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PALCO IT, s.r.o. | Industrial technologies | Posts | Other contributions | LED ballast description
LED driver description, reliability aspects and factors. Currently, the most using two types of topologies for LED drivers are: More simplier quasi resonant Flyback topology with implicit active Power Factor correction for the input power up to about 120W. Resonant LLC topology, more complicated for 200W+ of input power. Good filtering is mandatory incumbent on developer, by the way, there is a standard. Overvoltage protection (typicaly class D) with varistor, using is inconsistence. It may cast an evil shadow on your developed driver by frequently breaking of the fuse caused by overvoltage spikes in the electricity. This is mainly task for overvoltage protection of electricity of building as a complex. I will try to describe 5 points of the Flyback topology driver regarding to reliability or MTBF (mean time between failure) of the LED driver. Input electrolytic capacitors (BULK capacitors), the active PFC topology excludes them, this is the step on higher reliability. Snubber capacitor - don't use ceramic type - use the foil type. Secundary rectifier ideal is SiC diode, if you use schotky than overdimension twice regard the breakdown voltage (sum of the output voltage + reflected primary voltage it's divide input voltage by the transf. turn ratio), practicaly 200V+ reverse break down voltage of schotky diode is enough. Secondary electrolytic capacitors. Idealy using of polymer types 4 or 6 in parallel or tantalic types. Regard to the cost, if you use classical electrolytic capacitors, use min. 2pcs. (rather 4 pcs - this lower ripple current just by one capacitor) in parallel on 105°C and selected with Low ESR (eq.ser. resistor). Primary sensing is ideal (e.g. LT3799-1 suitable), becouse the optocoupler is excluded as a potential source of failure. In SMPS on the MosFET acts, as sum, 4 margins: Rectified input voltage (nominal 325V). Reflected secundary voltage (~Vout*Ntransformer). Leakage inductance spike, may suppressed by Snuber (50V-200V). 4. min. 10% (rather 20-30) of backup is "developer decorum". Definitely 900V breakdown voltage MosFET is necessary. The primary sensing has, of course, worse influence on the regulation loop of SMPS. On the other side, even a good current regulation +-2% of secondary sensing is depredated by the very long PI correction (below 100Hz) of the current part of regulation (in Constant Current mode) to suppress flickering LEDs caused mains frequency. Typical good result is +-5%. And there is a comparable with primary sensing (of course with good regulating techniques in control circuit).
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