Design Idea DI-130
TinySwitch®-III
Passive PFC LED Lighting Supply
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
LED Lighting
TNY279P
18 W
185-265 VAC
10 V
Flyback
Design Highlights
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.
•
•
•
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
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.
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 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.
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
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.
C8
2.2 nF
250 VAC
C6
C7
680 µF 680 µF
10 V, 1.8 A
35 V
35 V
D1, D2
R16
R11
D9
1N4007
100 kΩ
1/2 W
100 mΩ
SB580
C4
R14
5.1 kΩ
1 W
RTN
C16
1 nF
1 kV
6.8 µF
400 V
D6
1N4007
R5
C9
F1
3.15 A
1 kΩ
1/8 W
100 nF
50 V
R17
5.1 kΩ
1/8 W
R12
C12
220 pF
1 kV
L1
47 Ω
2200 µH
U1
1/2 W
C13
47 nF
275 VAC
C14
220 nF
275 VAC
D5
D13
UF4005
185-265
VAC
1N4007
+
L2
2200 µH
R15
10 Ω
1/2 W
LM358
D11
1N4148
TinySwitch-III
U2
R11
2 kΩ
D
S
TNY279P
1/8 W
VR1
12 V
EN/UV
BP/M
R10
51 Ω
D7
R6
R13
5.1 kΩ
1N4007
68 kΩ
C10
C17
C15
6.8 µF
400 V
1/8 W
C16
1 µF
50 V
D3, D4
1N4007
100 nF 100 µF
U4
TL431
2%
50 V
16 V
U3
PC817A
PI-4520-110606
Figure 1. Circuit Diagram of a Passive PFC Power Supply for Driving Lighting LEDs.
November 2006
DI-130
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