AC-DC｜Design

Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins

2019.04.11

Points of this article

・Components necessary for the different detection pins should be set according to the data sheet and the design manual.

・If noise enters the detection pins, erroneous operation and other problems may result. The addition of a capacitor or RC filter should be considered.

This article discusses components necessary for various detection pins used for a power supply IC to perform step-down and regulation control.

We begin by reviewing the functions of detection pins of the power supply IC, BD7682FJ. The BD7682FJ performs step-down and regulation by monitoring the requisite points through the FB pin, the ZT pin, and the CS pin.

The FB (feedback signal input) pin monitors the secondary-side output voltage, via a photocoupler, to enable regulation control. The ZT (zero current detection) pin detects, from the VCC windings, the fact that the supplied current has gone to zero when power accumulated in the coil is supplied to the secondary-side capacitor upon switch turn-off. And, the CS (primary-side current sensing) pin monitors the current of the switch (MOSFET), and also has an overcurrent limiting function.

As the overall operation, the switch on-time is controlled using the FB pin and CS pin, and turn-off is controlled using the ZT pin.

Components that are required for each of the pins are explained.

Current Sense Resistor：R19

R19 converts the current flowing to the primary side due to the switching transistor Q1 into a voltage. This voltage is monitored by the CS pin, and an output overload protection point is set. The value of R19 is set such that when the primary-side maximum current Ippk (0.66 A) flows, the CS pin overcurrent detection voltage is Vcs = 1 V.

→1.5Ω

Moreover, the loss P_R19 of the resistor R19 is

Considering pulse resistance, P_R19 is assumed to be 1 W or higher.

Pulse resistance may change with the structure of the resistor and the like even for the same power rating. The manufacturer of the resistor used should be consulted.

Resistor and Capacitor for CS Pin Noise Protection: R22, C13

When noise is not completely eliminated by the blanking function, this RC filter is added. Even when the filter is not necessary, it is recommended that R22 (value approx. 1 kΩ) be inserted as a surge countermeasure. The value of C13 is about 47 pF.

ZT Pin Voltage Setting Resistor: R21

R21 sets the bottom detection voltage at the ZT pin. The bottom detection voltage at the ZT pin is Vzt1 = 100 mV typ. (ZT pin voltage falling) and Vzt2 = 200 mV typ. (ZT pin voltage rising). From ZTOVP(min) = 3.30 V, as a rule Vzt is set to approximately 1 to 3 V. R20 was set to 100 kΩ as per ” Switch Setting Resistors for Overload Protection Points“. The number of windings is 8 turns for both Nd and Ns, as calculated in “Transformer T1 Design – 2“.

Upon setting,

→12kΩ

ZT_Pin Capacitor：C11

The capacitor C11 is used for ZT pin regulation and to adjust the bottom detection timing. It should be set while checking the ZT pin waveform and bottom detection timing. In this circuit example, it is set to 47 pF.

FB Pin Capacitor：C12

C12 is a capacitor for FB pin regulation. A value of approximately 1000 pF to 0.01 μF is recommended. In this circuit example, it is set to 2200 pF.

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