LM3450 Datasheet, PDF (12/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
www.ti.com
The output voltage has to be regulated slowly to ensure the converter ignores the twice line frequency ripple
present on the output. Therefore, the voltage loop containing the error amplifier should have a bandwidth at least
an order of magnitude slower (<20Hz is common). Sometimes the bandwidth is increased to improve transient
response, which is the case with off-line dimmable LED drivers. Though PF decreases with the increase in
bandwidth, high PF (>.95) is still possible.
i_L (t)
I_L-MAX
I_L
0
t
t_ON = DT_S t_OFF = (1-D)T_S
T_S
Figure 19. Basic CRM Inductor Current Waveform
CRM BACKGROUND
During critical conduction mode (CRM), a converter operates at the boundary of continuous conduction mode
(CCM) and discontinuous conduction mode (DCM). This is usually implemented as follows. The main switching
MosFET (Q_SW) is turned on and the inductor current rises to a peak threshold. Q_SW is then turned off and the
current falls until it reaches zero. At this point, Q_SW is turned on and the cycle repeats. Near zero voltage
switching, enabled by the inductor current return to zero, gives CRM topologies an efficiency improvement
compared to CCM topologies. Figure 19 shows the resulting inductor current waveform, where the average
inductor current (I_L) is half of the peak current (I_L-MAX).
In a CRM flyback PFC application, the rectified AC input is fed forward to the control loop, creating a sinusoidal
primary peak current envelope (I_P-pk) as shown in Figure 20. 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), represented by the red line in
Figure 20, can also be approximated as a sinusoid proportional to the duty cycle (D(t)):
I_in(t)
=
I_P-PK
x
2
D(t)
(3)
Since CRM operation is hysteretic and the input voltage is fed-forward, the input current shaping loop is as fast
as possible. Only the output voltage needs to be regulated with a narrow bandwidth error amplifier, which greatly
simplifies the system dynamics.
12
Submit Documentation Feedback
Product Folder Links: LM3450
Copyright © 2010–2013, Texas Instruments Incorporated