iru3055 International Rectifier Corp., iru3055 Datasheet - Page 12

• Manufacturer Part Number: iru3055
• Description: 5-bit Programmable 3-phase Synchronous Buck Controller Ic -
• Manufacturer: International Rectifier Corp.

IRU3055
Power MOSFET Selection
The IRU3055 is a controller for 3-phase synchronous
buck converter. For each phase, the average inductor
current will be one third of the total output current in an
ideal case, which will greatly alleviate the thermal man-
agement for power switch. In general, the MOSFET se-
lection criteria depends on the maximum drain-source
voltage, RMS current and ON resistance (R
both high side and low side MOSFET, a drain-source
voltage rating higher than maximum input voltage is nec-
essary. In the demo-board, 20V rating should be satis-
fied. The gate drive requirement for each MOSFET is
almost the same. If logic-level MOSFET is used, some
caution should be taken with devices at very low VGS
to prevent undesired turn-on of the complementary
MOSFET, which results a shoot-through circuit.
If output inductor current ripple is neglected, the RMS
current of high side switch is given by:
The RMS current of low side switch is given as:
In the demo board, RMS current of high side switch is:
RMS current of low side switch is:
For R
sible in order to get highest efficiency. The MOSFET
from International rectifier IRF3704S with a R
20V drain source voltage rating and 77A I
for high side MOSFET.
For a high input and low output case, the low side switch
conducts most of output current and handles most of
the thermal management. Two MOSFETs can be put in
parallel to further reduce the effect R
tion losses. In the demo-board, MOSFET from Interna-
tional Rectifier IRF3711S with R
110A I
dissipation includes conduction loss and switching loss.
The conduction loss for high side switch in each phase
can be estimated by the following equation:
The low side switch power dissipation is:
12
P
P
CON(HI)
CON(LO)
I
I
I
I
I
I
RMS(HI)
RMS(HI)
RMS(LO)
RMS(LO)
RMS(HI)
RMS(LO)
DS(ON)
D
is selected as synchronous MOSFET. The power
= R
=R
=
=
=
=
=
=
of MOSFET, it should be as small as pos-
DS(ON)
DS(ON)
(1.5/12)360/3 = 7.1A
D3I
(V
(1 - D)3I
(1 - V
(1 - 1.5/12)360/3 = 18.7A
OUT
3q3(I
3q3(I
OUT
/V
OUT
/m
IN
/V
)3I
OUT
OUT
OUT
IN
/m
)3I
OUT
/m)3(I
/m)3(I
/m
DS(ON)
OUT
/m
OUT
OUT
=6mV, 20V V
DS(ON)
/m)3(1-V
/m)3(V
---(9)
D
and conduc-
DS(ON)
is selected
DS(ON)
OUT
OUT
=9mV,
/V
DS
). For
/V
www.irf.com
IN
and
IN
)
)
Where q is the temperature coefficient of ON resistor of
MOSFET R
sheet (typically between 1 and 2).
In this example, the MOSFET IRF3704S is chosen to
be the high side switch with:
The conduction loss for high side MOSFET is given as:
Low side switch is configured with one IRF3711 with 6mV
R
The switching loss for MOSFET is more difficult to cal-
culate due to effect of the parasitic components, etc.
The switching loss can be estimated by the following
equation:
In this example, for low side MOSFET, the body diode is
turned on before MOSFET is on. Therefore, the switch-
ing losses for low side MOSFET is almost zero due to
zero voltage switching. For high side MOSFET, from data
sheet, we have:
The total power dissipation is:
DS(ON)
P
CON(HI)
R
q = 1.5 @ 1508C
P
P
P
Where:
V
is turned off.
tr is the rising time.
tf is the fall time.
F
I
turned off. It can be estimated by:
I
tr = 50ns
tf = 50ns
Select F
V
I
P
P
P
P
P
SW
SW
SW
S
DS(ON)
CON(LO)
CON(LO)
SW
DS(OFF)
DS(OFF)
SW(HI)
SW(HI)
D(HI)
D(HI)
D(LO)
. The conduction loss is calculated as:
is the switching frequency.
= I
= Peak Inductor Current = 24A
is the current in MOSFET when MOSFET is
= V
=9mV31.53(60/3)3(60/3)31.5/12=0.68W
= P
= 0.68W+2.16W = 2.84W
LOAD
= 12V3(50ns+50ns)3150KHz324A/2
= 2.1W
= 9mV
= 12V
DS(ON)
= 6mV31.53(60/3)3(60/3)3(1-1.5/12)
= 3.15W
DS(OFF)
is the Drain to Source voltage when switch
P
S
CON(HI)
/m + half of the ripple current
= 150KHz
CON(LO)
3(tr+tf)3F
and can be found in MOSFET data
+P
= 3.15W
SW(HI)
S
3I
SW
/2
08/13/02
Rev. 1.4