application switching

WHITE PAPER: WP011
Selecting eGaN® FET Optimal On-Resistance
• Diode losses will vary with die size, but this variation is neglected for the optimization process for simplicity. The diode losses will vary inversely to other charge
dependent losses with die size (will actually get smaller with increased die size), but this variation is assumed small in comparison to that of the charge dependent
losses.
• The current at turn-on and turn-off are assumed equal and the influence of inductor current ripple is ignored. To quantify the error of doing so, consider turn on at
I L
-I P
and turn off at I L
+I P
, then the turn-on/off losses are:
[
V ⋅ ( I + I ) V ⋅ ( I
k +
2
2
− I )
k
]
⋅Q
⋅ A ⋅ f
[
V ⋅ ( I ) V ⋅ ( I ) V ⋅ ( I ) V ⋅ ( I
= k + k + k −
2
2
2
2
)
k
]
[
V ⋅ ( I ) V ⋅ ( I ) ( )
]
= k + k − k ⋅Q
⋅ A ⋅ fSW
2
2
BUS
BUS
BUS
L
L
L
P
ON
ON
BUS
BUS
BUS
P
L
L
ON
OFF
P
OFF
OFF
BUS
P
SW , A
SW , A
ON
SW
BUS
P
OFF
⋅ Q
SW , A
⋅ A⋅
f
SW
So the error is an underestimation for eGaN FETs:
I
p
( kON
kOFF)
I
L
⋅ k
≈
1
3
I
I
p
L
≈
1
6
I
I
pp
L
for a peak to peak ripple = 30%, then error = 5% (even smaller for MOSFET, where k ON
and k OFF
values are almost equal.
• Error for soft-switching devices where Q G
is used on driver loss and incorrectly includes Q GD
. This error, Q GD
/Q G
is about 30% over estimation of soft switching gate
drive losses.
Table 3: State of the art MOSFETs normalized to 1 Ω typical R DS(ON)
for difference voltage ratings
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