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2020.10.21 AC/DC

Summary

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

This article concludes the 14-article series on "Design of a Secondary-Side Synchronous Rectifying Circuit to Improve the Efficiency of an AC/DC Converter".

These articles were prompted by the increasingly stringent demands in recent years for efficiency in AC/DC power supplies. One approach to improving the efficiency in an AC/DC converter is to change from the diode rectification method that is currently the most common design used to a synchronous rectification method, for which enhanced efficiency can be expected. However, a number of issues complicate the conversion of an AC/DC converter to synchronous rectification. Many AC/DC converters operating at up to medium power levels use the PWM flyback design, and operate in a continuous mode depending on conditions. If such devices were converted directly into a synchronous rectification design, normal control would not be possible in continuous mode operation; simultaneous turn-on of the primary-side switching element and the secondary-side rectifying element would occur, and the resulting through-current could cause element failure. For this reason, it is necessary to employ a measure such as adding a simultaneous-on prevention circuit, adopting a quasi-resonant design such that continuous mode operation does not occur, or using the device only in non-continuous mode operation.

In response to these issues, the BM1R001xxF series of secondary-side synchronous rectifying controller ICs was developed in order to facilitate synchronous rectification in diode rectification-based AC/DC converters. In these articles we have presented design examples that use these ICs to introduce synchronous rectification into AC/DC converters that employ diode rectification.

The key points of these articles are summarized below. Links to the articles are also included, and should prove useful as well.

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

  1. Introduction

    Key Points

    ・Improvement of AC/DC converter efficiency is essential given the increasingly rigorous standards being adopted in various countries.

    ・Use of secondary-side synchronous rectification in flyback AC/DC converters is plagues with various problems such as avoiding through-current states.

    ・Controller ICs are being developed to enable secondary-side synchronous rectification.

  2. Design Procedure

    Key Points

    ・The design procedure, in broad outline, is as follows.

    • 1. Design of the synchronous rectifying circuit section:Selection of MOSFETs for synchronous rectification, Controller IC selection, Selection of peripheral components
    • 2. Design of the shunt regulator circuit section
    • 3. Troubleshooting
    • 4. Characteristic evaluation
  3. IC Used in Design

    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.

  4. Power Supply Specifications and Replacement Circuit

    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.

  5. Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET

    Key Points

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

    ・At the beginning of the design process of modification for synchronous rectification, a MOSFET to replace the output-rectifying diode is selected.

    ・To determine the specifications of the replacement component, the currents, voltages, waveforms, and the like in the existing circuit are determined.

  6. Synchronous Rectification Circuit Section: Power Supply IC Selection

    Key Points

    ・The operation and various conditions of the existing circuit are confirmed, and the power supply IC to be used in the design is selected.

    ・The maximum ON time is set to prevent failure due to simultaneous ON operation of MOSFETs on the primary and secondary sides.

    ・The compulsion OFF time is calculated, and the relevant BM1R001xxF series model is selected.

    ・In the BM1R001xxF series, five models with different compulsion OFF times are provided.

  7. Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components--D1, R1, R2 at the DRAIN Pin

    Key Points

    ・The voltage at the DRAIN pin of the BM1R00147F controls the gate of the secondary-side MOSFET M2.

    ・The DRAIN pin detection level is as low as several millivolts, thus the slight surge voltage upon switching of the MOSFET M2 is erroneously detected.

    ・As a countermeasure, a resistor and diode are added to absorb surges at the DRAIN pin.

  8. Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-C1, R3 at MAX_TON Pin, and VCC Pin

    Key Points

    ・A capacitor and resistor are connected in series at the MAX_TON pin of the BM1R00147F to reduce noise.

    ・The above capacitor and resistor are also used for phase compensation at the MAX_TON pin, and so they must always be connected.

    ・In the case of a Low Side Type design, power can be easily supplied to the VCC pin of the BM1R00147F from the secondary-side VOUT pin.

    ・In the case of the High Side Type, a separate power supply must be provided by adding an auxiliary power supply circuit or by providing auxiliary windings on the transformer secondary side.

  9. Shunt Regulator Circuit Section: Selection of Peripheral Circuit Components

    Key Points

    ・The output voltage is set by setting the peripheral components of the BM1R00147F shunt regulator circuit section.

  10. Troubleshooting ①: Case When Secondary-Side MOSFET Suddenly Turns OFF

    Key Points

    ・The secondary side of a conventional isolated flyback converter is replaced, and therefore it is extremely important that actual operation be confirmed.

    ・If malfunctioning of the secondary-side MOSFET due to noise occurs, a ferrite bead can be added to the DRAIN pin line and the resistance value of the filter resistance can be increased.

  11. Troubleshooting ②: Case When Secondary-Side MOSFET Turns On Due to Resonance Under Light Loading

    Key Points

    ・When replacing the secondary side of a conventional isolated flyback converter, it is extremely important that actual operation be confirmed.

    ・Under light loading, the secondary-side MOSFET may turn on due to resonance operation; there are four countermeasures available.

    • 1) Decrease the value of the DRAIN pin-connected resistor R1
    • 2) Change to a model (IC) with a long compulsion off time
    • 3) Add a snubber circuit between the drain and source of the secondary-side MOSFET
    • 4) Decrease the transformer windings ratio Ns/Np

    ・For each countermeasure there are points to be noted, which involve tradeoffs.

  12. Troubleshooting ③: Case When, Due to Surge, VDS2 Rises to Above Secondary-Side MOSFET VDS Voltage

    Key Points

    ・Because the secondary side of a conventional isolated flyback converter is being replaced, it is extremely important that the actual operation be verified.

    ・There are cases in which surges may cause VDS2 to exceed the rated VDS of the secondary-side MOSFET; to deal with this, three countermeasures are available.

    • 1) Insert a capacitance between the drain and source of the secondary-side MOSFET
    • 2) Increase the gate resistance value of the primary-side MOSFET
    • 3) Reduce the windings ratio Ns/Np of the transformer to lower VDS2

    ・There are tradeoffs to be considered for each of these countermeasures.

  13. Comparison of Efficiency of Diode Rectification and Synchronous Rectification

    Key Points

    ・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.

  14. Points to Note Relating to PCB Layout

    Key Points

    ・In secondary-side synchronous rectification as well, nearly all the important points pertaining to PCB layout are based on the fundamentals of layout in switching power supply circuits.

Design Example for PWM Flyback Converter

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