LM3445 Application Note 2069 LM3445 - 230VAC, 6W- 15W Isolated Flyback... Literature Number: SNVA447B

LM3445 Application Note 2069 LM3445 - 230VAC, 6W- 15W Isolated Flyback... Literature Number: SNVA447B
LM3445
Application Note 2069 LM3445 - 230VAC, 6W- 15W Isolated Flyback LED Driver
Literature Number: SNVA447B
National Semiconductor
Application Note 2069
Montu Doshi
November 23, 2010
Introduction
Key Features
This demonstration board highlights the performance of a
LM3445 based Flyback LED driver solution that can be used
to power a single LED string consisting of 4 to 10 series connected LEDs from an 180 VRMS to 265 VRMS, 50 Hz input
power supply. The key performance characteristics under
typical operating conditions are summarized in this application note.
This is a four-layer board using the bottom and top layer for
component placement. The demonstration board can be
modified to adjust the LED forward current, the number of series connected LEDs that are driven and the switching frequency. Refer to the LM3445 datasheet for detailed instructions.
A bill of materials is included that describes the parts used on
this demonstration board. A schematic and layout have also
been included along with measured performance characteristics.
•
•
•
•
Drop-in compatibility with TRIAC dimmers
Line injection circuitry enables PFC values greater than
0.94
Adjustable LED current and switching frequency
Flicker free operation
Applications
•
•
•
•
Retro-fit TRIAC Dimming
Solid State Lighting
Industrial and Commercial Lighting
Residential Lighting
Performance Specifications
Based on an LED Vf = 3.4V
Symbol
Parameter
Min
Typ
Max
VIN
Input voltage
180 VRMS
230 VRMS
265 VRMS
VOUT
LED string voltage
13 V
20 V
27 V
ILED
LED string average current
-
350 mA
-
POUT
Output power
-
7.2 W
-
fsw
Switching frequency
-
67 kHz
-
Demo Board
LM3445 - 230VAC, 6W- 15W Isolated Flyback LED Driver
LM3445 - 230VAC, 6W- 15W
Isolated Flyback LED Driver
Dimming Characteristics
30126904
30126924
AN-2069
© 2010 National Semiconductor Corporation
301269
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AN-2069
LM3445 230VAC, 8W Isolated Flyback LED Driver Demo Board Schematic
30126901
Warning: The LM3445 evaluation board has exposed high voltage components that present a shock hazard. Caution must be taken when handling the evaluation
board. Avoid touching the evaluation board and removing any cables while the evaluation board is operating.
Warning: The ground connection on the evaluation board is NOT referenced to earth ground. If an oscilloscope ground lead is connected to the evaluation
board ground test point for analysis and the mains AC power is applied (without any isolation), the fuse (F1) will fail open. For bench evaluation, either
the input AC power source or the bench measurement equipment should be isolated from the earth ground connection. Isolating the evaliation board
(using 1:1 isolation line isolation transformer) rather than the oscilloscope is highly recommended.
Warning: The LM3445 evaluation board should not be powered with an open load. For proper operation, ensure that the desired number of LEDs are connected
at the output before applying power to the evaluation board.
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2
AN-2069
LM3445 Device Pin-Out
30126902
Pin Description 10 Pin MSOP
Pin #
Name
Description
1
ASNS
PWM output of the triac dim decoder circuit. Outputs a 0 to 4V PWM signal with a duty cycle proportional to the triac
dimmer on-time.
2
FLTR1
First filter input. The 120Hz PWM signal from ASNS is filtered to a DC signal and compared to a 1 to 3V, 5.85 kHz
ramp to generate a higher frequency PWM signal with a duty cycle proportional to the triac dimmer firing angle. Pull
above 4.9V (typical) to tri-state DIM.
3
DIM
Input/output dual function dim pin. This pin can be driven with an external PWM signal to dim the LEDs. It may also
be used as an output signal and connected to the DIM pin of other LM3445 or LED drivers to dim multiple LED
circuits simultaneously.
