How to select a LED driver to DIM LED Strip?

Introduction

LED dimming has advantages in terms of energy savings, enhanced ambience, improved light source efficacy/ lifetime…etc. The dimming performance is mainly determined by the LED source and driver. Generally speaking, the LED source can be simply divided into two categories: LED diodes only and LED diode with resistor. Sometimes LED source is designed as a module which could have DC-DC converter inside. However such complex module is not in the scope of article here. In case the LED source or module is made from LED diodes only, the common dimming method is realized by adjusting the amplitude of current fed into LED. Therefore the LED driver should be designed to meet such demand. Mean Well HLG families is featured with current adjusting and is controlled by external dimmer with signal of DC1-10V* or 10V pulse width modulation (PWM) or even just a simple resistor. Another category of LED source is LED diode with resistor which is simply so called LED strip. LED strip is widely used because its voltage is relatively fixed thanks to the present of resistor in serious with LED diode. Therefore user or installer can just take any constant voltage source / LED driver to power the LED strip. Yes LED strip is relatively easy to be powered by either 12 or 24V. But sometimes it would be tricky to perform the dimming. This article shows the consequence of LED strip dimming using driver with amplitude current adjusting. Some of the common dimming undesired behaviors are also discussed. The best and easiest way of dimming an LED strip is using a dimmable LED driver which has output PWM.

Common undesired behaviors when dimming an LED

It sounds to be an easy task to dim a light source. Actually there are several considerations needed taken into account in case a good and smoothly dimming performance is expected. Otherwise the undesired issues shown in the Table 1 could further lower the expectation.

LED behaviour and description

undesired dimming behaviour

Figure 1. Mean Well amplitude current adjusting LED driver HLG-100 connected to LED strip at different load (length) conditions: (a) full load(length) resulting in the best dimming performance; (b) 70% load(length) implying non-perfect dimming experience due to small dead travel; (c) 30% load(length) implying worst dimming result due to huge dead travel.

Output PWM resolving the dead travel

It may be criticized that the dead travel issue is gone in case the end application is always using the LED driver at full load. Furthermore it makes less sense to pay a high wattage driver while it is used only at much lower than its rated voltage. Yes it is a very straightforward thinking but may not be realistic. It is due to the fact that LED strip is quite often used where the length cannot be predicted precisely. For example, the decorative lighting used in a bar or restaurant. Therefore the solutions is used the output PWM LED driver to realize the dimming instead of changing current amplitude. The output light level is chopped depending on the duty cycle of dimming signal. Important parameters are the dimming resolution and output PWM frequency. Minimal dimming level should be low to 0.1% in order to achieve 8 bit dimming resolutions to fit most of applications. Output PWM frequency should be as high as possible to prevent the flickering issue shown in Table 1. According to the literature, it is suggested to keep at least higher than 1.25kHz in order to minimize the visual distractions*.

Conclusion

The best way to dim LED strip is using output PWM method which resolves the dead travel in the commonly seen dimming issues. Mean Well offer PWM series with range 40~120Watt featuring analog 3 in one dimming (digital one: DALI is on the way) and IP67 protection is the suitable driver for the LED strip applications.

Note: Reference: Designing to Mitigate the Effects of Flicker in LED Lighting by Brad Lehman and Arnold J. Wilkins 2014

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