Understanding Modern Power MOSFETs - Fairchild Semiconductor

67
Selecting Power MOSFETs for
Synchronous Buck Applications: Step 1
• Calculate the high side
conduction loss per mΩ of
R DS(ON) :
P
CONDHS
= I
2
OUT
V
×
V
OUT
3
10 −
×
• and the switching loss per nC
of gate charge:
68
P
SWHS
= V × I
IN
OUT
× F
IN
SW
⎛
× ⎜
⎝I
1
DRIVER
⎞
⎟
⎟×
⎠
9
10 −
• Calculate the low side conduction
loss per milliohm of R DS(ON) :
P
CONDLS
⎛ V
= ⎜ 1−
⎝ V
OUT
2
OUT
−3
• and approximate the switching
loss per nC of gate charge:
P
SWLS
≈ 1×
I
OUT
× F
IN
SW
⎞
⎟ × I
⎠
⎛
× ⎜
⎝I
1
DRIVER
× 10
⎞
⎟ × 10
⎠
Selecting Power MOSFETs for
Synchronous Buck Applications: Step 2
• We will consider two different examples to show how the operating
conditions affect the MOSFET choice
Sync. Buck data input
conduction loss per mOhm
Vin 5 V high side 0.06600 low side 0.03400
Vout 3.3 V
switching loss per nC.
Iout 10 A high side 0.00676 low side 0.00135
fsw 230000 Hz
Idrv 1.7 A
Sync. Buck data input
conduction loss per mOhm
Vin 12 V high side 0.04000 low side 0.36000
Vout 1.2 V
switching loss per nC.
Iout 20 A high side 0.04235 low side 0.00353
fsw 300000 Hz
Idrv 1.7 A
−9
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