Detailed method analysis of LED connection and low-power LED driver circuit design of constant current diode

Due to the low price, low heat generation and high luminous efficiency of low-power white LEDs, they have been widely applied to general lighting and landscape lighting. Therefore, there is an urgent need for the design and performance improvement of low-power LED driving power supplies. LED illuminators are usually powered by mains. Due to the low operating voltage of LEDs and low current, driving LEDs with mains to solve the buck and rectification problems requires higher efficiency, smaller size and lower cost. LED is a current-driven device whose brightness is proportional to the forward current. In order to ensure efficient and uniform LED illumination, the LED driver should be a constant current output. Therefore, designing an efficient, simple, and inexpensive LED driver has become a research hotspot. In this paper, a low-power LED driving circuit based on constant current diode is designed. The power frequency is supplied by the mains and the output current is constant.

When designing an LED lighting system, it is necessary to consider what kind of LED driver to use, and how to connect the LEDs. Only a reasonable matching design can ensure the normal operation of the LED. The low-power white LEDs typically have a forward voltage range of 2.8 to 4 V and an operating current of 15 to 20 mA. LED lamps for illumination are typically a plurality of such low power LEDs that are combined in series and parallel fashion, and these LEDs typically need to be matched to produce uniform brightness. It is also necessary to connect these LEDs together in a reasonable manner. The entire LED lamp group cannot be operated because one of the LED lamp beads is damaged.

A plurality of LEDs of the same type are connected in series, and the current flowing through each LED is equal. When the LEDs are in poor agreement, although the forward voltages of different LED beads are different, the current flowing through each LED is equal, and the brightness of each LED bead will be the same. LED series connection drive source output voltage requirements are large, the current must be constant below 20 mA. When one of the LEDs is disconnected due to poor quality, the entire LED lamp group will not light, which requires high quality and soldering process for the LED lamp bead.

All the LED beads are connected in parallel, and the driver needs to output a large current, and the output voltage is about 3 V. Parallel connection avoids the serious drawback of extinguishing the entire lamp group after one LED burns out. Since the luminous intensity of the LED is proportional to the operating current, there are certain differences between the parameters of the LED lamp bead, and the current flowing through each LED bead is inconsistent, which directly leads to uneven brightness of the LED. When driving parallel LEDs in constant current mode, the LED lamp beads should be connected in parallel as much as possible to prevent the current flowing through other LEDs from burning out due to the burning of several LEDs.

In order to improve reliability and uniformity of illumination, a cross-array connection method is proposed, and FIG. 1 is a diagram of an LED cross-array connection mode.

It can be seen from the figure that when the individual LEDs are short-circuited or open, the entire lamp group will not be extinguished. The cross-array connection method has the characteristics of simple circuit, stable brightness, high reliability, and low driving requirements.

2.1 Capacitor Buck Circuit
When the LED is powered by AC mains, it must be converted to low-voltage DC by AC/DC and DC/DC conversion. Currently, the step-down circuit mainly has a power-frequency transformer linear step-down circuit, a high-frequency switch circuit, and an IC-based Buck circuit, capacitor step-down circuit and so on. Considering the size and cost of the driving power supply, this paper uses a capacitive step-down circuit. Figure 2 shows the capacitor step-down circuit:

In Figure 2, the charge and discharge current of the non-polarity step-down capacitor C1 is IC=2πfCU0 (U0 is the AC voltage, f is the AC frequency), and the current I0 supplied from the step-down capacitor C1 to the load is actually the charge and discharge current IC flowing through C1. . When the load current is less than the charge and discharge current, the excess current will flow through the filter capacitor C2. If U0=220 V, f=50 Hz, IC=69C (IC is mA, C is μF). In order to ensure safe and reliable operation of the step-down capacitor, the withstand voltage should be greater than 2 times the mains voltage, so the step-down capacitor should use a monolithic capacitor with a withstand voltage of 630 V. R1 is a 1 MΩ discharge resistor. When the circuit is powered off, C1 is quickly discharged through R1, and D1—D4 is a full-wave rectifier bridge consisting of IN4007. In order to obtain better filtering effect, the capacity of the selected filter capacitor should satisfy RLC=(3~5)T/2 (RL is the load resistance, T is 0.02 s), and the withstand voltage should be greater than 1.1 2 U0 (U0 is the capacitor) Buck circuit output voltage). In principle, the larger the capacitance value is, the smoother the output voltage is, the smaller the ripple value is. However, as the capacitance increases, the volume generally increases. When considering the board area, a large-capacity filter capacitor should be selected.

2.2 Low-power LED driver circuit for mains supply The mains-powered low-power LED driver circuit based on constant current diode is shown in Figure 3:

In the figure, D5 and D6 are constant current diodes. The constant current diode used in this design is the 2DHL series of Guizhou Bo Yue Company. The 2DHL series of constant current diodes is a basic electronic device made of silicon.

It can be seen from the figure that the circuit enters a steady state at 100 ms. After stabilization, its output voltage is 25V and the output current is constant at 60 mA. The feasibility of a low-power LED driver circuit based on a constant current diode is basically verified.