Analyze the core problem of LED lighting power supply design

The problem that the life of the electrolytic capacitor does not match the LED

An important consideration for LED lighting is that the LED driver circuit and the LED itself should have a working life comparable. Although there are many factors that affect the reliability of the drive circuit, electrolytic capacitors have a crucial impact on overall reliability. In order to extend the operating life of the system, it is necessary to analyze the capacitance in the application in a targeted manner and select the appropriate electrolytic capacitor.

In fact, the effective operating life of an electrolytic capacitor is largely affected by the ambient temperature and the internal temperature rise caused by the ripple current acting on the internal impedance. Electrolytic capacitor manufacturers offer electrolytic capacitors rated for life based on exposure to the highest rated temperature environment and application of maximum rated ripple current. The typical capacitor rating life at 105 ° C may be 5000 hours, the actual working stress of the capacitor is lower than the rated level, and the effective working life is longer. Therefore, on the one hand, selecting an electrolytic capacitor with a long rated working life and capable of withstanding a high rated operating temperature can of course prolong the working life. On the other hand, depending on the actual stress and operating temperature, capacitors with lower rated operating temperature and rated life can still be selected to provide a lower cost solution; in other words, proper stress and operating temperature are considered in the design. , can effectively extend the working life of the electrolytic capacitor, so that it can better match the life of the LED.

For example, ON Semiconductor's off-line LED driver Green Point® reference design meets the ENERGY STAR solid-state lighting standard. Panasonic's ECA-1EM102 aluminum electrolytic capacitor is rated at 1000μF, 25V, 850mA, 2,000. Hours and 85 ° C. The available life of this capacitor exceeds 120,000 hours under the assumption of an ambient temperature of 50 °C. Therefore, the best way to make the LED driver circuit work under suitable temperature conditions and properly handle the heat dissipation problem can achieve the matching problem between the LED driver circuit and the LED working life.

In general, if electrolytic capacitors must be used in the LED driver circuit, efforts must be made to control the applied force and operating temperature of the capacitor to maximize the capacitor operating life in order to match the LED lifetime; on the other hand, the designer Electrolytic capacitors should also be avoided as much as possible.

Common causes and treatments for LED flashing

Usually the human eye can perceive a light flicker with a frequency of 70 Hz, above which it is not perceived. Therefore, in LED lighting applications, if the pulse signal appears at a frequency lower than the low frequency component of 70 Hz, the human eye will feel the flicker. Of course, in specific applications, there are a number of factors that can cause the LEDs to flicker. For example, in off-line low power LED lighting applications, one common power supply topology is an isolated flyback topology. For example, the GreenPoint® reference design of 8W off-line LED driver that meets the “Energy Star” solid-state lighting standard by ON Semiconductor, as the sinusoidal square wave power conversion of the flyback regulator does not provide constant energy to the primary bias, dynamic A power supply (DSS) circuit may activate and cause a light flash. In order to avoid this problem, the primary bias must be partially discharged in each half cycle, and accordingly, the magnitude of the capacitance and resistance constituting the bias circuit needs to be properly selected.

In addition, electromagnetic interference (EMI) filters are required even in LED driver applications that provide excellent power factor correction and support TRIAC dimming. The transient current caused by the TRIAC step excites the natural resonance of the inductor and capacitor in the EMI filter. If this resonant characteristic causes the input current to drop below the TRIAC holding current, the TRIAC will turn off. After a short delay, the TRIAC will normally turn on again, stimulating the same resonance. This series of events may be repeated multiple times during one half cycle of the input power waveform to form a visible LED flash. To address this issue, a key requirement for TRIAC dimming is the extremely low input capacitance of the EMI filter, which must be decoupled by TRIAC and wire-wound impedance. According to the formula, if the capacitance in the dimming module is reduced, the resistance of the resonant circuit can be increased, and in principle, the oscillation is suppressed and the desired circuit operation is resumed.

How does PWM dimming affect the life of LEDs?

The life of the LED itself is very long, PWM dimming does not damage the expected life of the LED; even because PWM dimming helps reduce the heat of the LED, it can actually help extend the life expectancy of the LED. Of course, in the system design, it is necessary to effectively detect and control the LED temperature to ensure reliable operation of the LED, reflecting its advantages of long life and low maintenance cost.

Potential problems in controlling LED brightness with TRIAC dimming

The current TRIAC dimming is a popular dimming method. TRIAC dimmers were originally designed for incandescent lamps, but leading suppliers such as ON Semiconductor have also introduced LED drivers that support TRIAC dimming, making them ideal for TRIAC dimming in LED lighting. However, this method also has its limitations, such as increasing circuit complexity, affecting power factor, and relatively limited dimming level. ON Semiconductor recently introduced the NCL30000 LED driver for residential and commercial LED lighting applications. The device uses a critical conduction mode (CrM) flyback architecture to provide a power factor greater than 0.95 in a single-stage topology, eliminating the need for DC-DC conversion sections and simplifying the circuit. This device provides extremely high efficiency even at low levels, meeting various specifications and overall system efficacy requirements. In addition, the device is compatible with leading-edge TRIAC dimmers and trailing-edge transistor dimmers. Depending on the dimmer used, the LED light output can be adjusted to less than 2%, providing excellent dimming performance.