Environmental Engineering Reference
In-Depth Information
a computer simulation to get approximate results without a substantial amount of
device characterization:
c
1
i
u
1
c
2
i
u
2
P
sw
¼
f
ð
þ
Þ
c
1
¼
0
:
012
c
2
¼
0
:
0042
(Hz, mJ/cycle)
ð
8
:
13
Þ
u
1
¼
1
:
25
u
2
¼
1
:
3
For the case of 100
C junction temperature and a fixed link voltage,
c
1
and
u
1
are
for turn-ON and
c
2
and
u
2
are for turn-OFF. For different link voltages or different
temperature the coefficients will need to be recomputed based on some device
characterization. A similar procedure will work for MOSFET devices [13].
8.2.3 Reverse recovery
In a hard switched converter with reactive current flow, there will be diode con-
duction when the transistor opposite the conducting diode is gated ON. Circuit
current will switch to the transistor gated ON, but an additional component of
current will flow through the switch in a shoot-through fashion until the diode is
commutated OFF. The diode current is quickly reversed, but persists, for a duration
of time necessary to sweep all the stored charge from the p-n junction. The time to
accomplish this is the reverse recovery time during which an amount of charge
Q
rr
is cleared. As with the active devices, the power dissipation in the diode during
reverse recovery is calculated as
P
rr
¼
fE
rr
ð
8
:
14
Þ
1
6
3
U
dc
2
E
rr
¼
t
rr
i
In many power electronic circuits, particularly those built with thyristors, it is
necessary to add snubbers across the active device to limit
dU
/
dt
,
dI
/
dt
or some
combinations. In the MOS controlled thyristor (MCT), it is necessary to add
dI
/
dt
snubbing in series with the anode to limit the rate of rise of current to prevent
device damage. Gate turn off thyrsitors (GTOs) also require
dU
/
dt
snubbers to limit
the rate of rise of voltage during forward recovery to allow the device sufficient
time to internally stabilize. If a snubber capacitor is used, its switching power
dissipation is calculated as shown in (8.15):
P
snub
¼
fE
snub
2
ð
8
:
15
Þ
1
2
C
3
U
dc
2
E
snub
¼
