NCP1606
http://onsemi.com
17
For the above example, this leads to:
R
OUT2
+
2.5 V
400 V * 2.5 V
@ 1.9 MW + 12.0 kW.
STATIC OVERVOLTAGE PROTECTION
If the OVP condition lasts for a long time, it may happen
that the error amplifier output reaches its minimum level
(i.e. Control = V
EAL
). It would then not be able to sink any
current and maintain the OVP fault. Therefore, to avoid any
discontinuity in the OVP disabling effect, the circuit
incorporates a comparator which detects when the lower
level of the error amplifier is reached. This event, called
“static OVP”, disables the output drives. Once the OVP
event is over, and the output voltage has dropped to normal,
then Control
rises above the lower limit and the driver is
re−enabled (Figure 35).
Figure 35. OVP Timing Diagram
Vout
Vcontrol
Icontrol
Dynamic OVP
Static OVP
IovpH
IovpL
Vout(nom)
Drive
V
EAH
V
EAL
NCP1606 Undervoltage Protection (UVP)
When the PFC stage is plugged in, the output voltage is
forced to roughly equate the peak line voltage. The
NCP1606 detects an undervoltage fault when this output
voltage is unusually low, such that the feedback voltage is
below V
UVP
(300 mV typ). In an UVP fault, the drive
output and error amplifier (EA) are disabled. The latter is
done so that the EA does not source a current which would
increase the FB voltage and prevent the UVP event from
being accurately detected. The UVP feature helps to
protect the application if something is wrong with the
power path to the bulk capacitor (i.e. the capacitor cannot
charge up) or if the controller cannot sense the bulk voltage
(i.e. the feedback loop is open).
Furthermore, the NCP1606 incorporates a novel startup
sequence which ensures that undervoltage conditions are
always detected at startup. It accomplishes this by waiting
approximately 180 ms after V
CC
reaches V
CC(on)
before
enabling the error amplifier (Figure 36). During this wait
time, it looks to see if the feedback (FB) voltage is greater
than the UVP threshold. If not, then the controller enters a
UVP fault and leaves the error amplifier disabled.
However, if the FB pin voltage increases and exceeds the
UVP level, then the controller will start the application up
normally.
Figure 36. The NCP1606’s Startup Sequence with
and without a UVP Fault
FB
Control
2.5 V
UVP
UVP Wait
UVP Wait
V
UVP
V
EAH
V
EAL
V
OUT(nom)
V
OUT
V
CC(off)
V
CC(on)
V
CC
UVP Fault is “Removed”
The voltage on the output which exits a UVP fault is
given by:
V
OUT
(UVP)
+
R
OUT1
) R
OUT2
R
OUT2
@ 300 mV
(eq. 13)
If R
OUT1
= 1.9 MW and R
OUT2
= 12.0 kW, then the V
OUT
UVP threshold is 48 V. This corresponds to an input voltage
of approximately 34 Vac.
Overcurrent Protection (OCP)
A dedicated pin on the NCP1606 senses the peak current
and limits the driver on time if this current exceeds
V
CS(limit)
. This level is 1.7 V (typ) on the NCP1606A and
0.5 V (typ) on the NCP1606B. Therefore, the maximum
peak current can be adjusted by changing R
SENSE
according
to:
I
peak
+
V
CS(limit)
R
SENSE
(eq. 14)
An internal LEB filter (Figure 37) reduces the likelihood
of switching noise falsely triggering the OCP limit. This
filter blanks out the first 250 ns (typical) of the current
sense signal. If additional filtering is necessary, a small RC
filter can be added between R
SENSE
and the CS pin.
