What Type of LED Driver You Need Constant Current vs Constant Voltage


Like regular diodes, light emitting diodes (LEDs) are constructed using a semiconducting material which has been “doped” with impurities in order to create a P-N (positive-negative) junction. Current flows easily from the P-side (anode) to the N-side (cathode), it’s known that LED drivers are considered constant current devices, so why do manufacturers offer constant voltage drivers for LEDs as well? How can we tell the difference between these two?

The reason for this is to give the light fixture designers a number of options in optimizing their system. If many strings of LEDs are used in series, the most efficient way to drive them is to use a constant current power supply and connect the LEDs directly across the terminals of the power supply. However, if strings of LEDs are connected in parallel, there may be an issue in trying to match the current in all the strings. A possible alternative to this would be to place an external component or active circuit to control the current. This may result in a slightly less efficient overall number of lumens per watt, but it allows the user to have full flexibility in ensuring that an identical current flows though many LED strings in parallel.

How do I know what type of LED driver I need?

If you take a look at high powered LEDs, one unique characteristic is the exponential relationship between the applied forward voltage to the LED and the current flowing through it. You can see this clearly from the electrical characteristics of the Cree XP-G2 below in Figure 1. When the LED is turned on, even the smallest 5% change in voltage (2.74V to 2.87V) can create a 100% increase in current driven to the XP-G2 as you can see at the red marks current went from 350mA to 700mA.

electrical characteristics of the Cree XP-G2

Now higher current does make the LED brighter, but it also will eventually over-drive the LED. See Figure 2 for Cree’s specifications of the maximum forward current and the de-rating curves in different ambient temperature conditions. In the example above we would still be alright driving the XP-G2 LED at 700mA, however, if you didn’t have a current limiting device, the current would continue to rise as temperature did. This would eventually push the current way above the limit…especially in hotter environments. The excess forward current would result in extra heat within the system, cut down on the LEDs lifespan, and eventually ruin the LED. We call this thermal runaway which is explained in more detail here. This is the reason the preferred method of powering high powered LEDs is with a constant current LED driver. With a constant current source, even as the voltage changes with temperature the driver keeps the current steady while not over driving the LED and preventing thermal runaway.

Cree specifications de-rating curves

When do I use a constant voltage LED driver?

The above example is with high powered LEDs and on a smaller scale as we only talked of using one LED. With lighting in the real world, it isn’t convenient or economical to build everything by hand from a single diode, LEDs are usually used together in series and/or parallel circuits to create the desired outcome. Fortunately for lighting designers, manufacturers have introduced many LED products to the market that have multiple LEDs already assembled together like LED rope light, LED strips, LED bars, etc.

The most common LED strips are designed with a group of LEDs in series with a current-limiting resistor in line with them. The manufacturers make sure the resistors are of the right value and in the right position so that the LEDs on the strips will be less prone to the variation of the voltage source as we talked of with the XP-G2. Since their current is already being regulated, all they need is a constant voltage to power the LED(s).

When LEDs or an array of LEDs are constructed like this they will typically state a voltage to be run at. So if you see that your strip takes 12VDC, don’t worry about a constant current driver, all you will need is a 12VDC constant voltage source as the current is already being regulated by on board circuitry that the manufacturer has built in.
We can see how each of these design modes can be used to implement a solution for different lighting applications.

Let’s show some cases:

• Application: advertising signage
• Three diodes in series per strip, with reverse bias protection diode and series limit resistor
• Current is controlled by

current control

— Where Vcc = supply voltage
— ΣVf = sum of forward voltages of LEDs
— R = series resistor

• Each strip consumes approx. 300 mA
• Customer required fixed 12 V output
• Solution provided with LDV100 series
• Or alternatively use the LDB60-12 V models. When used below a five ampere load, the unit operates in a constant voltage mode
Here we will show you how to choose right power supply for led strip

Case Study 2:

• Application: high bay lighting
• LED configuration of nine LEDs in series
• Vf of each LED will vary from 8 V to 10 V
• Customer drives the LEDs with 700 mA
• Pout = 700 mA (Forward current) * 90 (Vf max of nine diodes in series) = Minimum of 63 watts
• Voltage range of 72 to 90 volts
• Solution provided by LXC75 700 mA part
• Alternatively, an LDB product can be used to limit the current to 700 mA

Longjann Tech has a range of products available to the user to provide a solution for whatever the application requirements may be. We have constant current, constant voltage, and dimmable constant current models available in an extensive power range. These start at 5 watts and continue up to 600 watts.

Advantage of using a constant current LED driver
So when you’re building your own fixture or working with our high powered LEDs, it is of your best interest to use constant current drivers because:
1. They avoid violating the maximum current specified for the LEDs, therefore avoiding burnout/thermal runaway.
2. They are easier for designers to control applications, and help create a light with more consistent brightness.

Advantage of using a constant voltage LED driver
You use a constant voltage LED driver only when using an LED or array that has been specified to take a certain voltage. This is helpful as:
1. Constant voltage is a much more familiar technology for the design and installation engineers.
2. The cost of these systems can be lower, especially in larger scale applications.

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