www.powerint.com

Design Idea DI-63

with Output Cable-Drop Compensation

September 2003

DI-64

®

Application

Charger

TNY266P

6 V, 800 mA ±7%

Power Output

4.8 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Figure 1. TinySwitch-II Based Charger/Adapter Circuit Diagram.

Design Highlights

• Universal input, no-load consumption < 300 mW

•Meets CISPR-22 Class B EMI without a Y capacitor

• Extremely simple circuit, requires only 32 components!

• Circuit has output cable voltage drop compensation

• Ultra-low Leakage current: < 5

µA at 265 VAC input

Operation

Fusible resistor RF1 provides short-circuit fault protection and

limits start-up inrush current. Inductors L1 and L2 and capacitors

C1 and C2 form a low-cost pi (

π) filter that attenuates conducted

EMI.

Transformer (T1) has two shield windings (1-3 and 1-open)

which reduce the generation of EMI noise. Winding phasing

and D7 orientation let no secondary winding current flow when

the MOSFET in U1 is ON, so primary winding current stores

energy in the core of T1. When the MOSFET in U1 turns OFF,

the energy stored in T1 drives current out of the secondary

winding, forward biasing D7, charging C7 and developing/

maintaining the output voltage across C7. The V

the V

R4, R5 and the U2-LED determine the CC set point.

Resistors R7, R10 and diode D6 compensate for output-cable

voltage-drop. The CV portion of the output VI curve is flat,

at the end of the cable, because a current sense resistor (R10)

equal to the DC resistance of the output cable is in the voltage

feedback loop. Diode D6 implements temperature

compensation for the cable drop.

The combination of TinySwitch-II frequency jitter, the

output diode snubber (R6 and C5), the T1 shield windings and

careful primary clamp circuit component selection enable

compliance with CISPR-22, Class B conducted EMI limits,

without a Y-1 Safety capacitor. Eliminating the Y capacitor

gives this circuit very low (< 5

µA) AC leakage current.

This circuit is suitable for portable electronics chargers.

PI-3704-091903

85-265

VAC

L

N

6 V, 0.8 A

RTN

RF1

8.2 Ω

1 W

Fusible

D1-D4

1N4005 x 4

C8

6.8 µF

400 V

L1

1.0 mH

L2

Ferrite

Bead

C3

1.0 nF

500 V

C5

1 nF

500 V

D6

1N4148

D5

1N4007G

U1

TNY266P

U2

PC817D

C6

680 µF

10 V

C4

0.1 µF

50 V

D7

SB260

1

T1 EE13

L

10

9

R2

200 kΩ

R3

100 Ω

R5

1.35 Ω

2.0 W

R9

330 Ω

C7

100 µF

10 V

R8

160 Ω

R4

300 Ω

VR1

BZX79-

B5V1

R6

10 Ω

R7

120 Ω

R10

0.24 Ω

C2

6.8 µF

400 V

TinySwitch-II

Q1

2N3906

D

S

EN

BP

L3

Ferrite

Bead

2

1

1

3