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EE Times-India > EDA/IP

Failsafe multi-chip LED module driver design

Posted: 19 Sep 2014     Print Version  Bookmark and Share

Keywords:LED  current  SSL  MOSFET  BJT 

The operational life of an LED is dependent on how effectively the current flowing through is kept within the specified boundaries under all possible working conditions. For multi-chip LED modules, the tightly binned LEDs are arranged in strings having multiple LEDs connected in series, parallel, or series-parallel configurations, sharing a common constant current or voltage source, and each LED string is typically driven at a regulated current that is substantially equal among all of the LED strings. Although a small imbalance in string currents does not cause noticeable differences in brightness, parameters like the compound forward voltage (sum of VF of all LEDs connected in series), and its critical dependence on temperature and magnitude of forward current (IF) flowing through it, besides other process dependent variations, make the current balancing a tough task over the long period. Also, as a result of the failure of any LED(s), string(s), or the possibility of LEDs being leaky or less efficient due to hot spot formation, may further impose more burden on these and lead to shortening of life and ultimately catastrophic failure of the SSL and its associated driving source.

The motive behind developing this driver was to provide an efficient and fault tolerant workhorse especially for integrated LED modules comprising medium and high-power LEDs configured in series or series-parallel combination. Unlike Solid State Lights made out of discrete power LEDs, the integrated LED chip has no room for replacement/repair of any faulty LED if it becomes open, shorted, or leaky due to electrical or thermal stress encountered during its operation. The circuit handles such mishaps and isolates the faulty string(s) without posing any penalty on power budget or affecting other strings working normally.

The proposed design idea uses a technique which allows the parallel-driven LED strings to work within a specified regulated current range by precisely injecting a reference current (IB) into multiple current sensitive switches to which LED string are tied, and accordingly adjusts its magnitude under normal and anticipated electrical faults, which might occur during its life time, and thus provides fault tolerant protection from imbalance of current in LED strings during short circuit, leaky, or open circuit conditions. Unlike other circuits, the simplicity, cost effectiveness, and efficiency of the circuit facilitates many additional advantages besides its ingenuity.

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