High power LEDs produce more lumens per watt than previous lighting technologies and are replacing incandescent and CFL bulbs. When driving high power LEDs, the power consumption in the current limiting resistors can make this method impractical and unattractive. When driving ultra-efficient light sources, very efficient methods are desired. The most efficient way to drive high-current LEDs is to use a DC-DC converter with current feedback.
An LED driver is a stand-alone power supply that regulates the power required by an LED or an array of LEDs. Light emitting diodes are low energy, long life, low power consumption lighting devices and therefore require a dedicated power supply.
DC-DC converters are highly efficient power conversion circuits that use passive low-pass LC filters to smooth the switching action to a constant voltage. There are switched LED drivers that generate a constant current to bias the LED, usually by measuring the voltage across a small ohmic current sensing resistor connected in series with the LED or LED string. Although there are DC-DC converters designed for LED drivers, many conventional DC-DC converter controllers can be used to design LED drivers by using a current sense resistor in the feedback rather than a voltage divider.
A buck converter is a DC-DC converter that can effectively produce an output voltage below the input voltage. The energy conversion efficiency can be higher than 90%, depending on the components used and the power level. The buck converter makes it easy to drive LEDs from a voltage source that is higher than the forward voltage drop of the LED, such as a desktop power supply, a 12 V battery or a lithium-ion battery pack with several batteries in series.
Sometimes we want to drive LEDs from batteries for applications such as torches or battery packs with LEDs. The voltage of a Li-ion battery can vary between 2.5 V and 4.2 V, depending on the state of charge and the load. As the LED forward voltage is in the range of 2-3 V, the lithium-ion battery may not be able to drive the LED when it is almost depleted, which can shorten the effective battery life of LED lighting applications.
A good circuit for driving LEDs from Li-Ion batteries is the buck-boost. A buck-boost is a DC-DC converter that can produce an output voltage above or below the input voltage. Typically, the efficiency is good, but not as high as a buck or boost converter.
Boost converters are DC-DC converters that produce an output voltage higher than the input voltage. Boost converters are useful when the load voltage is always known to be higher than the input voltage. A useful application is to drive series-connected LEDs from a low-voltage source.
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