The key to optimizing the working life of LED bulbs
Previous LED bulb drivers required a large number of external components, high-priced isolation components, and special design strategies to prevent long-term aging problems (such as electrolytic capacitors). Now, the integration of the driver circuit makes the lamp more prone to reliability issues such as early failure or a reduction in MTTF (mean time between failures).
The MTTF is a measure of the time to first failure and is usually calculated as the FIT rate of each component in the circuit (per 1 billion hours of failure rate). Electrical isolation is required for safety reasons as the drive circuit reduces the large AC voltage (100Vac/220Vac) to a dc voltage that can power the LED.
In a typical electrically isolated AC/DC converter, a discrete optical isolator (or optocoupler) is used to provide feedback from the secondary side to the primary side controller, which converts an electrical signal into an optical signal. The signal passes through the light barrier and then back to the electrical signal (Figure 1). Because the FIT ratio of the optocoupler is higher than that of the semiconductor component, the MTTF specification of the entire circuit is lowered.
In addition, due to the effects of aging, the current transfer rate of the optocoupler may change with time and temperature. This change will affect the loop stability of the power supply and shorten the life of the LED driver circuit. While many LED lights and fixtures can operate at high PCB temperatures, designers must eliminate these weaknesses to achieve longer life.
Figure 2 shows iWatt's primary-side digital control technology, PrimAccurate. This patented solution uses real-time waveform analysis to detect LED current through the primary side of the isolation transformer without the need for direct feedback from the output while maintaining stringent string stringing. Constant current regulation.
Another benefit of this technology is internal feedback loop compensation, which simplifies the design and reduces the number of external components. In particular, it does not require optocouplers, which have the highest FIT ratio, which improves the reliability of the LED driver circuit. The overall reliability of the bulb.
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