Fluctuation AC input caused by LED flashing solution

In LED lighting systems, adding circuitry to improve PFC response time can help eliminate flicker caused by rapidly changing AC input voltages. If the power factor correction (PFC) block does not respond fast enough, ripple AC input can push the output voltage out of its normal range and cause the human eye to perceive changes in the illumination output. Adding some simple circuitry to the PFC block can help to improve response times and eliminate flicker problems.

The idea is to add overvoltage protection to prevent flickering at startup and then limit the AC on / off problem during normal operation by using automatic sensing to sense when the AC input is off. Prevent Overvoltage Conditions Add overvoltage cancellation (OVE) to PFC blocks to help make it easier to react to voltages that rise too fast, especially during startup. Without an OVE block, there will be a gap between the output voltage and the feedback control if the PFC is not responding in time. Since the output voltage reaches its goal, the feedback control attempts to lower the control value, but because it is too slow, too much energy may be generated. Add OVE function to solve this problem. When too much boost voltage goes beyond the target, the protection is started and the PFC converter makes a quick reaction. Figure 1 shows the output response of the PFC controller with and without an additional OVE circuit. OVE-equipped controllers effectively eliminate over-stress conditions at startup.

 

Figure 1: The OVP function is added to eliminate over-stress at startup

 

Automatic Detection of AC Absence Although the addition of the OVE function eliminates the flicker at startup, it is still possible that AC input voltage fluctuations may cause flickering during normal operation. In many cases, the PFC controller's supply voltage (Vcc) is provided by a separate power supply, such as a backup power supply, and a large capacitor is used to stabilize the operation of the IC. This setting can cause mismatch between PFC and IC operation. For example, if the AC line voltage accidentally turns off for two or three AC line cycles, the IC's Vcc may remain available but the PFC output voltage will decrease because there is no input power. As a result, the control loop attempts to compensate for the output voltage drop and the control voltage reaches its maximum. This condition lasts for as long as the AC input is missing, and is the same or worse than if the OVE function was not activated at startup. The output voltage fluctuates between overvoltage and regulation levels, causing significant flicker.

Adding a circuit that detects the presence of an input voltage and then connecting it to a circuit that eliminates overshoot can help solve the problem. It looks like the easiest way to check the input voltage is to detect the AC voltage directly, but identifying the AC line voltage can be a little more complicated. The input capacitance may change the AC input waveform, and the detection time may be delayed when the AC line voltage changes from zero to peak. In addition, testing the AC line voltage may require the assignment of an additional test pin, which may place a burden on the layout. Taking these factors into account, using indirect detection is actually a better approach.

When using a PFC converter operating in critical conduction mode (CRM), there is a relatively straightforward way to set indirect detection of AC line voltage. Zero current detection (ZCD) The timing signal, which reflects the combination of auxiliary and inductor windings, can be used to detect when the inductor current is zero. When the AC line voltage is missing, the non-ZCD signal causes the switching frequency to become the same as the low frequency oscillator, and the maximum on-time repeats. Evaluating the relationship between these signals makes it possible to understand whether an AC line voltage is present or not. During the disappearance of the AC input voltage, an internal automatic detection circuit counts the condition and triggers an overshoot cancellation. The circuit can be prepared to wait for the AC input voltage to be returned. Once the AC input voltage returns to normal, the OVE circuit can quickly cancel the output overvoltage condition. Figure 2 shows how it works. Figure 2: Output Response When AC Line Voltage is On and Off The pink waveform is the PFC output voltage and the blue waveform is the AC input voltage. The PFC output voltage shows the current overshoot through the AC input, even when the AC input is on and off. Conclusion During startup, using the OVE feature eliminates voltage fluctuations and the two-step method of detection and management prevents flickering when the AC voltage disappears instantaneously during normal operation.www.luxledchina.com