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illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S.

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Rev. A 11/06

Power Integrations

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Resistor R16 provides the bias current for U2. Resistor R10

setsthegain. CapacitorC22improvestransientresponsiveness.

CapacitorC11andresistorR9rolloffthehighfrequencygainof

U2. Capacitor C2 performs a soft-finish function that prevents

the 3.3 V and the 5 V output voltages from overshooting their

regulationsetpointsduringtheinitialpowerupoftheconverter.

Figure 3. Output Regulation vs. Input Line Voltage, Room

Temperature, Full Load.

Figure 2. Efficiency vs. Input Line Voltage at Full Load, Room

Temperature, 50 Hz Line.

All output windings areAC stacked and exhibit good cross

regulation, due to the tight coupling within transformer

T1. The –5 V output uses Zener diode VR1 as a post

regulator.

Key Design Points

• All secondary winding should be wound in layers

right next to each other, in order to assure the best

cross regulation.

• Use foil for 3.3 V and 5 V secondary windings for

the best cross regulation.

• The reflected output voltage (V

to ensure good output voltages cross regulation. If

tighter cross regulation is needed, a lower value of

V

power delivery.

• The configuration of U5, Q1, R17 and D6 increases

the speed of U5 and reduces pulse grouping.

Table 1. Transformer Design Parameters.

TIW = Triple Insulated Wire, NC = No Connect, FL = Flying Lead

TRANSFORMER PARAMETERS

Core Material

PC40EF25-Z or equivalent

Bobbin

EF25 Horizontal

Winding Details

1/2 Primary: 32T, 31 AWG

Bias: 9T, 0.45 mm

Secondary: Foil 2T, Foil 1T,

6T, 0.45 mm

×2 & 3T 0.25 mm

1/2 Primary: 32T, 31 AWG

Winding Order

(pin numbers)

Primary (3-2), Bias (5-4),

Secondary (9-10, 10-8, 7-8,

6-7), Primary (2-1)

Primary

Inductance

342

µH, ±10%

195

205

215

225

245

235

265

255

AC Input Voltage

95

85

75

65

55

45

35

25

190

200

210

220

240

250

230

270

260

AC Input Voltage (VAC)

110

105

100

95

90

85

80

3.3 V

5 V

-5 V

22 V

17.5 V