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2020.04.08 AC/DC

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

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

By the end of the previous article, selection of the required components and calculation of component values were completed. As the next step, we begin mounting the selected components on a printed circuit board and begin the task of checking the various characteristics and confirming that the design specifications have been met. From this point, we shall explain a number of problems that may be anticipated when checking characteristics, and measures to address these problems.

In "Design of a Secondary-Side Synchronous Rectification Circuit to Improve the Efficiency of an AC/DC Converter", the aim is to boost efficiency while maintaining the existing power supply specifications by the use of synchronous rectification on the secondary side of an isolated PWM flyback AC/DC converter circuit. Here, rather than using a new design, we opt for what is in a sense a modified design, with the primary side left as-is and the secondary side replaced with a circuit that uses a controller IC called the BM1R00147F. Hence it is extremely important to verify that the circuit as a whole operates properly as an AC/DC converter.

Problem ①:Case When Secondary-Side MOSFET Suddenly Turns OFF

There are cases in which a malfunction that turns the secondary-side MOSFET off occurs due to noise appearing in the voltage of the drain pin of the power supply IC. The diagram below shows operation in which VGS2, which normally should be on longer (indicated by the broken-line waveform), turns off after a short time due to noise.

Countermeasure ①-1: Insert a ferrite bead B1, increase the drain pin connection resistance R1

In response to Problem ①, "Countermeasure ①-1"* involves inserting a ferrite bead B1 for surge absorption and increasing the value of the filter resistor R1 to prevent malfunctions caused by noise. As the ferrite bead B1, a device with high impedance at low frequencies, for example the MPZ1608S102AT manufactured by TDK, is effective. The diagram below on the left shows the position of bead insertion and indicates the resistor R1 for adjustment and reference values. The diagram on the right shows the operation waveform with the countermeasure implemented. (*The designation "Countermeasure ①-1" is used to mean a first countermeasure to Problem ①.)

Points to be Noted

If the impedances of B1 and R1 are set too high, under light loading the secondary-side MOSFET may turn on due to resonance operation. Hence operation under light loading must be checked. Details of this phenomenon and countermeasures will be explained in the next article.

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 line and the resistance value of the filter resistance can be increased.

Design Example for PWM Flyback Converter

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