LED Driving Design Structure

LED modules are typically driven by one of the three structures listed below:
- 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 or tubes which have resistor or simple 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. The output current is controlled by the DC/DC converter only.  

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. In the following chapters, we will go through DC/DC working mechanism and potential problem when selecting a power supply with the wrong specification.  

DC/DC Converter

DC/DC converters have 3 types: Buck (step down conversion), Boost (step up conversion) and Buck-boost. No matter which converters they are, most of them 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:

Potential Compatibility Issue between CV Power Supply and DC/DC Converter

Off-the-shelf DC-driven LED tubes or 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.

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. 

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. More details are explained below: