Introduction to Digital Power Conversion
XMC4000/1000 Family
Modulation
Application Guide 75 V1.0, 2015-01
6.8 CCM / (DCM): PFC using Fixed-Off-Time (FOFFT)
A PFC in FOFFT mode, commutates the MOSFET by an equidistant, on-time pulse stream, where
each pulse length is the time it takes till the inductor current hits the Peak Current (PCC) level. With
this kind of control the inductor current ripples along the average current envelope, satisfying CCM,
except the DCM close to zero.
The Peak Current has to be aligned with the sinusoidal half-wave curve of input voltage, with for
example a feed-forward loop (not shown), sensing the input voltage |V
AC(t)
|.
For the output voltage control there must also be an output voltage sensing feed-back (This is implicit,
but not shown in the following figure):
Figure 60 Mode CCM / (DCM) – PFC using Fixed-Off-Time (FOFFT) – Example
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