NCP1216AFORWGEVB
http://onsemi.com
4
D
max
+ 2 @
75
f
op
Ǹ
(eq. 6)
The total area of the selected wire for primary and
secondary windings is a tradeoff between the desired output
power, allowable conduction losses in the windings and
thermal considerations. The current density in the
transformer winding can generally range from 2 to
3.5 A/mm
2
. If a cooling fan is used, the current density can
be increased.
The reset winding can be made with a single wire
technique, given the low magnetization current flowing
into it.
In some cases, a small air gap can be inserted into the
magnetic circuit of the forward transformer. This solution
brings the residual flux density Br to a lower value than
without a gap. The main drawback lies in the primary
inductance decrease which forces a higher magnetizing
current.
Output Inductor Design
The value of the output inductor selected depends on the
acceptable level of ripple current. For a small ripple current,
a large inductance is needed. On the other hand, when the
current ripple is high, large output capacitors must be used
to reduce the voltage ripple. In practice, it is usual to limit the
current ripple to about 10−20% of the average current of the
inductor. The maximum current ripple DI
max
in a forward
converter occurs at 50% duty cycle. Its value can be found
via equation (7):
DI
max
+
V
sec max
4 @ f
op
@ L
2
(eq. 7)
where:
V
sec
max
is the maximum secondary voltage
L
2
is the inductance of inductor L2
In the NCP1216A demo board, where a 100 mH inductor
is used, the maximum output ripple will be DI
max
= 2.0 A.
This is rather high, but the allowable dimensions of the
inductor limit a higher inductance value selection.
The values and types of output capacitors must be chosen
with respect to the maximum allowable output voltage
excursion as well as the RMS current that will flow in them.
Current Sense Transformer Design
The current sense transformer is used to reduce power
losses traditionally found in the standard current sense
resistor configuration. If a classical current sense resistor
were used in this application, the associated power loss
would be about 3.0 W. When the current sense transformer
is used, power losses are about 50 mW. The disadvantage of
this solution lies in the current error brought by the
magnetization current of current sense transformer. This
error is additive so it should accounted for and reduced.
A toroidal core with 38 turns of the secondary winding
was used in NCP1216A demo board. The primary winding
is created by one turn of isolated wire. The peak current I
2pk
of the current sense resistor can be obtained from equation 8:
I
2pk
+ I
1pk
@
1
N
s
* I
magpk
(eq. 8)
where:
I
1pk
is the peak current of the power switch
N
s
is the count of secondary turns
I
magpk
is the peak value of the magnetization current
Figure 2 shows the current sense transformer circuit. The
peak value of the magnetization current is given by
equation 9:
I
magpk
+
V
csth max
@ d
max
L
s
@ f
op
(eq. 9)
where:
V
csth
max
is the maximum threshold voltage of the
current sense input
L
s
is the inductance of the secondary winding
I2
RSENSE
D1
Imag
I1/Ns
T2
Ns Np
I1
Q1
Figure 2. Implementation of the Current Sense
Transformer
The value of the current sense resistor R
sense
can be
calculated by using equation 10:
R
sense
+
V
csth max
I
2pk
(eq. 10)
The NCP1216A Leading Edge Blanking circuit (LEB)
allows the designer to avoid using a RC network to suppress
voltage spikes during the switch turn-on event.