4
COFF
OFF time setting pin. A user set current and capacitor connected from the output to this pin sets the constant OFF
time of the switching controller.
5
FLTR2
Second filter input. A capacitor tied to this pin filters the PWM dimming signal to supply a DC voltage to control the
LED current. Could also be used as an analog dimming input.
6
GND
Circuit ground connection.
7
ISNS
LED current sense pin. Connect a resistor from main switching MOSFET source, ISNS to GND to set the maximum
LED current.
8
GATE
Power MOSFET driver pin. This output provides the gate drive for the power switching MOSFET of the buck
controller.
9
VCC
10
BLDR
Input voltage pin. This pin provides the power for the internal control circuitry and gate driver.
Bleeder pin. Provides the input signal to the angle detect circuitry as well as a current path through a switched
230Ω resistor to ensure proper firing of the triac dimmer.
3
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AN-2069
Bill of Materials
Designator
AA1
Description
Manufacturer
Printed Circuit Board
Part Number
RoHS
-
Y
U1
Triac Dimmable Offline LED Driver, PowerWise
National
Semiconductor
LM3445MM
Y
C1
Ceramic, X7R, 250VAC, 10%
Murata Electronics
North America
DE1E3KX332MA5BA01
Y
C2
Ceramic, Polypropylene, 400VDC, 10%
WIMA
MKP10-.033/400/5P10
Y
C3
CAP, CERM, 330pF, 630V, +/-5%, C0G/NP0, 1206
TDK
C3216C0G2J331J
Y
C4
Ceramic, X7R, 250V, X2, 10%, 2220
Murata Electronics
North America
GA355DR7GF472KW01L
Y
C5
CAP, Film, 0.033µF, 630V, +/-10%, TH
EPCOS Inc
B32921C3333K
Y
C6
CAP, CERM, 0.015µF, 500V, +/-10%, X7R, 1812
Vishay/Vitramon
VJ1812Y223KBEAT4X
Y
C7-DNP
CAP, CERM, 0.1µF, 630V, +/-10%, X7R, 1812
MuRata
GRM43DR72J104KW01L
Y
C8
CAP, CERM, 0.1µF, 630V, +/-10%, X7R, 1812
MuRata
GRM43DR72J104KW01L
Y
CAP, CERM, 1µF, 50V, +/-10%, X7R, 1210
MuRata
GRM32RR71H105KA01L
Y
C10
CAP, CERM, 0.47µF, 50V, +/-10%, X7R, 0805
MuRata
GRM21BR71H474KA88L
Y
C12
Aluminium Electrolytic, 680uF, 35V, 20%,
Nichicon
UHE1V681MHD6
Y
C13
CAP, CERM, 1µF, 35V, +/-10%, X7R, 0805
Taiyo Yuden
GMK212B7105KG-T
Y
C14
CAP, CERM, 0.1µF, 25V, +/-10%, X7R, 0603
MuRata
GRM188R71E104KA01D
Y
C15
CAP, TANT, 47uF, 16V, +/-10%, 0.35 ohm, 6032-28
SMD
AVX
TPSC476K016R0350
Y
C16
CAP, CERM, 0.47µF, 16V, +/-10%, X7R, 0603
MuRata
GRM188R71C474KA88D
Y
C17
CAP, CERM, 0.22µF, 16V, +/-10%, X7R, 0603
TDK
C1608X7R1C224K
Y
C18
CAP, CERM, 2200pF, 50V, +/-10%, X7R, 0603
MuRata
GRM188R71H222KA01D
Y
C20
CAP, CERM, 330pF, 50V, +/-5%, C0G/NP0, 0603
MuRata
GRM1885C1H331JA01D
Y
D1
DIODE TVS 250V 600W UNI 5% SMD
Littelfuse
P6SMB250A
Y
D2
Diode, Switching-Bridge, 600V, 0.8A, MiniDIP
Diodes Inc.
