STR5A162D datasheet(13/21 Pages) SANKEN

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STR5A100D Series

STR5A100D-DS Rev.1.3

SANKEN ELECTRIC CO.,LTD.

Aug. 08, 2014

11. Design Notes

11.1 External Components

Take care to use properly rated, including derating as

necessary and proper type of components.

VAC

VCC

CRD clamp snubber

D/ST

S/GND

5~8

D50

VOUT

(+)

C52

C51

R51

R52

(-)

Figure 11-1 Peripheral circuit of FB pin and VCC pin

 Output Electrolytic Capacitor

Apply proper derating to ripple current, voltage,

and temperature rise. Use of high ripple current and

low impedance types, designed for switch mode

power supplies, is recommended.

 FB Pin Peripheral Circuit and CRD Clamp

Snubber

Figure 11-1 shows the FB pin peripheral circuit.

The auxiliary winding voltage, VD is divided by

resistors (R3, R4 and R5) and induced into FB pin.

The FB pin voltage is controlled to be Feedback

Reference Voltage, VFB(REF) = 2.50 V.

The value of R5 is about 3.3 kΩ to 10 kΩ.

The value of R3 and R4 are calculated as follows:





)

(

)

REF

(

)

REF

(

OUT











(6)

where,

ND: Turns of auxiliary winding of transformer

NS: Turns of secondary side winding of transformer

VOUT: Output voltage

VF: Forward drop voltage of secondary side rectifier

diode D50

VFB(REF): Feedback Reference Voltage, 2.50 V

IFB(OP): Feedback Current, − 0.8 μA

In addition, the negative voltage is input to FB pin. As

shown in Figure 11-2, the negative voltage, VFW, of

VD is calculated as follows:

IN(AC)





(7)

where,

VIN(AC) : Input voltage

ND : Turns of D winding

NP : Turns of P winding

Auxiliary

winding

voltage, VD

VREV

VFW

Figure 11-2 The auxiliary winding voltage waveform

The absolute maximum rating of FB Pin Source

Current, IFB is − 10 mA. The value of R3 and R4 are

chosen so that the FB pin source current, IFB(FW),

satisfies Equation (8) considered about surge.

FB(FW)





)

(













(8)

There, the maximum input voltage substitutes in

VIN(AC).

R3, R4 and R5 should be adjusted in actual operation

condition.

The IC generates the feedback signal by sampled VD

waveform that is FB pin input signal. In order to

improve the accuracy of VD waveform sampling, it is

necessary to realize the ideal VD waveform for

reducing the peak and width of the surge component

and

suppressing

the

ringing.

Because

the

waveform depends on the waveform of the primary

winding P voltage, a clamp snubber circuit should be

added on the primary winding P. The method of

setting the value of the clamp snubber circuit is shown

in Section 10.5.

In order to maintain the sampling accuracy during

light load operation, an auxiliary switch diode

SARS05 should be used as D1 where the approximate

value of R2 is 220 Ω to 470 Ω. R2 should be adjusted

to obtain proper VD waveform in actual operation

condition.

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