A linear constant current LED drive circuit design

LED is a cold light source with low working voltage and high light efficiency. It is considered to be a new light source for lighting in the 21st century. However, there are two key problems that prevent LED lighting equipment from being widely used. One is the high price; the other is that the unstable control circuit greatly reduces the life of LED. According to statistics, about 70% of the failures of LED white light lighting fixtures are due to power supply problems, about 20% are due to circuit and structure problems, and less than 10% are due to the quality problems of the LED tube itself. Therefore, the choice of power management solution is also very important for energy saving, which requires the selection of the most suitable AC-DC driver in the drive circuit design. Therefore, a reliable and low-cost control circuit is the premise for the popularization of LED lighting.
From the electrical characteristics of LED, it can be seen that the average forward current of LED shows a large linear growth with the increase of forward voltage. After the LED is forward-conducted, a small change in its forward voltage will cause a large change in the LED current, and the current has a great influence on the junction temperature of the LED. Excessive current can easily cause the junction temperature of the LED lamp bead to rise and damage. In addition, from the optical characteristics of LED, it can be seen that as the forward current increases, the LED luminous flux increases, that is, the brightness increases. Therefore, in order to keep the LED brightness constant, it is necessary to ensure the stability of the LED forward current. Therefore, it is very important to design a reasonable driving power supply for LED lighting fixtures.
This paper proposes a LED linear constant current driving circuit, which has the characteristics of low cost, simple structure, high efficiency, small size, etc., and is very suitable for driving power supplies for indoor lighting LED lamps (such as LED fluorescent lamps).

1 LED linear constant current driving circuit
When using LED lamps, multiple lamp beads need to be connected in series or in parallel to work. Although the voltage required to drive multiple LEDs in parallel is lower, the forward voltage drop of each LED is different, so the brightness of each LED is different, unless a separate adjustment method is used to ensure that each LED has the same brightness. Therefore, the parallel method must ensure uniform brightness, which is more complicated to implement. The series method can ensure that the current flowing through each LED is the same and the brightness is consistent, which is a commonly used structure at present.
When the series driving method is adopted, if one or more LEDs fail and break the circuit (the short circuit has a small impact on the circuit and can be ignored), the circuit will break and cannot work normally. In order to avoid this defect, a voltage regulator can be connected in parallel at both ends of each LED (as shown in Figure 1). When a certain LED lamp bead is short-circuited, the parallel voltage regulator will be put into operation to ensure that the current of the series lamp bead remains unchanged. It should be noted that the voltage regulator value of the voltage regulator should be higher than the conduction voltage of the LED, otherwise the parallel voltage regulator will shunt part of the current and make the LED dim or even not light up.

This article adopts a series drive method, and its LED linear constant current control circuit is shown in Figure 2.

In the figure, Vz1, Vz2, VQ1, VQ2, R1, and R2 form a linear constant current source, which ensures that the current flowing through each white light LED is the same, so as to obtain uniform brightness. The LED driving power supply uses the mains direct rectification and filtering to obtain the DC working voltage for controlling the LED, without the need for boost or buck processing, so the power driving circuit is simple and the power efficiency is high. The LED used is a high-brightness white light LED (operating voltage range: 3.0~3.2V), and 94 LED lamp beads are used to form an LED fluorescent lamp.
Next, the working principle of the linear constant current source circuit is analyzed. The circuit adopts a complementary two-terminal constant current source structure, as shown in Figure 3.

Transistor VQ1, voltage regulator Vz1 and R1 form a constant current source, which provides a stable working current to voltage regulator Z2, while transistor VQ2, voltage regulator Vz2 and R2 form another constant current source to provide a stable working current to voltage regulator Vz1. Since the two constant current sources stabilize each other's voltage regulator working point, the stable voltages Vz1 and Vz2 and the total current flowing through the constant current unit no longer change, so the constant working current flowing through the LED can be guaranteed.
The relationship between each current and voltage in the figure is as follows:

The total working current flowing through the LED, ILED is:

The LED lamp beads we use are 0.06 W LEDs with a working current of 20mA. For the convenience of design, the selected circuit components are completely symmetrical, and the parameters are:

The constant current source circuit provides a working current of 20mA, which meets the requirements of the lamp beads used.

