DN490 - Offline LED Lighting Simplified: High Power Factor, Isolated LED Driver Needs No Opto-Isolators and is TRIAC Dimmer Compatible

Offline LED Lighting Simplified: High Power Factor, Isolated
LED Driver Needs No Opto-Isolators and is TRIAC Dimmer
Compatible – Design Note 490
Wei Gu
Introduction
As environmental concerns over traditional lighting increase and the price of LEDs decreases, high power LEDs
are fast becoming a popular lighting solution for offline
applications. In order to meet the requirements of offline
lighting—such as high power factor, high efficiency, isolation and TRIAC dimmer compatibility—prior LED drivers
used many external discrete components, resulting in
cumbersome solutions. The LT®3799 solves complexity,
space and performance problems by integrating all the
required functions for offline LED lighting.
opto-coupler. Its unique bleeder circuit makes the LED
driver compatible with TRIAC dimmers without additional
components. Open- and shorted-LED protection ensures
long term reliability.
No-Opto Operation
Figure 1 shows a complete LED driver solution. The LT3799
senses the output current from the primary side switch
current waveform. For a flyback converter operating in
boundary mode, the equation for the output current is:
IOUT = 0.5 • IPK • N • (1 – D)
IPK is the peak switch current, N is the primary to secondary turns ratio and D is the duty cycle. The IC regulates
The LT3799 controls an isolated flyback converter in
critical conduction (boundary) mode, suitable for LED
applications requiring 4W to over 100W of LED power. Its
novel current sensing scheme delivers a well-regulated
output current to the secondary side without using an
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trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
L2
750μH
90V
TO 270V
AC
L1
33mH
BR1
C3
0.22μF
C1
0.068μF
C2
0.1μF
R1
200Ω
R3
499k
R4
499k
R7
100k
R6
D2 20Ω
R8
100k
C4
C5 4.7pF
10μF
4:1:1
R13
2k
D3
VIN
DCM
VIN_SENSE
R5
3.48k
FB
R16
32.4k
100k
NTC
FAULT
BR1: DIODES, INC. HD06
D1:
CENTRAL SEMICONDUCTOR CMR1U-06M
D2, D3: DIODES INC. BAV20W
D4:
CENTRAL SEMICONDUCTOR CMR1U-02M
Z1:
FAIRCHILD SMBJ170A
Z2:
CENTRAL SEMICONDUCTOR CMZ5937B
T1:
COILCRAFT JA4429-AL
M1: FAIRCHILD FDPF15N65
R9
40.2k
Z1
C10
560μF
×2
D1
R16
20Ω
CTRL3
GATE
CTRL2
SENSE
CTRL1
VINTVCC
R10
15.4k
GND
FAULT CT COMP+
C6
0.1μF
20W
LED
POWER
Z2
M1
C9
4.7μF
RS
0.05Ω C8
2.2nF
COMP–
C7, 0.1μF
Figure 1. TRIAC Dimmable 20W Offline LED Driver Using the LT3799
05/11/490
1A
D4
R15
4.99k
LT3799
VREF
R18
100k
R4
100k
DN490 F01
the output current by adjusting the peak switch current
and the duty cycle through a novel feedback control.
Unlike other primary side sensing methods that need to
know input power and output voltage information, this
new scheme provides much better output current regulation since the accuracy is barely affected by transformer
winding resistance, switch RDS(ON), output diode forward
voltage drop and LED cable voltage drop.
High Power Factor, Low Harmonics
By forcing the line current to follow the applied sine-wave
voltage, the LT3799 achieves high power factor and
complies with IEC61000-3-2, Class C lighting equipment Harmonics Requirement. A power factor of one
is achieved if the current drawn is proportional to the
input voltage. The LT3799 modulates the peak switch
current with a scaled version of the input voltage. This
technique provides power factors of 0.97 or greater. A
low bandwidth feedback loop keeps the output current
regulated without distorting the input current.
TRIAC Dimmer Compatible
When the TRIAC dimmer is in the off state, it’s not completely off. There is considerable leakage current flowing
through its internal filter to the LED driver. This current
charges up the input capacitor of the LED driver, causing
random switching and LED flicker. Prior solutions added
a bleeder circuit, including a large, expensive high voltage MOSFET. The LT3799 eliminates the need for this
MOSFET or any other extra components by utilizing the
transformer primary winding and the main switch as the
bleeder circuit. As shown in Figure 2, the MOSFET gate
signal is high and the MOSFET is on when the TRIAC
is off, bleeding off the leakage current and keeping the
input voltage at 0V. As soon as the TRIAC turns on, the
MOSFET seamlessly changes back into a normal power
delivery device.
Open- and Shorted-LED Protection
The LED voltage is constantly monitored through the
transformer third winding. The third winding voltage is
proportional to the output voltage when the main switch
is off and the output diode is conducting current. In the
event of overvoltage or open-LED, the main switch turns
off and the capacitor at the CT pin discharges. The circuit
enters hiccup mode as shown in Figure 3.
In a shorted LED event, the IC runs at minimum frequency
before the VIN pin voltage drops below the UVLO threshold
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as the third winding can’t provide enough power to the
IC. The IC then enters its start-up sequence as shown
in Figure 4.
CTRL Pins and Analog Dimming
The LT3799’s output can be adjusted through multiple
CTRL pins. For example, the output current would follow
a DC control voltage applied to any CTRL pin for analog
dimming. Overtemperature protection and line brownout
protection can also be easily implemented using these
CTRL pins.
Conclusion
The LT3799 is a complete offline LED driver solution
featuring standard TRIAC dimming, active PFC and wellregulated LED current with no opto-coupler. This high
performance and feature-rich IC greatly simplifies and
shrinks offline LED driver solutions.
VIN
100V/DIV
VGATE
5V/DIV
2ms/DIV
DN490 F02
Figure 2. MOSFET Gate Signal and VIN
VOUT
20V/DIV
VSW
100V/DIV
100ms/DIV
DN490 F03
Figure 3. Output Open-Circuit Event
IOUT
2A/DIV
VSW
100V/DIV
100ms/DIV
DN490 F04
Figure 4. Output Short-Circuit Event
For applications help,
call (408) 432-1900, Ext. 3565
Linear Technology Corporation
dn490f LT/AP 0511 226K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2011
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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