[quote=redline]
Another question: I bought some LED drivers, 24 volt, 180 watt, voltage regulated.
Why are the larger wattage LED drivers all voltage regulated? instead of current regulated like the small drivers?

There are some CC high wattage drivers out there, but they are very, very expensive. Ive seen up to 400w ones. They are similar to HID electronic ballast. Often they comes together with a DMX unit, as mostly this equipment is used on discos and such, using RGB leds.

I dont know exactly the reason of almost nobody doing these type of devices. Although as the voltage increases the driver design becomes more complex and the components used are more expensive, any manufacturer should be able to do it without mayor problems. At least, if they dont seek for very high efficiencies.

I believe that manufacturers thinks that any that want to do arrays of high wattage have the knowledge to do a dedicated driver for the application with higher efficiency than any generic one, thus demand for high wattage generic CC drivers is not enough for mass production.

Along the last year, most manufacturers of power ICs have been releasing components designed specifically for high power LED arrays. They have integrated the current sense circuit and the FET with comparators, so often only needs an external capacitor, inductance and a resistance to complete the basic CC circuit. So a basic CC circuit is now easy to do yourself. Tuning it to take care of the LEDs (avoid transient spikes) and work with the higher efficiencies is more complicated, thought.


Quote Originally Posted by redline
Is the voltage regulation tight enough that you don't have to worry about excessive current swings. Or is it expected that you use secondary current regulation, like a resistor or a reg chip in each string?

I am currently using a resistor in each string since it is cheap and simple. I could make some small reg circuits if need be, but am lazy. I burn them in for a couple hundred hours, then I customize the resistance on each string to get desired current.
If you use a voltage regulation, you always need a secondary current regulation. But a resistance on each array is enough.

This secondary control is needed because the negative coefficient of electric resistance vs temperature of LEDs: Vf associated to a given If drops when chip temperature increases. Thus, if you drive the array controlling just the voltage, as the chips heat, for the same Vf, there is more If (current) flowing. As the relationship between Vf and If is exponential, little changes on Vf lead to huge changes of If.

As the resistor drops more voltage as higher is the current, it filter this effect and strongly minimize the current swing: as chips heats, for same Vf, If increases. But when If increases, the resistor drops more voltage, thus there is less voltage available for the leds, thus If increase is controlled.

So although not ideal (the ideal is to limit the current, not the voltage), a tight voltage supply with a resistor on each string works fine enough. And its by far the easiest way, and currently, the cheaper way, of driving high wattage arrays.

The problem of this way of driving leds is its inefficiency: the resistor burns power and rest efficiency to the circuit. Additionally, as LED's chips heats, their efficiency drops. By this effect is partially compensated by the fact of the reduced Vf at a given current (because power burned is Vf*If and light emitted only depends of If). When using resistors, you lose the efficiency gain by the dropping Vf, because what you do is burn those volts at the resistor: all the theoretical gain is wasted.

So as higher the resistance of the resistor used, higher filtering effect (lower If swing) but more power losses.

And when you add the loses of the PS converting AC to DC to the losses on the resistors, you may get perfectly an overall efficiency of the driver design at 70%, meaning that each watt used by the LEDs needs 1.4W. Way larger than any HID or floro ballast, which are about 10% (1.1W for 1W of bulbs consumption) and still less. And that way is difficult to get true electric savings.

So for experimenting arrays, constant voltage PS and resistor are easy and cheap, but expensive on the long run.

The better the thermal path of you arrays, the smaller resistances you may use and you will have lower power losses. When using a good thermal setup and using not high currents (Tj raises very fast with the increased current), it allow you to use resistances with way lower power losses (because less ohms). But obviously this will require the sum of the LEDs Vf is very close to the voltage supplied to the array, and that is not always possible.

In this scenario, if the sum of Vf is just a bit over the supplied voltage, maybe you could use the array without resistances at all. You wont have any control over current, but you can do a try an measure the current after temps are stabilized and if it falls on a good range, run it that way (the disadvantage of no using any resistor, or still small ones is in case one led fail and open: the increased current on the array may fry all it)