Introduction to FOT control

AN3112

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Doc ID 16820 Rev 3

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Introduction to FOT control

In the output power range already mentioned, 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 offer a solution to the problem. 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 just the on-time 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.

It is worth noting 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