Rev. B 02/13

6

LYT4211-4218/4311-4318

www.powerint.com

Application Example

25 W TRIAC Dimmable High Power Factor LED Driver

Design Example (DER-350)

The circuit schematic in Figure 8 shows a TRIAC dimmable high

power factor LED driver based on LYT4317E from the LYTSwitch

family of devices. The design is configurable for non-dimmable

only applications by simple component value changes. It was

optimized to drive an LED string at a voltage of 36 V with a

constant current of 0.7 A ideal for Lumens PAR lamp retro-fit

applications. The design operates over an input voltage range

of 90 VAC to 132 VAC.

The key goals of this design were compatibility with standard

leading edge TRIAC AC dimmers, very wide dimming range

(1000:1, 700 mA:0.7 mA), high efficiency (>85%) and high power

factor (>0.9). The design is fully protected from faults such as

no-load (open load), overvoltage and output short-circuit or

overload conditions and over temperature.

Circuit Description

The LYTSwitch device (U1- LYT4317E) integrates the power FET,

controller and start-up functions into a single package reducing

the component count versus typical implementations. Configured

as part of an isolated continuous conduction mode flyback

converter, U1 provides high power factor via its internal control

algorithm together with the small input capacitance of the

design. Continuous conduction mode operation results in

reduced primary peak and RMS current. This both reduces

EMI noise, allowing simpler, smaller EMI filtering components

and improves efficiency. Output current regulation is maintained

without the need for secondary-side sensing which eliminates

current sense resistors and improves efficiency.

Input Stage

Fuse F1 provides protection from component failures while RV1

provides a clamp during differential line surges, keeping the

peak drain voltage of U1 below the 725 V rating of the internal

power FET. Bridge rectifier BR1 rectifies the AC line voltage.

EMI filtering is provided by L1-L3, C1, C4, R2, R24 and R25

together with the safety rated Y class capacitor (CY1) that bridges

the safety isolation barrier between primary and secondary.

Resistor R2, R24 and R25 act to damp any resonances formed

between L1, L2, L3, C1 and the AC line impedance. A small

bulk capacitor (C4) is required to provide a low impedance

source for the primary switching current. The maximum value

of C2 and C4 is limited in order to maintain a power factor of

greater than 0.9.

LYTSwitch Primary

To provide peak line voltage information to U1 the incoming

rectified AC peak charges C6 via D2. This is then fed into the

VOLTAGE MONITOR pin of U1 as a current via R10. This

sensed current is also used by the device to set the line input

overvoltage protection threshold. Resistor R9 provides a

discharge path for C6 with a time constant much longer than

that of the rectified AC to prevent generation of line frequency

ripple.

The VOLTAGE MONITOR pin current and the FEEDBACK pin

current are used internally to control the average output LED

current. For TRIAC phase-dimming applications a 49.9 k

W

resistor (R14) is used on the REFERENCE pin and 2 M

W (R10)

on the VOLTAGE MONITOR pin to provide a linear relationship

between input voltage and the output current and maximizing

the dimming range.

Diode D3, R15 and C7 clamp the drain voltage to a safe level

due to the effects of leakage inductance. Diode D4 is

necessary to prevent reverse current from flowing through U1

for the period of the rectified AC input voltage that the voltage

across C4 falls to below the reflected output voltage (V

R6

360 k

Ω

PI-6875-101812

D

S

BP

V

R

FB

CONTROL

FL1

T1

RM8

12

1

FL2

10

11

R10

2 M

Ω

1%

R15

200 k

Ω

R9

510 k

Ω

1/8 W

R24

47 k

Ω

1/8 W

R1

510

1/2 W

R25

47 k

Ω

1/8 W

R2

47 k

Ω

1/8 W

L1

1 mH

L2

1 mH

R14

49.9 k

Ω

1%

1/16 W

R18

165 k

Ω

1%

1/16 W

R23

20 k

Ω

36 V,

700 mA

90 - 132

VAC

RTN

L

N

R19

20 k

Ω

1/8 W

C5

100 nF

50 V

R26

30

Ω

R20

39

Ω

1/8 W

R17

3 k

Ω

1/10 W

R27

10

Ω

1/10 W

D2

DFLU1400

C7

2.2 nF

630 V

D3

US1J

D5

BAV16

Q2

MMBT3904

D8

BAV21

Q1

X0202MA2BL2

D6

BAV21

D7

BYW29-200

D4

US1D

R22

1 k

Ω

1/10 W

BR1

MB6S

600 V

RV1

140 VAC

F1

5 A

C11

330

µF

63 V

C13

100 pF

200 V

CY1

470 pF

250 VAC

C12

330

µF

63 V

C8

47

µF

16 V

C15

100 nF

50 V

C14

10 nF

50 V

C9

56

µF

50 V

LYTSwitch

U1

LYT4317E

C6

2.2

µF

250 V

C1

220 nF

250 V

C4

100 nF

250 V

L3

5 mH

R8

100

Ω

1 W

C3

470 nF

50 V

C2

100 nF

250 V

D9

DFLU1400-7

Figure 8.

DER-350 Schematic of an Isolated, TRIAC Dimmable, High Power Factor, 90-132 VAC, 25 W / 36 V / 700 mA LED Driver.