118 TECHNOLOGY / LED
Figure 7: The ripple current from an LED driver connected to an LED.
PWM (100% flicker) driven LED system with a PWM frequency of up to 1KHz whereas an LED light source with a flicker frequency from 1KHz and only 5% flicker would not be seen by observers. For clarity an LED light source with 5% flicker is essentially a DC light source with 5% ripple current. However, figure 9 shows that the majority of subjects in the study would accept light sources which give rise to the stroboscopic effects irrespective of the flicker percentage after a flicker frequency of approximately 1500Hz.
The advantages of LED systems is the electronics required to control them are advancing at a tremendous pace and therefore it is now possible to see driver technologies that implement DC driving which are capable of dimming the LED output down to less than 1% without using PWM to control the LED current with long life and low ripple current. One such promising technology is the Digital Signal Processing technology incorporated into the iDrive range of LED drivers. The DSP technology directly controls the LED current at a switching frequency up to 120KHz and the RMS ripple current using long life (300,000 hour solid-state) capacitors is less than 5% as shown in figure 10. The good news is that several groups are looking into the effects of flicker including
Figure 8: The LRC results on human detection of stroboscopic effects.
the IEEE PAR1789 standards group which is recommending practices for modulating current in High Brightness LEDs for mitigating health risks to viewers. You can see more from this group on their website
www.grouper.ieee.org/groups/1789/. Also Nema discusses flickering within NEMA LSD 49-2010 entitled “Solid State Lighting for Incandescent Replacement—Best Practices for Dimming” available free at
www.nema. org/stds/
lsd49.cfm
How to avoid flicker when specifying projects • Consult a trained, experienced lighting designer to evaluate your lighting application sensitivity to flicker
• Consider flicker when specifying products for sensitive applications
• Ask manufacturers how their product modulates light output - LED driver
- Integral PWM dimming (what frequency & bit resolution) - DC dimming (what switching frequency)
- Behaviour during failure modes
• Be aware of system components which can cause flicker
- Mains dimmable switches and drivers - Step-down transformers connected to less than minimum load
Figure 9: The LRC results on human acceptability of stroboscopic effects.
• Understand the issue further and stay up to date
Conclusions
Artificial Light in general does not pose too much of a hazard. However certain lighting technologies offer the opportunity to improve quality of lighting from a human visual perspective by increasing the flicker frequency and flicker percentage. These aspects are really important but are generally never considered when installing lighting installation. For example, how many LED street light applications use LED drivers that have a flicker frequency of 100Hz – I don’t know the answer but I bet the specifiers of street lights have probably never specified they should be higher than 100Hz! Such a low frequency could cause serious implications when driving as stroboscopic effects could cause visual difficulties in estimating speed for example. Ideally, all light sources should try and maintain a flicker frequency > 30KHz and with a flicker percentage of less than 10%. This is definitely a major challenge for the LED lighting industry moving forward although the technologies are available to achieve this but not at the price the market currently wants.
Therefore, I suggest if you want a healthy LED lighting system for your clients invest in quality and procure a lighting system with high switching frequencies and preferably uses DC outputs with low ripple current rather than switched frequency outputs.
g.archenhold@
mondiale.co.uk
Geoff Archenhold has been seconded twice to the UK Government to support the Light- ing, LED and Photonics industry and currently helps LED companies develop business plans to raise investment from the finance com- munity. He is an active investor in LED driver and fixture manufacturers and a lighting en- ergy consultancy. The views expressed in this article are entirely those of Geoff Archen- hold and not necessarily those of mondo*arc.
Figure 10: The variation of ripple current in a DSP enabled LED driver.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148