Introduction to Digital Power Conversion
XMC4000/1000 Family
PWM Generation
Application Guide 28 V1.0, 2015-01
4.7 Fixed ON-Time with Frequency Limit Control
In power switch control with FOT PWM, it is mandatory to have pulse rate limiting add-ons in the loop,
ensuring a minimum of off-time to be fed back by the conversion process in each FOT start request.
Another extreme is maximum off-time.
Both extremes will require ‘f
max–min
timer’ add-ons in the loop.
Figure 18 FOT Control with Frequency Limits Supervision
The FOT Timer (Slice1)
Assume that this single-shot FOT timer works in a CRM or DCM mode PFC controller. In each switch
cycle the timer waits for a start request in the control loop, after a certain off-time. Then it will switch
on the inductor current for a fixed time, to rise from zero again, on any of the following conditions:
ZCD AND f
max
-period is due // CRM, FOT density OK
An early ZCD event AND before expired f
max
-period // DCM, FOT pulse delayed
f
min
-period expires AND there was no ZCD event in the meantime // DCM, FOT at ‘Time-Out’
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