application switching

WHITE PAPER: WP011
Selecting eGaN® FET Optimal On-Resistance
Note that not all devices will have all these loss components, e.g. a synchronous buck converter would have practically no turn-on or turn-off losses in the synchronous
rectifier. Furthermore, to optimize multiple devices in a converter, the losses stemming from the interaction between devices also need to be considered (e.g.
the diode reverse recovery losses of one device may be dissipated in another FET. This occurs in circuits such as synchronous buck converters where synchronous FET
related losses are dissipated in the control FET, but by optimizing the control FET only, this loss component will remain unchanged. Thus for optimization purposes,
this issue is resolved by considering all the losses induced by a device to be relevant for its sizing, regardless of where the power is dissipated.
Die Size Optimization
By considering each of these device loss components in eq. (1) in turn, some conclusions can be drawn:
• The conduction losses (item 1) are frequency independent
• Commutation loss (items 2a and 2b) are both frequency and load current dependent and can be combined as follows:
where:
k k ON
+ k OFF
P
k
= ON
V
2
RG
=
V − V
BUS
COMM
= k ⋅QSW , A
DR
PL
⋅ A⋅
I
R
L
⋅ f
SW
G
k
OFF
= and Q
SW, A
= QGD,
A
+ QGS2, A
VPL
(2)
• Loss components in items 3, 4 and 5 are all frequency dependent, but current independent and can be combined. While Q RR
is current related, MOSFET vendors
neglect to present their characteristic adequately over current, temperature and di/dt to accurately calculate these losses:
(
Q
)
2
OSS,
A
P
CHARGE
= ⋅ VBUS
+ Q
G,A
⋅ VDR
+ QRR
A
⋅ VBUS
⋅ A⋅
f
,
SW
(3)
• Diode losses, item 6, are assumed die size independent (only a weak function of die size) and neglected for the optimization process.
If we now define two new variables ΔI EQ
and ΔI EQRR
as:
∆I
EQ
Q
=
OSS , A
⋅ VBUS
+ 2
V
BUS
⋅ Q
⋅ k ⋅ Q
G,A
SW,
A
⋅ V
DR
(4a)
∆I
EQRR
=
2 ⋅ Q
k ⋅ Q
RR,
A
SW , A
(4b)
Then combining eq. (2) and eq. (3) and substituting eq. (4a) and (4b), the switching losses are:
P
SW
=
=
=
[
V
(
Q
k ⋅ Q ⋅ I +
2
2
⋅ V + Q ⋅ V + Q
)]
⋅ V
[
V
V
k ⋅ Q ⋅ I +
2
2
k ⋅ Q ⋅ ( ∆I
+ ∆I
)]
⋅ A⋅
f
[
V
2
k ⋅ Q
]
⋅ ( I + ∆I
+ ∆I
) ⋅ f ⋅ A
= P
BUS
BUS
BUS
SW,
A
⋅ A
SW,
A
SW,
A
SW , A
L
L
L
OSS,
A
BUS
EQ
BUS
SW,
A
EQRR
G,A
EQ
SW
DR
EQRR
RR,
A
BUS
SW
⋅ A ⋅
f
SW
(5)
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