Because light-emitting diodes (LEDs) are a current-driven DC device, they require a converter so they can be powered from an AC voltage. LED drivers have different operating characteristics from the typical AC to DC, and DC to DC power supplies used for most electronic equipment. They are often a constant current output rather than a constant voltage output.

The 63110A load module has a unique LED mode for LED driver test.

Chroma has created the 63110A electronic load module with an LED operating mode for simulating LED loading. Key features include programmable LED operating resistance (Rd), programmable internal resistance (Rr) for simulating LED ripple current, and fast response for pulse-width modulation (PWM) dimming test. The 63110A has 16-bit precision voltage and current measurement with dual-range; two voltage ranges of 0-100 Vdc and 0-500 Vdc; and two 100-W loads in one load module – up to 500 Vdc and up to 2 A loads can be paralleled for high current and power applications.

The 63110A LED load allows users to set the LED driver's output voltage and current, and the electronic load can then simulate the LED’s loading characteristics. The LED’s forward voltage and operating resistance can also be set to further adjust the loading current and ripple current to better simulate an LED. The 63110A design has increased bandwidth to allow for PWM dimming testing.

What Makes LED Load Characteristics Unique

Figure 1.

As shown on the V-I curve in Figure 1, the LED has a forward voltage (Vf) and an operating resistance (Rd).

LED drivers are usually tested by either using LEDs; using resistors for loading; or using electronic loads in constant resistance (CR) mode, or constant voltage (CV) mode.

However, the above loading methods each have distinct disadvantages. First, those manufacturers who use LEDs as a load run into problems with the aging of the LEDs. Different LED drivers may require different types of LEDs or a different number of LEDs. This makes it inconvenient for mass production testing. Second, resistive or linear loads cannot simulate the Vf and Rd coefficient of an LED.

When using a typical electronic load to test LED drivers, the CR and CV mode settings are used. These settings can only test stable operation, and therefore are unable to simulate PWM dimming/intensity control.

Comparing the Actual LED Characteristics to the Electronic Load

The current waveform from an actual LED is shown in Figure 2. The current waveform from 63110A's LED mode load function is shown in Figure 3. Both figures show the start-up voltage and current of the LED driver with an LED versus the Chroma 63110A electronic load in LED mode and they are very similar.

The dimming current waveform of an LED is shown in Figure 4. The dimming current waveform when using 63110A as an LED load is shown in Figure 5.

Chroma’s 63110A LED load will calculate and simulate the LED characteristics from the operating voltage (Vo), operating current (Io), and operating resistor (Rd) coefficient settings. As shown in the diagram to the right, Vo and Io are not the real loading values.

Io is determined by the LED driver, and if the Io varies from the setting value then Vo will also vary. For example, if the Io setting is 100 mA, but the LED driver output 110 mA, then Vo will also increase. This is different from the standard CC and CV modes.

Lighting Technology Magazine

This article first appeared in the August, 2011 issue of Lighting Technology Magazine.

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