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 an 8W off-line LED driver that meets the ENERGY STAR solid-state lighting standard, since the sinusoidal square wave power conversion of the flyback regulator does not provide constant energy to the primary bias, dynamic self-powered (DSS) The circuit may activate and cause a flash of light. 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. During a one-half cycle of the input power waveform, this series of events may be repeated multiple times to form a visible LED flash. To address this issue, a key requirement for TRIAC dimming is that the input capacitance of the EMI filter is extremely low and the capacitor must be decoupled by the TRIAC and the winding 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.