An LED Driver Design for High Currents

blockHigh-brightness LEDs are being used in luminaires for commercial downlighting, and industrial, architectural, exterior, and emergency lighting.

To produce more lumens, some LED designs require forward currents to increase. Typical LED forward currents range from 350 mA to 1000 mA; a few of the latest LED require up to 3000 mA.

To address this high current approaching 3 Amps, some driver solutions are utilizing innovative switching.

The majority of LED driver solutions available are based on standard voltage-regulator, using a fixed frequency, current-mode-control buck converter. Control schemes tend to be complex, often having two loops: an outer-loop to regulate the current control and an inner-loop to provide the peak current control. This control technique typically requires external compensation components.

Brightness control is difficult to achieve. Most solutions rely on pulse width modulation frequencies in excess of 200 Hz to avoid flicker. A good dynamic range for brightness control requires the ability to modulate to duty cycles as low as 10%. With a 200 Hz signal, this means the driver has to support the ability to turn on and off in a period of 0.1 × 1/200 seconds = 0.5 ms. As some driver solutions have a soft-start feature in the region of several ms, this can restrict the dynamic control range.

The voltage in the current-control loop can be as high as 1.2 V, which has a large impact on the power dissipation. When driving a single LED with a forward voltage (Vf) of 3.5V, the efficiency drops by 34%, even before other losses are considered.

The diagram shows a circuit built around a driver designed for driving high-current LEDs. With a simplified control scheme, the component count includes two resistors, two capacitors, one Schottky diode, and the power inductor (L). An output capacitor is not required because the peak-to-peak current ripple can be as low as 10% of the maximum LED current. This meets the majority of high bay LED lighting applications.

The design uses an inner-loop that controls the current by sensing the voltage developed across the sense resistor (R2) during the recirculation diode (D) conduction phase. No outerloop is required, removing the need for external compensation components.

Soft-start is not used in the design, so expect the life expectancy of the driver will be less than drivers that do provide soft-start, and as such the driver design is best deployed in applications not requiring repeated starting or on-off switching.

Portions excerpted from a whitepaper by Peter Tod

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