NCP1379
http://onsemi.com
20
R
zcd
) R
opu
R
opl
+ −
N
p,aux
V
IN
* V
OPP
V
OPP
(eq. 2)
Where:
N
p,aux
is the auxiliary to primary turn ration: N
p,aux
= N
aux
/ N
p
V
IN
is the DC input voltage
V
OPP
is the negative OPP voltage
By selecting a value for R
opl
, we can easily deduce R
opu
using Equation 2. While selecting the value for R
opl
, we
must be careful not choosing a too low value for this resistor
in order to have enough voltage for zero−crossing detection
during the off−time. We recommend having at least 8 V on
ZCD pin, the maximum voltage being 10 V.
During the off−time, ZCD pin voltage can be expressed as
follows:
V
ZCD
+
R
opl
R
ZCD
) Ropl
ǒ
V
auz
* V
d
Ǔ
(eq. 3)
We can thus deduce the relationship between R
opl
and
R
zcd
:
R
ZCD
R
opl
+
V
aux
* V
d
* V
ZCD
V
ZCD
(eq. 4)
Design example:
• V
aux
= 18 V
• V
d
= 0.6 V
• N
p,aux
= 0.18
If we want at least 8 V on ZCD pin, we have:
R
ZCD
R
opl
+
V
aux
* V
d
* V
ZCD
V
ZCD
+
18 * 0.6 * 8
8
+ 1.2
(eq. 5)
We can choose: R
zcd
= 1 kW and R
opl
= 1 kW.
For the over power compensation, we need to decrease the
peak current by 37.5% at high line (370 Vdc). The
corresponding OPP voltage is:
V
OPP
+ 0.375 V
ILIM
+ −300 mV
(eq. 6)
Using Equation 2, we have:
R
ZCD
) R
opu
R
opl
+ −
N
p,aux
V
IN
* V
OPP
V
OPP
(eq. 7)
+
−0.18 370 *
(
−0.3
)
(
−0.3
)
+ 221
Thus,
R
opu
+ 221
Ropl
* R
ZCD
+ 221 1k * 1k + 220 kW
(eq. 8)
OVERVOLTAGE PROTECTION / BROWN−OUT
NCP1379 combine brown−out and overvoltage detection
on the pin Fault.
S
R
Q
Q
VCC
S
R
Q
Q
grand
reset
DRV
OVP/BO
HV−BULK
+
−
IBO
noi s e de l a y
VBO
BO reset
+
−
Vclamp
VOVP
noi s e de l a y
Rc l a mp
CS c omp
Rbou
Rbol
Dz
VDD
La tc h
Clamp
7
Figure 36. Brown−out and Overvoltage Protection
In order to protect the power supply against low input
voltage condition, the pin 7 permanently monitors a fraction
of the bulk voltage through a voltage divider. When this
image of bulk voltage is below the V
BO
threshold, the
controller stops switching. When the bulk voltage comes
back within safe limits, the circuit restarts pulsing. The
hysteresis for the brown−out function is implemented with
a high side current source sinking 10 mA when the
brown−out comparator is high (V
bulk
> V
bulk(on)
)
In order to avoid having a too high voltage on pin 7 if the
bulk voltage is high, an internal clamp limits the voltage.
In case of over voltage, the zener diode will start to
conduct and inject current inside the internal clamp resistor
R
clamp
thus causing pin 7 voltage to increase. When this
voltage reaches V
OVP
, the controller latches−off and stays
latched. The controller will be reset if V
CC
falls bellow
V
CC(reset)
or if a brown−out occurs (Figure 37).
