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L6565
the L6565 puts a limit on the minimum OFF-time of the switch. This is done by blanking the triggering block of
the ZCD circuit as mentioned before. The duration of the blanking time (3.5µs min.) is a function of the error
amplifier output VCOMP, as shown in the diagram of figure 6.
If the load current and the input voltage are such that the switch OFF-time falls below the minimum blanking
time of 3.5µs, the system will enter the "Frequency Foldback" mode, a sort of "ringing cycle skipping" illustrated
schematically in figure 14.
Figure 14. Frequency foldback: ringing cycle skipping as the load is progressively reduced
In this mode, uneven switching cycles may be observed under some line/load conditions, due to the fact that
the OFF-time of the MOSFET is allowed to change with discrete steps (2·Tv), while the OFF-time needed for
cycle-by-cycle energy balance may fall in between. Thus one or more longer switching cycles will be compen-
sated by one or more shorter ones and vice versa. However, this mechanism is absolutely normal and there is
no appreciable effect on the performance of the converter or on its output voltage.
t
V
DS
TFW
T
BLANKmin
TV
t
V
DS
T
BLANK
Pin = Pin'
(limit condition)
Pin = Pin'' < Pin' Pin = Pin''' < Pin''
t
V
DS
T
BLANK
Figure 15. Frequency Foldback: qualitative
frequency dependence on power
throughput
Further load reductions involve lower values for
VCOMP, which increases the blanking time. There-
fore, more and more ringing cycles will be skipped.
When the load is low enough, so many ringing cycles
need to be skipped that their amplitude becomes
very small and they can no longer trigger the ZCD cir-
cuit. In that case the internal starter of the IC will be
activated, resulting in burst-mode operation: a series
of few switching cycles spaced out by long periods
where the MOSFET is in OFF state.
Voltage Feedforward block (see fig. 17b):
The power that QR flyback converters with a fixed
overcurrent setpoint (like fixed-frequency systems)
are able to deliver changes with the input voltage
considerably. With wide-range mains, at maximum
line it can be more than twice the value at minimum
line, as shown by the upper curve in the diagram of
figure 16. The L6565 has the Line Feedforward func-
tion available to solve this issue.
Figure 16. Typical power capability change vs.
input voltage in ZVS QR flyback
converters
f
sw
Pin
BURST MODE
without frequency foldback
with frequency foldback
Vin fixed
Vin
Vinmin
P
inlim
@ V
in
Pinlim @ Vinmin
11.522.533.54
0.5
1
1.5
2
2.5
system optimally
compensated
system not
compensated
