onsemi FAN6921MRMY: Advanced Critical Conduction Mode PFC and Quasi-Resonant Current Mode Controller

In the pursuit of higher efficiency and greater power density in modern switch-mode power supplies (SMPS), the integration of advanced power factor correction (PFC) and a highly efficient DC-DC conversion stage is paramount. The onsemi FAN6921MRMY represents a significant leap forward by combining a Critical Conduction Mode (CrM) PFC controller with a Quasi-Resonant (QR) Current Mode PWM controller in a single monolithic IC. This integration is specifically engineered to meet stringent energy efficiency standards like ENERGY STAR and EU CoC, making it an ideal solution for adapters, open-frame supplies, and LED lighting drivers.

The first stage of the IC is dedicated to Power Factor Correction. Operating in Critical Conduction Mode (also known as Transition Mode), the PFC stage ensures the input current waveform closely follows the input voltage waveform, achieving a near-unity power factor. This mode offers an optimal balance by enabling operation at the boundary between continuous and discontinuous conduction modes. This results in significantly lower Total Harmonic Distortion (THD) compared to discontinuous mode designs and eliminates the reverse recovery losses associated with continuous mode designs. The integration of a dedicated multiplier and a voltage loop error amplifier enhances linearity and provides robust regulation of the PFC output voltage, even under wide-ranging load and input line conditions.

The second stage is a quasi-resonant current mode PWM controller for the flyback converter. Quasi-resonant operation is a key feature for maximizing efficiency. By detecting the valley of the drain-source voltage waveform on the power MOSFET (through an auxiliary winding), the controller ensures the switch is turned on at the moment of minimum voltage (Valley Switching). This technique dramatically reduces switching losses and minimizes electromagnetic interference (EMI). The current mode control architecture provides inherent cycle-by-cycle current limiting, simplified feedback loop compensation, and excellent load transient response.

A critical built-in feature for safety and reliability is the non-latch protection (NLP) mechanism. The IC incorporates comprehensive protection functions including over-voltage protection (OVP), over-load protection (OLP), and abnormal over-current protection (AOCP). Unlike a latch-off response that requires a power cycle to reset, the non-latch protection feature allows the system to attempt a restart automatically after a fault condition is cleared, enhancing the robustness and user-friendliness of the end product.

Furthermore, the controller includes an internal active filter for the PFC stage, which eliminates the need for an external sense resistor and RC filter network. This not only reduces the external component count and board space but also improves noise immunity and overall system reliability.

ICGOODFIND Summary: The onsemi FAN6921MRMY is a highly integrated, feature-rich combo controller that expertly marries an efficient CrM PFC with a QR flyback PWM controller. It is engineered to achieve high power factor, low THD, and superior efficiency through valley switching, all while offering robust non-latch protection and a reduced BOM count for compact, reliable, and high-performance power supplies.

Keywords: Power Factor Correction (PFC), Quasi-Resonant Control, Critical Conduction Mode, Valley Switching, Non-Latch Protection.