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PassFET Bias Circuit
The passFET (Q1) is used in its linear region to stand-off the line voltage from the LM3448 regulator. Both
the V
CC
startup current and discharging of the EMI filter capacitance for proper phase angle detection are
handled by Q1. Therefore Q1 has to block the maximum peak input voltage and have both sufficient surge
and power handling capability with regards to its safe operating area (SOA). The design equations are,
(28)
(29)
(30)
Note that if additional TRIAC holding current is to be sourced through Q1, then the transistor will need to
be sized appropriately to handle the additional current and power dissipation requirements.
Auxiliary Winding Bias Circuit
For high efficiency during steady-state operation, an additional winding is used to establish an auxiliary
voltage V
AUX
used to provide a V
CC
bias voltage. A minimum value of 13V is recommended for V
AUX
. An
auxiliary transformer turns ratio n
AUX
and corresponding turns calculation is used to size the primary
auxiliary winding N
A
,
(31)
(32)
The minimum primary bias supply capacitance (C14||C15), given a minimum V
CC
ripple specification at
twice the line frequency f
2L
, is calculated to keep V
CC
above UVLO at the worst-case current,
(33)
Input Capacitance
The input capacitor of the flyback (C2) has to be able to provide energy during the worst-case switching
period at the peak of the AC voltage input. C2 should be a high frequency, high stability capacitor (usually
a metallized film capacitor, either polypropylene or polyester) with an AC voltage rating equal to the
maximum input voltage. C2 should also have a DC voltage rating exceeding the maximum peak input
voltage + half of the peak to peak input voltage ripple specification. The minimum required input
capacitance is calculated given the same ripple specification,
(34)
Output Capacitance
C12 should be a high quality electrolytic capacitor with a voltage rating greater than the specified over-
voltage protection threshold V
OVP
. Given the desired voltage ripple, the minimum output capacitance is
calculated,
(35)
19
SNOA555C–April 2011–Revised May 2013 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver
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