ISL6721EVAL3Z Intersil, ISL6721EVAL3Z Datasheet - Page 4

• Manufacturer Part Number: ISL6721EVAL3Z
• Description: EVAL BOARD 3 FOR ISL6721
• Manufacturer: Intersil

Specifications of ISL6721EVAL3Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The gate charge loss is about 23mW according to
Equation 11. The total MOSFET losses calculate out to be
about 1.15W.
Secondary Side Circuit Design
The output rectifiers need to have a low V
operating current to minimize the power loss, short
reverse recovery period, and an appropriate peak
breakdown voltage rating. The breakdown voltage can be
estimated as shown in Equation 12.
Equation 12 yields a minimum voltage breakdown of
voltage levels of 93.7V for the 12V rectifier, and 136.6V
for the 18V diode selection. However, to account for the
large di/dt spikes and the reflected resonant reset
voltages observed, a 200V Schottky rectifier was
selected for both outputs. A D2PAK package was selected
for its low thermal resistance characteristics.
Assuming a junction temperature of +100°C, the forward
drop on the diodes is about 0.48V according to the
datasheet of the device. The full load diode loss is about
1.2W on the 12V output, and 0.48W on the 18V output
rectifiers.
A ring of about 20MHz was observed on the diode
waveforms. An RC snubber is needed on the 12V output
across the freewheeling diode to provide sufficient
damping. Based on the body capacitance of the diode,
the snubber resistor can be selected as shown in
Equation 13.
A 20Ω resistor was selected for this application. The
snubber capacitor was chosen based on the formula
in Equation 14.
An 820pF capacitor is used in this application. A 1Ω
resistor is used to handle the power loss given by
Equation 15.
V
P
P
R
P
R
C
gate
=
sn
sw
sn
sn
=
105.6
⎛
⎜
⎝
≅
=
≅
=
=
V
------------------------------------------------------
2 π
------------------------------------ -
π f
C V
in
Q
V
•
•
max
ds
•
g
•
max
ring
max
2.58
---------- - 300
0.42
•
2
diode
1
f
•
ring
•
•
N
--------------- - V
•
1
•
R
V
N
sec
I
----------------------------- -
gs
•
sn
inRMS
•
p
C
F
1
D
•
sw
diode
=
×10
–
max
F
sw
----------------------------------------------
π 20MHz
max
=
f
3
⎞
⎟
⎠
•
•
=
•
820pF 52.8
=
(
--------------------------------------- -
1.3
7.8nC 10 300
•
30
------------------------------------------------------------- -
2 π
4
F
1
•
sw
+
2 6
25
•
•
•
•
•
•
20Ω
)
20MHz
(
------------------ -
×10
tr
2 x
+
•
2
•
1
–
tf
•
=
9
)
300
796pF
•
×10
500pF
×10
Application Note 1491
f
3
at the output
3
=
=
23.5m
=
0.69W
(EQ. 15)
(EQ. 10)
(EQ. 11)
(EQ. 13)
(EQ. 14)
15.91Ω
(EQ. 12)
W
The output filter inductor selection is pretty
straightforward. Since the volt-second product across the
inductor is constant, the output inductance can be
calculated with either the on time or the off time.
Assuming DI to be the minimum load at which the output
is critically continuous at low line, the inductance value
can be calculated as in Equation 16, where V
inductor voltage, V
winding, V
The border for continuous mode of conduction was set at
25% of full load on the 12V output and 20% on the 18V
output at low line. Using Equation 16, the inductor values
were chosen to be 39µH and 180µH respectively. These
are off-the-shelf drum core, surface-mount shielded
inductors. Care was taken to accommodate operation
under an overcurrent condition before shutdown occurs.
This is reflected in the current rating of the inductors
selected.
Two important factors to consider while selecting the
output capacitor would be the ripple current rating and
the ESR. The worst-case pk-pk ripple current can be
calculated from Equation 17 corresponding to high line
voltage or minimum duty cycle. Using the inductance
values selected, and providing a 50% margin, the
maximum RMS ripple current, ΔΙ/√3 calculates out to be
about 0.98A on the 12V output and about 0.32A on the
18V rail. Using these numbers for RMS ripple current
seen by the inductor and the maximum DCR numbers
from the datasheet, the total I
chokes can be estimated at about 0.81W.
In order to keep the voltage ripple below 50mV, the
maximum ESR of the capacitor can be calculated from
Equation 17.
Providing a margin factor of 2 to compensate for increase
in ESR over-temperature, the maximum allowable ESRs
for the 12V and the 18V capacitors are 24mW and 88mW
respectively. Since the outputs are regulated, the output
voltage ratings on the capacitors selected are 16V and
25V respectively. Low ESR OSCONs were selected with
high ripple current ratings, to account for the nature of
the continually switching loads. Ceramic capacitors were
added to share the stress on the filter capacitors, and for
high frequency decoupling. The power losses estimation
can be summarized in Table 1, and correspondingly the
full load efficiency is about 87% at low line.
ESR
L
=
≤
⎛
⎜
⎝
----------------------------------------------------- -
V
---------------------- - V
VoltageRipple
------------------------------------------
RippleCurrent
IN
N
•
P
OUT
N
S
ΔI
–
is the output voltage.
f
–
S
V
o
is the voltage across the secondary
⎞
⎟
⎠
•
D
-------------- -
F
max
sw
2
R losses across the
L
is the
(EQ. 17)
(EQ. 16)
AN1491.0