2 VCC Self-Bias at Different AC Line and Load Conditions

The VCC self-biasing circuit is designed to perform energy harvesting based on both input and output conditions. As shown in Figure 2-1, the circuit achieves energy harvesting from the switching node capacitor during QR flyback operation. Under specific conditions, such as low AC line input, when the reflected output voltage equals or exceeds the VBUS input voltage, and no energy stored in the Coss capacitor, energy harvesting from the inductor occurs, as shown in Figure 2-1. A small portion of the primary switching current is directed to the VCC capacitor through an internal path. Consequently, the UCG28826 seamlessly enables energy harvesting from both the capacitor and inductor under varying operational conditions.

Figure 2-1 VCC Capacitor Energy Harvest

To verify the VCC voltage is sustained across every switching cycle, a potential challenge arises during extended periods without switching events, such as in open-load conditions or burst mode, where the non-switching interval can extend beyond 40ms. During such prolonged intervals, additional energy is required to maintain the VCC voltage. The UCG28826 addresses this by sourcing VCC bias from the high-voltage (HV) pin when the VCC voltage drops below 5.6V, as shown in Figure 3-1. However, this method can be less efficient, particularly when the HV pin voltage is significantly higher than the VCC voltage. To optimize efficiency, charge the VCC capacitor when the HV pin voltage is close to the VCC level, typically around the zero-crossing points of the AC line cycle. This approach minimizes energy loss and enhances VCC charging efficiency, which improves no-load power consumption performance. Failure to implement this strategy can adversely affect the overall power efficiency of the system under light or no-load conditions. Because of this, properly configuring the value of the VCC capacitor is important.