Design Idea DI-122

TinySwitch-III

13 W (17.2 W Peak) Non-Isolated Power Supply

for White Goods: <150 mW No-load Power Consumption

Figure 1. TinySwitch-III 13/17.2 W White Good Power Supply.

DI-122

January 2007

Application

Device

Power Output Input Voltage

Output Voltage

Topology

White Goods TNY279P 13 W / 17.2 W Pk 85-265 VAC

-5 V, 2.0 A / -12 V, 0.25 A / 0.6 A Pk

Flyback

Design Highlights

• Lowcost,lowcomponentcountsolution

• ImpedancefreepathfromNeutraltooutputRTN,ideal

forapplicationsdrivingtriacs

• <150mWno-loadconsumptionat230VACinput

• Bothoutputsexhibitgoodcrossregulation

• Maximumambienttemperatureofupto70°C

• MeetsCISPR-22ClassB/EN55022BEMIlimits

withoutaninputcommonmodechoke

Operation

The non-isolated flyback converter shown in Figure 1 uses a

TNY279toprovidetwooutputvoltages:-5Vat2Aand-12V

at250mAcontinuousand600mApeak.

The MOSFET integrated within U1 switches the primary of

transformer T1. Each time it turns on, the primary current

rampsuntilitreachesaninternalcurrentlimitandtheMOSFET

is turned off. During the off time, the energy stored in T1

transfers to the secondary where it is rectified and filtered by

D4 and C11 and D5 and C9. The primary side RCD clamp

(D2,C6,R1andR2)limitsthepeakDRAINvoltagespike

caused by transformer leakage inductance.

Feedback is taken directly from the -5 V output. Once the

output exceeds the voltage defined by the V

ratingofVR1,Q1isbiasedon.Thispullscurrentoutofthe

EN/UV pin of U1 and disables the next switching cycle. By

adjusting the number of enabled to disabled cycles, output

regulationismaintained.

ThevalueofR7setsthebiascurrentthroughVR1,operating

theZenerclosertoitsspecifiedtestcurrent.ResistorR6limits

the base current of Q1, R5 and C8 roll-off of high frequency

gain and R3 limits the current out of the EN/UV pin.

DiodeD3ensurestheEN/UVpinisclampedto>-0.3Vwith

respect to the SOURCE pin of U1.

Power Integrations E-Shield™ transformer construction

techniques, the RCD clamp, a simple π filter (C4, C5 and

L1), small X-type (C1) and Y-type (C2) capacitors and the

frequencyjitterfunctionoftheTinySwitch-IIIfamily, provide

good EMI margin (see Figure 3). Using a one square inch

metal shield, located underneath U1 and T1 (connected

to the SOURCE pin of U1) can further reduce EMI noise

generation.

®

C9

470

µF

25 V

C11

2200

µF

10 V

C12

47

µF

16 V

1

10

8

7

3

RTN

Line

Safety Earth

Ground

Neutral

-5 V, 2 A

-12 V, 250 mA

(600 mA pk)

C10

47

µF

25 V

C8

100 nF

50 V

C7

100 nF

50 V

C6

1 nF

1 kV

D1

1N4007

D4

SB530

D5

SB3100

D3

1N5817

VR1

BZX79-B4V7

4.7 V, 2%

Q1

PN2222A

D2

1N4007GP

C1

47 nF

275 VAC

C4

33

µF

400 V

C5

33

µF

400 V

C2

1 nF

250 VAC

F1

3.15 A

PI-4498-112906

D

S

EN/UV

BP/M

U1

TNY279P

TinySwitch-III

L1

1000

µH

L3

3.3

µH

T1

EF20

R3

4.7 k

Ω

R6

5.1 k

Ω

R4

47

Ω

R7

620

Ω

*One square inch metal shield (connected to the S pin of U1) under U1 and T1 eliminates the need for C2.

**For high-line only operation (180 VAC input, minimum), 10

µF (400 V) capacitors can be used for C4 and C5.

**

*

**

R5

1 k

Ω

R1

200 k

Ω

0.5 W

R2

200

Ω