Four-wheel drive LED street lighting power supply

LED street lights are a very important application in LED lighting. Under the premise of energy saving and power saving, the trend of LED street lights replacing traditional street lights is becoming more and more obvious. There are many designs of LED street light power supplies on the market. Early designs paid more attention to the pursuit of low cost; recently, a consensus has gradually formed that high efficiency and high reliability are the most important.

This article mainly proposes suitable architectures for the application of several different LED street lights and analyzes their advantages and disadvantages so that readers can find the most suitable solution based on the specific situation and the type of street light being designed.

Solution 1: Direct AC input, and constant current control for 6 LED strings

Among the solutions introduced in this article, this one should be the one with the highest efficiency and the lowest circuit cost (Figure 1). The primary side circuit is directly controlled by an optocoupler to adjust the output voltage. Compared with other traditional solutions, this solution has less switching loss. The voltage of CS is fixed at 0.25V, and constant current control is performed on each of the 6 strings of LEDs. The IC will detect the position of FB and fix the LED string with the lowest voltage at 0.5V. At this time, due to the different sum of the Vf values ​​of each string of LEDs, the voltage drop will fall on the MOS tube, resulting in some losses. If the LED is generally screened by BIN for Vf, the loss should be controlled within 2%, which is less than the general switching loss. The advantages of this solution are high efficiency and low cost, and the disadvantages are AC input and more R&D costs. This solution is suitable for street lamps that can be directly input by AC.

Solution 2: DC or battery input, constant current control for 6 LED strings

It adopts a multi-string boost structure design. The LED driving method is similar to the previous one, except that the AC input is changed to DC or battery input (Figure 2). As long as the appropriate MOS tube is selected for the low-voltage side sensor design, a considerable number of LEDs can be connected in series. Compared with the AC input solution, its design is simpler. However, due to the additional boost switch, the efficiency is relatively low. The advantages of the solution are simple design and low circuit cost, and the disadvantage is low efficiency. It is suitable for street lights with solar cells or input through adapters.

Solution 3: Single-string step-down structure

Some manufacturers still prefer to use a single-string design, which has the advantages of easy maintenance and modular design. Street lamps of different powers can use the same light bar. As long as the panel is replaced and different numbers of light bars are plugged in, street lamps of various powers can be combined. But its disadvantage is that each string requires an independent power module, which is more 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, the maximum number of LEDs in the string is 14. If you want to design a 20W light bar, you need to use 700mA LEDs. In order to achieve the highest efficiency, it is necessary to adjust the input voltage according to the number of LEDs, that is, the output voltage of the adapter. Taking 10 LEDs as an example, if you want to achieve the highest efficiency, you must adjust the input voltage to about 42V.

The advantages of this solution are high efficiency of step-down structure, single string design, and flexible configuration. The disadvantages are high circuit cost and the number of LED series connections is limited by the IC withstand voltage. It is suitable for street lights input through adapters.

Option 4:

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 withstand voltage of the IC, but is determined by the MOS, so more LEDs can be connected in series. 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 changes in the input voltage, allowing the street light to maintain the same brightness when the battery is fully loaded or running out of power.

The advantage of this solution is that the number of LEDs in series is not limited by the IC voltage resistance, but the disadvantage is that the circuit cost is higher and the efficiency is slightly lower than the step-down structure. It is suitable for solar street lights.

The following table compares and ranks the advantages of four power supply design schemes for LED street lights.