2 Test experimental results and analysis
The test wiring is shown in Figure 2. The voltage is changed by adjusting the autotransformer to simulate the change of the grid voltage. By simulating the fluctuation of grid voltage, the working characteristics of this constant current control circuit in the actual grid are tested to observe whether the working current of the LED will fluctuate greatly with the fluctuation of the external voltage, and the efficiency of this circuit under different voltages and the junction temperature of the LED are actually tested.
2.1 Characteristics of LED forward current changing with input voltage
Figure 4 is the relationship curve between the working current of the LED lamp string and the power supply voltage.

When the power supply voltage Ui increases from 220V to 250V, the rectified DC voltage changes from 310V to 350V, while the working current of the LED lamp string, If, changes from 20mA to 21.5 mA, which only changes by 1.5 mA. Therefore, it can be seen that the circuit has a good constant current effect, ensuring the basic stability of the brightness of the LED eye lamp.
2.2 The influence of input voltage change on the drive circuit parameters
Figures 5 and 6 show the changes of the DC voltage applied to the LED working circuit, the voltage applied to the LED lamp string, and the voltage borne by the constant current source as the input voltage changes. From Figures 5 and 6, it can be seen that after the input voltage is rectified, the DC voltage formed is divided by the LED lamp string and the linear constant current source. The more the divided voltage on the LED lamp string, the higher the efficiency of the input voltage. Therefore, through the analysis of Figure 6, it can be seen that the power efficiency of this LED drive circuit is high.

As can be seen from Figure 5, when the power input voltage is between 200 and 220 V, most of the DC voltage Ui (DC) after rectification and filtering is loaded on both ends of the LED, and the voltage division of the constant current control circuit is small; it can also be seen from Figure 6 that when the power input voltage is less than or equal to 220 V, the power efficiency is relatively high, reaching 98.31% at 220 V.
When the power input voltage Ui (AC) is higher than 220 V, as the input voltage increases, the voltage U (led) across the LED remains basically unchanged (about 291 V), and the increased input voltage is basically borne by the voltage U (constant) of the constant current control circuit, which ensures that the forward voltage of a single LED remains basically unchanged (3.1 V), is in a constant power working state, and the current and light emission remain stable.
2.3 The influence of input voltage change on LED junction temperature
The light decay of LED is closely related to the junction temperature. When the junction temperature rises, the forward voltage decreases, and the decrease in junction voltage leads to an increase in current. The increased current in turn causes the junction voltage to decrease, forming a vicious cycle. Junction temperature is an important cause of light decay. The higher the junction temperature, the earlier the light decay occurs and the shorter the life. Therefore, the change of junction temperature is also an important indicator for examining a driving circuit. Here, the method in the literature is used to measure the junction temperature:

where Tj(LED) is the LED junction temperature, TO is the test ambient temperature (20℃), VO is the initial forward voltage of the LED, VT is the forward voltage of the LED after thermal equilibrium (measured after 1 hour), and K is the temperature coefficient of the LED (-2mV/℃). The specific measurement is to change the input voltage. Under different input voltages, let the LED lamp cool down fully and measure the total voltage across the LED. This voltage is U(led initial). After 1 hour, measure the total voltage of the LED again under the same input voltage. This voltage is U(led end). Divide U(led initial) by the total number of LEDs to get the initial forward voltage VT of a single LED. Divide U(led end) by the total number of LEDs to get the forward voltage VO of a single LED after thermal equilibrium. Substitute it into formula (2) to obtain the junction temperature of the LED. The experimental results are shown in Figures 7 and 8.

As can be seen from Figures 7 and 8, the power input voltage Ui (AC) is different, but the voltage across the LED remains basically unchanged. After one hour, the voltage change is also very small. In this period of time, the junction temperature is low, and the junction temperature of a single LED remains basically unchanged or changes slightly. Therefore, this constant current source control circuit can ensure that the junction temperature of the LED is basically stable and small, which can reduce light decay and effectively improve the service life of the LED fluorescent lamp.

3 Conclusions
The circuit structure given in this paper is simple and reliable. The innovation lies in the application of the complementary constant current circuit to the constant current drive circuit of the LED. The experimental results show that when the input of the circuit changes from 200 to 250 V, the driving current changes by only 1.5 mA, and the LED junction temperature is lower than 37°C. These are the two most prominent key parameters affecting the operation of LED lamp beads, indicating that the circuit can provide a good working condition for the LED. In addition, the experimental results show that the circuit has a high power efficiency under normal mains power supply conditions (220 V). The circuit has been used on a 1.2 m LED fluorescent tube with good results. In the design process, attention should be paid to matching the circuit parameters according to the working current of the specific circuit.