LNK403-409/413-419
Input Stage
Application Example
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, R16 and R17 together with
the safety rated Y class capacitor (C7) that bridges the safety
isolation barrier between primary and secondary. Resistor R16
and R17 act to damp any resonances formed between L1, L2,
C1 and the AC line impedance. A small bulk capacitor (C2) is
required to provide a low impedance source for the primary
switching current. The maximum value of C1 and C2 is limited in
order to maintain a power factor of greater than 0.9.
14 W TRIAC Dimmable High Power Factor LED Driver
Design Example
The circuit schematic in Figure 7 shows a TRIAC dimmable high
power-factor LED driver based on LNK406EG from the
LinkSwitch-PH family of devices. It was optimized to drive an
LED string at a voltage of 28 V with a constant current of 0.5 A
( 5%) ideal for PAR lamp retro-fit applications. The design
operates over a universal input voltage range of 90 VAC to
265 VAC but provides the specified output current tolerance
over a line voltage range of 90 VAC to 132 VAC (this is configurable
for high-line only applications by simple component value changes).
LinkSwitch-PH Primary
To provide peak line voltage information to U1 the incoming
rectified AC peak charges C3 via D2. This is then fed into the
VOLTAGE MONITOR pin of U1 as a current via R2 and R3. This
sensed current is also used by the device to set the line input
overvoltage and undervoltage protection thresholds. Resistor
R1 provides a discharge path for C3 with a time constant much
longer than that of the rectified AC to prevent generation of line
frequency ripple.
The key goals of this design were compatibility with standard
leading edge TRIAC AC dimmers, very wide dimming range
(1000:1, 500 mA:0.5 mA), high efficiency (>85%) and high power
factor (>0.9). The design is fully protected from faults such as
no-load, overload and output short-circuit conditions and over
temperature.
Circuit Description
The LinkSwitch-PH device (U1) 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.
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 kW
resistor (R4) is used on the REFERENCE pin and 4 MW (R2+R3)
on the VOLTAGE MONITOR pin to provide a linear relationship
between input voltage and the output current and maximizing
the dimming range. Resistor R4 also sets the internal line input
undervoltage and overvoltage protection thresholds.
C8
C10
R15
20 kΩ
330 µF 330 µF
28 V, 500 mA
50 V
50 V
11
1
FL1
L3
1000 µH
D8
MBRS4201T3G
R1
D2
DL4007
VR1
P6KE200A
FL2
3
240 kΩ
R9
750 kΩ
1%
1/2 W
RTN
R18
510 Ω
1 W
D3
UF4007
R8
150 Ω
C5
D6
DL4936
R7
10 kΩ
22 µF
50 V
BR1
2KBP06M
600 V
R16
1 kΩ
2
R2
2 MΩ
1%
R10
750 kΩ
1%
T1
RM8
F1
3.15 A
L
R3
2 MΩ
1%
D4
UF4002
L1
1000 µH
C3
C2
C1
C11
D5
90 - 265
VAC
1 µF
400 V
100 nF
47 nF
220 nF
630 V
RV1
275 VAC
1N4148
630 V
275 VAC
L2
R5
3 kΩ
1000 µH
R6
162 kΩ
1%
D
S
V
D7
VR2
ZMM5245B-7
15 V
N
D1
CONTROL
LinkSwitch-PH
U1
LNK406EG
BAV21WS-7-F
DL4002
BP
R17
1 kΩ
VR3
ZMM5259B-7
39 V
R12
Q3
MMBT3904
R
FB
15 Ω
1%
Q2
IRFR310
C6
15 nF
50 V
R4
Q1
FMMT558
49.9 kΩ
C12
1 µF
50 V
C13
100 nF
50 V
C4
10 µF
16 V
R13
130 Ω
1/2 W
1%
R19
1 kΩ
R11
2.4 MΩ
R20
10 kΩ
C7
2.2 nF
250 VAC
PI-5997-061510
Figure 7. Schematic of an Isolated, TRIAC Dimmable, High Power Factor, Universal Input, 14 W LED Driver.
5
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Rev. D 08/11
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