Design Idea DI-130

TinySwitch-III

Passive PFC LED Lighting Supply

Figure 1. Circuit Diagram of a Passive PFC Power Supply for Driving Lighting LEDs.

DI-130

November 2006

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

LED Lighting

TNY279P

18 W

185-265 VAC

10 V

Flyback

Design Highlights

• Very high efficiency: ≥82%

• Low component count: only 40 parts

• No common-mode choke required to meet EN55022B

conducted EMI requirements

• Valley Fill circuit allows supply to meet IEC61000-3-2

THD limits

• ON/OFF control rejects the high line ripple voltage

caused by the Valley Fill (THD correction) circuit

Operation

The flyback converter shown in Figure 1 uses a member of

the TinySwitch-III family (U2, a TNY279P) to provide up

to 1.8 A of load current to six, high-intensity, Luxeon LEDs

(the LXHL series).

The output voltage is slightly below the forward voltage

drop of the LEDs. Therefore, when the LEDs are connected

to the supply, it operates in constant current (CC) mode. If

the LEDs are disconnected from the supply, Zener diode

VR1 provides voltage feedback, which regulates the output

voltage at about 13.5 VDC. A 100 mΩ resistor (R11) senses

the output current and an Opamp (U1) drives the optocoupler

(U3), which provides feedback to U2. The TinySwitch-III

family of devices regulate by disabling or skipping MOSFET

switchingcycles. Astheloadcurrentreachesthecurrentlimit

set-point threshold, U1 drives U3 on. The photo-transistor

in U3 pulls current out of the EN/UV pin of U2, causing it to

skip switching cycles. Once the output current drops below

the current limit set-point threshold, U1 stops driving U3,

which stops pulling current out of the EN/UV pin of U2,

and switching cycles are enabled again. The TL431 (U4)

provides a reference for U1 to compare against the voltage

drop across R11.

The output diode (D9) is located in the lower leg of the

transformer (T1) secondary winding to reduce EMI noise

generation. An RCD clamp (R16, C4 and D13) protects the

drain node of the MOSFET from the flyback voltage spike.

The Valley Fill circuit (D5, D6, D7, C15, C16 and R15)

limits the values of the third and fifth harmonics of the line

frequency current, which enables this supply to meet the

requirements forTotal Harmonic Distortion (THD) specified

in IEC61000-3-2.

ThefrequencyjitteringfunctioninU2,ashieldwindinginT1

and a Y class capacitor (C8) across T1 reduce the generation

of conducted EMI so that a simple pi filter (C13, L1, L2 and

C14) allows the supply to meet EN55022B limits.

®

PI-4520-110606

D

S

EN/UV

BP/M

TinySwitch-III

U2

TNY279P

U3

PC817A

U1

R14

5.1 k

Ω

R15

10

Ω

1/2 W

R16

100 k

Ω

1/2 W

R12

47

Ω

1/2 W

R10

51

Ω

R11

2 k

Ω

1/8 W

R11

100 m

Ω

1 W

R17

5.1 k

Ω

1/8 W

R5

1 k

Ω

1/8 W

R13

5.1 k

Ω

L2

2200

µH

L1

2200

µH

F1

3.15 A

C13

47 nF

275 VAC

C16

6.8

µF

400 V

C16

1

µF

50 V

C8

2.2 nF

250 VAC

C4

1 nF

1 kV

C10

100 nF

50 V

C15

6.8

µF

400 V

C6

680

µF

35 V

C9

100 nF

50 V

C12

220 pF

1 kV

C7

680

µF

35 V

2

8

4

1

1

10

6

2

3

3

10 V, 1.8 A

RTN

185-265

VAC

C17

100

µF

16 V

C14

220 nF

275 VAC

D6

1N4007

D5

1N4007

D13

UF4005

U4

TL431

2%

VR1

12 V

LM358

D11

1N4148

D1, D2

1N4007

D3, D4

1N4007

D7

1N4007

D9

SB580

R6

68 k

Ω

1/8 W