Design Idea DI-116

TinySwitch-III

28 W Multiple Output Supply with <50 mW

No-Load Consumption

Figure 1. 25 W, 28 W pk Multiple Output Supply Using TNY280P.

DI-116

September 2006

Application

Device

Power Output Input Voltage

Output Voltage

Topology

DVD / Set-top Box

TNY280P 25 W (28 W pk)

90-265 VAC

5 V / 12 V / -12 V / -24 V

Flyback

Design Highlights

• Simple, low cost, low parts count solution

• Low input power at no-load: <50 mW at 265 VAC

• Low standby power consumption: <90 mW input with

5 V, 5 mA load

• High efficiency: >74% at 90 VAC

• >10 dBµV EMI margin to EN55022B conducted EMI

limits, even with output connected to safety earth ground

• Simple output overvoltage latching shutdown circuit

protects load under fault conditions

• On-time extension provides 28 ms hold-up time

Operation

TheDVD orsettopboxsupplyshowninFigure1wasdesigned

as a Flyback converter using a TNY280P device (U1). In this

circuit, the 10 µF value of C4 selects the increased current

limit level of U1, which allows the supply to deliver up to

28 W of peak power during startup or load transients (opening

the DVD tray).

The internal MOSFET in U1 conducts current through the

primary winding of T1 during each enabled switching cycle.

When the primary current reaches the MOSFET current

limit, it turns off and the energy in T1 is transferred to the

secondary.

The primary clamp (D5, VR3, R1, R2 and C3) limits the

maximum peak drain voltage below the 700 V breakdown

voltage of the internal MOSFET.

Outputovervoltageprotection(OVP)isprovidedbyU2,R12,

R13,VR1 andVR2. If the feedback loop were to open circuit

due to a failure of U3, VR1 or VR2 would turn on U2 causing

the current into the BP/M pins of U1 to exceed 6.5 mA, and

U1 to latch off. The values of VR1 and VR2 determine the

output voltage at which this occurs. Once triggered, cycling

the AC power resets the OVP latch.

To reduce no-load input power and increase light-load

efficiency, resistor R3 feeds the supply current for U1 from

the auxiliary bias winding on the transformer.

D

S

EN/UV

BP/M

PI-4416-090606

U1

TNY280P

NC

2

3

8

6

7

10

9

4

5

TinySwitch-III

R2

200

Ω

1/2 W

R1

560

Ω

R3

100 k

Ω

R7

10

Ω

R4

10

Ω

R3

16

Ω

R8

56

Ω

R9

1 k

Ω

R10

3 k

Ω

R15

115 k

Ω

1%

R13

1 k

Ω

R12

100

Ω

U2-B

PC817D

U3-A

U2-A

U3-B

PC817D

R11

10

Ω

R14

15 k

Ω

1%

R16

10 k

Ω

1%

R6

10

Ω

C2

47

µF

400V

C3

1 nF

1 kV

C4

10

µF

50 V

C5

1

µF

100 V

C12

2.2

µF

50 V

C10

2200

µF

10 V

C7

680

µF

16 V

L2

3.3

µH

L1

10 mH

L3

3.3

µH

C8

220

µF

16 V

VR3

BZY97C200

200 V

D9

BAV20

D5

FR107

VR4

BZX79-

B8V2

8.2 V

VR1

1N5244B

14 V

U4

TL431

2%

VR2

BZX79-B4V7

4.7 V 2%

D10

BAV20

D8

SB530

D7 SB360

D6

BAV21

EE25

T1

C13

100 nF

50 V

C16

220 nF

50 V

C15

100 nF

50 V

C18

100 nF

50 V

C17

1

µF

50 V

C11

220

µF

10 V

C6

330 pF

100 V

C18

1 nF

250 VAC

C9

1 nF

50 V

C14

2.2

µF

50 V

C1

47

µF

400V

D1

1N4937

D2

1N4937

D3

1N4007GP

90-265

VAC

+12 V,

1.15 A

+5 V,

2.1 A

RTN

RTN

-12 V,

20 mA

-24 V,

15 mA

F1

2A

RV1

275 VAC

D4

1N4007GP

®