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IR3710MTRPBF
switching power loss because the output current flow
through the lower MOSFET’s body diode during the
dead time stores some minority charges. When the
upper MOSFETs turn on, it has to carry this extra
current to remove the minority charges. The reverse
recovery power loss can be found in equation 8.
P Qrr = Q rr ? V IN ? F S (8)
By combining the P SW and P Qrr , the total switching
power loss of the upper MOSFETs is much greater
than its conduction loss. International Rectifier
MOSFET datasheets has separated the gate charge
Inductor selection involves meeting the steady state
output ripple requirement, minimizing the switching
loss of upper MOSFETs, transient response and
minimizing the output capacitance. The output
voltage includes a DC voltage and a small AC ripple
component due to the low pass filter which has
incomplete attenuation of the switching harmonics.
Neglecting the inductance in series with output
capacitor, the magnitude of the AC voltage ripple is
determined by the total inductor ripple current flow
through the total equivalent series resistance (ESR)
of the output capacitor bank.
of Q GS1 and Q GS2 so that the designer can calculate
the switching power loss. Therefore, selection of the
upper MOSFETs should consider those factors.
Δ I =
V OUT
L
? ( 1 ? D ) ? Ts =
V OUT ? ( V IN ? V OUT )
V IN ? L ? F s
(12)
Otherwise, the converter losses degrade the system
efficiency and may exceed the thermal constraints.
The main power loss of lower MOSFETs is the
conduction loss because its on-time is in the range of
90% of the switching period. The switching power
loss of lower MOSFETs can be negligible because
their body diode voltage drops are in the range of 1V.
Equation 9 shows the conduction power loss
calculation. T S is inversely proportional to fs, and T OFF
is the on-time of the lower MOSFETs. R DS(on)
increases approximately 30% with temperature.
One can use equation 12 to find the inductance. The
main advantage of small inductance is increased
inductor current slew rate during a load transient,
which leads to small output capacitance requirement
as discussed in the Output Capacitor Selection
section. The draw back of using smaller inductances
is increased switching power loss in upper
MOSFETs, which reduces the system efficiency and
increases the thermal dissipation as discussed in the
Power Loss section.
P COND = I RMS_COND 2 ? R DSON ?
T OFF
Ts
(9)
Input Capacitor Selection
The main function of the input capacitor bank is to
? ?
1 ? Δ I ?
Where : I RMS_COND = I OUT ? 1 - D ? 1 +
?
3 ? I OUT ?
2
provide the input ripple current and fast slew rate
current during the load current step up. The input
capacitor bank must have adequate to handle the
The driver power loss is a small factor when heavily
loaded but it can be significant contributor of
degradation to the converter efficiency in light load.
Equation 10 shows the driver power loss relating to
the total gate charge of upper and lower MOSFETs
and switching frequency.
total RMS current. Figure 21 shows a typical input
current. Equation 13 shows the RMS input current.
The RMS input current contains the DC load current
and the inductor ripple current. As shown in equation
12, inductor ripple current is unrelated to the load
current. The maximum RMS input current occurs at
the maximum output current. The maximum power
P COND =
1
Ts
Ts
∫ V DR ? I GDr ? dt = V DR ? Q GTotal ? F S (10)
0
dissipate in the input capacitor equals the square of
the maximum RMS input current times the input
capacitor’s total ESR.
The low frequency and core losses are main factors
of the total power loss of an inductor. Low frequency
loss of an inductor is caused by the resistance of
copper winding. The copper loss of the winding is
shown in equation 11. The core loss of an inductor
depends on the B-H loop characteristic, volume and
frequency. This data can be obtained from the
inductor manufactures.
P DCR = I RMS 2 ? DCR (11)
Figure 21. Typical Input Current Waveform.
Where : I RMS = I OUT ? 1 + ? ? ?
?
∫
1 ? Δ I ?
Inductor Selection
1 Δ I ?
3 ? I OUT ?
2
I IN_RMS =
1
Ts
?
Ts
f 2 ( t ) ? dt = I OUT ? D ? 1 +
0
? ? ? (13)
3 ? I OUT ?
2
Page 13 of 20
www.irf.com
IR Confidential
4/26/10
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