Lighting in Design Q2 2018

In cases where the forward voltage of the LED module is just outside the output voltage range of the driver, the LED module may turn on and off a few times before remaining off. At this stage one might ask, “What about the wattage ... surely the wattage of the driver has to match the wattage of the LED module?” The answer is that this is not necessarily the case. If one has selected a driver that offers the correct operating current and output voltage range one will notice that the wattage of the LED solution is lower than or equal to the wattage of the driver. If the wattage of the LED solution is higher than the wattage of the driver the matching of operating cur- rent and output voltage has not been done correctly and one needs to review the choice of the driver. Other requirements If the above steps have been followed, hopefully one has found several suitable drivers. Now it is time to look at other (mostly practical) requirements to narrow the selection. Additional requirements can be:

generate light.The forward voltage of an LED mod- ule is the sum of the forward voltages of the single LEDs that make up that module or panel (this is in case the LEDs are connected in series). As there are tolerances in the forward voltage of single LEDs (partly because the Vf of an LED slightly increases over lifetime), the forward voltage of an LED mod- ule/panel is usually given as a voltage range , not a single value. As with the forward current, the forward voltage is temperature dependent. Besides offering a fixed operating current any con- stant current driver offers an output voltage range. When selecting an LED driver, the output voltage range of the driver must cover the full forward volt- age range of the LED module (see Figure 1). If the forward voltage of an LED module falls outside the output voltage range of a driver, the following will happen: - Forward voltage LED solution > output voltage range driver: LEDmodule will not work/no light. - Forward voltage LED solution < output voltage range driver: Risk of unstable behaviour of the LED module/fashing.

Comparison of… Value in light module

Value in LED Driver Output current

Detailed procedure

(1) Current

I max.

=

- Determine forward current of LED solution. - Check whether LED driver can be operated with the same output current. - Check whether max. dc forward current of LED module is greater than or equal to output current of LED driver (incl. tolerances). CAUTION! The max dc forward current can be temperature dependent, check datasheet of LED solution.

≥

Max. dc forward current

Output current + tolerances

(2) Voltage

Min. forward volt- age

>

Min. output voltage

- Check whether voltage range of LED solution is completely within the voltage range of the driver.

CAUTION!The forward voltage is temperature depen- dent and increases over lifetime.

Max. forward voltage

<

Max. output volt- age

NOTICE! To ensure full dimming performance, the forward voltage of the LED module at min. dim level must be greater than or equal to the min. output voltage of the driver. - Determine the forward voltage of the LED mod- ule at lowest dim level. In cases where there is no data available for the LED module at lowest dim level: take the min. forward voltage minus 20% as an approximation. Check whether the forward voltage of the LED module is greater than or equal to the min. output voltage of the driver.

Min. forward voltage @ min. dimlevel

>

Min. output volt- age

Figure 1: Scheme offering guidelines on how to check compatibility between LED solution and LED driver.

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LiD Q2 - 2018