NCP1339
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26
Over Power Compensation (OPP)
The power delivered by a QR flyback stage is an
increasing function of the bulk voltage, V
bulk
. It is however
desirable to clamp the power delivery to limit the stress on
the power components that can otherwise be excessive
during transient or fault conditions.
An integrated overpower circuit provides a relatively
constant output power across bulk voltage, V
bulk
.
Practically, the maximum peak current is made a decreasing
function of the bulk voltage. The direct measure of the V
bulk
high−voltage rail would cause losses in the sensing network
and hence alter the standby efficiency.
Instead, the auxiliary winding voltage (V
AUX
) is used.
During power−switch on−time, V
AUX
provides a negative
voltage that is a V
bulk
portion (input voltage scaled down by
the primary to auxiliary winding turns ratio) as shown in
Figure 55. The negative voltage applied to the pin is referred
as V
OPP
. The maximum internal current setpoint (V
CS(OPP)
)
is the sum of V
OPP
and peak current sense threshold, V
ILIM1
.
The current setpoint is calculated using Equation 7.
V
CS(OPP)
+ V
ILIM1
) V
OPP
(eq. 7)
That is that:
V
CS(OPP)
+ V
ILIM1
*
ǒ
N
AUX
N
P
@ V
BULK
Ǔ
(eq. 8)
AUX
BULK
P
N
V
N
⎛
−
⎢
⎝
Figure 55. Auxiliary Winding Voltage Waveform
⎛
⎢
⎝
V
AUX
(V)
For example, (V
OPP
= −0.25 V) results in a current
setpoint of 0.55 V. In general, V
OPP
is selected in the range
of −200 mV at the highest line level. Refer to application
notes for more details.
V
CS(OPP)
+ 0.8−0.25 + 0.55 + 68.75% @ 0.8 + 68.75% @ V
ILIM1
The OPP pin is not designed to operate below –250 mV
which corresponds to a 31.25% decrease of the maximum
current limit. If a lower voltage happens to be applied, the
internal ESD diode that clamps OPP pin negative voltages
may turn on and lead to carriers injection within the die. To
avoid possible resulting disturbance, care must be taken to
limit the current sourced by the diode below 2 mA. If the
circuitry of Figure 56 is used, a conservative condition is:
V
AUX,max
R
OPP1
w −2 mA å R
OPP1
w*
V
AUX,max
2m
(eq. 9)
Finally, please note that another comparator internally
fixes the maximum peak current set point to V
ILIM1
. Hence,
even if the OPP pin is adversely biased above 0 V, the current
setpoint remains clamped to 0.8 V typically.
For optimum performance over temperature, we
recommend keeping the low−side OPP resistor below 3 kW.
Current Setpoint
As explained in this operating description, the current
setpoint is affected by several functions. Figure 56
summarizes these interactions. As shown by this figure, the
current setpoint is FB/4. However, this value is limited by
the following functions:
• This level is clamped during the soft−start phase. The
setpoint is actually limited by a clamp level ramping
from 0 to 0.8 V within 4 ms.
• It is also limited by the OPP function: during the
on−time, a negative voltage is applied to the OPP pin.
This voltage is summed with a 0.8−V voltage reference
to form the actual maximum setpoint (see OPP section).
