Comparison of solutions for four kinds of LED street lights
LED street light is a very important application in LED lighting. Under the premise of saving energy and electricity, the trend of LED street lamps replacing traditional street lamps is becoming more and more obvious. There are many designs of LED street light power supply on the market. The early design paid more attention to the pursuit of low cost; in the near future, a consensus has gradually formed, and high efficiency and high reliability are the most important.
This article is mainly for the application of several different LED street lights, proposes a suitable architecture, and analyzes its advantages and disadvantages, so that readers can find the most suitable solution according to the specific situation and the type of street lights designed.
Option 1: Direct AC input, constant current control for 6 strings of LEDs respectively
Among the several schemes introduced in this article, this one should be the one with high efficiency and less circuit cost at present (Figure 1). Directly use the optocoupler to perform retrospective control on the primary side circuit to adjust the output voltage. Compared with other traditional schemes, this scheme has less switching loss. The CS voltage is fixed at 0.25V, and the 6 strings of LEDs are respectively controlled by constant current. The IC will detect the position of FB and fix the string of LEDs with less voltage at 0.5V. At this time, since the sum of the Vf values of each string of LEDs is different, the generated voltage drop will fall on the MOS tube, resulting in some losses. If it is an LED that is generally screened for Vf and BIN, the loss should be controlled within 2%, which is less than the general switching loss.
The advantages of this scheme are high efficiency and low cost, but the disadvantage is that AC input requires more research and development costs. This solution is suitable for street lights that can be directly input with AC.
Option 2: DC or battery input, constant current control for 6 strings of LEDs respectively
It adopts a multi-string boost structure design, and the LED driving method is similar to the previous one, except that the AC input is changed to DC or the battery input (Figure 2). In the design of low-voltage side sensing, as long as appropriate MOS tubes are selected, a considerable number of LEDs can be connected in series. Compared with the AC input scheme, its design is simpler. However, the efficiency is relatively low due to the addition of a boost switch.
The advantages of this scheme are simple design and low circuit cost, but the disadvantage is low efficiency. It is suitable for solar cells or street lights with input via adapter.
Option 3: Single series step-down structure
Some manufacturers still prefer to use a single-string design, which has the advantage of easy maintenance and modular design. Street lights of different powers can use the same light bar, as long as the panel is replaced and a different number of light bars are inserted, various street lights of different powers can be combined. But its disadvantage is that each string requires an independent power module, which is expensive, and the step-down structure will limit the number of LEDs to the withstand voltage of the IC. In the example shown in Figure 3, there are at most 14 LEDs in series. If you want to design a 20W light bar, you need to use 700mA LEDs. In order to achieve high efficiency, the input voltage, ie the output voltage of the adapter, must be adjusted for the number of LEDs. Take 10 LEDs as an example, if you want to achieve high efficiency, you must adjust the input voltage to about 42V.
The advantages of this scheme are that the step-down structure has high efficiency, single-string design, and configuration is more flexible. It is suitable for street lights input through an adapter.
Scheme 4: RT8480 with single-string boost structure
For the same single-string design, the boost structure (Figure 4) will be less efficient than the buck structure, but the number of LEDs in series is no longer limited by the IC's withstand voltage, but is determined by the MOS, so more LEDs can be connected in series LED. Since the output voltage of most solar cells is not high, solar street lights are more suitable for using a boost structure. The constant current design of the current mode can make the output current less affected by the change of the input voltage, so that the street lamp can maintain the same brightness when the battery is fully loaded or when it is about to run out of power.
The advantage of this scheme is that the number of LEDs in series is not limited by the IC withstand voltage, but the disadvantage is that the circuit cost is higher and the efficiency is slightly lower than that of the step-down structure. It is suitable for solar street lights.
