Traditionally, flyback topology is used in high-density AC-DC power conversion, but bottlenecks have appeared in efficiency and power density. The new active clamp flyback (ACF) topology can improve performance and reduce power consumption while minimizing the size and weight of the power converter.
ACF topology provides smaller size and higher energy efficiency
Modern power supply applications pursue more compact volume and higher efficiency, which makes power supply designers tend to choose high frequency applications more and more. Switching power supply using high frequency operation can reduce the size of the transformer and then increase the power density of the power supply. With the development of high-voltage FET and the emergence of wide-bandgap semiconductor devices such as GaN/SiC, new ACF topologies are playing a role in high-density power conversion applications.
The ACF topology utilizes the energy stored in the parasitic components to achieve zero-voltage switching (ZVS) rather than dissipating it in the snubber circuit. Active clamping operation produces waveforms without spikes due to parasitism, resulting in better performance of electromagnetic interference (EMI) than other traditional techniques. The classical flyback topology with RCD buffer has reached its efficiency and power density limit, while ACF topology solves the efficiency problem and reduces EMI radiation.
AC-DC active clamp flyback PWM controller with high energy efficiency
The NCP1568, introduced by onsemi, is a highly integrated AC-DC PWM controller for implementing an active clamp flyback (ACF) topology. NCP1568 uses a proprietary variable frequency algorithm to realize ZVS of MOSFET under various line and load conditions. ZVS function realizes high frequency operation, improves the power density of power converter, and realizes high energy efficiency.
ACF operation simplifies EMI filter design to avoid interfering with other sensitive circuits in the system. The NCP1568 integrates a high voltage startup circuit, a powerful low side driver, and a 5V logic level driver for active clamping FETs (including SJ FETs and GaN FETs).
NCP1568 also has a multi-mode operation and conversion function from ACF mode to discontinuous conduction mode (DCM) to meet regulatory requirements all over the world. The NCP1568 can further realize skip in standby mode, thus generating excellent standby power consumption. NCP1568 is a programmable control solution, supporting quiet skip and programmable frequency. Proprietary adaptive ZVS allows high frequency operation while reducing EMI. Multi-mode operation improves light load efficiency. A combination of flexible control solution and user-programmable function allows NCP1568 to be used with super junction MOSFET (silicon) and gallium nitride (GaN) FET, has the minimum frequency clamping and mute jump, eliminates audible noise, and can operate at a programmable frequency from 100 kHz to 1 MHz.
Meet the application fields with high requirements for power density
The NCP1568 supports built-in adaptive dead time for main and active clamped FETs, peak current mode control with built-in slope compensation option and flexible control solution and programmability, allowing configuration using external silicon or GaN FET, customer programmable option to transition to DCM, integrated frequency foldback with the minimum frequency clamping, the maximum performance in standby mode, standby power less than 30 mW, integrated 700 V high voltage and startup circuit, supporting AC line brownout detection.
The NCP1568 features a low-side 0.85 A/1.5 A source /sink current driver, a 65 mA/150 mA active clamp driver output oscillator, and an internal soft-start timer with four options. In terms of protection functions, it has dedicated FLT pins compatible with thermistors, adjustable over-power protection (OPP), options to support automatic recovery and latched of various faults, and internal thermal shutdown function.
NCP1568 is suitable for a variety of applications, including AC-DC adapters, industry, telecommunications, lighting and other application fields with high power density requirements. It is suitable for AC/DC power conversion, USB Power Delivery and Qualcomm Quick Charge applications. Common end products include high-density notebook and mobile adapters, USB Type-C power supply, server auxiliary power supply and industrial power supply.
Multiple evaluation boards speed up the development of various applications
onsemi also introduces a variety of evaluation boards for different applications, including NCP1568DC48WGEVB, a wide range DC input constant voltage power supply from 48 W, 120 V to 400 V, which is suitable for server auxiliary power supply and other DC input applications requiring low height, high efficiency and high power density. NCP1568DC48WGEVB adopts ACF topology, with onsemi's NCP1568 ACF controller, NCP51530A half-bridge driver, NCP4306 synchronous rectifier controller and secondary NTMFS6B03 synchronous MOSFET, which can achieve more than 94% efficiency at full load.
NCP1568PD60WGEVB is an NCP1568 USB-PD 60W evaluation board, which is matched with the NCP1568 AC-DC PWM controller to realize ACF topology. This evaluation board can be used to evaluate 60W USB PD applications. Another STR-ACF-12V100WPSU-GEVB evaluation board, which can convert NCP1568 100W output AC to DC in a Strata development environment, provides an easy-to-use evaluation board for industrial use and general-purpose power supply.
Conclusion
ACF topology has become the main design architecture for high-density power conversion applications, which can meet high energy efficiency requirements, low EMI, compact size, etc. It can be widely used in power converters using wide-bandgap semiconductor devices such as GaN/SiC, and has great development potential in market application. onsemi's NCP1568 AC-DC ACF PWM controller, with high integration and complete functionality, will be the best partner to develop these products.
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