iru3055 International Rectifier Corp., iru3055 Datasheet - Page 13

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

Heat Sink Selection
The criteria of selecting heat sink is based on the maxi-
mum allowable junction temperature of the MOSFETs.
That is:
The maximum heat-sink-to-air thermal resistance is cal-
culated as:
In this example, the MOSFET is mounted in the PCB
board with more than 1" square PCB board. Therefore,
the junction temperature for MOSFET can be calculated
as:
Where R
with MOSFET on 1" square PCB board and it is avail-
able from MOSFET data sheet.
For MOSFET IRF3704S with D2 package, R
W. Assume ambient temperature is T
side MOSFET, the junction temperature is given as:
For low side MOSFET, IRF3711s, the maximum junc-
tion temperature can be calculated as:
This is the worst case. For conservative consideration,
two IRF3711 can be put in parallel.
Rev. 1.4
08/13/02
T
Where:
T
P
R
R
R
T
of MOSFET, for example 1508C.
R
T
358C + 2.84W3408C/W = 1498C
358C + 3.15W3408C/W = 1618C
A
A
J(MAX)
J
D
uJC
uCS
uSA
uSA
= T
+ P
= The Ambient Temperature
= Power Dissipation of each MOSFET
< (T
= The Thermal Resistance from junction to case
uJA
= the thermal resistance from case to heat sink
= the heat-sink-to-air thermal resistance
A
D
+ P
= maximum allowable junction temperature
3(R
is the junction-to-ambient thermal resistance
J(MAX)
D
3R
uJC
-T
+R
A
uJA
)/P
uCS
D
+R
-R
uJC
uSA
+R
) < T
uCS
J(MAX)
A
=358C. For high
uJA
= 408C/
www.irf.com
Input Filter Selection
The selection criteria of input capacitor are voltage rat-
ing and the RMS current rating. For conservative consid-
eration, the capacitor voltage rating should be 1.5 times
higher than the maximum input voltage. The RMS cur-
rent rating of the input capacitor for multi-phase con-
verter can be estimated from the above Figure 12.
First, determine the duty cycle of the converter (V
The ratio of input RMS current over output current can
be obtained. Then the total input RMS current can be
calculated. From this figure, it is obvious that a multi-
phase converter can have a much smaller input RMS
current, which results in a lower amount of input capaci-
tors that are required.
For high current applications, multiple bulk input capaci-
tors in parallel may be necessary. Some electrolytic
capacitors, such as Panasonic HFQ series, Sanyo MV-
WX or equivalent may be put in parallel to provide a large
current. In addition, ceramic bypass capacitors for high
frequency de-coupling are recommended. Furthermore,
some small ceramic capacitors should be put very close
to the drain of the high side MOSFET and source of the
low side switch to suppress the voltage spike caused by
parasitic circuit parameters.
For high current applications, a 1mH input inductor is
recommended to slow down the input current transient.
I
RMS(IN)
I
OUT
Figure 12 - Normalized input RMS current vs.
0.5
0.4
0.3
0.2
0.1
0
0
0.1
duty cycle.
Three-
Phase
0.2
D
0.3
Single-
Phase
IRU3055
Two-
Phase
0.4
O
0.5
/V
13
IN
).