HD06-T
Y
D3
Diode, Silicon, 1000V, 1A, SOD-123
Comchip Technology CGRM4007-G
Y
D4
Diode, Schottky, 100V, 1A, SMA
STMicroelectronics
STPS1H100A
Y
Diode, Zener, 13V, 200mW, SOD-323
Diodes Inc
DDZ13BS-7
Y
Diode, Zener, 36V, 550mW, SMB
ON Semiconductor
1SMB5938BT3G
Y
Diode, Schottky, 100V, 150 mA, SOD-323
STMicroelectronics
BAT46JFILM
Y
Fuse, 500mA, 250V, Time-Lag, SMT
Littelfuse Inc
0443.500DR
Y
H1, H2, H5, H6 Standoff, Hex, 0.5"L #4-40 Nylon
Keystone
1902C
Y
H3, H4, H7, H8 Machine Screw, Round, #4-40 x 1/4, Nylon, Philips
panhead
B&F Fastener Supply NY PMS 440 0025 PH
Y
C9, C11
D5, D10
D6
D7, D8, D9
F1
J1, J2
Conn Term Block, 2POS, 5.08mm PCB
Phoenix Contact
1715721
Y
L1, L2
Inductor, Radial Lead Inductors, Shielded, 4.7mH,
130mA, 12.20ohm, 7.5mm Radial,
TDK Corporation
TSL0808RA-472JR17-PF
Y
Terminal, 22 Gauge Wire, Terminal, 22 Guage Wire
3M
923345-02-C
Y
Q1
MOSFET, N-CH, 600V, 200mA, SOT-223
Fairchild
Semiconductor
FQT1N60CTF_WS
Y
Q2
Transistor, NPN, 300V, 500mA, SOT-23
Diodes Inc.
MMBTA42-7-F
Y
Q3
MOSFET, N-CH, 650V, 800mA, IPAK
Infineon
Technologies
SPU01N60C3
Y
Q4
MOSFET N-CH 100V 170MA SOT23-3
Diodes Inc.
BSS123-7-F
Y
LED+, LED-,
TP7, TP8
R1
RES, 221 ohm, 1%, 0.25W, 1206
Vishay-Dale
CRCW1206221RFKEA
Y
R2, R7
RES, 200k ohm, 1%, 0.25W, 1206
Vishay-Dale
CRCW1206200KFKEA
Y
R3, R8
RES, 309k ohm, 1%, 0.25W, 1206
Vishay-Dale
CRCW1206309KFKEA
Y
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4
Description
Manufacturer
Part Number
RoHS
R4, R12
RES, 10k ohm, 5%, 0.25W, 1206
Vishay-Dale
CRCW120610K0JNEA
Y
R5-DNP
RES, 680 ohm, 5%, 1W, 2512
Vishay/Dale
CRCW2512680RJNEG
Y
R6
RES, 820 ohm, 5%, 1W, 2512
Vishay/Dale
CRCW2512820RJNEG
Y
R10
RES, 430 ohm, 5%, 1W, 2512
Vishay/Dale
CRCW2512430RJNEG
Y
R11
RES, 49.9k ohm, 1%, 0.125W, 0805
Vishay-Dale
CRCW080549K9FKEA
Y
R13
RES, 33.0 ohm, 1%, 0.25W, 1206
Vishay-Dale
CRCW120633R0FKEA
Y
R14
RES, 10 ohm, 5%, 0.125W, 0805
Vishay-Dale
CRCW080510R0JNEA
Y
R15
RES, 10.0k ohm, 1%, 0.1W, 0603
Vishay-Dale
CRCW060310K0FKEA
Y
R16
RES, 280k ohm, 1%, 0.1W, 0603
