VREF
+
-
LM3450A FLYBACK
+
VREF
+
-
EMI + BRIDGE
LM3409HV BUCK
Design Information
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9
Design Information
The following section explains how to design using the LM3450A power factor controller and phase
dimming decoder. Refer to AN-1953 LM3409HV Evaluation Board (
SNVA390
) for a detailed design
procedure of the LM3409HV secondary stage and to the LM3450/A LED Drivers with Active Power Factor
Correction & Phase Dimming Decoder (
SNVAS681
) data sheet for specific details regarding the function
of the LM3450A device. All reference designators refer to the Simplified Evaluation Board Schematic. Note
that parallel and series resistances are combined in one schematic symbol for simplification. To improve
readability of this design document, each subsection is followed by a list of Definitions for new terms used
in the calculations.
Section 11
, showing all components and connectors, is found at the end of this
document as well as a Bill of Materials for each assembly version.
Figure 11. Two-Stage PFC LED Driver
9.1
1
ST
Stage - CRM Flyback
The first stage of the evaluation board shown in
Figure 11
is a critical conduction mode (CRM) flyback
converter controlled with the LM3450A. CRM converters operate at the boundary of continuous conduction
mode (CCM) and discontinuous conduction mode (DCM). CRM is implemented by turning on the main
switching FET (Q3) until the primary current rises to a peak threshold. Q3 is then turned off and the
current falls until a zero crossing is detected. At this point, Q3 is turned on and the cycle repeats.
In the CRM flyback PFC application, the rectified AC input is fed forward to the control loop, yielding a
sinusoidal peak current threshold. This peak threshold creates a sinusoidal primary peak current envelope
I
P-pk
as shown in
Figure 12
. The secondary peak current envelope I
S-pk
will simply be a scaled version of
the primary according to the turns ratio of the transformer. Assuming good attenuation of the switching
ripple via the EMI filter, the average input current I
IN
(t), shown in red, can also be approximated as a
sinusoid. Since the input current has the same shape and phase as the input voltage, high power factor
(PF) can easily be achieved.
8
AN-2150 LM3450A Evaluation Board
SNVA485B – June 2011 – Revised May 2013
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