UCC28910_15 Datasheet, PDF (23/53 Pages) Texas Instruments – High-Voltage Flyback Switcher with Primary-Side Regulation and Output Current Control

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UCC28910
UCC28911
SLUS769C – JULY 2013 – REVISED MARCH 2015
Feature Description (continued)
9.3.5 Valley Switching
The UCC28910 and the UCC28911 utilize valley switching to reduce switching losses in the MOSFET and
minimize the current spike at the FET turn on. The UCC28910 operates in valley switching in almost all load
conditions until the V_DS ringing is diminished. By switching at the lowest V_DS voltage the MOSFET turn on dV / dt
is minimized which is a benefit to reduce EMI.
Referring to Figure 28, devices will operate in a valley switching mode in most load conditions to switch-on at the
lowest available V_DS voltage. According to the load it is established a minimum switching period. The MOSFET is
switched-on at the first valley that occurs after this minimum period is elapsed. With this control scheme the
device can be turned-on at the first valley that occurs after transformer demagnetization or it can skip some
valleys before turn-on operating in this case in the so called valley skipping mode. Valley switching is maintained
during constant current regulation to provide improved efficiency and EMI benefits in constant current operation.
If for some reason no valley is detected at the end of the t_ZTO time the MOSFET turns-on. In order to guarantee
discontinuous mode operation at least the first valley needs to be detected or the Mosfet is not turned-on (see
Figure 28).
VDS
Valley Skipping
Valley Skipping
t
MINP
Minimum allowed period
Minimum allowed period
tZTO
tZTO
tZTO
t
ZCD
~ 150 ns
Switch-On
t
Figure 28. Valley Skipping
In very light-load or no-load condition the V_DS ringing amplitude is very low and not easy to detect, moreover with
very low ringing amplitude there would be no benefit in valley switching so in this condition the valley switching is
disabled (see Figure 29). The device switch on the MOSFET as soon the time t_ZTO is elapsed. The t_ZTO timer
starts as soon as the minimum switching period is elapsed.
VDS
No Valley Switching in Light Load
MINP
Minimum Allowed Period
Minimum Allowed Period
tZTO
tZTO
t
Figure 29. Valley Switching Disable at Light Load
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