www.fairchildsemi.com
AN-9750
High-Power Factor Flyback Converter for LED Driver with
FL7732 PSR Controller
Introduction
This highly integrated PWM controller, FL7732, provides
several features to enhance the performance of low-power
flyback converters. The proprietary topology enables
simplified circuit design for LED lighting applications. By
using single-stage topology with primary-side regulation, a
LED lighting board can be implemented with few external
components and minimized cost, without requiring an input
bulk capacitor and feedback circuitry. To implement high
power factor and low THD, constant on-time control
utilizes an external capacitor connected at the COMI pins.
Precise constant-current control regulates accurate output
current across changes in input voltage and output voltage.
The operating frequency is proportionally changed by the
output voltage to guarantee DCM operation with higher
efficiency and simple design. FL7732 provides protection
functions such as open-LED, short-LED and over-
temperature protection. The current-limit level is
automatically reduced to minimize the output current and
protect external components in short-LED condition.
This application note presents practical design consideration
for an LED driver employing Fairchild Semiconductor
PWM PSR controller FL7732. It includes designing the
transformer, selecting the components, and implementing
constant current regulation. The step-by-step design
procedure helps engineers design a power supply. The
design procedure is verified through an experimental
prototype converter. Figure 1 shows the typical application
circuit of primary-side controlled flyback converter using
FL7732 created in the design example.
Figure 1. Typical Application Circuit
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 5/15/12
www.fairchildsemi.com
AN-9750
APPLICATION NOTE
Basic Operation
Generally, Discontinuous Conduction Mode (DCM)
operation is preferred for primary-side regulation because it
allows better output regulation. The operation principles of
DCM flyback converter are as follows:
V
IN.peak
n:1
I
D
D
-
V
D
+
+
V
OUT
-
Lm
I
M
I
DS
C
O
Mode I
During the MOSFET turn-on time (t
ON
), input voltage
(V
IN.pk
) is applied across the primary-side inductor (L
m
).
Then, drain current (I
DS
) of the MOSFET increases linearly
from zero to the peak value (I
pk
), as shown in Figure 2.
During this time, the energy is drawn from the input and
stored in the inductor.
Q
+
V
DS
-
Figure 3. Mode I: Q[ON], D[OFF]
Mode II
When the MOSFET (Q) is turned off, the energy stored in
the inductor forces the rectifier diode (D) to be turned on.
V
Gate
MODE I
MODE II
MODE III
V
DS
nV
OUT
Figure 4. Mode II: Q[OFF], D[ON]
V
IN
I
M
I
DS
I
D
I
DS
I
D
I
O
Figure 5. Mode III: Q[OFF], D[OFF]
V
A
N
A
V
O
N
S
t
DIS
t
S
While the diode is conducting, output voltage
(V
OUT
),
together
with diode forward-voltage drop
(V
F
),
is applied across the
secondary-side inductor and diode current
(I
D
)
decreases
linearly from the peak value
(I
pk
N
P
/N
S
)
to zero. At the end of
inductor current discharge time
(t
DIS
),
all energy stored in the
inductor has been delivered to the output.
Figure 2. Basic Function of DCM Mode Flyback
Mode III
When the diode current reaches zero, the transformer
auxiliary winding voltage begins to oscillate by the
resonance between the primary-side inductor (L
m
) and the
effective capacitor loaded across MOSFET (Q).
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 5/15/12
www.fairchildsemi.com
2
AN-9750
APPLICATION NOTE
Constant Current Regulation
The output current (I
O
) can be estimated by using the peak
drain current (I
pk
) of MOSFET and discharging time (t
DIS
) of
inductor current because output current (I
O
) is same as the
average of the diode current (I
D
) in steady state. The output
current estimator identifies the peak value of the drain
current with a peak-detection circuit and calculates the
output current using the inductor discharging time and
switching period (t
S
). This output information is compared
with an internal precise reference to generate error voltage
(V
COMI
), which determines the duty cycle of the MOSFET in
Constant Current Mode. With Fairchild’s innovative
TRUECURRENT
®
technique, the constant output current
can be precisely controlled.
Io
½
N
1
t
DIS
1
VCS
P
N S RSENSE
2
t
S
OSC
V
OUT
Linear Frequency
Controller
Freq.
6
V
S
VS
Figure 8. Linear Frequency Control
(1)
TRUECURRENT
®
calculation makes a precise output
current prediction.
