Rainbow Electronics MAX16841 - Manual (Page 10)

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MAX16841
Controller IC for Dimmable Offline LED Lamps
Resistors R16 and R20
The average current in resistor R16 is the average input
current of the buck converter.
If P
OUT
is the output power, then the input power is given by:
OUT
IN
IN
IN
M
M INrms
IN
CS
CS
P
LP IN Lmax
P
P
P
I
2 V
V 2 V
I R20 0.1V
R16
10µA
V 80%
R
IL
I I 0.5 I
=
η
× π
=
Ч
= Ч
Ч +
=
Ч
=
= + Ч ∆
I
LP
is the switch peak current. Maximum peak in the
switch current occurs at the peak level of the highest
input voltage.
V
CS
is 2.2V. Allow 80% margin for tolerances.
Inductor Selection
For optimum efficiency, inductor L5 must be operated in
continuous-conduction mode.
The current in the inductor would be at its maximum level
at peak of the highest input voltage. LED string voltage
is assumed constant. Calculate the duty cycle at peak of
the highest input voltage.
LED
INmax
V
D
2 V
=
×
The percentage peak-to-peak ripple is considered
between 30% and 60% of the inductor current. Assuming
60% peak-to-peak inductor current ripple, the maximum
inductor current is given by:
OUT
Lmax
LED
P
I
2 V
× π
=
×
The minimum value of the inductor is given by:
( )
INmax LED
Lmax SW
2 V V D
Lmin
0.6 I f
Ч − Ч
=
Ч Ч
Figure 2 shows a PFC triac, dimmable, isolated (flyback
topology) offline LED driver.
Here the current through the Q4 MOSFET is controlled.
Current through Q4 is the same as the input current
of the flyback converter. The input-side circuitry is the
same as in the nonisolated buck LED driver that was
previously described. During startup, the device is
powered up from Q2, R10, R11, and D8. Bootstrap from
the bias winding on the transformer turns off the Q2
MOSFET, thus saving power from high-voltage line. Here
the switching threshold programmed by R15, R16, and
R18 can be lower than the LED string voltage.
Output-side electrolytic capacitors C8 and C9 are used
for filtering the double-line frequency current ripple in
LED current.
During an open LED condition, the voltage across the
output capacitors increases and is reflected on the bias-
winding side.
Once the bias-winding voltage goes above 22.5V (typ),
NDRV is disabled and the Q4 MOSFET turns off.
Choose the transformer turns ratio based on the voltage
rating of the MOSFET. Use the following expression to
calculate primary-secondary turns ratio:
DSmax INmax
PS
LEDmax
0.8 V V
N
V
× −
=
where:
N
PS
is the primary-secondary turns ratio
V
DSmax
is the voltage rating of the Q4 MOSFET
V
INmax
is the maximum peak input voltage
V
LEDmax
is the maximum voltage of the LED string
Factor 0.8 is taken into account for the voltage spikes,
due to transformer-leakage inductance.
Use the following equation to calculate bias-secondary
turns ratio:
AS
LEDmax
18V
N
V
=
where N
AS
is the bias-secondary turns ratio and 18V is
the bias voltage for the device.