IR3622MTRPBF Datasheet, PDF (20/31 Pages) International Rectifier – HIGH FREQUENCY 2-PHASE, SINGLE OR DUAL OUTPUT SYNCHRONOUS STEP DOWN CONTROLLER WITH OUTPUT TRACKING AND SEQUENCING

IR3622MPbF
Power MOSFET Selection
The IR3622 uses two N-Channel MOSFETs per
channel. The selection criteria to meet power
transfer requirements are based on maximum
drain-source voltage (V_DSS), gate-source drive
voltage (V_gs), maximum output current, On-
resistance R_DS(on), and thermal management.
The MOSFET must have a maximum operating
voltage (V_DSS) exceeding the maximum input
voltage (V_in).
The gate drive requirement is almost the same
for both MOSFETs. Logic-level transistor can be
used and caution should be taken with devices at
very low gate threshold voltage (V_gs) to prevent
undesired turn-on of the complementary
MOSFET, which results a shoot-through current.
The total power dissipation for MOSFETs
includes conduction and switching losses. For
the Buck converter the average inductor current
is equal to the DC load current. The conduction
loss is defined as:
P_cond
= (upperswitch)=
I2
load
∗ R_ds(on) ∗ D ∗ϑ
P_cond
= (lowerswitch)=
I2
load
∗ R_ds(on) ∗(1 − D)∗ϑ
ϑ = R_ds(on) temperatuer dependency
The R_DS(on) temperature dependency should be
considered 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, IRF6622 is selected for control
FET and IRF6629 is selected for synchronous
FET. These devices provide low on resistance in
a compact Direct FET package.
The MOSFETs have the following data:
ControlFET(IRF6622):
V_ds = 25V,Q_g =18.7nC @10V_gs
R_ds(on) = 6.3mΩ@V_gs =10V
SyncFET(IRF6629):
V_ds = 25V,Q_g = 51nC@10V_gs
R_ds(on) = 2.1mΩ@V_gs =10V
The conduction losses will be: P_con=1.1W/Phase
The switching loss is more difficult to calculate,
even though the switching transition is well
understood. The reason is the effect of the
parasitic components and switching times during
the switching procedures such as turn-on / turn-
off delays and rise and fall times. The control
MOSFET contributes to the majority of the
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switching losses in synchronous Buck converter.
The synchronous MOSFET turns on under zero
voltage conditions, therefore, the turn on losses
for synchronous MOSFET can be neglected.
With a linear approximation, the total switching
loss can be expressed as:
P_sw
= ^{Vds(off )}
2
* ^{tr + tf}
T
* I_load
Where:
- - - (13A)
V _ds(off) = Drain to source voltage at the off time
t_r = Rise time
t_f = Fall time
T = Switching period
I_load = Load current
The switching time waveforms is shown in
figure18.
VDS
90%
10%
VGS
td(ON)
tr td(OFF)
tf
Fig. 18: switching time waveforms
From IRF6622 data sheet:
tr = 13ns
tf = 14ns
These values are taken under a certain condition
test. For more details please refer to the IRF6622
data sheet.
By using equation (13A), we can calculate the
switching losses. P_sw=2.8W
The reverse recovery loss is also another
contributing factor in control FET switching
losses. This is equivalent to extra current
requires to remove the minority charges from
synchronous FET. The reverse recovery loss can
be expressed as:
P_Qrr = Q_rr * t_rr * F_s
Q_rr : ReverseRecoveryCharge
t_rr : ReverseRecoveryTime
F_s : SwitchingFrequency
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