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Design Idea DI-34

TinySwitch-II

5 W Universal Adapter

May 2003

DI-34

®

Application

Adapter

TNY266P

5 V

Power Output

5 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Design Highlights

• Simple design: low parts count (23)

• <300 mW no-load without transformer bias winding

• Frequency jittering reduces EMI - meets EN55022 Class B

with no Y capacitor: very low primary-to-secondary leakage

current

•

±7% output tolerance with simple Zener reference

• High Frequency (132 kHz) operation allows small, low cost

EE16 transformer

• Built-in thermal shutdown protection

• ON/OFF digital regulation: No analog control loop to

compensate

Operation

Figure 1 is a simple flyback converter utilizing several

TinySwitch-II built-in features such as switching frequency

jittering, thermal shutdown and auto-restart fault protection.

Typical applications are wall mount adapters and other AC/DC

applications requiring very low cost and small size. The ability

to meet EMI requirements with no Y capacitor makes this

circuit ideal for applications requiring low primary-to-secondary

leakage currents.

Figure 1. TinySwitch-II – TNY266P, 5 V, 1 A, 5 W Charger-Adapter Power Supply.

The TinySwitch-II frequency jitter feature and transformer

construction allow use of a simple filter (L1, L2, C1 and C2)

to meet EMI requirements with L2 used to attenuate high

frequency EMI in the radiated spectrum (>30 MHz). The

circuit also uses a low cost RCD clamp (C7, D5, R3, and R4).

Diode D5 is a Glass Passivated (GP) general purpose diode,

allowing partial recovery of the leakage energy by

recirculating clamp current during the D5 reverse recovery

time, thus improving efficiency. Resistor R3 reduces EMI by

limiting the peak D5 reverse current and thus softening its

recovery characteristic.

The combined voltage drops of Zener diode (VR7) and

optocoupler diode (U2) set the output voltage. Resistor R7

biases VR7 according to its specification. The TinySwitch-II

feedback current is independent of load allowing

±7% output

voltage tolerance with this simple Zener reference.

Due to the digital nature of the TinySwitch-II control scheme,

no loop compensation is required. In addition, the current

transfer ratio (CTR) of the optocoupler is not critical and no

DC gain setting resistor in series with VR7 is required.

®

PI-3326-051303

D

S

BP

EN

N

L

5 V, 1.0 A

RTN

RF1

8.2 Ω

2 W

D1

1N4005

D2

1N4005

D3

1N4005

D4

1N4005

C1

6.8 µF

400 V

L1

1.0 mH

D5

1N4005 GP

U1

TNY266P

L2

Bead

C3

0.1 µF

50 V

U2

PC817A

VR7 BZX79C3V9, 2%

1

4

3

2

5

8

C4

1000 µF

10 V

R7

150 Ω

R4

200 kΩ

1/2 W

D6

1N5822

T1

EE-16

C7

1.0 nF

1 kV

L3

3.3 µH

1.0 A

C5

470 µF

10 V

C2

6.8 µF

400 V

TinySwitch

NC

R3

100 Ω

85 - 265

VAC