Design Idea DI-91

TinySwitch-III

12 W Universal Input CV Adapter

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Figure 1. TinySwitch-III 12 W, Universal Input, CV Adapter.

DI-91

®

January 2006

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

General Purpose

TNY278P

12 W

85-265 VAC

12 V

Flyback

D

S

S

BP/M

EN/UV

L1

1 mH

D1

1N4007

RV1

275 VAC

F1

3.15 A

D2

1N4007

C1

6.8

µF

400 V

C6

1

µF

60 V

C2

22

µF

400 V

C10

1000

µF

25 V

C5

2.2 nF

250 VAC

C11

100

µF

25 V

+12 V, 1 A

85-265

VAC

RTN

J4

J3

J1

J2

C7

100 nF

50 V

U1

TNY278P

C4

10 nF

1 kV

VR1

P6KE150A

NC

8

6

4

T1

2

5

1

3

D5

1N4007GP

D6

UF4003

D7

BYV28-200

U2

PC817A

VR2

1N5255B

28 V

VR3

BZX79-C11

11 V

C7 is configurable to adjust

U1 current limit, see circuit

description

*R5 and R8 are optional

components

R5*

3.6 M

Ω

R3

47

Ω

1/8 W

R4

2 k

Ω

1/8 W

R6

390

Ω

1/8 W

R7

20

Ω

R8*

21 k

Ω

1%

R1

1 k

Ω

R2

100

Ω

D3

1N4007

D4

1N4007

L2

Ferrite Bead

3.5 × 7.6 mm

PI-4244-021406

†

†

TinySwitch-III

Design Highlights

• No-load consumption <50 mW at 265 VAC (R8 fitted)

• Active mode efficiency >75% (CEC standard: ≥71%)

• BP/M pin capacitor value selects MOSFET current

limit (I

• Output OVP function senses overvoltage on the output

through the primary bias winding

• Accurate (±5%), auto-recovering, hysteretic thermal

shutdown function maintains safe PCB temperatures

• Auto-restart function protects against output short

circuits and open feedback loops

• > 3.2 mm package pin creepage: reliable operation in

high humidity and high pollution environments

Operation

This universal input, 12 V, 1 A, flyback converter was

designed around a member of the TinySwitch-III IC family,

the TNY278P. Because of the many features integrated into

the device family, the supply only requires 31 through-hole

components (no SMT), which enables a simple, single-sided

PCB layout to be used.

The use of an E-shield™ in the transformer, a low-noise drain-

node clamp (R2, C4 and D5), a single Y capacitor (C5) and the

deviceʼs switching frequency jitter function enable a low-cost

pi (π) filter (C1, L1 and C2) to attenuate conducted EMI enough

toprovidemorethan10dBofmargintoEN550022requirements

(see Figure 4). The clamp circuit Zener (VR1) only conducts

briefly during startup and at worst-case overload, just before

the auto-restart function is activated.

Although TinySwitch-III family members do not need a bias

windingtooperate,onewasusedinthisdesigntotakeadvantage

of the ICʼs output OVPdetection function. If an open loop fault

causes an overvoltage on the output, VR2 will conduct. When

the current into the BP/M pin exceeds 5.5 mA, an internal latch

triggers and disables MOSFET switching (see Figure 3), until

ACpowerisremovedandreapplied(toresetthelatch). Resistor

R3 limits the current through VR2, while R3 and R7 attenuate

the portion of the leakage inductance spike that appears in the

bias winding.