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NCP1027
Improving the Precision in Auto-Recovery OVP
Given the OVP variations the internal trip current
dispersion incur, it is sometimes more interesting to
explore a different solution, improving the situation to the
cost of a minimal amount of surrounding elements.
Figure 37 shows that adding a simple Zener diode on top
of the limiting resistor, offers a better precision since what
matters now is the internal 8.7 V V CC breakdown, plus the
Zener voltage. A resistor in series with the Zener diodes
keeps the maximum current in the V CC pin below the
maximum rating of 15 mA just before tripping the OVP.
V CC
R limit
D1
Ground
+
L aux
Figure 37. A simple Zener diode added in
parallel with R limit , allows for a better
precision OVP.
Over Power Compensation
Over Power Compensation or Protection (OPP)
represents a way to limit the effects of the propagation
delay when the converter is supplied from its highest input
voltage. The propagation delay naturally extends the
power capability of any current-limited converter.
Figure 38 explains why. The main parameter is the on
slope, that is to say, the pace at which the inductor current
grows-up when the power switch closes. For a flyback
controller, the slope is given by:
Figure 38. Internal logic blocks take a certain amount
of time before shutting off the driving pulses in
presence of an overcurrent event.
At low line, S on is relatively low and does not bother the
final peak value. The situation differs at high line and
induces a higher peak current. Therefore, the power supply
output power capability increases with the input voltage.
Let us a take a look at a simple example. Suppose the peak
current is 700 mA:
L p = 1.0 mH
V in lowline = 100 Vdc
V in highline = 350 Vdc
Son + in
V
Lp
(eq. 4)
I peak,max = 700 mA
t prop = 100 ns
Pout + 1 I2peak, final FSWLp h
where L p is the transformer magnetizing/primary
inductance and V in , the input voltage.
As the internal logic takes some time to react, the switch
gate shutdown does not immediately occur when the
maximum power limit is detected (just before activating
the overload protection circuit). Clearly speaking, it can
take up to 100 ns for the NCP1027 current sense
comparator to propagate through the various logical gates
before reaching the power switch and finally shutting it off.
This is the well-known propagation delay noted t prop .
Unfortunately, during this time, the current keeps growing
as Figure 38 depicts. The peak current will therefore be
troubled by this propagation delay. The formula to obtain
the final value is simply:
(eq. 6)
2
Where: F sw is the switching frequency and h the efficiency.
Usually h is bigger in high line conditions than in low line
conditions. This formula is valid for a Discontinuous
Conduction Mode flyback.
From Equation 5, we can calculate the final peak current
in both conditions:
I peak,final = (100/1m) x 100n + 700m = 710 mA at low line.
I peak,final = (350/1m) x 100n + 700m = 735 mA at high line.
From Equation 6, we can have an idea of the maximum
output power capability again, in both conditions with
respective low and high line efficiency numbers of 78%
and 82% for instance:
Ipeak, final + in t prop ) Ipeak, max
0.735
V
Lp
(eq. 5)
P out,lowline = 0.5
P out,highline = 0.5
0.71 2
2
1m
1m
65k
65k
0.78 = 12.8 W
0.82 = 14.4 W
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