LED Driver Design

This is where high-quality LEDs paired with good thermal management and robust drive electronics have real value. This article proposes an electrical drive solution that balances higher initial cost with long life – at least as long as the LEDs themselves.

DC bus voltages

It often takes 100 or more 1W LEDs to put out the thousands of lumens needed for a streetlamp. One way to drive 100 LEDs would be a single series chain.

This ensures an equal current through every LED, and since light output is proportional to current, it is the best way to guarantee equal light output from each device. The problem is that the DC voltage could easily be 400V. Such a voltage could be lethal, and also requires large, expensive components.

A second way to arrange 100 LEDs would be to use multiple parallel strings with a lower DC voltage. Well-known, cost-effective topologies such as the flyback converter make good AC-DC stages (often called “offline converters”) because they can combine the step-down function with galvanic isolation and power factor correction (PFC).

DC bus voltages of 60V or less are common due to the 48V used in telecom applications and due to safety regulations such as the IEC’s definition of a safety extra low voltage. Because it is not as low as the logic voltages of digital circuitry and yet not as high as rectified offline voltage, a 48V distribution voltage is often referred to as an intermediate DC bus.

Driver topologies

A DC-DC converter is the natural choice for the final stage in an LED power supply. The LED requires a DC current, so the voltage output is also DC.

The intermediate DC bus concept allows the designer to use cost-effective, non-isolated DC-DC converters because the previous stage has taken care of rectification, PFC and isolation.

Within non-isolated converters there are three main types:
1. Step-down, or buck
2. Step-up, or boost
3. Step-up/down, or buck-boost.

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