LM3450 datasheet(13/28 Pages) NSC

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FIGURE 3. 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 im-

plemented as follows. The main switching MosFET (Q

SW) is

turned on and the inductor current rises to a peak threshold.

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 improve-

ment compared to CCM topologies. Figure 3 shows the re-

sulting inductor current waveform, where the average induc-

tor current (I

L) 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 (I

P-pk) as shown in Figure 4. 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 4, can also be approximated as a sinusoid pro-

portional to the duty cycle (D(t)):

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.

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FIGURE 4. CRM Flyback Current Waveforms

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