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2019.09.25 AC/DC

IC Used in Design

Design of a Secondary-Side Synchronous Rectifying Circuit to Improve the Efficiency of an AC/DC Converter

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In this article, an example of design of synchronous rectification in an AC/DC converter with diode rectification, using the BM1R001xxF series of secondary-side synchronous rectifying controller ICs, is explained. A brief summary of the BM1R001xxF series of ICs used in this design is also given.

IC Used in Design: BM1R001xxF Series

The BM1R001xxF series are essentially synchronous rectifying controller ICs to implement synchronous rectification in the secondary-side output stage of an AC/DC converter. The lineup consists of five products, BM1R00146F to BM1R00150F, and the "compulsion OFF times", which are the mask times to prevent MOSFET gate turn-on on the secondary side by a resonance waveform occurring at the DRAIN terminal, are different for each model. Details are explained in the section on IC selection, but these different models are provided because an appropriate compulsion OFF time must be selected based on the conditions of the circuit in which synchronous rectification is implemented. The products are provided in simple, compact SOP8 packages.

The main internal function blocks are the shunt regulator block and the synchronous rectifying controller block. The shunt regulator features low current consumption and high precision, and can reduce standby power consumption by decreasing the current in the control circuit. The synchronous rectifying controller supports all modes from discontinuous to critical to continuous mode operation, and can be used in PWM converters as well. During continuous mode operation, operation is possible without primary-side switching synchronization signal input, and the number of components and space used can be reduced.

Moreover, the shunt regulator and the synchronous rectifying controller are constructed in separate chips. By making the shunt regulator independent, both high-side switching and low-side switching can be accommodated. In addition, the synchronous rectifying controller alone can be used. Thus use in many different applications is possible.

The operating power supply voltage range is wide, extending from 2.7 V to 32 V, for use in applications with various outputs. Through adoption of high-voltage 120 V processes, drain voltages can be monitored directly.

The key features, specifications, and applications are summarized below.

Key Features Key Specifications
  • ・Internal low-power shunt regulator reduces power consumption in standby mode
  • ・Synchronous rectifying switch supports both high- and low-sides
  • ・High-voltage process: drain terminal voltage rating of 120 V
  • ・Wide input operating voltage range: 2.7 V to 32 V
  • ・Supports various driving methods including PWM and quasi-resonant control
  • ・Primary-side signal input unnecessary when in continuous mode
  • ・Internal overvoltage protection (OVP) for SH_IN and SH_OUT terminals
  • ・Internal thermal shutdown (TSD)
  • ・Internal auto-shutdown function
  • ・SOP8 package supporting flow soldering
  • ・Input voltage range::2.7V to 32V
  • ・Operating circuit current (no switching): 800 µA (Typ)
  • ・Circuit current (during auto-shutdown): 120 µA (Typ)
  • ・Drain monitor terminal voltage rating: 120 V
  • ・Operating temperature range:-40℃ to +105℃
Applications
  • ・General AC/DC converters
  • ・Charger/adaptor
  • ・TV

As indicated in the “Design Procedure”, it is recommended that an appropriate model be selected from the BM1R001xxF series on the basis of the specifications of the circuit in which synchronous rectification is implemented.

Key Points:

・The BM1R001xxF series consists of five models, with different compulsion OFF times.

・Packages are simple and compact SOP8 packages.

・A shunt regulator features low current consumption and high precision, and can reduce power consumption in standby mode by reducing the control circuit current.

・Synchronous rectifying controllers support all modes, from discontinuous to critical to continuous, and can also be used in PWM converters.

Design Example for PWM Flyback Converter

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