NCP1219PRINTGEVB
http://onsemi.com
3
Transformer
The turns ratio, N, is chosen to minimize the voltage
stresses placed on main switch, Q5, and the secondary diode,
D12. N is calculated using Equation 1,
N
+
N
S
N
P
+
k
C
@
ǒ
V
out
)
V
f
Ǔ
BV
DSS
@
k
D
*
V
OS
*
V
bulk(max)
(eq. 1)
where N
S
is the number of turns on the secondary winding,
N
P
is the number of turns on the primary winding, k
c
is the
clamp voltage ratio, V
out
is the regulated output voltage, V
f
is the forward voltage drop of the secondary rectifying
diode, BV
DSS
is the breakdown voltage of the main switch,
k
D
is the derating factor of the main switch, V
os
is the clamp
voltage overshoot, and V
bulk(max)
is the maximum DC bulk
voltage supplying the controller. Using a 650 V MOSFET
with a derating factor of 0.8 and a clamp voltage ratio, k
c
, of
1.6 yields a turns ratio of 0.303. This maintains sufficient
margin for the voltage rating of the MOSFET.
The power components for the flyback topology can be
selected for operation in either discontinuous conduction
mode (DCM) or continuous conduction mode (CCM).
Measuring the tradeoffs of the two modes at the power level
required for this design, the transformer is designed to make
a transition between DCM and CCM at low line and a load
current of 1.6 A. This ensures that the converter operates in
DCM at nominal load. The critical primary inductance,
L
P(crit)
, to cause this transition is calculated using
Equation 2.
L
P(crit)
+
h
@
V
bulk(min)
2
@
ǒ
V
out
)
V
f
N
Ǔ
2
@
f
OSC
@
V
out
@
I
out(crit)
@
ǒ
V
bulk(min)
)
V
out
)
V
f
N
Ǔ
@
ǒ
V
out
)
V
f
N
)
h
@
V
bulk(min)
Ǔ
(eq. 2)
where f
osc
is the switching frequency of the controller, and
I
out(crit)
is the load current at which the transition between
DCM and CCM occurs. By operating in the transition
between DCM and CCM, the secondary RMS current is
minimized, reducing the requirements on the transformer
and output capacitor. For the evaluation board design, with
a transition occurring at I
out
= 1.6 A, the primary inductance
is 350
m
H.
Sense Resistor
To calculate the value of the current current sense resistor,
R
sense
, the peak current of the primary winding of the
transformer must first be calculated. The energy storage
relationship is used to determine the peak primary current,
calculated using Equation 3.
I
peak
+
2
@
P
out
L
P(crit)
@
f
OSC
@
h
Ǹ
(eq. 3)
Using the specified peak output power to calculate the
peak primary current:
2
@
48 W
350
m
H
@
65 kHz
@
85%
Ǹ
+
2.23 A
The NCP1219 has a current limit comparator reference
voltage, V
ILIM
, of 1 V, typical. R
sense
, is calculated using
Equation 4.
R
sense
+
V
PWM
I
peak
(eq. 4)
This results in a value of 449 m
W
for R
sense
(R51||R52||R53||R54). A 430 m
W
resistor is chosen for
sufficient margin to deliver the peak output power.
The primary rms current, I
L(rms)
is needed in order to
calculate the power dissipation in the R
sense
. First, the
maximum duty ratio, D
max
, is calculated using Equation 5.
D
max
+
V
out
V
out
)
N
@
V
bulk(min)
(eq. 5)
The maximum duty ratio determines the change in
primary current,
Δ
I
L
, as shown in Equation 6.
D
I
L
+
V
bulk(min)
@
D
max
L
pri
@
f
OSC
(eq. 6)
Finally,
Δ
I
L
is used to calculate I
L(RMS)
as in Equation 7.
I
L(RMS)
+
D
max
@
ǒ
I
peak
2
*
I
peak
@
D
I
L
)
D
I
L
2
2
Ǔ
Ǹ
(eq. 7)
The power dissipated in the sense resistor is then calculated
using Equation 8.
P
Rsense
+
I
L(RMS)
2
@
R
sense
(eq. 8)
The power rating of the resistor is chosen to handle the
maximum power dissipation. For this design, the worst case
peak power dissipation is calculated to be 400 mW. Four
1206 surface mount resistors in parallel are chosen to
dissipate the power. Note that this is the worst case power
dissipation calculated assuming a continuous output current
of 2 A. For normal operating conditions (I
out
= 1.25 A), the
power dissipation is 208 mW.
