How to design a LED lighting application

Published : 06/24/2020 10:30:00

Within LED lighting projects great care must be taken in selecting the correct power supply, with suitable specifications to meet the needs of the application. By analyzing in more detail how LED modules are generally handled it is possible to better understand where design difficulties may arise and choosing the right power supply. Here are the three most common structures:



  • A constant current (CC) LED driver connects to LED directly, luminance controlled by LED driver’s output current
  • A constant voltage (CV) LED power supply, typically 12V or 24V, connects to LED strips which have resistor or current limiting circuit on the LED PCB; the resistor or current limiter defines the current and luminance
  • Cascade connection of a CV power supply and a DC-to-DC converter (DC/DC) and then to the LED board



For the first two structures, power supplies are connecting to the LED without active conversion in between. The selection of power supply is relatively simple, either the right current or the right voltage should be selected. The third driving structure includes an active DC/DC in between which could bring certain compatibility concerns.



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Progetto di illuminazione LED d'interni



Types of DC/DC converters



To better understand how to identify the potential criticalities seen above, it is necessary to focus on the main types of DC/DC converters on the market:

  • Buck (step down conversion)
  • Boost (step up conversion)
  • Buck-boost

Most of the DC/DC converters have a wide operating range of input voltage. Within this input range, DC/DC converter will convert the input power to the desired output voltage and/or current to the load. If the load is fixed, the input power will also be fixed. Since the power is the same when input voltage is high, then input current will be low, and vice versa. Power conversion formula is shown below:



Formula di conversione della potenza di alimentatori led



Compatibility problem between CV power supply and DC/DC converter



Off-the-shelf DC-driven LED spotlight normally contains such converters inside. On the manual of those products normally only a fixed input voltage is specified, e.g. “24VDC input”. When applying this voltage to the lamp, it will certainly work; however, the actual working range might be much wider and lower than this rated input voltage due to the DC/DC converter used inside.



Progetti di illuminazione LED d'interni



To explain the phenomenon better, we take Mean Well LDD-350H DC/DC converter as an example. LDD-350H is a DC/DC converter that provides 350mA constant current to its load. The input voltage range is 9~56VDC, and the output range, which is related to LED forward voltage, is 2~52VDC. Since it is a step-down converter, the input voltage should be 3V higher than the output voltage according to the specification. This means, if the LED is 9V, LDD-350H can work at any of the input voltage between 12~56VDC. A lamp with this LED and converter built-in might very likely specify its input voltage as“24VDC input” as 24V power supply are commonly available in the market.



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With insufficient information, the system installer might overlook the potential compatibility issue and suffer from startup failure such as blinking, darker light output, or simply no light at all. The issue is related to the transient response at power on and the behavior of DC/DC converter.

In the next article will be explained the possible situations of incompatibility between power supplies and converters and the solutions to overcome this problem.



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