LED flashing solution caused by fluctuating AC input

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

The idea is to add overvoltage protection to prevent flickering during startup and then limit AC on/off problems during normal operation by using automatic detection to sense when the AC input is off. Preventing overvoltage conditions from adding overvoltage relief (OVE) to the PFC block helps to more easily react to voltages that rise too fast (especially during startup). Without the OVE block, if the PFC does not respond in time, there will be a gap between the output voltage and the feedback control. Since the output voltage reaches its target, the feedback control attempts to lower the control value, but because it is too slow, it may generate too much energy. Adding OVE functionality can solve this problem. When too much boost voltage exceeds the target, the protection starts and the PFC converter reacts quickly. Figure 1 shows the output response of the PFC controller with and without an additional OVE circuit. The controller with OVE effectively eliminates the stress of overpressure conditions at startup.

Figure 1: Adding the OVP function eliminates overvoltage stress at startup

Automatic detection of AC does not exist Although the addition of the OVE function can eliminate flicker at startup, it is still possible that AC input voltage fluctuations cause flicker during normal operation. In many cases, the PFC controller's supply voltage (Vcc) is provided by an independent 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 a mismatch between PFC and IC operation. For example, if the AC line voltage is accidentally disconnected 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 value. This condition continues as long as the AC input is missing, which is the same or worse than the condition in which no OVE function occurred at startup. The output voltage fluctuates between the overvoltage level and the regulation level, causing significant flicker.

Adding a circuit that detects the presence of an input voltage and then connecting the block to an overshoot-eliminating circuit can help solve the problem. It seems that the easiest way to check the input voltage is to directly detect the AC voltage, but identifying the AC line voltage can be a bit complicated. The input capacitance may change the AC input waveform. When the AC line voltage changes from zero to peak, there may be a delay in the detection time. In addition, detecting the AC line voltage may require specifying an additional sense pin that may burden 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 up indirect detection of the AC line voltage. A zero current sense (ZCD) timing signal, reflecting the combination of auxiliary and inductive 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 continues to repeat. Evaluating the relationship between these signals makes it possible to know if the AC line voltage is present. During the disappearance of the AC input voltage, the internal auto-detection circuit is used to count the condition and trigger overshoot cancellation to prepare the circuit for waiting for the AC input voltage to return. Once the AC input voltage returns to normal, the OVE circuit can quickly offset the output overvoltage condition. Figure 2 shows how it works. Figure 2: Output response pink waveform is the PFC output voltage when the AC line voltage is on and off, 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 turned on and off. Conclusion During the startup process, the OVE function can eliminate voltage fluctuations. In normal operation, the two-step method of detection and management can prevent flicker when the AC voltage disappears instantaneously.