Design Information
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The primary bias supply shown in
Figure 13
enables instant turn-on through Q1 while providing an
auxiliary winding for high efficiency steady state operation. The two bias paths are each connected to V
CC
through a diode (D8, D9) to ensure the higher of the two is providing V
CC
current. The LM3450A BIAS pin
helps to ensure that the auxiliary winding is always providing V
CC
during normal operation.
Since there is optical isolation, a secondary bias supply is also desirable. This is accomplished with
another auxiliary winding, diode (D4), and capacitance (C4, C5) which creates another flyback output that
scales with the regulated output (similar to the auxiliary primary bias winding). To ensure secondary bias
regulation is closely coupled to the regulated flyback output, the output winding is tapped to provide the
secondary bias output.
It is also advantageous to linear regulate down to approximately 9V, from the 12V bias supplies, for every
opto-isolator supply rail (V
POP1
, V
OP1
, V
OP2
) . This will stabilize the opto-isolator rail over the entire operating
range, preventing noise coupling into COMP and the dimming input of the LM3409.
The primary and secondary bias outputs for both versions of the board are set to 12.5V at the nominal
input voltage. The turns calculations (referred to the output) for the primary auxiliary winding and the tap
point for the secondary winding are:
(17)
The minimum primary bias supply capacitance is calculated, given a minimum V
CC
ripple specification, to
keep V
CC
above UVLO at the worst-case current:
(18)
14
AN-2150 LM3450A Evaluation Board
SNVA485B – June 2011 – Revised May 2013
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