Rev. B 09/15

4

LYT2002-2005

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Applications Example

Circuit Description

This circuit shown in Figure 4 is configured as a primary-side

regulated flyback power supply utilizing the LYT2004E from the

LYTSwitch-2 family of ICs. This type of LED driver design is typical

for an external ballast application where safety isolation is required

while power factor correction is not. The output can drive an LED

load from 48 V to 22 V with a constant output current of 180 mA ±5%

across input range of 90 VAC to 265 VAC and ambient temperature

range of 0 ºC to 60 ºC. It has an average efficiency of >86% and

<30 mW no-load input power measured at nominal input voltages

(i.e. 115 VAC and 230 VAC). This design easily meets the most

stringent current energy efficiency requirements.

Input Filter

AC input power is rectified by bridge diode BR1. The rectified DC is

filtered by the bulk storage capacitors C1 and C2. Inductor L1, L2,

C1 and C2 form a pi (π) filter, which attenuates conducted differen-

tial-mode EMI. Resistors R1 and R2 placed across the inductors

damp the Q to improve frequency noise filtering without reducing low

frequency noise attenuation. A small value Y capacitor (C7) across

the transformer was used to reducer common-mode noise currents.

The fuse F1 provides protection against catastrophic failure. This

can be replaced by a fusible resistor for cost reduction but should be

suitably rated (and typically a wire wound type) to withstand the

instantaneous dissipation experienced during input capacitor charging

when first connected to the AC line.

LYT2004 Primary

The LYTSwitch-2 family (U1) incorporates the power switching device,

oscillator, CC/CV control engine, start-up, and protection functions.

The integrated 725 V power MOSFET provides a large drain voltage

margin in universal input AC applications, increasing reliability and

also reducing the output diode voltage stress by permitting the use of

higher transformer turns ratios. The device is completely self-

powered from the BYPASS pin and decoupling capacitor C4.

The optional bias supply formed by D2 and C5 and R6 provides

operating current to U1 via resistor R9. This reduces the no-load

consumption from 200 mW to less than 30 mW. The bias supply also

increases light load efficiency.

The rectified and filtered input voltage is applied to one side of the

primary winding of T1. The other side of the transformer’s primary

winding is driven by the integrated power MOSFET in U1. The

leakage inductance drain voltage spike is limited by an RCD-R clamp

consisting of D1, R3, R4, R5, and C3.

Output Rectification

The output from the transformer is rectified by D3, a 1 A, 400 V

ultrafast recovery type diode (for higher efficiency), and filtered by

C6. In this application C6 was sized to meet a (typical) ripple

requirement of less than 10% without the need for an additional LC

post filter.

PI-7280-051414

D

S

FB

BP

L1

3.9 mH

C1

12

µF

400 V

C4

1

µF

50 V

C5

1

µF

50 V

C2

12

µF

400 V

C6

100

µF

63 V

C7

1 nF

500 V

U1

LYT2004E

LYTSwitch-2

C3

470 pF

1 kV

R2

3.9 k

Ω

1/8 W

R5

560

Ω

R4

560

Ω

R8

6.98 k

Ω

1%

1/8 W

R9

12 k

Ω

1/8 W

R7

60.4 k

Ω

1%

1/8 W

R6

2

Ω

1/8 W

T1

EE19

9

5

10

1

2

R10

1.2 M

Ω

D1

S1ML

D2

BAV21W-7-F

4

D3

US1G

R3

220 k

Ω

L2

3.9 mH

22 V - 48 V,

180 mA

TP3

RTN

TP4

R1

3.9 k

Ω

1/8 W

F1

2 A

90 - 265

VAC

L

TP1

N

TP2

BR1

B10S-G

1000 V

Figure 4. Energy Efficient 8.6 W LED Power Supply (>86 % Average Efficiency, <30 mW No-load Input Power).