When output voltage decreases, secondary diode conduction
time is increased and the linear frequency control lengthens
the switching period, which retains DCM operation in the
wide output voltage range, as shown in Figure 8. The
frequency control also lowers primary rms current with
better power efficiency in full-load condition.
nVo
Lm
t
DIS
T
n
3
V
O
4
Lm
4
t
3
DIS
4
T
3
3
n V
O
5
Lm
5
t
3
DIS
5
T
3
Figure 6. Detection for TRUECURRENT
®
Calculation
Figure 9. Primary and Secondary Current
BCM Control
The end of secondary diode conduction time is possibly over
a switching period set by linear frequency control. In this
case, FL7732 doesn’t allow CCM and the operation mode
changes from DCM to BCM. Therefore, FL7732 originally
eliminates sub-harmonic distortion in CCM.
Figure 7. TRUECURRENT
®
Calculation Principle
Linear Frequency Control
As mentioned above, DCM should be guaranteed for high
power factor in flyback topology. To maintain DCM in the
wide range of output voltage, frequency is linearly changed
by the output voltage in linear frequency control. Output
voltage is detected by the auxiliary winding and resistive
divider connected to the VS pin, as shown in Figure 7.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 5/15/12
www.fairchildsemi.com
3
AN-9750
APPLICATION NOTE
Protections
The FL7732 have several self-protection functions, such as
over-voltage protection, over-temperature protection, and
pulse-by-pulse current limit. All the protections are
implemented as Auto-Restart Mode.
Open-LED Protection
FL7732 protects external components, such as diode and
capacitor at secondary side, in open-LED condition. During
switch-off, the
V
DD
capacitor is charged up to the auxiliary
winding voltage, which is applied as the reflected output
voltage. Because the
V
DD
voltage has output voltage
information, the internal voltage comparator at the
V
DD
pin
can trigger output over-voltage protection (OVP), as shown
in Figure 9. When at least one LED is open-circuited, output
load impedance becomes very high and output capacitor is
quickly charged up to
V
OVP
x
N
S
/ N
A
. Then switching is shut
down and the V
DD
block goes into “Hiccup Mode” until the
open-LED condition is removed, as shown in Figure 10.
Internal
Bias
V
DD
good
VDD 4
V
OVP
+
-
Figure 11. Waveforms at Open-LED Condition
Short-LED Protection (OCP)
In case of short-LED condition, the switching MOSFET and
secondary diode are usually stressed by the high powering
current. However, FL7732 changes the OCP level in the
short LED condition. When V
S
voltage is lower than 0.4V,
OCP level changes to 0.2V from 0.7V, as shown in Figure
12 so that powering is limited and external components
current stress is relieved.
S
V
DD
good
R
Q
Shutdown Gate Driver
Figure 10. Internal OVP Block
Under Voltage Lockout (UVLO)
The turn-on and turn-off thresholds are fixed internally at
16V and 7.5V, respectively. During startup, the V
DD
capacitor must be charged to 16V. The V
DD
capacitor
continues to supply V
DD
until power can be delivered from
the auxiliary winding of the main transformer. V
DD
is not
allowed to drop below 7.5V during this startup process. This
UVLO hysteresis window ensures that V
DD
capacitor
properly supplies V
DD
during startup.
Figure 12. Internal OCP Block
Figure 13 shows operational waveforms at short-LED
condition. Output voltage is quickly lowered to 0V right
after the LED-short event. Then, the reflected auxiliary
voltage is also 0V making V
S
voltage less than 0.4V. 0.2V
OCP level limits primary-side current and
V
DD
hiccups up
and down in between UVLO hysteresis.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 5/15/12
www.fairchildsemi.com
4
AN-9750
APPLICATION NOTE
Over-Voltage Protection (OVP)
The OVP prevents damage in over-voltage conditions. If the
V
DD
voltage exceeds 23V at open-loop feedback condition,
the OVP is triggered and the PWM switching is disabled. At
open-LED condition, V
DD
reaches V
DD_OVP
. Then, auto-
restart sequence causes a delay, limiting output voltage.
Over-Temperature Protection (OTP)
The built-in temperature-sensing circuit shuts down PWM
output if the junction temperature exceeds 150°C. There is
hysteresis of 10°C.
Figure 13. Waveforms at Short-LED Condition
At short-LED condition,
V
S
is low due to low output voltage.
Then, OCP level is changed to 0.2V to reduce output current.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 5/15/12
www.fairchildsemi.com
5
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