CrM Boot PFC and High Frequency Quasi-Resonant Flyback Controller

The NCP1945 is a combination IC which integrates power factor correction (PFC) and quasi−resonant flyback functionality for designing high-performance off-line USB-PD and USB Type-C power converters. The NCP1945 has optimized circuitry for a Boost PFC with features to ensure near-unity power factor while optimizing efficiency at all loads. The PFC operates in critical conduction mode (CrM) until the power drops below a threshold level at which time innovative Valley Synchronized Frequency Fold-back (VSFF) method is used. Using the VSFF the PFC stage enters the discontinuous conduction mode (DCM) with a dead-time which increases as the load decreases (frequency foldback) reducing switching losses. The NCP1945 also supports a quasi-resonant (QR) current-mode flyback stage featuring proprietary valley-lockout circuitry to ensure stable valley switching down to the 6th valley, then transitions to frequency foldback mode to reduce switching losses. When the load decreases further, the NCP1945 QR section enters skip mode to manage the power delivery with special minimum peak current modulation circuitry. The minimum peak current modulation circuitry reduces the switching frequency quickly improving light load performance with high frequency designs. The QR circuitry automatically senses reflected output voltage and adjusts features or optimal performance including minimum peak current in light loads, current limiting circuitry to ensure a constant output current limit regardless of programmed output voltage, or nameplate output power. To help ensure converter ruggedness, the NCP1945 implements several key protective features such as internal brownout detection, a non-dissipative Overpower Protection (OPP) for constant maximum output power regardless of input voltage, a latched over voltage and NTC-ready overtemperature protection through a dedicated pin, and line removal detection to safely discharge the X2 capacitors when the line is removed.