AN2811

General circuit description

Doc ID 14904 Rev 1

9/25

Referring to the schematic diagram in Figure 5, fuse1 is the input fuse to prevent hazards if

the system current exceeds the fuse rating. D1 is the bridge rectifier to convert AC to DC.

The filter is formed by C11, L2 and C3 that are used to attenuate the high frequency

harmonic interference. T1 is the flyback transformer and U1 is formed by the PWM controller

and output MOSFET. The auxiliary winding (pin 5 and 6) and diode D5 provide bias supply

for each control circuit. The output stage includes D6, C9 and C13. R9 and R12 are the

output current sense resistors providing current sense signal. These are connected in

parallel to share the power dissipation.

The constant-current control circuit consists of U2, Q1, U1 and some passive components.

The output current sense signal feeds to OP amp in U2. R7 and C7 consist of the

compensation network for the output signal of U2 in order to properly drive Q1. The 0.3 V

reference voltage on pin 6 of U2 is obtained by voltage divider R11 and R10. The collector

junction of Q1 is connected to the feedback pin of U1 and completes the feedback loop.

The output voltage is indirectly monitored by the auxiliary winding (pin 5 and 6 of T1) and

feedback to pin 3 of U1 through R1 and R4. Once the voltage at pin 3 of U1 exceeds 3 V, U1

shuts down, then enters the auto-restart mode. Thanks to U1, which includes an overload

protection function, if the LED is absent in the application (no load), this solution provides a

safeguard. The LED and the application board are both fully protected.

3.2

Bill of material

Table 2.

Bill of material

Name

Value

Rated

Type

C1

1 nF

25 V

Ceramic cap [0603]

C2, C8

100 nF

25 V

Ceramic cap [0603]

C3, C11

2.2

μF

400 V

Al elcap CAPPR3.5-8X12

C4

56 nF

25 V

Ceramic cap [0603]

C5, C12

10

μF

25 V

Al elcap CAPPR2-5X11

C6

2.2 nF

25 V

Ceramic cap [0603]

C7

12 nF

25 V

Ceramic cap [0603]

C9

220

μF

16 V

Al elcap CAPPR3.5

-8X11.5

C10

470 pF

25 V

Ceramic cap [0603]

C13

1

μF

25 V

Ceramic cap [0805]

D1

MB6S PKG30 E3

1 A 600 V bridge rectifier

Vishay

D3

BAT46JFILM

Small signal Schottky diode STMicroelectronics [SOD323]

D5

STTH1R06A

1 A 600 V ultrafast rectifier

STMicroelectronics [SMA]

D6

STPS2H100A

2 A 100 V Schottky rectifier

STMicroelectronics [SMA]

Fuse1

500 mA 250 V

Fuse_5_8.5*8_Bel

L2

LPS3314-105ML

1 mH, 0.1 A

Inductor, Coilcraft L_LP3314

Q1

BC817-40

NPN general-purpose

transistor

[SOT-23]

R1

24

0 k

Ω

1%

Resistor [0603]