Technical Information Site of Power Supply Design

2019.10.23 AC/DC

Power Supply Specifications and Replacement Circuit

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

In the previous article, the BM1R001xxF series IC that will be used in the design was summarized. From this article we embark on the actual design process.

To reiterate, the purpose of this design is to use the BM1R001xxF series of secondary-side synchronous rectifying controller ICs to perform synchronous rectification in an AC/DC converter that uses diode rectification. Hence we start with the circuit and specifications of a diode-rectification AC/DC converter, and assume that these specs are inherited when modifying the design for synchronous rectification.

Example of Power Supply Specifications and Replacement Circuit

In this example, the rectifying diode and the shunt regulator section in the secondary-side diode rectification circuit of the AC/DC converter below are replaced with a BM1R001xxF series IC.

<AC/DC Converter Power Supply Specifications>

  • ・Input voltage(VIN): 400 Vdc
  • ・Output voltage(VOUT): 5 V
  • ・Output current(IOUT): 10 A
  • Power supply type: Isolated PWM flyback converter
  • ・Switching frequency: 130 kHz

The circuit diagram for the AC/DC converter is shown below.

The area highlighted in orange is the part to be replaced; here DOUT is the rectifying diode, and U1 and the resistors are the shunt regulator section. PC1 is an opto-coupler (photocoupler), an isolating element to feed back the output voltage to the primary side with isolation. It is not replaced, since it is also necessary even in the circuit with the BM1R001xxF series IC substituted.

Next, we present the substituted circuit. The shaded areas indicate substituted circuit sections.

There are two substitution methods. On the left is the low-side type of substitution in which the switch (MOSFET) is positioned on the low side (GND line) of the transformer; on the right is the high-side type, in which the switch (MOSFET) is located on the high side (VOUT line) of the transformer. Both types will be explained, and ultimately the designs of both types of circuits will be completed and evaluated.

In the above circuits, rectifying diodes are replaced with MOSFETs, and shunt regulators are replaced with the shunt regulator within an IC. However, it must be said that there is a slight increase in the number of external components. Hence when using general secondary-side synchronous rectifying ICs, the standby power consumption is always worsened by the amount of the IC circuit current. However, these ICs incorporate shunt regulators that reduce the IC current consumption, so that efficiency can be greatly improved without worsening standby power consumption, and in addition the components of the shunt regulator section can be eliminated.

Key Points:

・In this design example, a diode-rectification AC/DC converter is modified for synchronous rectification.

・There are low-side and high-side types of modification for synchronous rectification.

・There is a slight increase in the number of external components, but efficiency improvement, and in particular high efficiency when in standby, is a goal of AC/DC converters, and so synchronous rectification is advantageous.

Design Example for PWM Flyback Converter

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