AN3142 datasheet(4/44 Pages) STMICROELECTRONICS

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Introduction to FOT control

AN3142

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Doc ID 17005 Rev 3

Introduction to FOT control

In this area where the TM/CCM usability boundary is uncertain, a third approach that

couples the simplicity and affordability of TM operation with the high-current capability of

CCM operation may be a solution to the dilemma. Generally speaking, FF PWM is not the

only alternative when CCM operation is desired. FF PWM modulates both switch ON and

OFF times (their sum is constant by definition), and a given converter operates in either

CCM or DCM depending on the input voltage and the loading conditions. Exactly the same

result can be achieved if the on-time only is modulated and the off-time is kept constant, in

which case, however, the switching frequency is no longer fixed (

Figure 3 and Figure 4).

This is referred to as “fixed-off-time” (FOT) control. Peak-current-mode control can still be

used.

An important factor is that FOT control does not need a specialized control IC. A simple

modification of a standard TM PFC controller operation, requiring just a few additional

passive parts and no significant extra cost, is all that is needed.

Figure 3.

Basic waveforms for fixed

frequency PWM

Figure 4.

Basic waveforms for fixed-off-time

PWM

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