PFC type and principle analysis

PFC type and principle analysis
1, Passive PFC (Passive Correction)
1. Principle and Implementation
Compensate for phase shift caused by inductive/capacitive loads and suppress harmonics through passive components such as inductors and capacitors. For example:
Inductance compensation: Connect a power frequency inductor in series between the rectifier bridge and the filtering capacitor, and use its "current non abrupt change" characteristic to smooth the current pulse and reduce waveform distortion.
Valley filling circuit: Adopting a capacitor parallel structure, it releases stored energy when the input voltage is low, prolongs the conduction time of the rectifier diode, and improves current continuity.
2. Characteristics
The power factor is increased to 0.7-0.8, but harmonics cannot be completely eliminated.
Large size, low efficiency, only suitable for low-power scenarios below 75W.
2, Active PFC (Active Correction)
1. Core principle
Using high-frequency switching devices (such as MOSFETs) to dynamically adjust the input current waveform to be in the same frequency and phase as the voltage, a typical topology is a Boost boost circuit.
Boost circuit function: Boost the rectified pulsating DC to a stable high voltage (such as 400V), and adjust the duty cycle of the switch tube through PWM to force the current to be sinusoidal.
Control logic: Adopting dual closed-loop control of voltage loop and current loop, real-time monitoring of input voltage and current to ensure waveform matching.
2. Technological evolution
Gallium Nitride (GaN) Applications: For example, the nano micro NV6127 chip integrates a 650V voltage resistant switch tube, supports 2MHz high-frequency switching, improves heat dissipation performance and efficiency, and is suitable for scenarios such as servers and fast charging.
Interlocking parallel technology: Multiphase PFC parallel reduces current ripple and optimizes low voltage output efficiency.
3. Advantages
Power factor>0.95, harmonic distortion (THD)<5%.
Supports wide voltage input (90-264V), suitable for medium to high power devices.
3, Other classifications and principles
1. Single stage PFC
Integrating PFC and DC-DC conversion functions, only one switch tube and control circuit are needed, simplifying the design but with poor dynamic response, suitable for low-cost compact devices.
2. Two stage PFC
Pre stage Boost+PFC: realizes current waveform correction, and post stage DC-DC regulates output voltage, with high accuracy but complex circuit.
Typical applications: Scenarios that require high input current quality, such as medical power supplies and industrial frequency converters.