IRU3048
7
Rev. 1.7
09/12/02
www.irf.com
For Di1=25% of I1, we get L1=9.9mH
For Di2=25% of I2, we get: L2=5.7mH
Panasonic provides a range of inductors in different val-
ues and low profile for large currents.
For L1 choose ETQP6F102HFA (10.2mH, 4A)
For L2 choose ELLATV6R8M (6.8mH, 4A)
Power MOSFET Selection
The selections criteria to meet power transfer require-
ments is based on maximum drain-source voltage (VDSS),
gate-source drive voltage (VGS), maximum output cur-
rent, On-resistance RDS(ON) and thermal management.
The MOSFET must have a maximum operating voltage
(VDSS) exceeding the maximum input voltage (VIN).
The gate drive requirement is almost the same for both
MOSFETs. Caution should be taken with devices at very
low VGS to prevent undesired turn-on of the complemen-
tary MOSFET, which results a shoot-through current.
The total power dissipation for MOSFETs includes con-
duction and switching losses. For the Buck converter
the average inductor current is equal to the DC load cur-
rent. The conduction loss is defined as:
The total conduction loss is defined as:
The RDS(ON) temperature dependency should be consid-
ered for the worst case operation. This is typically given
in the MOSFET data sheet. Ensure that the conduction
losses and switching losses do not exceed the package
ratings or violate the overall thermal budget.
For this design, IRF7313 is a good choice. These de-
vices provide low on-resistance in a compact SOIC 8-
Pin package.
The MOSFETs have the following data:
The total conduction losses for channel 1 is:
The total conduction losses for channel 2 is:
The control MOSFET contributes to the majority of the
switching losses in synchronous Buck converter. The
synchronous MOSFET turns on under zero-voltage con-
dition, therefore the turn on losses for synchronous
MOSFET can be neglected. With a linear approxima-
tion, the total switching loss can be expressed as:
Figure 4 - Switching time waveforms.
From IRF7313 data sheet we obtain:
These values are taken under a certain condition test.
For more detail please refer to the IRF7313 data sheet.
By using equation (6), we can calculate the switching
losses.
PCON1 = 1.1W
PCON2 = 1.1W
IRF7313
VDSS = 30V
ID = 5.2A @ 708C
RDS(ON) = 46mV @ VGS=4.5V
q = 1.5 for 1508C (Junction Temperature)
Where:
VDS(OFF) = Drain to Source Voltage at off time
tr = Rise Time
tf = Fall Time
T = Switching Period
ILOAD = Load Current
PSW = 3 3 ILOAD ---(6)
VDS(OFF)
2
tr + tf
T
2
2
PCOND(Upper Switch) = ILOAD3RDS(ON)3D3q
PCOND(Lower Switch) = ILOAD3RDS(ON)3(1 - D)3q
q = RDS(ON) Temperature Dependency
PCON(TOTAL)=PCON(Upper Switch)q+PCON(Lower Switch)q
IRF7313
tr = 13ns
tf = 26ns
PSW1 = 187.2mW
PSW2 = 78mW
V
DS
V
GS
10%
90%
t
d
(ON)
t
d
(OFF)
t
r
t
f
