Datasheet

t
LK_RESET
t
SMPL
VS ring(p-p)
t
DM
UDG-12202
UCC28710, UCC28711
UCC28712, UCC28713
UCC28714, UCC28715
SLUSB86 –NOVEMBER 2012 www.ti.com
In primary-side control, the output voltage is sensed on the auxiliary winding during the transfer of transformer
energy to the secondary. As shown in Figure 14 it is clear there is a down slope representing a decreasing total
rectifier V
F
and resistance voltage drop (I
S
R
S
) as the secondary current decreases to zero. To achieve an
accurate representation of the secondary output voltage on the auxiliary winding, the discriminator reliably blocks
the leakage inductance reset and ringing, continuously samples the auxiliary voltage during the down slope after
the ringing is diminished, and captures the error signal at the time the secondary winding reaches zero current.
The internal reference on VS is 4.05 V. Temperature compensation on the VS reference voltage of -0.8-mV/°C
offsets the change in the output rectifier forward voltage with temperature. The resistor divider is selected as
outlined in the VS pin description.
Figure 14. Auxiliary Winding Voltage
The UCC28710 family includes a VS signal sampler that signals discrimination methods to ensure an accurate
sample of the output voltage from the auxiliary winding. There are however some details of the auxiliary winding
signal to ensure reliable operation, specifically the reset time of the leakage inductance and the duration of any
subsequent leakage inductance ring. Refer to Figure 15 below for a detailed illustration of waveform criteria to
ensure a reliable sample on the VS pin. The first detail to examine is the duration of the leakage inductance reset
pedestal, t
LK_RESET
in Figure 15. Because this can mimic the waveform of the secondary current decay, followed
by a sharp downslope, it is important to keep the leakage reset time less than 600 ns for I
PRI
minimum, and less
than 2.2 µs for I
PRI
maximum. The second detail is the amplitude of ringing on the V
AUX
waveform following
t
LK_RESET
. The peak-to-peak voltage at the VS pin should be less than approximately 100 mV
p-p
at least 200 ns
before the end of the demagnetization time, t
DM
. If there is a concern with excessive ringing, it usually occurs
during light or no-load conditions, when t
DM
is at the minimum. The tolerable ripple on VS is scaled up to the
auxiliary winding voltage by R
S1
and R
S2
, and is equal to 100 mV x (R
S1
+ R
S2
) / R
S2
.
Figure 15. Auxiliary Waveform Details
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Product Folder Links: UCC28710, UCC28711 UCC28712, UCC28713 UCC28714, UCC28715