TPS40140 Datasheet, PDF (44/65 Pages) Texas Instruments – DUAL OR 2-PHASE, STACKABLE CONTROLLER

TPS40140
DUAL OR 2-PHASE, STACKABLE CONTROLLER
SLUS660A – SEPTEMBER 2005 – REVISED JULY 2006
www.ti.com
6.1.4 Step 4: MOSFET Selection
The MOSFET selection determines the converter efficiency. In this design, the duty cycle is very small so
that the high-side MOSFET is dominated with switching losses and the low-side MOSFET is dominated
with conduction loss. To optimize the efficiency, choose smaller gate charge for the high-side MOSFET
and smaller R_DS(on) for the low-side MOSFET.
The RENESAS RJK0305 and RJK0301 are selected as the high-side and low-side MOSFETs
respectively. To reduce the conduction loss, two RJK0301 components are used.
The power losses in the high-side MOSFET is calculated with the following equations:
The RMS current in the high side MOSFET is show in Equation 8.
Ǹ ^{ǒ Ǔ I}SWrms ⁺
D
I_OUT2
)
I_RIPPLE2
12
+ 7.07 A
(8)
The R_DS(on) is 13 mΩ when the MOSFET gate voltage is 4.5 V.
The conduction loss is:
P_SWcond + ^ǒI_SWrms^Ǔ2 R_DS(on) (sw) + 0.65 W
(9)
The switching loss is:
Ipk × Vin × f × R × (Qgd + Qgs )
Psw =
sw
drv
sw
sw _{= 0.26W}
sw
Vgtdrv
(10)
The calculated total loss in the high-side MOSFET is:
P_SWtot + P_SWcond ) P_SWsw + 0.91 W
(11)
The power losses in the low-side SR MOSFET is calculated by:
The RMS current in the low side MOSFET is shown in Equation 12.
Ǹ ^{ǒ Ǔ I}SRrms ⁺
(1 * D)
I_OUT2
)
I
2
RIPPLE
12
+ 18.7 A
(12)
The R_DS(on) is 4mΩ when the MOSFET gate voltage is 4.5V.
The total conduction loss in the two low-side MOSFETs is:
P_SRcond + ^ǒI_SRrms^Ǔ2
R_DS(on)
N
(sr)
+
0.7
W
(13)
N is the number of MOSFET. Here, it is equal to 2.
The total power loss in the body diode is:
P = 2 × I × t × V × f = 0.77W
diode
OUT d
f sw
(14)
So the calculated total loss in the SR MOSFET is:
P_SRtot + P_SRcond ) P_DIODE + 1.47 W
(15)
6.1.5 Step 5: Peripheral Component Design
44
DESIGN EXAMPLES
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