AC-DC｜Design

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

2020.02.05

Points of this article

・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.

table of contents

In succession to the preceding article, we explain the selection of peripheral circuit components for the BM1R00147F power supply IC that was previously selected. Here, we focus on C1 and R3 at the MAX_TON pin and on the VCC pin.

Peripheral Circuit Component Selection–C1, R3 at the MAX_TON Pin

As the MAX_TON pin voltage, 0.4 V (Typ) is output. In order to alleviate the effect of switching noise, a capacitor C1 and a resistor R3 are connected in series.

C1 and R3 also are used for phase compensation at the MAX_TON pin, and so they must always be connected. Recommended values are about 1000 pF for C1 and about 1 kΩ for R3.

Peripheral Circuit Component Selection–VCC Pin

The VCC pin is the power supply pin for the IC. Operation begins at VCC=2.3 V (Typ) or above, and the IC shuts down at VCC=2.25 V (Typ) or lower.

In this design example, the secondary-side output VOUT is 5 V, and so in the case of the Low Side Type, power can be fed as in the VCC power supply method example 1 on the right. This is the simplest power supply method.

In the case of the High Side Type, the secondary-side GND and the IC GND pin SR_GND are not common, and so a separate power supply must be provided by adding an auxiliary power supply circuit as in VCC power supply method example 2, or by providing auxiliary windings on the transformer secondary side as in VCC power supply method example 3.

For stable IC operation, a VCC pin capacitor CVCC of 1 μF or above should be selected. If an extremely large capacitance is used, the startup time is lengthened, so caution should be exercised. In this design example, a 10 μF capacitor is employed.

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