Usually the human eye can perceive the flicker of light at a frequency of 70 Hz, but not above that frequency. Therefore, in the application of LED lighting, if the frequency of the pulse signal is lower than 70 Hz, the human eye will feel the flicker. Of course, in specific applications, there are many factors that may cause LED lights to flicker. For example, in off-line low power LED lighting applications, a common power supply topology is isolated flyback topology. GreenPoint & reg, an 8W off-line LED driver that meets the "Energy Star" solid-state lighting standard, is taken as an example. As the sinusoidal square wave power conversion of flyback regulator does not provide constant energy for primary bias, the dynamic self-powered (DSS) circuit may activate and cause flicker. In order to avoid this problem, the primary bias must be able to discharge in each half cycle. Correspondingly, the capacitance and resistance of the bias circuit need to be properly selected.
In addition, electromagnetic interference (EMI) filters are required even in LED drive applications that provide excellent power factor correction and support TRIAC dimming. Transient current caused by TRIAC step will excite the natural resonance of inductance and capacitance in EMI filter. If the resonant characteristic causes the input current to drop below the TRIAC maintenance current, the TRIAC will be turned off. After a short delay, TRIAC is usually turned on again, which excites the same resonance. During a half cycle of the input power waveform, the series of events may be repeated many times, resulting in visible LED scintillation. To cope with this problem, a key requirement of TRIAC dimming is that the input capacitance of EMI filter is very low, and this capacitance can be decoupled through TRIAC and winding impedance. According to the formula, if the capacitance of the dimming module is reduced, the resistance of the resonant circuit can be increased. In principle, the oscillation can be suppressed and the desired circuit can be restored.