Constant Current vs. Constant Voltage Drivers

When it comes to wiring LED COB circuits, series configurations with constant current drivers are generally the easiest way to go – especially if you’re new to electronics. Since LEDs are semi-conductors and operate a little differently than most basic circuits, it’s far easier to simply drive them at their rated current with a constant current driver than it is to try and produce that current by providing a constant voltage. If you check the datasheet and find that your LED COB has a typical forward voltage of 36V at 2400mA, you don’t want to strive to give the LED 36 volts, you want to strive to give it 2400mA at whatever voltage that current happens to occur at.

Constant Current Drivers

Constant current drivers are simple. If you go constant current, you’ll usually be wiring your COBs in series if you’re using multiple chips (for more info on series vs. parallel wiring, check this post out). In series configurations, the voltage dropped across each COB adds up, but the current flowing through each COB is the same (this is opposite of parallel). So, if you have a 1,400mA driver and your COB datasheet tells you that your COB will have a voltage of 34.5V across it when 1,400mA of current is flowing through it, then you can multiply this voltage by however many COBs you have for your total circuit voltage drop. If you had 2 in series, your voltage drop would be 69V.

Constant current drivers provide a steady current, but only between certain voltage ranges. The figure you’re looking for is called “constant current range” and this will tell you what the minimum and maximum voltages are that the driver can produce its rated current within. If, for example, your driver put out 1,400mA between 50V and 100V, the 2 COBs above would be within this range since they’re 69V total, and both of them would get the full 1,400mA. The driver will automatically adjust its output voltage to match the voltage drop of the LEDs on the circuit, as long as they’re within the proper range.

The constant current range of an HLG-185H-C1400 is 71V to 143V.

An example of a series circuit where you’d use a constant current driver.

Constant Voltage Drivers

If you’re using multiple chips on a constant voltage driver, you’ll usually be wiring them in parallel. In this configuration, the voltage across every COB is the same, but current can vary. If your driver is set to output 36V, then every light on the circuit that’s wired in parallel will have 36V across it. In order to predict how much current a COB will pull at a certain voltage, you will have to check its data sheet.

You might find that at 36V, your COB pulls 2,400mA of current. If you have 2 of these COBs on a 36V constant voltage system, your driver would need to be able to supply at least 4.8A of current. If it can do more, that’s fine. The COBs will only pull what their I-V curve dictates, depending on what voltage you run them at. Unfortunately, there is no easy way to calculate current/voltage for an LED, because the current voltage relationship in every LED is different – this is why you need to check the data sheet for their expected values.

With CV drivers, you need to be very careful when setting the voltage. Something very important worth noting is that you can only manually set the output voltage on the “A” and “C” versions of the HLG drivers. If you get the “B” version, your output voltage will not be directly adjustable. So, as an example, if you wanted to run your COBs at 35.5V to hit a certain level of current, you would be better off purchasing something like the HLG-240H-36A, not the HLG-240H-36B. If you get the A version, you could adjust this particular driver’s output voltage anywhere between 33.5V and 38.5V, as shown by the VOLTAGE ADJ. RANGE in the data sheet below. If you get the “B” version, the driver would put out 36V exactly, until you dim it with an external potentiometer.  You can use this external potentiometer to achieve your target voltage in a roundabout way (dimming the driver will lower the current, and, as a result, the voltage across the COBs), but I prefer to have direct control over the voltage with the built in potentiometer on the “A” version.

Remember, a small change in voltage can cause the LEDs to draw a disproportionate amount of current. Setting your driver to run at 36V rather than 35.5V could cause each COB to draw hundreds of milliamps more than they would at 35.5V. For a guide on choosing the proper CV driver for your system, read this post.

 

A parallel circuit where you’d use a constant voltage driver.

Why Current Matters More

When working with LEDs, it’s best to focus solely on getting the proper current to each LED. Think of forward voltage as a by-product of the current you supply. Why? Because current is what determines how bright an LED is. Revisiting the example above, let’s say you have 2 of the same COBs, and the data sheet states they have a typical forward voltage of 36V, measured at 2400mA. If you were to use a constant-voltage driver and provide 36 volts to each of the COBs in parallel, you may find that they draw significantly different amounts of current. This is due to the fact that every LED has a slightly different inherent current-voltage curve, and small changes in voltage produce large swings in current.

