but this does not mean that LED can only operate at these nominal currents, and it does not mean that LED is a constant current device. For example, 1W and 3W Cree LED are same model, current increases from 350mA to 700mA, and power increases from 1W to 3W, so this LED can run at any value between 350-700 ma.

To get a deeper understanding of this problem, we first need to know current-voltage characteristics of LEDs.

02 LED current-voltage characteristics

An LED is called a light emitting diode in Chinese, so it is a diode in itself. Its current-voltage characteristics are very similar to common diode current-voltage characteristics. It's just that curve is usually steep. For example, volt-ampere characteristics of a 20 mA straw hat LED are shown in Figure 1.

Fig. 1. Volt-ampere characteristics of a low-power LED

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

Fig. 2. Volt-ampere characteristics of a powerful 1 W LED

03 Voltage characteristic temperature coefficient

Until now, many people thought that LED current is also fixed when voltage is fixed, so constant voltage and constant current are same. In fact, current-voltage characteristic of an LED is not fixed, but changes with temperature, so voltage is fixed, and current is not constant, but changes with temperature. This is because LED is a diode and its current-voltage characteristic has a negative temperature coefficient.

Figure 3. Series resistance can only reduce influence of temperature, but not eliminate it

04 Several LEDs are connected in parallel, can I use a constant voltage source?

Due to discrete nature of current-voltage characteristics of LEDs, not only current-voltage characteristics of LEDs of same power from different manufacturers differ, but also current-voltage characteristics of LEDs of same type produced by the same manufacturer are also different.

Figure 4. Variation of current-voltage characteristics of LEDs from different manufacturers and one manufacturer

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

05 After multiple LEDs are connected in parallel and powered by a constant voltage source, can different series resistors be used to balance current?

This is possible at room temperature, but cannot be maintained after raising temperature. This problem is shown in Figure 6. The current-voltage characteristics of an LED at room temperature are represented by a solid line. The current-voltage characteristics of two LEDs are slightly different in slope. When using a constant voltage Vo power supply, different resistors can be selected to obtain same forward current Io. But as temperature rises, its current-voltage characteristic shifts to left, as shown by dotted line. Since it is still original constant voltage and original resistance, current at this time became I1 and I2. Not equal to original Io.

Figure 5. A series resistor can maintain a constant current at room temperature, but cannot maintain a balance of currents after a rise in temperature.

06 After connecting N LEDs in series, if a constant voltage source is used for power, temperature effect (increase in current caused by temperature increase) will be increased by N times< /p >

This is due to fact that after connecting all LEDs in series, current-voltage characteristics of each LED are connected in series along voltage axis.

Fig. 6. Several series-connected LEDs

Equivalent to superimposing multiple volt-ampere characteristics at DC point, temperature rises after power-on and all volt-ampere characteristics shift to left.

After temperature rises, N I–V characteristics shift to left, which increases current strength N times. If a constant current source is used for power, then after temperature rises, current can be kept constant at level of Io.

07 When connecting multiple LEDs in series using a DC power supply, temperature effect of current-voltage characteristic can be used to estimate degree of rise in junction temperature

In many applications (eg fluorescent lamps and street lamps) many LEDs are often connected in series. At present, influence of temperature coefficient of LEDs is more obvious. Because when a constant current source is used for power, its action is equivalent to superimposing current-voltage characteristics of each LED on voltage axis. When temperature rises to 60 degrees, current-voltage characteristic will shift to left by 0.12 V. If 10 LEDs are connected in series, then all current-voltage characteristics will shift to left, and total shift will reach 1.2 V. . This is a pretty impressive number. Conversely, this characteristic of an LED can also be used to measure its junction temperature. For example, there is a combination of 10 LEDs in series and 3 in parallel. After being connected to a DC source, measured forward voltage drop drops from 32.3V to 30.6V. Changes to 1.7V. Then we can assume that transition temperature rise is 1.7/10/0.002=85 degrees

08 How to balance currents of each string in a series-parallel circuit when DC is applied

If you are using a constant current source to power only one string of LEDs, this is of course most ideal option. However, if you want to power multiple strings of LEDs connected in parallel, how can you ensure that current in each string is same? Yes, if a constant current source is used to power multiple strings of LEDs connected in parallel, current of each string must be different due to discrete nature of LED's volt-ampere characteristic. Actually, because each string of LEDs is unlikely to have a low forward voltage in one string and a higher forward voltage in other string. Instead, it will be relatively evenly distributed, so current won't vary much between strings.

09 In a DC Serial-Parallel Circuit, How to Avoid Problems Caused by a Damaged LED

If only two strings are connected in parallel, and one of LEDs in one string is broken (open circuit), then not only will that string not light up, but all current will flow to other string, causing other string's current to double times more, and it will quickly break. To prevent a broken string from lighting up, you can use all-parallel string method, i.e. any one from each string is connected in parallel with LEDs in same position in other strings. Thus, if any of them is broken (open circuit), only this one goes out, and rest of LEDs remain on. But if there are only two strings of LEDs connected in parallel and one of LEDs is open, then current will flow to other LED connected in parallel with it and its current will double, so that life of this LED is not long, and will soon burn out if burned out. is an open circuit, then all LEDs will not light up, but damage to other LEDs is not serious, because. they do not work for a long time in an overcurrent condition. To reduce impact on other LEDs after a particular LED is damaged, it is hoped that more LED strings connected in parallel, better. On fig. 7 shows a diagram of 3 rows of 5 parallels and parallel parallel rows. At this time, if a certain LED is faulty, total current is distributed to remaining 4 LEDs, and total current is distributed to all parallel LEDs in each row, and current distributed to low forward voltage LED will be larger. But not too much harm.

Fig. 7. Three strings of LEDs and five parallels

Each LED in three strings and five parallels is connected in parallel with LEDs in same position in other strings, and only current of that string is divided into remaining 4 paths, while other strings remain same. like before. If an LED is shorted instead of open when it is open, then other LEDs in that row will not light up. Of course, to avoid this phenomenon, it is best to connect voltage regulator tube in parallel with each LED, rather than connect each string in parallel. At this time, any LED is faulty (open circuit), voltage regulator tube is on, and current sharing ratio changes very little. A short circuit means that one less LED emits light.

Figure 8. Each LED connected in parallel with a Zener tube After using this method, there is no need to parallel same line.

A summary of above description could be as follows:

Is it true that a constant voltage source is useless in LED lighting? Not at all.

10 Using DC Switching Power Supply Plus DC Module to Mains Power LED Street Light

A switched-mode AC/DC power supply is required in any mains powered system. There are two power supply methods, one of which is to add a DC feedback control circuit to switching power supply to provide constant current output. However, most of this method can only output one strong current, and accuracy of constant current is not high. Another option is to use a DC power supply at front and add many DC modules at back. This solution features DC high flexibility and high accuracy.