Application information HVLED805
16/29 Doc ID 18077 Rev 1
After the first few cycles initiated by the starter, as the voltage developed across the auxiliary
winding becomes large enough to arm the DMG circuit, MOSFET’s turn-on will start to be
locked to transformer demagnetization, hence setting up QR operation.
The starter is activated also when the IC is in CC regulation and the output voltage is not
high enough to allow the DMG triggering.
If the demagnetization completes – hence a negative-going edge appears on the DMG pin –
after a time exceeding time T
BLANK
from the previous turn-on, the MOSFET will be turned
on again, with some delay to ensure minimum voltage at turn-on. If, instead, the negative-
going edge appears before T
BLANK
has elapsed, it will be ignored and only the first negative-
going edge after T
BLANK
will turn-on the MOSFET. In this way one or more drain ringing
cycles will be skipped (“valley-skipping mode”, Figure 13) and the switching frequency will
be prevented from exceeding 1/T
BLANK
.
Note: That when the system operates in valley skipping-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 of one ringing cycle, while the OFF-time needed for
cycle-by-cycle energy balance may fall in between. Thus one or more longer switching
cycles will be compensated by one or more shorter cycles 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.
5.4 Constant voltage operation
The IC is specifically designed to work in primary regulation and the output voltage is
sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier
diode.
Figure 14 shows the internal schematic of the constant voltage mode and the external
connections.
Figure 13. Drain ringing cycle skipping as the load is progressively reduced
Pin = Pin'
(limit condition)
P
in
= P
in''
< P
in'
P
in
= P
in'''
< P
in''
t
DS
T
FW
T
osc
T
V
T
ON
t
V
DS
T
osc
t
V
DS
T
osc
