LED lighting - characteristics

The LED lighting technology presents the particularity of being the first technology to allow the development of appropriate and effective solutions for all applications of functional lighting, unlike earlier technologies.

To better understand why the use of LED lighting can result in these remarkable efficiency gains, basic terminology needs to first be explained. Listed below are definitions of the key terms in use :

• LED (Light Emitting Diode) - A diode type semiconductor which emits light when a current passes through it. LED semiconductor materials convert electrical energy into visible electromagnetic radiation (i.e., into light).
• LED component - The substrate and primary optical unit of the light assembly. The purpose of the LED component is to protect the semiconductor and to conduct the heat generated from LED to dissipation systems.
• LED module - The assembly of one or more LED components with optical, mechanical and thermal elements.
• LED luminaire - A complete system consisting of a LED module, a housing, an optical reflector, wiring, connectors, joints, heat dissipation system (heat sink or fan), and for most cases the driver
• Driver - An electronic device which can convert the electric power of a low-voltage AC electrical network into electric power appropriate for the LED luminaire (direct voltage and current). The driver may be external or internal to the luminaire. A driver can power one or more luminaires. Light dimming function can be embedded in this device (1-10 V control, DALI control, ...)

Fig. N42 – A LED is just one small element within a larger construct that is sold commercially as a lamp or an assembled luminaire

Inrush and steady-state currents

When a LED luminaire is first energized, a variable current is required by the luminaire during the first second time interval, and then the current stabilizes as soon as rated operating conditions are reached. Three transient fundamental events occur during the start up phase: the power supply of the luminaire, the start of the driver, and the powering of the LED module (light is on). Then the luminaire transitions to the steady state operating condition.

In the initial moments after a luminaire is energized, a significant transient current appears (can be up to 250 times the rated current depending of the characteristics of products) due to the capacitors used to perform the power factor correction (the power factor of LED luminaires is generally greater than 90%, since the luminaire driver includes a power factor correction stage). The duration of this transient current is less than 1 millisecond (ms). When the luminaire is powered on, the current will be at its highest level when the voltage angle is 90° (in that case, the voltage is at its peak value of 325 V for a 230 Volt AC network). When switching on at zero angle voltage, the inrush current is far smaller.

Once the inrush current has passed, a time range of between 100 ms and 1.5 seconds elapses. During this time, the driver is initialized (power supply for electronic control circuits, are energized, for example). The current consumed during this phase is less than the rated current.

Once the driver is initialized, the LED module is energized and light appears. An overload of about twice the rated current occurs during the initial period of power supply of the module containing the LEDs. Fig. N43 illustrates the various stages involved in energizing the luminaire. Note that state 4 in Fig. N43 represents the steady state operating condition.

[a] State 1: Initial Power supply,
[b] State 2: Driver starting,
[c] State 3: Powering the LED module,
[d] State 4: Steady state operating conditions

Fig. N43 – Illustration of four states of a LED being energized:

In the steady-state condition, the current consumed by LED luminaires is not perfectly sinusoidal. The total harmonic distortion of current (THDI) ranges between 10% and 20%. Given that the rated currents of LED luminaires are low, the impact of these currents on network voltage is slight. Measurements in various industrial plants powered by the public low-voltage power supply system (on which the short-circuit impedance is low) show that the total voltage harmonic distortion (THDV) is generally less than 3%. According to the IEC 61000-2-4 standard relating to the compatibility levels of voltage tolerances, if the THDV is less than 5% (class 1 electromagnetic environment), the network is considered sound and compliant.

Common mode currents

Definition: when currents flow without close-by opposing currents, the unopposed portion of current is referred to as common mode current. Common mode currents can result in radiation which can then result in interference or distortion.

How LED technology deals with this challenge? In the following example, measurements were performed by first energizing 20 luminaires that were isolated from earth. Given the configuration, the leakage current could only be looped back via the protective earth (PE) conductor of the power cable. The current flowing in this conductor at the energizing stage is presented below (see Fig. N44)

Fig. N44 – Depiction of earth leakage current test results

For switching on at zero voltage, the leakage current is practically zero.

The frequency of the transient current is high (about 100 kHz).

At the steady state stage, for the 20 luminaires isolated from earth, the leakage current value measured at 50 Hz was about 2 mA.