NCP1075(2012) Ver la hoja de datos (PDF) - ON Semiconductor
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NCP1075 Datasheet PDF : 26 Pages
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NCP1072, NCP1075
Vnom * VCC(clamp)
Vstby * VCC(min)
Itrip
v Rlimit v
ICCskip
(eq. 2)
Where:
Vnom is the auxiliary voltage at nominal load
Vstby is the auxiliary voltage when standby is entered
Itrip is the current corresponding to the nominal operation. It
thus must be selected to avoid false tripping in overshoot
conditions. Always use the minimum of the specification for
a robust design, i.e. Itrip < IOVP.
ICCskip is the controller consumption during skip mode.
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.
VCC(min) is the level above which the auxiliary voltage must
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.
If a further improvement on standby efficiency is
concerned, it is good to obtain VCC around 8 V at no load
condition in order not to re−activate the internal clamp
circuit.
Figure 24. A More Detailed View of the NCP107X Offers Better Insight on How to Properly Wire an Auxiliary
Winding
Since Rlimit shall not bother the controller in standby, e.g.
keep VCC to above VCC(min) (7.2 V maximum), we
purposely select a Vnom well above this value. As explained
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
(Rlimit also drops voltage in standby...). Plugging the values
in Equation 2 gives the limits within which Rlimit shall be
selected:
13
* 8.4
6m
v
Rlimit
v
8 * 7.2
0.36m
that is to say: 0.77 kW < Rlimit < 2.2 kW.
If we design a 65 kHz power supply delivering 12V, then
the ratio between auxiliary and power must be: 13 / 12 =
1.08. The OVP latch will activate when the clamp current
exceeds 6 mA. This will occur when Vauxiliary grows−up
to:
1. 8.4 + 0.77k x (6m + 0.8m) ≈ 13.6 V for the first
boundary (Rlimit = 0.77 kW)
2. 8.4 + 2.2k x (6m +0.8m) ≈ 23.4 V for the second
boundary (Rlimit = 2.2 kW)
Due to a 1.08 ratio between the auxiliary VCC and the
power winding, the OVP will be seen as a lower overshoot
on the real output:
1. 13.6 / 1.08 ≈ 12.6 V
2. 23.4 / 1.08 ≈ 21.7 V
As one can see, tweaking the Rlimit value will allow the
selection of a given overvoltage output level. Theoretically
predicting the auxiliary drop from nominal to standby is an
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