Vishay-Dale
CRCW0603280KFKEA
Y
R17
RES, 475k ohm, 1%, 0.1W, 0603
Vishay-Dale
CRCW0603475KFKEA
Y
R18
RES, 49.9k ohm, 1%, 0.1W, 0603
Vishay-Dale
CRCW060349K9FKEA
Y
R19
RES, 10 ohm, 5%, 0.1W, 0603
Vishay-Dale
CRCW060310R0JNEA
Y
R20
RES, 1.91k ohm, 1%, 0.1W, 0603
Vishay-Dale
CRCW06031K91FKEA
Y
R21
RES, 2.70 ohm, 1%, 0.25W, 1206
Panasonic
ERJ-8RQF2R7V
Y
R22
RES, 10.7 ohm, 1%, 0.125W, 0805
Vishay-Dale
CRCW080510R7FKEA
Y
R23
RES, 324k ohm, 1%, 0.1W, 0603
Vishay-Dale
CRCW0603324KFKEA
Y
RT1, RT2
Current Limitor Inrush, 60Ohm, 20%, 5mm Raidal
Cantherm
MF72-060D5
Y
T1
FLBK TFR, 2.07 mH, Np=140T, Ns=26T, Na= 20T
Wurth Elektornik
750815040 REV 1
Y
Terminal, Turret, TH, Double
Keystone Electronics 1502-2
Y
Varistor 275V 55J 10mm DISC
EPCOS Inc
Y
TP9, TP10
VR1
5
S10K275E2
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AN-2069
Designator
AN-2069
Transformer Design
Mfg: Wurth Electronics, Part #: 750815040 Rev. 01
30126909
Parameter
Test Conditions
Value
D.C. Resistance (3-1)
20°C
1.91 Ω ± 10%
D.C. Resistance (6-4)
20°C
0.36 Ω ± 10%
D.C. Resistance (10-13)
20°C
Inductance (3-1)
10 kHz, 100 mVAC
0.12 Ω ± 10%
2.12 mH ± 10%
Inductance (6-4)
10 kHz, 100 mVAC
46.50 µH ± 10%
Inductance (10-13)
10 kHz, 100 mVAC
74.00 µH ± 10%
Leakage Inductance (3-1)
100 kHz, 100 mAVAC (tie 6+4, 10+13)
18.0 µH Typ., 22.60 µH Max.
Dielectric (1-13)
tie (3+4), 4500 VAC, 1 second
4500 VAC, 1 minute
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Turns Ratio
(3-1):(6-4)
7:1 ± 1%
Turns Ratio
(3-1):(10:13)
5.384:1 ± 1%
6
AN-2069
Demo Board Wiring Overview
30126903
Wiring Connection Diagram
Test Point
Name
I/O
Description
TP8, TP10
LED +
Output
LED Constant Current Supply
Supplies voltage and constant-current to anode of LED string.
TP7, TP9
LED -
Output
LED Return Connection (not GND)
Connects to cathode of LED string. Do NOT connect to GND.
J1-1
LINE
Input
AC Line Voltage
Connects directly to AC line or output of TRIAC dimmer of a 230VAC system.
J1-2
NEUTRAL
Input
AC Neutral
Connects directly to AC neutral of a 230VAC system.
