LYT4211-4218/4311-4318
Application Example
25 W TRIAC Dimmable High Power Factor LED Driver
Design Example (DER-350)
peak drain voltage of U1 below the 72± 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 R2±
together with the safety rated Y class capacitor (CY1) that bridges
the safety isolation barrier between primary and secondary.
Resistor R2, R24 and R2± 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.
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.
LYTSwitch Primary
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 (>8±5) 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.
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.
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.
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 (R14) is used on the REFERENCE pin and 2 MW (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, R1± 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 (VOR).
Input Stage
Fuse F1 provides protection from component failures while RV1
provides a clamp during differential line surges, keeping the
C13
100 pF
200 V
R26
30 Ω
C11
C12
63 V
D9
D2
DFLU1400
36 V,
R23
20 kΩ
330 µF 330 µF
DFLU1400-7
700 mA
63 V
12
1
FL1
R24
D7
47 kΩ
R9
510 kΩ
1/8 W
C7
2.2 nF
630 V
BYW29-200
R15
200 kΩ
1/8 W
BR1
MB6S
600 V
FL2
10
D6
RTN
BAV21
R20
39 Ω
1/8 W
C9
C5
100 nF
50 V
D3
US1J
56 µF
50 V
11
R10
C1
220 nF
250 V
R1
510
1/2 W
T1
RM8
2 MΩ
R19
20 kΩ
1%
1/8 W
D4
C2
C4
C6
R6
US1D
100 nF
250 V
100 nF 2.2 µF
360 kΩ
250 V 250 V
L3
5 mH
D5
R2
R25
47 kΩ
1/8 W
L1
1 mH
L2
1 mH
BAV16
47 kΩ
D8
1/8 W
R17
3 kΩ
1/10 W
R18
165 kΩ
1%
BAV21
D
S
V
1/16 W
CONTROL
LYTSwitch
U1
LYT4317E
BP
RV1
140 VAC
F1
5 A
Q1
Q2
MMBT3904
R
FB
R14
C15
100 nF
50 V
X0202MA2BL2
90 - 132
VAC
C3
470 nF
50 V
L
N
49.9 kΩ
1%
1/16 W
R27
R22
1 kΩ
1/10 W
C14
10 nF
50 V
R8
100 Ω
1 W
10 Ω
C8
47 µF
16 V
1/10 W
CY1
470 pF
250 VAC
PI-6875-101812
Figure 8. DER-350 Schematic of an Isolated, TRIAC Dimmable, High Power Factor, 90-132 VAC, 25 W / 36 V / 700 mA LED Driver.
6
Rev. B 02/13
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