Design Idea DI-117

TinySwitch-III

15 W, 12 V Adapter with <100 mW No-Load Consumption

DI-117

September 2006

Design Highlights

• Simple, low cost, low parts count solution

• Low input power at no-load: <100 mW at 265 VAC

• High efficiency >81% at 90 VAC

• Meets CEC requirements for active mode efficiency

(79 % vs. 73.5 % requirement)

• >10 dBµV margin to EN55022B conducted EMI limits

• Simple, primary-side output overvoltage latching shut-

down protects load under fault conditions

• Low line frequency leakage current <10 µA

Operation

The TNY279 (U1) in the 8-pin DIP package selected for the

flyback design in Figure 1 is ideal for adapter applications.

The arrangement of the four SOURCE pins on one side of

the package allows a small metal heatsink to be inserted.

This allows the device to operate in a sealed adapter with an

external ambient of 40 °C.

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, the controller turns it off, and the energy

in T1 is transferred to the output. Schottky diode D7 and

capacitor C8 rectify and filter the output. Inductors L3 and

capacitor C9 attenuate the switching ripple on the output.

Capacitor C3 selects the standard current limit of U1. For

higher efficiency, the next larger (TNY280P) device may

be used. In this case, the value of C3 would be changed to

1 µF, selecting the reduced current limit for the larger device,

but no other circuit changes would be required.

The primary clamp (D5, R1, C5, R5 and R6) limits the

maximum peak drain voltage below the 700 V breakdown

voltage of U1's internal MOSFET. Resistor R1 dampens the

high-frequency ringing of the T1 leakage inductance.

®

Application

Device

Power Output Input Voltage Output Voltage Topology

Adapters / General Purpose TNY279P

15 W

90-265 VAC

12 V

Flyback

Figure 1. 15 W, 12 V Output Supply Using TNY279P.

D

S

S

BP/M

EN/UV

L5

3.3

µH

L4

1 mH

D1

1N4005GP

F1

3.15 A

D2

1N4005

C1

10

µF

400 V

C7

1

µF

50 V

C5

1 nF

1 kV

C4

33 pF

1 kV

C10

220 nF

50 V

C2

22

µF

400 V

C8

680

µF

25 V

C6

68 pF

250 VAC

C11

1 nF

100 V

C9

220

µF

25 V

+12 V, 1.25 A

90-265

VAC

RTN

C3

100 nF

50 V

U1

TNY279P

10

9

NC

5

T1

4

1

3

D5

FR107

D6

1N4148

D7

SB580

U2-A

PC817A

U2-B

VR3

BZX79-B6V8

6.8 V

U3

LM431

R4

47

Ω

EF20

R3

10 k

Ω

R1

100

Ω

R5

240 k

Ω

R2

100

Ω

R10

3 k

Ω

R12

10 k

Ω

1%

R11

39 k

Ω

1%

R9

1 k

Ω

R8

56

Ω

R7

20

Ω

R6

240 k

Ω

D3

1N4005GP

D4

1N4005

L3

Ferrite Bead

3.5 × 7.6 mm

PI-4417-051106

TinySwitch-III