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Design Idea DI-67

Factor Corrected (PFC), 17 W LED Driver

October 2003

DI-67

®

Application

LED Arrays

TOP246F

700 mA (16-24 V)

Power Output

17.6 W max

Input Voltage

Output Current

Topology

Device

108-132 VAC 60 Hz

Flyback

Figure 1. Isolated Constant Current PFC LED Driver.

Design Highlights

• 700 mA of output current regulated to within

± 5%

• Power factor: > 0.98, THD:

≤ 9.6%

• No output current overshoot at power-on

• Temperature range: -40

°C to +80 °C

• Harmonics comply with IEC61000-3-2, Edition 2.1

• Conducted EMI complies with CISPR-22 B

Operation

This low-cost, TOP246F based PFC LED Driver takes

advantage of built-in TOPSwitch-GX features. This supply is

current (not voltage) regulated, except in the case of no-load.

Without a load, the output voltage is limited to about 30 V

(max), by R6 and VR2. Configured as a flyback converter, this

circuit operates in the discontinuous conduction mode. It can

deliver an average of 700 mA (1 A at the peak of output ripple)

over a 16 V to 24 V range, which makes it ideal for driving high

current LED arrays, such as the Luxeon 12 UP LED Ring.

The DC input capacitor (C3, 100 nF) was sized so that its

voltage approaches zero at the AC input zero crossings (see the

middle waveform of Figure 2).

A 220

µF capacitor (C6) was chosen for the bias supply to

minimize the 120 Hz ripple current into the current source (Q1),

which provides control current to U1. The output rectifier (D9)

must be rated for an average of 3.5 A. Capacitor C7’s value (680

µF) sets the magnitude of the output ripple current to 600 mA

peak to peak, at 120 Hz.

Resistors R7, R8, R9 and U2’s LED set the 700 mA average

current limit. The U2 phototransistor drives the current source

(Q1) and the PFC-loop filter capacitor (C8, 100

µF). Capacitor

C8 was sized to provide the low loop bandwidth necessary for

high power-factor. R4 is the power-off discharge path for C6

and C8.

The CONTROL pin bypass capacitor (C5, 1

µF) is just large

enough to allow smooth start-up of the output load current, and

yet small enough to prevent output current overshoot. A larger

value of C5 would increase start-up delay time.

D1-D4

IN4004

L2

150 uH

C1

100 nF

X2 CAP

C2

100 nF

X2 CAP

C3

100 nF

C7

680 uF

35 V

C6

220 uF

25 V

C8

100 uF

25 V

C5

1.0 uF

R6

100 Ω

5%

R4

10 kΩ

5%

R2

499 Ω

1%

R3

1.00 kΩ

1%

R7

100 Ω

1%

R8 1.50 Ω, 1%

R9 10.0 Ω, 1%

R1

10.0 kΩ

1%

R5

3.01 kΩ

1%

C4

2.2 nF

Y CAP

T1

D5

UF4005

D7

1N4148

D6

UF4005

D9

BYV28-200

D8

UF4002

1

23

14

8

7

4

3

5

2

VR1

P6KE200A

VR2

1N5254B

L1

27 mH

RV1

150 V

F1 T1A

250 V

108-132 VAC

60 Hz

700 mA

16-24 V

RETURN

U1

TOP246F

U2

PC81711NSZ

Q1

PN2907A

N

L

D

S

C

L

F

X

CONTROL

TOPSwitch-GX

+

6

PI-3746-110303