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| 型號： | AN5374S |
| 英文描述： | Consumer IC |
| 中文描述： | 消費性IC |
| 文件頁數(shù)： | 3/5頁 |
| 文件大?。?/td> | 82K |
| 代理商： | AN5374S |
3/5
AN5370 Application Note
www.dynexsemi.com
Fig.5 Stored charge (max)
Fig.6 Series diodes
0
5
10
15
20
25
30
1
10
100
Rate of decay of forward current - (A/
μ
s)
S
r
μ
C
T
case
= 150
C
I
F
= 1A
I
F
= 2A
I
F
= 3A
I
F
= 5A
I
F
=10A
I
F
= 20A
T
2
T
1
TA
(T
2
–
T
1
)
0
I
F
I
R1
I
R2
P
1
P
2
Fast diode
recovers
Slow diode
recovers
b) avalanche power in fast diodes
a) current
criteria above it is not possible for all diodes to avalanche,
therefore the current in the series string will always be less than
the current at which the diode with the highest leakage current
enters the avalanche region. ( see fig. 3)
The fact that avalanche diodes in a series string can be allowed
to operate with voltage sharing determined by the diodes, makes
it unnecessary to use static sharing resistors.
b) Dynamic
The dynamic sharing is concerned with the reverse voltage
appearing across the series string when the current in the diodes
is being commutated. The diodes conduct a small current in the
reverse direction (fig. 4) at the end of the commutation and when
the current snaps off a voltage is induced in the commutating
circuit imposing a reverse voltage across the diodes.
The integral of the reverse current is called the stored charge and
is expresses in micro-coulombs. The limit case curve for the
MZ04xxW diode is shown in figure 5. The stored charge is
dependent upon the commutating current, the rate of commutation
and the diode junction temperature. Diodes taken from production
will show a spread of stored charge, the fast diode having a low
stored charge and the slow diode having a high stored charge.
The reverse currents of the fast and slow diodes could be as
shown in fig.6. In order to simplify the analysis, a triangular
current with an instantaneous snap-off at the peak reverse
current is assumed. A possible condition exists where the
applied voltage is sufficiently high for the fast diodes to be
subjected to the avalanche voltage at snap-off (time T1). The fast
diode starts the avalanche regime at time T1 with a current IR1
and finishes the regime at time T2 with a current IR2. The
instantaneous avalanche power at IR1 and IR2 is P1 and P2
respectively.
P1 = I
R1
x V
(AB)R
+
1) ..............................................(7)
P2 = I
R2
x (V
(AB)R
+
2) ...............................................(8)
The
values represent the increase in voltage in the avalanche
region. From the analysis given in the appendix it follows that the
avalanche rating of the diodes in a series string, if no dynamic
sharing capacitors are used, must be :
Avalanche Power = I
R2
x (V
(AB)R
+
2), Watts
for a time
...........(9)
TA =
√
(1/(2.di/dt) x (
√
Qmax - Qmin /
√
Qmax),
μ
sec. .....(10)
Where Qmax & Qmin are the maximum and minimum diode
stored charge in
μ
C and di/dt is the rate of change of commutation
current in Amps/
μ
s.
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