Shanghai Arcata Electronic Inc.

      Constant voltage or current?  
  The first question developers face is almost always the choice of constant-current or -voltage supply. There are two fundamental types of LED arrays — those which run off a constant voltage (CV) and those which run off a constant current (CC) — and they're materially different inside. A constant voltage array will contain devices to limit the current from going too high when the LEDs get hot, such as resistors or constant current resistors (CCRs) or even a switching DC-DC regulator of some kind. By contrast, a constant current LED array will have LEDs connected in series and perhaps several of these strings connected in parallel.

You'll want to choose a constant voltage array under two circumstances:

1) You have yet to determine your LED array and the application is one where you don't know exactly how many LED strings will be hung on that supply or what the current draw will be (i.e., cove lighting).

2) The LED array is of the constant voltage variety and therefore has a fixed range of current for that fixed output voltage. In this instance, you'll need to ensure that the driver you select is the right voltage and that the allowable output current range is higher than the gross estimated current draw of your LED loads.

If you are aware of the current draw needed to match application light-level requirements, you'd probably choose a constant current array because it's usually the most efficient arrangement. If your LED array requires a constant current, then you'll need a CC LED driver. This type of driver will only have a certain range of voltages which it can drive; there will be a minimum voltage and a maximum voltage permissible. You need to ensure that your LED array has a voltage requirement that falls inside this permissible range.

The table reviews AC-to-DC driver topologies and their pros, cons, and applications. There is no wrong or right approach to driver selection. Instead, you need to match topology to the application requirements. Now let's address some specific issues.

  What is LED Driver Lifetime?  
  Driver lifetime should also be a major concern. If the temperature of your LED array is properly controlled, it should produce more than 70% of its initial light output after 50,000 hours. Obviously, you'd like your LED driver to last equally as long.

The lifetime of an LED driver is determined by the lifetime of the individual electronic components inside. The weak link, in particular, is the electrolytic capacitors, which are like little batteries. The electrolyte inside is typically a gel that gradually evaporates over the life of the component. The evaporation rate depends upon the temperature inside the driver — which, in turn, correlates to the external temperature on the driver case. Higher operating temperatures speed evaporation and hence shorten the life of the capacitor.

On the label of most LED drivers there is a small circle called the hotspot or "Tc point." This is usually the hottest point on the can and is used to determine the can temperature. The manufacturer will supply a temperature that must not be exceeded if the UL approval of the product is to remain valid. However, be aware that if you use the driver close to this limiting temperature, its operating lifetime will typically be shorter than if operated at a lower temperature. The driver manufacturer is able to supply curves that correlate the lifetime of the driver to its hotspot temperature. Fig. 3 provides an example of the curve for a typical LED driver.

  What is PF?  
  This is of major concern to utilities as it represents a difference between the power actually delivered to a facility and the power detected by a meter that determines the bill to a facility. Low power factor is costly to a utility. A conventional standard for PF is 0.9 or above; anything lower may result in a penalty assessed by the utility in the form of a multiplier on your electric bill. If PF isn't mentioned in a driver specification, the default is referred to as normal power factor and implies any PF below 0.9. The actual spec could be as low as 0.4 in some of the least expensive lighting products. While PF is generally meaningless in a residential setting, careful attention needs to be paid when installing large volumes of normal-power-factor products in industrial or commercial applications.  
  What is Output ripple?  
   It's straightforward to make an LED driver that has essentially no output current ripple by building it with two power conversion stages; a first stage generates a stable power supply and a second stage then generates the output current. A two-stage design has two control chips and two lots of high-frequency transformers inside and is more expensive. The cost of a driver can be significantly reduced by using just one power conversion stage for both power factor correction (PFC) at the input and for controlling the output current. The tradeoff is that now either the PFC is less perfect or sometimes as much as a 50% ripple at twice the line frequency is introduced into the output.