Blog de la compañía Recycle ON AC-DC Power Conversion:Offline Controller/Regulator,PFC Controller,Secondary Side Controller

Recycle ON AC-DC Power Conversion:Offline Controller/Regulator,PFC Controller,Secondary Side Controller

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I. Offline Controllers/Regulators

The offline controller is the primary core control chip in isolated AC-DC power supplies. It operates directly within the bus voltage environment following rectification of high-voltage mains power, without the need for a preceding low-voltage power supply. It serves as the core drive unit for mainstream offline switching topologies—such as flyback, forward and active-clamped flyback—and acts as the central control hub for power system start-up and shutdown, power regulation and basic protection. Its primary function is to control the conduction and turn-off of the primary-side power switching devices, thereby achieving the isolated conversion of AC energy and the regulation of power transmission.

1. Core Operating Principles

The offline controller utilises pulse width modulation (PWM), pulse frequency modulation (PFM) or a hybrid modulation mode to adjust the duty cycle and switching frequency of the power switching devices in real time, thereby controlling the energy storage and release processes in the primary winding of the transformer. By acquiring primary-side current sampling signals and auxiliary winding feedback voltage signals, and taking load variations into account, the chip dynamically adjusts switching parameters: reducing the switching frequency and minimising losses under light loads, whilst increasing power output under heavy loads, thereby achieving stable primary-side power control in conjunction with closed-loop feedback. Some high-performance offline controllers support multi-mode adaptive switching between quasi-resonant (QR), discontinuous conduction mode (DCM) and continuous conduction mode (CCM), effectively reducing switching losses and improving conversion efficiency across the entire load range.

2. Core Functions and Features

Offline controllers integrate the full suite of core functions for primary-side power supply control, forming the foundation for stable power supply operation. Firstly, constant voltage/constant current (CV/CC) control supports a primary-side feedback (PSR) architecture, enabling precise output voltage and current limiting without the need for a secondary-side optocoupler, thereby simplifying the circuit structure and enhancing system reliability; secondly, multi-mode energy efficiency optimisation accommodates a wide input voltage range and supports adaptive frequency and duty cycle adjustment to meet energy-saving standards such as Energy Efficiency Class 6; Thirdly, comprehensive protection mechanisms, with built-in over-voltage, under-voltage, over-current, over-temperature and short-circuit protection, which can rapidly latch the chip’s output to prevent the destruction of power devices; fourthly, a minimalist peripheral architecture, integrating a high-voltage start-up circuit and power supply module, eliminating the need for an additional auxiliary power supply and significantly reducing PCB area and material costs.

3. Mainstream Topologies and Application Scenarios

Offline controllers are primarily suited to isolated AC-DC topologies of small to medium power; the mainstream application is the flyback topology, whilst they are also compatible with advanced architectures such as active-clamped flyback and quasi-resonant flyback. Typical chips, such as the FSL206MR, UCC28740 and STACF01B, are widely used in small-to-medium power applications such as mobile phone adaptors, chargers, power supplies for small household appliances, industrial low-voltage power supply modules and LED driver power supplies. Among these, quasi-resonant off-line controllers enable valley switching, significantly reducing electromagnetic interference (EMI) and switching losses, making them the mainstream choice for mid-to-high-end off-line power supplies.

II. Power Factor Correction (PFC) Controller

In conventional AC-DC power supplies without PFC, the rectification and filtering circuit causes severe distortion of the input current waveform, generating a large amount of harmonics. This not only reduces the utilisation efficiency of grid power but also pollutes the grid and interferes with surrounding electronic equipment. As a dedicated control chip for the front-end of AC-DC power supplies, the power factor correction (PFC) controller’s core function is to correct the input current waveform, ensuring that the input current is highly synchronised with the input voltage in terms of both phase and waveform. This improves the power factor, reduces total harmonic distortion (THD), and meets the grid compliance standards of various countries.

1. Core Operating Principle

PFC controllers primarily operate in conjunction with a boost topology, achieving waveform correction through dual closed-loop control logic: the voltage closed-loop monitors the output bus voltage in real time to stabilise the DC output voltage of the front-end stage; the current closed-loop continuously samples the input current, dynamically adjusting the conduction time of the power switching transistors to force the input current to follow the variations of the mains-frequency sinusoidal voltage waveform, thereby eliminating current distortion. This control method enables the power factor of the power supply to be increased to over 0.95, significantly suppressing harmonic interference and improving the efficiency of grid energy utilisation.

2. Classification of Main Operating Modes

Based on differences in conduction modes, PFC controllers are primarily divided into two categories, suited to different power applications:

The first is Critical Conduction Mode (CRM/TM) PFC, represented by the LD7597 series. The switching device turns on at the instant the inductor current reaches zero, resulting in no flyback losses, a simple circuit structure and excellent THD performance. It is primarily suited to small and medium-power power supplies of up to 100W and is widely used in applications such as LED lighting and small household appliances.

