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AC/DC

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

Design Procedure

In " Design of a Secondary-Side Synchronous Rectifying Circuit to Improve the Efficiency of an AC/DC Converter", an example of the synchronous rectification design of an AC/DC converter with diode rectification using the BM1R001xxF series of secondary-side synchronous rectifying controller ICs will be explained. We begin by describing the design procedure.

Design Procedure

An example of synchronous rectification design of an AC/DC converter with diode rectification using a BM1R001xxF series secondary-side synchronous rectifying controller IC is presented.

The essence of this design is replacement of the rectifying diodes and shunt regulator on the secondary side of the AC/DC converter with the BM1R001xxF series secondary-side synchronous rectifying controller IC, to improve the efficiency of the AC/DC converter. The design process generally follows the procedure outlined below.

  1. Design of the synchronous rectifying circuit part
     1-1. Selection of MOSFETs for synchronous rectification
     1-2. Controller IC selection
     1-3. Selection of peripheral components
  2. Design of the shunt regulator circuit part
  3. Troubleshooting
  4. Characteristic evaluation

The circuit designs in 1 and 2 are summarized. Primary-side design is not included in this design example.

1. Summary of design of the synchronous rectification circuit part
1-1. Selection of MOSFETs for synchronous rectification

  • ・Initially, the MOSFETs used for synchronous rectification, which replace the rectifying diodes, are selected.
  • ・Using as criteria the reverse voltage VR and forward current IF of the rectifying diodes, the maximum drain-source voltage, peak current, losses due to Ron, maximum allowable dissipation of the package, and other factors are considered to select the MOSFETs used for replacement.
  • ・As matters to be checked, operation is always confirmed in a state in which it is incorporated into the product, and heat sinks and other heat dissipation methods are also studied as necessary.

1-2. Controller IC selection

  • ・The BM1R001xxF series adopts compulsion OFF time control to enable use with various power sources.
  • ・Using the previously verified reverse voltage VR and forward current IF occurring in the rectifying diodes and by calculating the maximum ON-time tMAX_ON of the secondary-side MOSFET, corresponding ICs are selected.
  • ・The BM1R001xxF series can be used with synchronous rectifying MOSFETs on either the low side or the high side, and so both circuits are considered.

1-3. Selection of peripheral components

  • ・In order to prevent erroneous detection due to the surge voltage occurring at the time of MOSFET switching, a component to use as a countermeasure at the drain pin is selected.
  • ・Settings for the feed circuit to the IC power supply pin VCC

2. Summary of design of the shunt regulator circuit part

  • ・The BM1R001xxF series incorporates a low-power, high-precision shunt regulator, to reduce the shunt regulator-related power consumption.
  • ・The shunt regulator is completely separated from the synchronous rectifying controller within the IC, so that even in high side-type flyback applications, the possibility of GND reference use of the shunt regulator is considered.
  • ・It is also possible not to use the shunt regulator within the IC, and the circuit resulting in this case is examined.

The next article will describe the design example and explain the BM1R001xxF series product used in the design.

Key Points:

・The design procedure, in broad outline, is as follows.
 1. Design of the synchronous rectifying circuit part:
  Selection of MOSFETs for synchronous rectification, Controller IC selection, Selection of peripheral components
 2. Design of the shunt regulator circuit part
 3. Troubleshooting
 4. Characteristic evaluation


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