Comparison of Efficiency of Diode Rectification and Synchronous Rectification


Points of this article

・The efficiency of a replacement synchronous rectifying circuit is clearly higher than the efficiency of the conventional secondary-side diode rectifying circuit that is replaced.

・There is almost no difference in efficiency for high-side and for low-side type synchronous rectification methods.

・The dominant factor affecting efficiency is the difference between the loss (VF) in the diode used for diode rectification and the loss (VDS) in the MOSFET employed for synchronous rectification.

In this article, data is presented comparing the efficiency of an AC-DC converter based on diode rectification prior to replacement of the secondary side with the efficiency of an AC-DC converter in which the BM1R00147F secondary-side synchronous rectifying power supply IC has been used to replace the secondary side.

Comparison of the Efficiencies of a Secondary-Side Diode Rectifying AC-DC Converter and a Secondary-Side Synchronous Rectifying AC-DC Converter

In this chapter, we have presented a design case with the objective of improving the efficiency of an existing AC-DC converter with secondary-side diode rectification by using synchronous rectification on the secondary side, in the form of a power supply IC for synchronous rectification. Here, we describe the results of efficiency measurements using three evaluation boards–one for the secondary-side diode rectification prior to substitution, and one each for the low-side type and high-side type rectification after substitution of the BM1R00147F IC. Measurement conditions were an input voltage of 400 V DC, output voltage of 5 V DC, and output current ranging from 0 to 10 A.

The graph on the left shows the efficiency over the entire range of the output current (Iout). The orange line denotes the efficiency of the diode rectification prior to substitution. The blue and red lines represent the efficiency after substitution with synchronous rectification, with the blue and red plots standing for low-side type and high-side type rectification respectively; because the efficiencies for the latter are essentially the same, the high-side type red line is hidden behind the blue line representing the low-side type. From this graph also it is seen that efficiency is lower for the orange line, representing diode rectification. On the right is shown another graph, with the vertical axis expanded.

As a result, it can be confirmed that for a maximum load of 10 A, the efficiency of the secondary-side diode rectification method prior to substitution is 77.3%, whereas the efficiencies after substitution are 81.3% (low-side) and 81.6% (high-side), for an improvement of 4%.

The dominant cause of this difference in efficiency is the difference in losses of the secondary-side rectifying diode and of the MOSFET that replaces it. The secondary-side rectifying diode used is generally an FRD (fast recovery diode) or an SBD (Schottky barrier diode) or the like. VF for such diodes used at the power levels of general cases are typically 0.5 to 1 V, and because the simple conduction loss is VF×Iout, upon assuming 1 V for VF, the conduction loss when Iout = 10 A is calculated to be 10 W. On the other hand, the conduction loss for the replacement MOSFET is Ron×Iout2, which is 0.4 W for Ron = 4 mΩ (from the MOSFET specifications), or about 1/25 of the value for the diode.

Of course all the other loss factors, such as switching losses, must also be added to obtain the actual efficiency, and the above simple comparison is not itself sufficient. However, it should help the reader to see how losses in the secondary-side rectifying element are dominant. Hence given the current state of affairs in which the VF characteristics of diodes themselves cannot be improved significantly, it can be said that secondary-side synchronous rectification is one promising option for greatly improving the efficiency of an AC-DC converter that uses secondary-side diode rectification.

As reference, circuit diagrams and BOM for the evaluation boards used are shown below. The efficiencies presented here are results obtained in these evaluations, and it should be understood that actual efficiencies will differ depending on variations in the characteristics of components used and differences in board layouts and the like.

The next article will explain points to be noted regarding board layouts.

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