Demo Board Assembly
30126905
Top View
30126906
Bottom View
7
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(Note 1, Note 2, Note 3)
Efficiency vs. Line Voltage
Original Circuit
0.83
8 LEDs
Efficiency vs. Line Voltage
Modified Circuits
10 LEDs
0.97
0.81
EFFICIENCY
EFFICIENCY
0.82
6 LEDs
0.80
0.79
4 LEDs
0.93
Mod C (10 LEDs)
0.89
Mod B (8 LEDs)
0.85
0.81
0.77
0.73
0.78
0.68
Original (6 LEDs)
Mod A (4 LEDs)
0.77
0.64
0.76
180 190 200 210 220 230 240 250 260
0.60
180 190 200 210 220 230 240 250 260
INPUT VOLTAGE (VRMS)
INPUT VOLTAGE (VRMS)
30126910
30126914
LED Current vs. Line Voltage
Original Circuit
LED Current vs. Line Voltage
Modified Circuits
600
650
LED CURRENT (mA)
LED CURRENT (mA)
450
550
4 LEDs
550
6 LEDs
350
250
8 LEDs
150
500
450
Mod C (10 LEDs)
Mod B (8 LEDs)
400
350
300
250
200
150
10 LEDs
Original (6 LEDs)
Mod A (4 LEDs)
100
180 190 200 210 220 230 240 250 260
50
180 190 200 210 220 230 240 250 260
INPUT VOLTAGE (VRMS)
INPUT VOLTAGE (VRMS)
30126911
30126915
Power Factor vs. Line Voltage
Original Circuit
Power Factor vs. Line Voltage
Modified Circuits
0.956
10 LEDs
0.97
0.952
8 LEDs
0.93
0.944
4 LEDs
0.940
0.936
POWER FACTOR
0.948
POWER FACTOR
AN-2069
Typical Performance Characteristics
6 LEDs
0.932
0.89
0.85
0.81
0.77
0.73
Mod C (10 LEDs)
Mod B (8 LEDs)
Original (6 LEDs)
Mod A (4 LEDs)
0.928
0.68
0.924
0.64
0.920
180 190 200 210 220 230 240 250 260
0.60
180 190 200 210 220 230 240 250 260
INPUT VOLTAGE (VRMS)
INPUT VOLTAGE (VRMS)
30126912
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30126916
8
Output Power vs. Line Voltage
Modified Circuits
12
10
9
8
25.0
22.5
10 LEDs
8 LEDs
OUTPUT POWER (W)
OUTPUT POWER (W)
11
AN-2069
Output Power vs. Line Voltage
Original Circuit
4 LEDs
7
6 LEDs
6
5
4
20.0
Mod B (8 LEDs)
17.5
15.0
Mod C (10 LEDs)
12.5
10.0
7.5
5.0
3
2.5
2
180 190 200 210 220 230 240 250 260
Original (6 LEDs)
0.0
180 190 200 210 220 230 240 250 260
Mod A (4 LEDs)
LINE VOLTAGE (VRMS)
INPUT VOLTAGE (VRMS)
30126913
30126917
Line Voltage and Line Current
(VIN = 230VRMS, 6 LEDs, ILED = 350mA)
Output Voltage and LED Current
(VIN = 230VRMS, 6 LEDs, ILED = 350mA)
30126918
30126920
Ch1: Line Voltage (100 V/div); Ch3: Line Current
(20 mA/div); Time (4 ms/div)
Ch1: Output Voltage (10 V/div); Ch3: LED Current
(100 mA/div); Time (4 ms/div)
Power MOSFET Drain and ISNS (Pin-7) Voltage
(VIN = 230VRMS, 6 LEDs, ILED = 350mA)
FLTR2 (Pin-5) and ISNS (Pin-7) Voltage
(VIN=230VRMS, 6 LEDs, ILED = 350mA
30126921
30126922
Ch1: Drain Voltage (100V/div); Ch4: ISNS Voltage
(500 mV/div); Time (4 µs/div)
Ch1: FLTR2 Voltage (200 mV/div); ISNS Voltage
(200 mV/div); Time (4 µs/div)
Note 1: Original Circuit (6 LEDs operating at 350mA): R21 = 2.7Ω; Modification A (10 LEDs operating at 375mA): R21 = 1.8Ω; Modification B (8 LEDs operating
at 350mA): R21 = 2.2Ω; Modification C (4 LEDs operating at 315mA): R21 = 3.9Ω
Note 2: The output power can be varied to acheive desired LED current by interpolating R14 values between the maximum of 3.9 Ω and minimum of 1.8 Ω
Note 3: The maximum output voltage is clamped to 36 V. For operating LED string voltage > 36 V, replace D6 with suitable alternative
9
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AN-2069
PCB Layout
30126907
Top Layer
30126908
Bottom Layer
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10
The LED driver is designed to accurately emulate an incandescent light bulb and therefore behave as an emulated
resistor. The resistor value is determined based on the LED
string configuration and the desired output power. The circuit
then operates in open-loop, with a fixed duty cycle based on
a constant on-time and constant off-time that is set by selecting appropriate circuit components. Like an incandescent
lamp, the driver is compatible with both forward and reverse
phase dimmers.
