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Four design schemes and comparison of LED street lamp power supply

2021-02-24

ALLTOP

Under the premise of saving energy and electricity, the trend of LED street lamps replacing traditional street lamps is becoming more and more obvious. On the market, there are many designs of LED street lamp power supplies.

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u003cbru003e Early design paid more attention to the pursuit of low cost; in the near future, consensus has gradually formed that high efficiency and high reliability are the most important. LED street lights are a very important application in LED lighting.

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u003cbru003e This article proposes a suitable architecture for several different LED street lamp applications, 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 lamp designed.

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u003cbru003e Solution 1 Direct AC input, and constant current control for 6 strings of LEDs

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u003cbru003e Among the several schemes introduced in this article, this one should be the one with the highest efficiency and the lowest circuit cost (Figure 1). Directly use the photocoupler to backtrack the primary side circuit and adjust the output voltage.

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u003cbru003e Compared with other traditional schemes, this scheme has less switching loss. Fix the voltage of CS at 0.25V, and perform constant current control on the 6 strings of LEDs.

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u003cbru003e IC will detect the position of FB and fix the LED string with the lowest voltage at 0.5V. At this time, because the sum of the Vf values u200bu200bof the LED strings is different, the voltage drop generated will fall on the MOS tube, causing some loss.

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u003cbru003e If it is an LED that is generally screened for Vf by BIN, the loss should be controlled within 2%, which is less than the general switching loss.

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u003cbru003e 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 scheme is suitable for street lamps that can be directly input by AC.

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u003cbru003e Scheme 2 DC or battery input, constant current control for 6 strings of LEDs respectively

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u003cbru003e It uses a multi-string boost structure design, the LED drive method is similar to the previous one, the difference lies in the change from AC input to DC or battery input (Figure 2).

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u003cbru003e For the design of low-voltage side sensing, as long as you choose an appropriate MOS tube, you can string a lot of LEDs.

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u003cbru003e Compared with the AC input scheme, its design is relatively simple. However, the efficiency is relatively low due to an additional boost switch.

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u003cbru003e The advantage of this scheme is simple design and low circuit cost, but the disadvantage is low efficiency. It is suitable for solar batteries or street lights input through an adapter.

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u003cbru003e Solution three single string step-down structure

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u003cbru003e Some manufacturers still like to use the 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 plugged in, street lights of different powers can be combined.

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u003cbru003e But its disadvantage is that each string requires an independent power module, which is costly, and the step-down structure will limit the number of LEDs to the IC's withstand voltage.

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u003cbru003e In the example shown in Figure 3, there are up to 14 LEDs in string. If you want to design a 20W light bar, you need to use a 700mA LED.

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u003cbru003e In order to maximize the efficiency, the input voltage must be adjusted for the number of LEDs, which is the output voltage of the adapter. Take 10 LEDs as an example, if you want to achieve the highest efficiency, you must adjust the input voltage to about 42V.

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u003cbru003e The advantages of this solution are higher efficiency of the step-down structure, single string design, and more flexible configuration. The disadvantage is that the circuit cost is higher and the number of LEDs in series is limited by the IC withstand voltage. It is suitable for street lights input through an adapter.

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u003cbru003e Solution four single string boost structure RT8480

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u003cbru003e 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 it can be connected in series More LEDs.

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u003cbru003e Since the output voltage of most solar cells is not high, solar street lights are more suitable for using boost structure.

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u003cbru003e The constant current design of the current mode can make the output current less affected by the change of the input voltage, and the street lamp can maintain the same brightness when the battery is fully loaded or almost out of power.

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u003cbru003e The advantage of this scheme is that the number of LEDs in series is not limited by the IC withstand voltage. The disadvantage is that the circuit cost is higher and the efficiency is slightly lower than that of the buck structure. It is suitable for solar street lights.

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u003cbru003e The following table compares and ranks the advantages of the four power supply design schemes for LED street lights.

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