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NCP1027
20- 8.7 v Rlimit v 12- 8 , that is to say : 1.8 k W t R limit
t 4 k W .
Plugging the values in Equation 3 gives the limits within
which R limit shall be selected:
6m 1m
If we design a 65 kHz power supply delivering 12 V, then
the ratio between auxiliary and power must be: 12/20 = 0.6.
The OVP latch will activate when the clamp current
exceeds 6.0 mA. This will occur when Vauxiliary grows up
to:
1. 8.7 + 1.8 k (6 m + 1.8 m) ≈ 23 V for the first
boundary ( R limit = 1.8 k W ).
2. 8.7 + 4 k (6 m + 1.8 m) ≈ 40 V for the second
boundary ( R limit = 4.0 k W ).
Due to a 0.6 ratio between the auxiliary V CC and the
power winding, the auxiliary OVP will be seen as a lower
overshoot on the real output:
1. 23 0.6 ≈ 13.8 V
2. 40 0.6 ≈ 24 V
As one can see, tweaking the R limit value will allow the
selection of a given overvoltage output level. Theoretically
predicting the auxiliary drop from nominal to standby is an
almost impossible exercise since many parameters are
involved, including the converter time constants. Fine
tuning of R limit thus requires a few iterations and
experiments on a breadboard to check the auxiliary voltage
variations but also the output voltage excursion in fault.
Once properly adjusted, the fail-safe protection will
preclude any lethal voltage runaways in case a problem
would occur in the feedback loop.
> 30 ms
Figure 35. The burst frequency becomes so low
that it is difficult to keep an adequate level on the
auxiliary V CC .
Figure 36 describes the main signal variations when the
part operates in auto-recovery OVP.
Figure 36. If the V CC current exceeds a certain threshold, an auto-recovery protection is
activated and protects the design.
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