Secondly, Continuous Conduction Mode (CCM) PFC, where the inductor current remains continuously conductive, resulting in low ripple and stable power output. This is suitable for medium-to-high-power AC-DC power supplies of 200W and above, and is commonly used in industrial power supplies, charging points and server power supplies, offering higher load stability and lower harmonic interference.

3. Core Value and Application Requirements

PFC controllers are essential components in high-power AC-DC power supplies. Safety and energy efficiency standards in most countries explicitly stipulate that AC-powered equipment rated at 100W or above must be equipped with a power factor correction circuit. In addition to optimising grid quality, PFC controllers stabilise the voltage at the primary bus, mitigating the impact of mains voltage fluctuations on the downstream conversion circuit. This enhances the power supply’s overall immunity to interference and output stability, whilst reducing the operational load on the downstream controller and power devices.

III. Secondary-Side Controller

The secondary-side controller is installed on the isolated secondary side of the high-frequency transformer and serves as the core for precise control at the output stage of an AC-DC power supply. Unlike the coarse-tuning control of the primary-side offline controller, the secondary-side controller focuses on high-precision voltage regulation, high-efficiency rectification and optimisation of load transient response. It thoroughly resolves the issues of insufficient feedback regulation accuracy and response lag on the primary side, making it a core component of high-end, high-precision and high-efficiency AC-DC power supplies.

1. Core Classification and Operating Principles

Secondary-side controllers are primarily divided into two categories: secondary voltage regulators and synchronous rectification controllers, which complement each other’s functions and operate in concert:

The secondary voltage regulator directly samples the output voltage and current signals; after error amplification and compensation calculations, the signal is fed back to the primary-side offline controller via optocoupler isolation, dynamically adjusting the primary-side switching parameters to achieve precise closed-loop voltage regulation. Compared to primary-side PSR feedback, secondary-side feedback (SSR) offers more direct sampling and faster response times, significantly optimising load transient response characteristics whilst reducing output voltage ripple and regulation deviation.

The synchronous rectification controller (representative model: NCP4305) is specifically designed to drive secondary-side rectifier MOSFETs, replacing traditional, inefficient Schottky diode rectification. By precisely detecting the voltage threshold of the transformer’s secondary winding, the chip enables adaptive switching on and off of the MOSFETs. It is compatible with a variety of topologies, including DCM, CCM, QR flyback and LLC, significantly reducing secondary rectification losses and markedly improving the overall conversion efficiency of the power supply, particularly in high-current output applications.

2. Key Advantages and Technical Features

Firstly, it offers ultra-high regulation accuracy. By directly sampling the output signal, it avoids the errors associated with primary-side auxiliary winding feedback; voltage regulation and load regulation can be maintained within ±1 per cent, meeting the power supply requirements of precision electronic equipment. Secondly, it is highly energy-efficient; the synchronous rectification architecture reduces secondary rectification losses by over 60 per cent, making it a key technology for high-power, high-efficiency power supplies; Thirdly, it offers excellent dynamic response, enabling rapid reaction to sudden load changes and voltage fluctuations whilst suppressing output voltage spikes and ripple; fourthly, it features a high degree of functional integration, with built-in protection against output overvoltage, overcurrent, short circuits and overheating, allowing independent fault protection at the output and forming a dual-protection system in conjunction with primary-side protection.

3. Suitable Applications and Architectural Pairings

Secondary-side controllers are predominantly used in mid- to high-end, high-precision, high-current AC-DC power supply applications, including fast-charging adapters, server power supplies, precision industrial power supplies, and new energy vehicle power supplies. Conventional, low-power, simple power supplies may adopt a primary-side PSR architecture to eliminate the need for a secondary-side controller, thereby reducing costs; whereas power supplies with higher requirements for output accuracy, efficiency and transient performance generally adopt a three-stage architecture comprising ‘front-end PFC + primary-side offline main controller + secondary-side precision control’, balancing grid compliance, conversion efficiency and output stability.

IV. Collaborative Architecture and Selection Comparison of the Three Controller Types

A complete, high-performance AC-DC power supply system follows a coordinated control logic of ‘PFC pre-stage optimisation + primary offline conversion + secondary-side precision voltage regulation’, with the three types of controllers each fulfilling their respective roles and enhancing performance at every stage: the PFC controller is responsible for optimising input power quality, addressing harmonic distortion and power factor issues; the offline controller handles primary energy conversion and power transmission, enabling basic power supply start/stop functions and coarse voltage regulation; the secondary-side controller is responsible for precise correction and efficiency enhancement at the output, ensuring the final power supply quality.

In terms of selection and adaptation: for low-power, low-cost applications (≤30W), an offline controller alone may be used to simplify the circuit and control costs; for medium-to-low-power compliance scenarios (30W–100W), an ‘offline controller + simplified PFC controller’ architecture is used to meet basic energy efficiency and grid standards; for mid-to-high-end, high-precision scenarios (≥100W), a complete ‘PFC controller + offline main controller + secondary-side controller’ architecture is employed to achieve high-efficiency, high-precision and highly stable power supply output.