NON-DIMMING PERFORMANCE
In steady state, the LED string voltage is measured to be 20.5
V and the average LED current is measured as 350 mA. The
100 Hz current ripple flowing through the LED string was
MEASURED EFFICIENCY AND LINE REGULATION (6 LEDS, NO TRIAC DIMMER)
VIN (VRMS)
IIN (mARMS)
PIN(W)
VOUT (V)
ILED (mA)
POUT (W)
180
31.73
5.35
19.67
221.64
4.36
Efficiency (%) Power Factor
81.4
0.9375
190
33.39
5.96
19.85
244.82
4.86
81.5
0.9394
200
35.11
6.61
20.04
269.16
5.39
81.6
0.9493
210
36.85
7.30
20.22
294.82
5.96
81.6
0.9493
220
38.53
8.01
20.40
321.26
6.55
81.8
0.9451
230
40.18
8.75
20.56
348.70
7.17
82.0
0.9463
240
41.75
9.50
20.74
375.70
7.79
82.0
0.9477
250
43..39
10.30
20.90
404.82
8.46
82.1
0.9490
260
45.07
11.14
21.05
434.48
9.15
82.0
0.9500
TRIAC dimmer and measuring the corresponding input and
output parameters, the dimming performance of the demonstration board driving 6 LEDs is summarized in the table
below.
DIMMING PERFORMANCE
The LED driver is capable of matching or exceeding the dimming performance of an incandescent lamp. Using a simple
rotary TRIAC dimmer, smooth and near logarithmic dimming
performance is achieved. By varying the firing angle of the
MEASURED DIMMING PERFORMANCE
VIN (VRMS)
VO (V)
ILED (mA)
POUT (W)
229.39
20.51
343.1
7.04
220.47
20.35
320.8
6.53
210.24
20.16
294.8
5.94
199.05
19.98
266.8
5.33
190.32
19.80
245.8
4.87
180.33
19.61
222.7
4.37
170.51
19.42
200.1
3.89
156.39
19.31
187.4
3.62
149.11
19.15
171.6
3.29
140.35
18.97
154.0
2.92
129.61
18.75
133.1
2.50
119.7
18.53
115.3
2.14
110.17
18.33
99.1
1.82
100.55
18.11
83.5
1.51
90.75
17.87
68.8
1.23
79.72
17.59
53.1
0.93
70.42
17.34
40.8
0.71
60.91
17.08
30.1
0.81
49.94
16.77
19.8
0.33
45.04
16.64
16.0
0.27
11
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AN-2069
measured to be 194 mApk-pk at full load. The magnitude of the
ripple is a function of the value of energy storage capacitors
connected across the output port and the TRIAC firing angle.
The ripple current can be reduced by increasing the value of
energy storage capacitor or by increasing the LED string voltage. With TRIAC dimmers, the ripple magnitude is directly
proportional to the input power and therefore reduces at lower
LED current.
The LED driver switching frequency is measured to be close
to the specified 67 kHz. The circuit operates with a constant
duty cycle of 0.21 and consumes near 8.75 W of input power.
The driver steady state performance for an LED string consisting of 6 series LEDs without using a triac dimmer is
summarized in the following table.
Experimental Results
AN-2069
30126924
Dimming Characteristics
CURRENT THD
of the fundamental current (as shown in the following figure)
and therefore meets the requirements of the IEC 61000-3-2
The LED driver is able to achieve close to unity power factor
Class-3 standard.
