NCP1060, NCV1060, NCP1063, NCV1063
www.onsemi.com
26
Figure 47. Primary Inductance Current
Evolution in CCM
3. Lateral MOSFETs have a poorly doped
body−diode which naturally limits their ability to
sustain the avalanche. A traditional RCD clamping
network shall thus be installed to protect the
MOSFET. In some low power applications, a
simple capacitor can also be used since
V
drain,max
+
V
in
)
N
@
ǒ
V
out
)
V
f
Ǔ
)
I
peak
@
L
f
C
tot
Ǹ
(eq. 3)
where L
f
is the leakage inductance, C
tot
the total
capacitance at the drain node (which is increased by
the capacitor you will wire between drain and
source), N the N
P
:N
S
turn ratio, V
out
the output
voltage, V
f
the secondary diode forward drop and
finally, I
peak
the maximum peak current. Worse case
occurs when the SMPS is very close to regulation,
e.g. the V
out
target is almost reached and I
peak
is still
pushed to the maximum. For this design, we have
selected our maximum voltage around 650 V (at V
in
= 375 Vdc). This voltage is given by the RCD clamp
installed from the drain to the bulk voltage. We will
see how to calculate it later on.
4. Calculate the maximum operating duty−cycle for
this flyback converter operated in CCM:
d
max
+
N
@
ǒ
V
out
@
V
f
Ǔ
N
@
ǒ
V
out
@
V
f
Ǔ
)
V
in,min
(eq. 4)
+
1
1
)
V
in,min
N
@
(V
out
@
V
f
)
+
0.44
5. To obtain the primary inductance, we have the
choice between two equations:
L
+
ǒ
V
in
@
d
Ǔ
2
f
sw
@
K
@
P
in
(eq. 5)
where K
+
D
I
L
I
Lavg
and defines the amount of ripple we want in CCM (see
Figure 47).
•
Small K: deep CCM, implying a large primary
inductance, a low bandwidth and a large leakage
inductance.
•
Large K: approaching DCM where the RMS losses are
worse, but smaller inductance, leading to a better
leakage inductance.
From Equation 6, a K factor of 1 (50% ripple), gives an
inductance of:
L
+
(127
@
0.44)
2
60k
@
1
@
5
+
10.04 mH
D
I
L
+
V
in
@
d
L
@
f
sw
+
127
@
0.44
10.04m
@
60k
+
92.8 mA peak to peak
The peak current can be evaluated to be:
I
peak
+
I
avg
d
)
D
I
L
2
+
49.2 m
0.44
)
92.8 m
2
+
158 mA
On I
L
, I
Lavg
can also be calculated:
I
Lavg
+
I
peak
*
D
I
L
2
+
158m
*
92.8m
2
+
111.6 mA
6. Based on the above numbers, we can now evaluate
the conduction losses:
I
d,rms
+
d
@
ǒ
I
peak
2
*
I
peak
@
D
I
L
)
D
I
L
2
3
Ǔ
Ǹ
+
0.44
@
ǒ
0.158
2
*
0.158
@
0.0928
)
0.0928
2
3
Ǔ
Ǹ
+
57 mA
If we take the maximum R
DS(on)
for a 125
°
C
junction temperature, i.e. 34
W
, then conduction
losses worse case are:
P
cond
+
I
d,rms
2
@
R
DS(on)
+
110 mW
7. Off−time and on−time switching losses can be
estimated based on the following calculations:
P
off
+
I
peak
@
ǒ
V
bulk
)
V
clamp
Ǔ
@
t
off
2T
SW
+
0.158
@
(127
)
100
@
2)
@
10n
2
@
16.7
m
+
15.5 mW
(eq. 6)
Where, assume the V
clamp
is equal to 2 times of reflected
voltage.
