)

2

( )

t

D

I

PK

P

x

-

(t

I

in

=

t

iL (t)

IL-MAX

IL

0

TS

tON = DTS

tOFF = (1-D)TS

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.

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 (QSW) is turned on and the inductor current rises to a peak threshold. QSW is then turned off and the

current falls until it reaches zero. At this point, QSW 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 (IL) is half of the peak current (IL-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 (IP-pk) as shown in Figure 20. The secondary peak current envelope (IS-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 (IIN), represented by the red line in

Figure 20, can also be approximated as a sinusoid proportional to the duty cycle (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

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