(P.F. ~ 0.94) which meets Energy Star requirements. This
design also exhibits low current harmonics as a percentage
30126923
Current Harmonic vs. EN/IEC61000-3-2 Class C Limits
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12
AN-2069
Circuit Operation With Rotary
Forward Phase Triac Dimmer
Input waveforms at full brightness setting
Output waveforms at full brightness setting
30126925
30126928
Ch1: Input Voltage (100 V/div); Ch3: Input Current
(20 mA/div); Time (4 ms/div)
Ch1: Output Voltage (10 V/div); LED Current
(100 mA/div); Time (4 ms/div)
Input waveforms at half brightness setting (90° firing angle) Output waveforms at half brightness setting (90° firing angle)
30126926
30126929
Ch1: Input Voltage (100 V/div); Ch3: Input Current
(20 mA/div); Time (4 ms/div)
Ch1: Output Voltage (10 V/div); LED Current
(100 mA/div); Time (4 ms/div)
Input waveforms at minimum brightness setting
Output waveforms at minimum brightness setting
30126927
30126930
Ch1: Input Voltage (100 V/div); Ch3: Input Current
(20 mA/div); Time (4 ms/div)
Ch1: Output Voltage (10 V/div); LED Current
(100 mA/div); Time (4 ms/div)
13
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AN-2069
Electromagnetic Interference (EMI)
The EMI input filter of this evaluation board is configured as
shown in the following circuit diagram.
30126931
FIGURE 1. Input EMI Filter and Rectifier Circuit
In order to get a quick estimate of the EMI filter performance,
only the PEAK conductive EMI scan was measured and the
data was compared to the Class B conducted EMI limits published in FCC – 47, section 15.(Note 4)
30126932
FIGURE 2. Peak Conductive EMI scan per CISPR-22, Class B Limits
Note 4: CISPR 15 compliance pending
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AN-2069
ILED = 348 mA
# of LEDs = 6
POUT = 7.2 W
The results are shown in the following figures.
Thermal Analysis
The board temperature was measured using an IR camera
(HIS-3000, Wahl) while running under the following conditions:
VIN = 230 VRMS
30126933
FIGURE 3. Top Side Thermal Scan
30126934
FIGURE 4. Bottom Side Thermal Scan
15
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AN-2069
pin, the on-time can be made to be constant. With a DCM
Flyback, Δi needs to increase as the input voltage line increases. Therefore a constant on-time (since inductor L is
constant) can be obtained.
By using the line voltage injection technique, the FLTR2 pin
has the voltage wave shape shown in Figure 6 on it with no
triac dimmer in-line. Voltage at VFLTR2 peak should be kept
below 1.25V. At 1.25V current limit is tripped. C11 is small
enough not to distort the AC signal but adds a little filtering.
Although the on-time is probably never truly constant, it can
be observed in Figure 7 how (by adding the rectified voltage)
the on-time is adjusted.
Circuit Analysis and Explanations
INJECTING LINE VOLTAGE INTO FILTER-2 (ACHIEVING
PFC > 0.94)
If a small portion (750mV to 1.00V) of line voltage is injected
at FLTR2 of the LM3445, the circuit is essentially turned into
a constant power flyback as shown in Figure 5.
30126937
FIGURE 6. FLTR2 Waveform with No Dimmer
For this evaluation board, the following resistor values are
used:
R3 = R8 = 309 kΩ
R20 = 1.91 kΩ
Therefore the voltages observed on the FLTR2 pin will be as
follows for listed input voltages:
For VIN = 180VRMS, VFLTR2, Pk = 0.78V
For VIN = 230VRMS, VFLTR2, Pk = 1.00V
For VIN = 265VRMS, VFLTR2, Pk = 1.15V
Using this technique, a power factor greater than 0.94 can be
achieved without additional passive active power factor control (PFC) circuitry.
30126935
FIGURE 5. Line Voltage Injection Circuit
The LM3445 works as a constant off-time controller normally,
but by injecting the 1.0VPk rectified AC voltage into the FLTR2
30126936
FIGURE 7. Typical Operation of FLTR2 Pin
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16
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Notes
17
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LM3445 - 230VAC, 6W- 15W Isolated Flyback LED Driver
Notes
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