Now, if you were to instead use a constant-current driver, and provide each of these COBs with exactly 2400mA in series, both lights should be outputting their rated luminous flux (or close to it) and you’re all set. If you measured the voltage drop across each of the COBs, you would likely find that the voltage across them is different by a small amount – maybe only by 10mV, or maybe 100mV. It doesn’t matter though – as long as the current is equal in each COB, you’re good to go. The driver will adjust its output voltage as required to maintain its rated current.

I-V Curves in LEDs

Every LED has its own special current-voltage relationship. Even the same models from the same brand will respond differently to changes in voltage and current (though these differences will be small if the LEDs are binned well with tight tolerances). You cannot use basic formulas to predict the behavior of an LED at a given voltage or current since you don’t know the resistance – you must refer to its I-V curve in the datasheet, which represents the ideal, typical response of that LED.

When you look at the current-voltage curve of a COB (take the CXB3590 curve below for example), you can see that very small changes in voltage produce large changes in current. An increase of just half a volt can produce an increase of 300mA in current! When you’re running COBs near their limit, a few hundred milliamps of extra current can be problematic.

Thermal Runaway and The Effect of Temperature on the I-V Curve

You’ll also notice that temperature plays an important role in the voltage-current relationship of LEDs. As shown above, there can be a full volt of difference in the forward voltage of an LED with a case temperature of 25 degrees compared to an LED with a case temperature of 85 degrees. Now, how would temperature affect a constant voltage driver vs. a constant current driver?

Let’s say your heat sinks are actively cooled (there’s a fan attached with air blowing over the fins), and your power supply that runs the fans dies, leaving the heat sinks unable to dissipate heat efficiently. With the heat sinks not functioning properly, the temperature in your COBs is going to rise – as an example, we’ll say that the temperature of the case rose from 85 degrees to 105 degrees.

If you’re using a constant voltage driver with your COBs in parallel, you’re going to run into trouble faster. Say you were driving your LEDs at a constant voltage of 36V. Based on the graph, at 85 degrees, your COBs were drawing approximately 2600mA. At 105 degrees, they’re now pulling 2700mA and are practically at the end of the line for their max current – this increased current is creating more heat, which causes more current draw. This is called thermal runaway, and can lead to the destruction of the COB. To prevent thermal runaway, in a constant-voltage circuit, you would need to have some method of limiting the current in the circuit, like putting a resistor in series with every COB.

Now, what would happen in a series circuit with a constant current driver if the case temperature of the COBs increased from 85 degrees to 105 degrees? Well, if you were running at a constant current of 2400mA, the voltage across the COBs would decrease from 35.75V to 35.5V, but, obviously, the current would remain the same. You would not see the COB make the situation any worse, as it would not draw more current. If the heat sink was able to get by and dissipate enough heat to maintain a COB case temperature of 105 degrees, the COBs should live long enough for you to find the problem and get the fans working again.

In Summary

Though constant voltage is gaining popularity, constant current drivers are still easier to work with. Here are the main points:

  • CC drivers are simple because there are no adjustments required. Just make sure your circuit voltage is within your driver’s constant current range.
  • If you go with a CV driver, you’ll need to make sure that you set the voltage output very carefully. Small changes in voltage produce big changes in current, and vice versa. If your power supply is providing 36.5V instead of 36V, you could be getting hundreds of milliamps of current more than you expected.
  • LEDs have unique I-V curves that are always moving around due to temperature, and for this reason, it’s impossible to know exactly how much current will flow if you provide a constant voltage.
  • Constant voltage supplies powering COBs in parallel make the COBs susceptible to thermal runaway, and I’ve even seen LED manufacturers (Bridgelux, in particular) advise against using constant voltage to power their COBs right in their documentation (see picture below). In my experience, as long as you aren’t driving the COB near max and your heat sink is doing its job, you should be fine.

So, if you were wondering which is the best way to go, I would suggest starting with constant current until you’re more familiar with the gear and how it all works together.

 

2 Comments

  1. so whats up with cobkits only selling low voltage drivers for parallel only applications?when I looked for a driver through them I saw nothing that I would want to use. Maybe thermal runaway and burn outs makes them more money.

    • LEDGardener

      December 24, 2016 at 10:36 am

      It looks like all the drivers on that site that are not constant current are “constant voltage + constant current”.

      This means that the driver will operate in constant voltage mode up until the point where the load draws the max current the driver can supply. Once it hits max current, it switches to constant current mode, where it holds current at max.

      Edit: After playing with these CV drivers, I’ve found them to work pretty well. Even though they do produce enough current to potentially cook individual COBs, it’s unlikely that any one COB will draw that much.

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