Lighting applications increasingly use LED technology because of its long life, low power and other benefits. However LEDs are semiconductor devices with specific requirements for either constant current (CC) or constant voltage (CV) power supplies (drivers), depending on their packaging. Below, we explain how to choose between these operational modes.
An LED, or Light Emitting Diode, behaves in many ways like any other diode. It allows current to flow easily when forward biased, but much less so when reverse biased. In traversing from the anode to the cathode through the LED’s N-type and P-type semiconductor materials, the electrons cross an energy gap. The difference in energy is released as a photon of light, with a frequency dependent on the size of this energy difference. This allows LED manufacturers to ‘tune’ the band gap energy and therefore the wavelength of light emitted, by varying the composition of the LED’s semiconductor materials.
For any given temperature, the forward voltage across an LED remains constant, independent of the current flowing through it. Therefore, a LED’s light output, and the power it dissipates, is controlled by the amount of current allowed through it. Fig. 1 shows the current/luminous intensity characteristic for a high-flux Cree LED.
Accordingly, any installation that uses directly-connected LEDs must use a constant current (CC) driver, with a current level set for the required brightness. For example a series-connected string of 12 LEDs with forward voltage 3.5V at 350 mA would require an applied voltage of 3.5 x 12 = 42V at 350mA. Two such strings connected in parallel would require 42V at 700mA.
To power 2 strings in parallel a driver such as Sunpower’s PCC70040 could be used; it would operate in constant current mode, maintaining a consistent 700mA output current, while its output voltage settles at 42V.
However, some LEDs are available as preconfigured light strips complete with driver circuits that manage the LED CC requirements. These strips require a CV driver such as Sunpower’s PCV2460 which delivers a stable, regulated voltage regardless of AC input disturbances or load variation.
Some of Sunpower’s CV LED power supplies can switch automatically between CV and CC modes. This allows for more flexible system designs that can handle all types of LED installation.