NCP1608BOOSTGEVB
http://onsemi.com
6
The value of C
bulk
is selected as 68
m
F to reduce
V
ripple(peak-peak)
to less than 15 V. This results in a peak
output voltage of 406.25 V, which is less than the peak output
OVP detection voltage (421 V).
The NCP1608 includes undervoltage protection (UVP).
During startup, C
bulk
charges to the peak of the ac line
voltage. If C
bulk
does not charge to a minimum voltage, the
NCP1608 detects an UVP fault. The UVP detection voltage
is calculated using Equation 12:
V
out(UVP)
+
V
UVP
@
ǒ
R
out1
@
R
out2
)
R
FB
R
out2
@
R
FB
)
1
Ǔ
(eq. 12)
V
out(UVP)
+
0.31
@
ǒ
4 M
@
25.5 k
)
4.6 M
25.5 k
@
4.6 M
)
1
Ǔ
+
49 V
The UVP feature protects against open loop conditions in
the feedback loop. If the FB pin is inadvertently floating
(perhaps due to a bad solder joint), the coupling within the
system may cause V
FB
to be within the regulation range (i.e.
V
UVP
< V
FB
< V
REF
). The controller responds by delivering
maximum power. The output voltage increases and over
stresses the components. The NCP1608 includes a feature to
protect the system if FB is floating. The internal pull-down
resistor (R
FB
) ensures that V
FB
is below the UVP threshold
if the FB pin is floating.
If the FB pin floats during operation, V
FB
begins
decreasing from V
REF
. The rate of decrease depends on R
FB
and the FB pin parasitic capacitance. As V
FB
decreases,
V
Control
increases, which causes the on time to increase until
V
FB
< V
UVP
. When V
FB
< V
UVP
, the UVP fault is detected
and the controller is disabled. The sequence is depicted in
Figure 6.
Figure 6. UVP Operation if Loop is Opened During
Operation
Loop is Opened
UVP Fault
Ct
(offset)
V
EAH
V
UVP
V
REF
V
FB
V
Control
V
out
V
out
V
CC
V
CC(off)
V
CC(on)
DESIGN STEP 6: Size the Power Components
The power components are sized such that there is
sufficient margin to sustain the currents and voltages applied
to them. At minimum line input voltage and maximum
output power the inductor peak current is at the maximum,
which causes the greatest stress to the power components.
The components are referenced in Figure 3.
1. The inductor peak current (I
L(peak)
) is calculated
using Equation 13:
I
L(peak)
+
2
Ǹ @
2
@
P
out
h
@
Vac
(eq. 13)
I
L(peak)
+
2
Ǹ @
2
@
100
0.92
@
85
+
3.62 A
The inductor rms current (I
L(RMS)
) is calculated using
Equation 14:
I
L(RMS)
+
2
@
P
out
3
Ǹ @
Vac
@
h
(eq. 14)
I
L(RMS)
+
2
@
100
3
Ǹ @
85
@
0.92
+
1.48 A
2. The output diode (D) rms current (I
D(RMS)
) is
calculated using Equation 15:
I
D(RMS)
+
4
3
@
2
Ǹ @
2
p
Ǹ
@
P
out
h
@
Vac
@
V
out
Ǹ
(eq. 15)
I
D(RMS)
+
4
3
@
2
Ǹ @
2
p
Ǹ
@
100
0.92
@
85
@
400
Ǹ
+
0.75 A
The diode maximum voltage is equal to V
OVP
(421 V)
plus the overshoot caused by parasitic contributions. For this
evaluation board, the maximum voltage is 450 V. A 600 V
diode provides a 25% derating factor. The MUR460
(4 A/600 V) diode is selected for this design.
3. The MOSFET (M) rms current (I
M(RMS)
) is
calculated using Equation 16:
I
M(RMS)
+
2
3
Ǹ @
ǒ
P
out
h
@
Vac
Ǔ
@
1
*
ǒ
2
Ǹ @
8
@
Vac
3
@
p
@
V
out
Ǔ
Ǹ
(eq. 16)
I
M(RMS)
+
2
3
Ǹ @
ǒ
100
0.92
@
85
Ǔ
@
1
−
ǒ
2
Ǹ @
8
@
85
3
@
p
@
400
Ǔ
Ǹ
+
1.27 A
The MOSFET maximum voltage is equal to V
OVP
(421 V) plus the overshoot caused by parasitic
contributions. For this evaluation board, the maximum
voltage is 450 V. A 560 V MOSFET provides a 20% derating
factor. The SPP12N50C3 (11.6 A/560 V) MOSFET is
selected for this design.
4. The current sense resistor (R
sense
) limits the
maximum inductor peak current of the MOSFET
and is calculated using Equation 17:
R
sense
+
V
ILIM
I
L(peak)
(eq. 17)
Where V
ILIM
is specified in the NCP1608 datasheet.
