In this project, we’re going to design a LED driver for an adjustable white balance LED panel that we will be designing in my next article. The LED panel will be using high color rendering index LEDs with cool, neutral and warm white balances, therefore this driver board will incorporate 3 LED drivers. There’s also a low-cost microcontroller and a couple of slide potentiometers to control the brightness and white balance.
This project is open source, you can find the project files over on my GitHub released under the permissive MIT License. This allows you to do what you wish with the design files, at your own risk, including using the design in part or whole for commercial purposes.
LED Driver Design Constraints
My drive for creating this project is two-fold: replace some failing studio light panels I use for filming, and to add additional lighting above my electronics work area. This immediately set a couple of requirements and constraints on the project.
Input power needs to be able to come from a V-Mount cinema battery, with a nominal voltage of 14.8V from four lithium polymer cells, or from an AC adapter. 15V high efficiency AC adapters are readily available, so my DC input will be planned around a nominal 15V input voltage. The peak output wattage of readily available, low cost 15VDC output power adapters is around 50W. This caps our maximum peak voltage and wattage for the driver.
The LEDs for the panel will need to be easy to source, high Color Rendering Index (CRI) and I want to have the highest efficiency possible to maximize battery run time. I was hoping for LEDs with a CRI of 95 or higher, as this would be perfect for filming assembly work - however, at my regular component suppliers they were either expensive, too inefficient or not in a package I was looking to work with or did not have all the white balance options within the series. Therefore, I settled for one of the most efficient series of high quality LEDs, the Samsung LM281B+ series. These LEDs have a forward voltage of 2.8V and are most efficient at 65mA of drive current, creating an impressive 165 lm/W according to the datasheet. If both the LED datasheet and the marketing claims of my existing light panels can be believed, this makes these LEDs almost three times more efficient than my existing panels which use high CRI through hole LED bulbs.
Selecting an LED Driver
We have everything we need to select a driver now that the input supply and LEDs have been locked down. To maintain high efficiency, the design will need to be a switched mode supply - so the major choice that comes next is whether to go with a boost or a buck supply. Initially, I favored a boost driver, to run more LEDs in series and keep the current low. Unfortunately, I couldn’t find anything that was in stock, efficient and worked with my constraints.
After simulating various drivers, I settled on the Diodes Inc AL8863 regulator. It is a buck regulator which would allow me to drive four LEDs in series. The warm white and cool white channels will have 20 LED strings in parallel, with the neutral white having 28 LED strings in parallel. This is 14.5W per warm/cool channel, and 20.3W for the neutral white channel - plus losses. While this total is over the 50W we specified, the microcontroller will manage the total current mix for the drivers based on its brightness and color balance slider input, thereby ensuring the peak current is not exceeded.
Having 3 driver channels allows the neutral white to be used as the primary light source, with the cool or warm channels allowing the color temperature of the panel to be fine tuned to match ambient light conditions ensuring natural fill lighting from the panel.
Basic Driver Circuit
The Diodes Inc AL8863 LED driver is relatively new, and very well featured so it was surprising to find it exceptionally well stocked. The datasheet has less details on implementation than most other drivers I’ve worked with, however there is a fantastic spreadsheet which takes care of all the calculations for you which is impressively easy to use.
In this project, we’re going to design a LED driver for an adjustable white balance LED panel that we will be designing in my next article. The LED panel will be using high color rendering index LEDs with cool, neutral and warm white balances, therefore this driver board will incorporate 3 LED drivers. There’s also a low-cost microcontroller and a couple of slide potentiometers to control the brightness and white balance.
This project is open source, you can find the project files over on my GitHub released under the permissive MIT License. This allows you to do what you wish with the design files, at your own risk, including using the design in part or whole for commercial purposes.
LED Driver Design Constraints
My drive for creating this project is two-fold: replace some failing studio light panels I use for filming, and to add additional lighting above my electronics work area. This immediately set a couple of requirements and constraints on the project.
Input power needs to be able to come from a V-Mount cinema battery, with a nominal voltage of 14.8V from four lithium polymer cells, or from an AC adapter. 15V high efficiency AC adapters are readily available, so my DC input will be planned around a nominal 15V input voltage. The peak output wattage of readily available, low cost 15VDC output power adapters is around 50W. This caps our maximum peak voltage and wattage for the driver.
The LEDs for the panel will need to be easy to source, high Color Rendering Index (CRI) and I want to have the highest efficiency possible to maximize battery run time. I was hoping for LEDs with a CRI of 95 or higher, as this would be perfect for filming assembly work - however, at my regular component suppliers they were either expensive, too inefficient or not in a package I was looking to work with or did not have all the white balance options within the series. Therefore, I settled for one of the most efficient series of high quality LEDs, the Samsung LM281B+ series. These LEDs have a forward voltage of 2.8V and are most efficient at 65mA of drive current, creating an impressive 165 lm/W according to the datasheet. If both the LED datasheet and the marketing claims of my existing light panels can be believed, this makes these LEDs almost three times more efficient than my existing panels which use high CRI through hole LED bulbs.
Selecting an LED Driver
We have everything we need to select a driver now that the input supply and LEDs have been locked down. To maintain high efficiency, the design will need to be a switched mode supply - so the major choice that comes next is whether to go with a boost or a buck supply. Initially, I favored a boost driver, to run more LEDs in series and keep the current low. Unfortunately, I couldn’t find anything that was in stock, efficient and worked with my constraints.
After simulating various drivers, I settled on the Diodes Inc AL8863 regulator. It is a buck regulator which would allow me to drive four LEDs in series. The warm white and cool white channels will have 20 LED strings in parallel, with the neutral white having 28 LED strings in parallel. This is 14.5W per warm/cool channel, and 20.3W for the neutral white channel - plus losses. While this total is over the 50W we specified, the microcontroller will manage the total current mix for the drivers based on its brightness and color balance slider input, thereby ensuring the peak current is not exceeded.
Having 3 driver channels allows the neutral white to be used as the primary light source, with the cool or warm channels allowing the color temperature of the panel to be fine tuned to match ambient light conditions ensuring natural fill lighting from the panel.
Basic Driver Circuit
The Diodes Inc AL8863 LED driver is relatively new, and very well featured so it was surprising to find it exceptionally well stocked. The datasheet has less details on implementation than most other drivers I’ve worked with, however there is a fantastic spreadsheet which takes care of all the calculations for you which is impressively easy to use.
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