Fail-Safe Multi-Chip LED Module Driver
The service life of an LED depends on how effectively the current flowing through it remains within specified limits under all possible operating conditions. For multi-chip LED modules, tightly packed LEDs are arranged in strings, with 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 with a steady current that is common among all LEDs. The strings are basically equal. While small imbalances in string current will not result in noticeable differences in brightness, parameters such as the composite forward voltage (the sum of the VF of all LEDs in series) and its critical influence on temperature and the magnitude of the forward current (IF) flowing through it Dependencies, as well as other process-related changes, make current balancing a long-term and difficult task. Additionally, failure of any LEDs, strings, or LEDs that may leak or be inefficient due to hotspot formation may further burden these LEDs and lead to shortened lifespan and eventual catastrophic failure of the SSL and its associated driving sources. .
The motivation behind developing this driver was to provide an efficient and fault-tolerant workhorse, especially for integrated LED modules containing medium and high power LEDs in series or series-parallel configurations. Unlike solid-state lights made from discrete power LEDs, any faulty LED cannot be replaced/repaired if the integrated LED chip opens, shorts, or leaks due to electrical or thermal stress encountered during its operation. This circuit handles such incidents and isolates the faulty string without causing any loss to the power budget or affecting other strings that are functioning properly.
The proposed design concept uses a technique to keep parallel-driven LED strings within a specified regulated current range by injecting a reference current (IB ) into multiple current-sensitive switches connected to the LED string and adjusting its amplitude accordingly. Operates under normal and expected electrical faults that may occur during its lifetime, thereby providing fault-tolerant protection against current imbalance in the LED string in the event of a short circuit, leakage, or open circuit. Unlike other circuits, the simplicity, cost-effectiveness, and efficiency of this circuit have many other advantages besides ingenuity.
The circuit consists of three LED strings S1, S2 and S3, each with three series-connected 3W white LEDs connected to a MOSFET/BJT based constant current sink (CCS). The current flowing into each string (IC1…IC3) is determined by a constant current IB, resulting in V GS , which is applied simultaneously to the gates of all MOSFETs. Constant current IB sets the gate-source voltage V GS to T4, T7 and T10. Once the potential drop across R5 approaches 600mV, T3 (we are only considering string 1) limits the current through the sink, thus reducing the gate drive.
Another function is integrated into each current sink by wiring T5 in parallel with T3. Normally T5 remains off until the drain voltage rises close to Vcc, creating enough voltage on R6 to drive T5 into saturation, thus de-energizing the MOSFET by connecting the gate to ground.
The position of R* controls the magnitude of the constant current (IB) in the voltage controlled current source (VCCS) configured around T1, T2 and (R1+R*) and accordingly generates the appropriate VGS to drive the MOSFET, but in addition, It also provides analog current dimming functionality. The Ref node in the schematic has been provided as an emergency shutdown function, disabling all strings by applying a voltage close to Vcc, pushing the constant current IB to zero. IB is equal to:
IB = (Vcc-Ve) / (R1+R*)
Performance: Normal string current has been set to around 1A for given component values. As can be seen, the bad-case current typically remains around 1A unless the string current drops to a few mA or becomes negligible under extreme leakage, short-circuit or open-circuit conditions. For a given component value, IB can vary from 0.4mA to 1.4mA. If necessary, the series current can be increased by appropriately reducing the detection resistor R5. The LED and power MOSFET must be mounted on a suitable heat sink/metal core PCB to avoid thermal runaway.
Short Circuit Protection: If any LED in a particular string is short circuited, the SSL will continue to function normally, however, if the entire string is short circuited, the potential of the MOSFET drain will rise to Vcc and in this case the corresponding MOSFET will be disabled, Force the drain current to zero.
Prevents LED leakage: If any LED in a particular string leaks, it will continue to function normally, however, if the entire string leaks, the drain of the MOSFET will rise to Vcc, in which case the MOSFET will be disabled, forcing the drain The current is zero.
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