Failsafe multichip LED module driver
The operational life of an LED depends upon how effectively the current flowing through it is kept within the specified limits 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.
The circuit consists of three LED strings, S1, S2, & S3, each having three series-connected 3W white LEDs connected to MOSFET/BJT based constant current sinks (CCS). The current (IC1…IC3) flowing into each string is determined by the constant current IB, generating VGS, which is applied simultaneously to the gates of all the MOSFETs. The constant current IB sets the gate to source voltage VGS to T4, T7, & T10. T3 (we’ll just consider String 1) limits the current through the sinks as soon as the potential drop across R5 approaches 600mV, thus reducing gate drive.
Another feature has been incorporated into each current sink by wiring T5 in parallel with T3. Normally T5 remains off until drain voltage rises close to Vcc, producing sufficient voltage across R6 to drive T5 into saturation to de-energize the MOSFET by grounding the gate.
The position of R* controls the magnitude of the constant current (IB) in a voltage controlled current source (VCCS) configured around T1, T2 & (R1+R*) & accordingly generates the appropriate VGS to drive MOSFETS, but additionally it provides an analog current dimming feature. The Ref node in the schematic has been provided as an emergency shut off feature to disable all the strings by applying a voltage close to Vcc, pushing constant current IB to zero. IB is equal to:
IB = (Vcc-Ve) /(R1+R*)
Performance: The normal string current has been set around 1A for the given component values. It can be seen that the worst case current normally remains around 1A, except cases where string current falls to a few mA or becomes negligible during extreme leaky, short, or open circuit conditions. IB can be varied from 0.4mA to 1.4mA for the given component values. If needed, the string current can be increased by reducing the sense resistor R5 appropriately. The LEDs and power MOSFETs must be mounted on suitable heat sinks/metal core PCBs to avoid thermal runaway.
Protection against short circuit: If any LED(s) in a particular string gets short circuited, the SSL would continue to work normally, however, if the entire string gets short circuited, the potential at the drain of the MOSFET will rise to Vcc and in such condition the respective MOSFET will be disabled, forcing drain current to zero.
Protection against leaky LEDs: If any LED(s) in a particular string gets leaky, it would continue to work normally, however, if entire string gets leaky, the drain of the MOSFET will rise to Vcc and in such condition the MOSFET will be disabled, forcing drain current to zero.