LED lighting industry has been evolving for tens of years and there are always questions about “isolated and non-isolated” , which one is better? In the past, this kind of debate only existed in the low and mid power designs but nowadays, the question goes on to high power applications like sports lighting and harbor lighting.
My answer is that we can only say that which one is more suitable for certain applications or luminaire designs. Both of them have the reasons to be utilized in the market. And this article provides some ideas for luminaire designers on how to choose between the two kinds of LED drivers.
The difference between non-isolated and isolated LED driver
This figure below shows the details about the two structures. The key difference is about the isolation transformer which insulate the output from the high AC input voltage.
And the table below shows more about the electrical differences.
|Topology||Buck,boost,buck-boost etc.||Flyback,forwardhalf bridge LLC etc.|
|Complexity||Low||Mid and high|
|Output Voltage||High||Mid and low|
|Output Safety||Unsafe||Relatively safe, safe SELV or class 2 can be achieved|
|Luminaire Design||More insulation needed||Easy|
Isolated dimming and isolated LED drivers
Both concepts can be easily mixed together when we are talking about “Isolated” since this word can be used for describing both dimming or LED driver itself. They are totally different concepts. Isolated dimming means that the dimming interface like DALI, 0-10V or PWM signals are insulated from input, output or physical earth. So no matter the LED driver output is safe or not, at least the dimming controllers which are connected directly to the luminaires are safe. And that is the reason isolated dimming should be employed by users. According to IEC61347-1, all the luminaires no matter non-isolated LED drivers are used or not, the dimming interface or circuitry has to be isolated separately. The electric strength level of input, output, case and dimming interface is shown as below table.
How to choose and design with non-isolated LED drivers.
It is not easy to find a right non-isolated LED driver for high power design, the following has to be considered carefully:
a.Efficiency and thermal. Non-isolated LED drivers have the advantage efficiency compared with conventional isolated LED drivers. But it does not mean efficiency is not important. Better efficiency means lower power loss and less heat generation thus the LED LED driver can have longer life and higher reliability with better thermal situation. Checking the case temperature at Tc point is mandatory for a good luminaire design.
b.Dimming performance. Using a lower cost LED driver does not mean we have to sacrifice performance. Smooth dimming with either 0-10V or DALI or DMX control compatibility is important for stadium lights, especially the fast dimming response.
c.Find the right input voltage range for different countries.
d.Try to use the high LED voltage to achieve the best efficiency.
e.Utilize the LED board with higher insulation. 3kV insulation voltage is suggested.
f.Do adequate insulation for the whole luminaire to make sure that all the points that can be touched by human body are safe and able to pass insulation test.
uPowerTek’s high power non-isolated LED drivers are certified with UL, ENEC, CB and RCM, and they can be good choices for the high power applications.
How do you solve the glow issue for grow light when dimming off?
The growers now more and more realized that the very little light we call it “glow” in this article can impact the yield of plant greatly as the famous professor Dr. Bruce Bugbee indicated in his course. So understand why the “glow” happens and how to avoid the “glow” is important for luminaire designers.
Why the “glow” happens
There are two ways to turn off the lights, one is to cut AC input off and the other is to “dim off” using the dimming control signals. Simply cutting off the AC input is surely able to prevent the light from glow because there is not energy path to the LED at all, but it is not in our discussion range since this is not a “smart” or “modernized” way to deal with grow light industry.
Today we only talk about the glow in the “dim off” status with AC input connected to the luminaires. There are two reasons as below:
a.Output voltage of the LED driver is not low enough during the “dim off”. There are a lot of LED driver using lower output voltage to low level to realize the dim-off.
b.The LED chips and the capacitors including the Y capacitor inside driver and parasitic capacitors between LED and earth form a current path to make the glow happen. As the figure shows below, the line input AC voltage gives the energy through CY1, Cp1 and Cp2 to the LED. CY1 is the Y capacitor and Cp1/Cp2 is the parasitic capacitor between LED and LED board heatsink which is connected to earth. For isolated drivers, CY1 is much bigger than Cp1 and Cp2, so the issue is not serious in most of the cases. But for non-isolated drivers, CY1 is almost zero, thus the common mode voltage is actually directly applied to Cp1 and Cp2, and when Cp1 is not equal to Cp2, there can be voltage difference between Vout+ and Vout- thus makes the LED glow.
How to avoid the “glow”
Knowing the reason of the glow, we are able to solve the problem by the following ways.
a.Do NOT use the way of lowering output voltage to dim off because different LED behaves differently and we don’t know by how much voltage, the LED may conduct. So the eliminate the possibility, we need to totally shutdown the LED driver rather than just lowering down the output voltage. Also there is another benefit that the standby power is much lower because the driver is in “sleep mode”. But the other result is that the cost of the driver is higher by adding such kind of circuitry inside the driver.
b.To avoid the common mode leakage current through the parasitic capacitors, we have two ways, either disconnect the earth or disconnect the input/output during the dim-off status. Obviously we are not allowed to disconnect the earth by safety standards with AC line connected. So the only way to have to do is to disconnect the input or output to LED by an internal relay. Also the drawback is the higher cost.
Check uPowerTek Glow-free non-isolated LED Drivers
Making sure of the darkness for cannabis and vegetable grow is so important that the LED driver manufacturers like uPowerTek is paying great efforts to the designs to avoid “glow” in off status. For the luminaire designers, you have to make sure that the luminaire is fully tested in the dark room before finalizing the product. Please feel free to let us know if you have any questions.