Capacitor step-down LED landscape lighting driver solution

Against the backdrop of advocating green environmental protection and energy conservation at home and abroad, along with the significant progress in LED lighting efficiency, lifespan and light color in recent years, semiconductor light emitting diodes (LEDs) have been widely used in landscape lighting, digital traffic information display, traffic signage, outdoor full-color display screens, and indoor and outdoor lighting with low illumination requirements due to their advantages of energy saving, long life, simple driving, and a wide variety of light colors.

In view of the current application status of LED in urban lighting, the author uses power electronic rectification and constant current technology to design and produce an LED landscape lighting lamp, which has the characteristics of low cost, reliable operation, long life, simple control, and can be expanded according to actual needs.

1 Low power LED driving solution

At present, two types of drive circuits are widely used in low-power LED products, namely constant current drive and voltage stabilization drive. The current output by the former circuit is constant, and the output voltage changes with the load; the latter output voltage is fixed, and the output current changes with the increase or decrease of the load (number of LEDs). According to the power input and output circuit forms, it can also be divided into isolated drive and non-isolated drive. The former is represented by switching power supply, while the latter mainly includes capacitor step-down and constant current/constant voltage IC. Its related performance, power consumption, cost and main applications are shown in Table 1.

Table 1 Comparison of low power LED drive solutions

2 LED landscape lighting circuit design

LED landscape lighting is to achieve the alternating lighting and gradual control of RGBLED. The circuit design is shown in Figure 1. The circuit uses 3181 integrated chip as the controller. Its related functions and pins refer to the literature [4]. A total of 36 low-power LEDs are controlled, using 12 RGB three-primary color LEDs each, and the driving scheme adopts capacitor step-down.

Among them, C1 and C5 are two identical capacitors connected in parallel, which play the role of voltage reduction and current limiting; the rectifier bridge composed of 4 lN4007s rectifies the input AC voltage; the filter capacitor C4 is used to filter out the AC component in the rectified output voltage to make the voltage smoother; the Schottky diode VD5 stabilizes the voltage at point a; VD6 provides a stable voltage for the control IC; R3 and R4 are current limiting resistors; R2 and C2 provide the AC input frequency for the control IC and the AC sampling frequency for the integrated IC; the transistors VT1, VT2, and VT3 are used to control the current change of the LED branch.

Figure 1 LED landscape lighting circuit

2.1 Selection of step-down capacitor value

The current I0 provided to the load through the step-down capacitors C1 and C5 (represented by C in the following text) is actually the charge and discharge current lc. When the load current, I0, is less than the charge and discharge current of c, the excess current will flow through the filter capacitor C4 and other branches.

Where Ui is the effective value of input AC voltage.

f--AC frequency.

For the 60 mA current I0 consumed by the load, at least a step-down capacitor value of 0.87uF is required. Because a high-voltage Zener diode is added in Figure 1 to step down the voltage, and the diode also consumes a large current when working. In addition, to ensure that the capacitor C works reliably, its withstand voltage should be greater than 2 times the power supply voltage. Therefore, two 684/630 V capacitors should be selected to work in parallel.

2.2 Rectification and filtering circuit

According to the literature [7], the maximum voltage that a single diode on the rectifier bridge can withstand is URM = √2Ui (Ui is the effective value of the input voltage) = 318.4 V. Therefore, the commonly used rectifier diode lN4007 (URM = 1000 V, IF = 1 A) is selected.

In order to obtain a smooth load voltage at the output end, RLC is generally taken as ≥ (3~5) T/2, where RL is the load impedance and T is the input signal period (0.02 s), and C is ≥ 24.38 uF.

In principle, the larger the capacitance value, the smoother the output voltage and the smaller the ripple value. However, as the capacitance increases, its volume generally increases as well. Considering the area of ​​the circuit board, a 47 uF/160 V electrolytic capacitor is used.

2.3 System power circuit design

Because the circuit requires two sets of voltages, one for driving three strings of LEDs, and one for providing working power (5 V) for the integrated IC. The voltage regulation value of VD5 is directly related to the number of LEDs driven in a single string. In the previous article, it is required to drive 12 LEDs in a single string, and it is assumed that the forward voltage drop Uf of the red LED is 2 V, while that of the green and blue LEDs is 3.2 V for calculation. The voltage Ua at point a in Figure 1 is ΣU+UR, where ΣU is 38.4 V and UR is the voltage drop on the balancing resistor. Therefore, the voltage regulation value of the voltage regulator diode VD5 should be greater than 38.4 V. Considering the voltage margin and the cost and convenience of obtaining components, VD5 uses a 50 V/1 W high-voltage voltage regulator diode, and VD6 uses a 5.1 V/0.5 W voltage regulator diode. According to the literature [1], the current limiting resistor R3 is selected as 62Ω/2 W, and R5 is 4.7 kΩ and 22 kΩ respectively.

In order to make the brightness of each color LED consistent, it is necessary to connect a balancing resistor in series with each LED string. The value of the balancing resistors R14 and R12 connected in series with the blue and green LEDs is UR/I (where UR=11.6V; I is the current flowing through the branch LED, which is 20 mA). Then R=580Ω. The actual balancing resistor R14=R12=680Ω. Similarly, R9=1.4kΩ.
2.4 System circuit cost estimation

To further illustrate the cost advantage of this circuit, the author lists the main component types and prices in the circuit in Table 2 based on the price report of the component supplier.

Table 2 LED landscape light driver circuit cost estimate.

As can be seen from Table 2, the cost of making the entire power supply is about 5.3 yuan; if other solutions are used, the price of only the small-power switching power supply is 7 to 15 yuan.

3 Trials and tests

Taking the LED underground lamp (a kind of landscape lighting decoration) as an example, the above circuit was welded on a single-sided universal welding board for testing. All circuits were connected, 36 LED lamp beads were inserted and welded on the disc lamp board with a diameter of 112 mm, the driving circuit was installed at the bottom of the underground lamp, and epoxy potting glue (waterproof and moisture-proof) was filled in all gaps. When the AC power supply is turned on, the colors of the LED strings of various colors change alternately according to the pre-set color. After the plastic cover is covered, the colorful color changes synthesized by the three primary colors of RGB can be seen, which basically meets the design requirements.

The relevant parameters of the circuit were tested using Huzhou Shengpu SPL2000 electric light source light color and electricity comprehensive analysis system. The results are as follows:

When the input voltage UAC is 220 V, the total current is 53 mA, the actual power consumption of the circuit is 1.4 W, and the power factor is 0.385.

To further verify the reliability of the circuit, the author conducted a 1000h electrical aging test on the product. The results showed that the circuit can provide stable and reliable driving for LEDs.

4 Conclusion

At present, although semiconductor light emitting diodes (LEDs) have many problems to be solved in conventional lighting technology, such as low brightness, prominent heat dissipation problems, and low white light life, they have been widely used in landscape lighting where brightness requirements are not high.

This paper designs and manufactures an LED driving circuit for landscape lighting in urban lighting. After long-term electrical aging tests, it is confirmed that the circuit has a simple structure, reliable operation, and low cost. It is particularly suitable for low-end market applications in the context of LED lamp beads occupying a large proportion of the total cost. The number of LED lamp beads in each string can also be adjusted according to actual needs, and the applicability is relatively wide. The number of LED lamp beads in each string can be adjusted arbitrarily within a certain range by changing the voltage value of the high-voltage voltage regulator. It is also possible to use a power resistor instead of a voltage regulator to make the LED light string and the integrated circuit not share the same ground. However, the flexibility and scalability of the physical circuit must be considered in the early stage of PCB board design.