NCP1072 ONSEMI [ON Semiconductor], NCP1072 Datasheet - Page 14

• Manufacturer Part Number: NCP1072
• Description: High-Voltage Switcher for Low Power Offline SMPS
• Manufacturer: ONSEMI [ON Semiconductor]

Where:
V
V
I
thus must be selected to avoid false tripping in overshoot
conditions. Always use the minimum of the specification for
a robust design, i.e. I
I
keep V
purposely select a V
before, experience shows that a 40% decrease can be seen on
auxiliary windings from nominal operation down to standby
mode. Let’s select a nominal auxiliary winding of 13 V to
offer sufficient margin regarding 7.2 V when in standby
(R
in Equation 2 gives the limits within which R
selected:
that is to say: 0.77 kW < Rlimit < 2.2 kW.
the ratio between auxiliary and power must be: 13 / 12 =
trip
CCskip
nom
stby
Since R
limit
If we design a 65 kHz power supply delivering 12V, then
V
nom
is the current corresponding to the nominal operation. It
Figure 24. A More Detailed View of the NCP107X Offers Better Insight on How to Properly Wire an Auxiliary
is the auxiliary voltage when standby is entered
is the auxiliary voltage at nominal load
also drops voltage in standby...). Plugging the values
* V
CC
is the controller consumption during skip mode.
limit
I
trip
CC(clamp)
to above V
shall not bother the controller in standby, e.g.
13 * 8.4
6m
nom
trip
v R
well above this value. As explained
< I
v R
CC(min)
limit
OVP
limit
v
.
V
v
stby
(7.2 V maximum), we
8 * 7.2
0.36m
I
* V
CCskip
CC(min)
limit
shall be
(eq. 2)
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Winding
14
This number decreases compared to normal operation since
the part in standby does almost not switch. It is around
0.36 mA for the 65 kHz version.
V
be maintained to keep the controller away from the dynamic
self supply mode (DSS mode), which is not a problem in
itself if low standby power does not matter.
concerned, it is good to obtain V
condition in order not to re−activate the internal clamp
circuit.
1.08. The OVP latch will activate when the clamp current
exceeds 6 mA. This will occur when Vauxiliary grows−up
to:
power winding, the OVP will be seen as a lower overshoot
on the real output:
selection of a given overvoltage output level. Theoretically
predicting the auxiliary drop from nominal to standby is an
CC(min)
If a further improvement on standby efficiency is
Due to a 1.08 ratio between the auxiliary V
As one can see, tweaking the R
1. 8.4 + 0.77k x (6m + 0.8m) ≈ 13.6 V for the first
2. 8.4 + 2.2k x (6m +0.8m) ≈ 23.4 V for the second
1. 13.6 / 1.08 ≈ 12.6 V
2. 23.4 / 1.08 ≈ 21.7 V
boundary (
boundary (
is the level above which the auxiliary voltage must
R
R
limit
limit
= 0.77 kW)
= 2.2 kW)
CC
limit
around 8 V at no load
value will allow the
CC
and the