Today, when LEDs are changing very quickly and constantly evolving, choosing the right LED and LED array can require quite difficult and smart selections. When selecting a LED driver, definitions and standards of which are still debatable, is added to this already complex decision process, the situation can become quite more complicated. As part of the LED driver selection process, the selection of a constant-current or constant-voltage driver, matching the LED circuit with its topology, dimming and flickering considerations, life requirements, single or multiple output needs, and the controllability functions, often require the help from a professional lighting designer to be able to correctly interpret concepts such as the selection of a constant-current or constant-voltage driver. In this article, we have tried to address some of the concepts that we come across when making a selection in this regard.
Constant-current or constant-voltage?
The first question that the lighting designers face is usually will be on selecting a constant-current or constant-voltage source. In general, constant-voltage LED modules and drivers can be preferred for applications such as hidden ceiling lighting, where it is not known how many LED arrays will be used or what the current to be drawn will be. However, if the current value required to meet the light level requirement of the application is known, selecting constant-current option shall be the most correct decision.
“The life of an LED driver is determined by the service life of each electronic component inside.”
Service Life
The expected service life of the driver to be used is one of the most important parameters. We all know that a lighting fixture is accepted to come to the end of its service life when it loses 30 percent of its initial light output. This requires, for example, that the LEDs to be used in an LED fixture that is planned to operate for 5 years fall to 70 percent brightness at the temperature to which they are exposed only at the end of 50,000 operating hours. Naturally, we expect that the LED driver to be used together will also withstand working for 50,000 hours or more. The life of an LED driver is determined by the service life of each electronic component inside. The “weakest link” here is the electrolytic capacitors.
The electrolyte inside the capacitor is usually a gel that evaporates gradually over the service life of the component. This evaporation rate depends on the temperature inside the driver, which in turn depends on the external temperature on the drive box that it is exposed to. Higher operating temperatures accelerate evaporation and therefore shorten the service life of the capacitor. Most LED drivers have a small circle on their labels, called the “Tc point”. This is usually the hottest point of the box and is used to measure the box temperature. The manufacturer ensures that the temperature at this point remains below the highest value allowed by the standards. However, when the driver is used at a point close to this limiting temperature threshold, its service life will be much shorter than when it is operated at a lower temperature. For this reason, when choosing a driver, we recommend that you definitely check the curves that associate the service life of the driver with the Tc temperature, on the product datasheet. As an example, the life curves of the Lifud 30YM series are shown below.
“When choosing a driver, we recommend that you definitely check the curves that associate the service life of the driver with the Tc temperature, on the product datasheet.”
Dimmability
If your application requires the use of a dimmable driver, you should consider both dimming and energy saving when selecting the correct driver. First of all, understanding human perception of light helps us a lot when determining the necessary characteristics. For example, in many applications, dimming the light by 50 percent is not perceived by most people as a significant dimming, while reducing it to 10 percent is considered as only a small amount of dimming. Therefore, in order to create a perceivable dimming effect, the product must be able to be dimmed up to 1 percent. In fact, this value should be reduced up to 0.1 percent in some places of application such as movie theaters. Of course, one should not get caught up in the idea that dimming the light by more than 10% is meaningless, since, for instance, dimming an LED to 10 percent brightness allows you to save more than 90 percent of electricity. Therefore, even at an imperceivable levels, dimming has a tremendous impact on energy saving.
Flicker
Flicker refers to the rapid and repeated changes in light intensity that causes the light to appear unstable and intermittent. Researches have revealed that vibration in lighting causes headache, eye strain and stress symptoms even when it is not visible to the eye, and triggers photosensitive epilepsy and migraine attacks at some frequencies. So, when selecting a driver, the first question that comes forward is whether the vibration ratio will be too low or not. It is too difficult to answer this question right now, since a universally accepted standard has not been established yet. Therefore, in a project that carries concerns about vibration, it seems useful to choose the product with the lowest vibration percentage (flicker percentage), also taking into account the related costs. Studies have revealed that when the vibration percentage is less than “0.0333 × vibration frequency”, provided that the vibration frequency is above 90 Hz, there is no effect of vibration on health, and when the vibration percentage is less than “0.08 × vibration frequency”, it falls into the low risk group. Based on these studies, we recommend that the vibration percentage should be below 8 percent (100Hz x 0.08) if a product with a vibration frequency of 100Hz is to be used at offices and fine craftsmanship areas, and below 3.3 percent (100Hz x 0.033) if it is to be used in critical areas such as treatment centers.
“Since the power factor represents the difference between the power actually supplied to a facility (apparent power) and the power detected by the meter that calculates a facility’s bill (active power),
power factor is a very important aspect for the electricity supply companies.”
THD (Total Harmonic Distortion)
Distortion of the sinusoidal wave shape has the potential to have dangerous consequences such as overheating in electrical devices, or even causing a fire in substations and switchgear plants. Given the growth in the number of nonlinear loads of electrical devices, THD is increasingly becoming even more important. Today, a THD below 20 percent is generally considered acceptable, while a THD below 10 percent is extremely a good value.
PF (Power Factor)
Since the power factor represents the difference between the power actually supplied to a facility (apparent power) and the power detected by the meter that calculates a facility’s bill (active power), power factor is a very important aspect for the electricity supply companies. Since a low power factor increases the reactive power, it causes a high cost for the electricity distributor. Therefore, the use of reactive force above the values determined by the administration may leave enterprises in the position of paying penalties. Although a power factor above 0.9 is generally considered standard and sufficient, values below it may lead to penalties for electricity users other than residential users. When selecting a driver, if you notice that the PF value is not specified in the technical data of the driver the PF value should be considered as below 0.9 and
it can reduce up to 0.4. Therefore, the use of such products should definitely be avoided, particularly in commercial or industrial applications. In addition to these parameters, we will also try to address factors such as input voltage range, efficiency, IP class, operating temperature, commissioning time, instantaneous surge voltage resistance, inrush current etc. in our future issues.
