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MC3423
V in
(11A)
V out
gate drive signal. A center?gate?fire SCR has more di/dt
capability than a corner?gate?fire type, and heavily
DC
Power
Supply
+
C out
OV
Sense
overdriving (3 to 5 times I GT ) the SCR gate with a fast
<1.0 m s rise time signal will maximize its di/dt capability.
A typical maximum number in phase control SCRs of less
V in
DC
Power
Supply
*
C out
+
OV
Sense
(11B)
V out
than 50 A(RMS) rating might be 200 A/ m s, assuming a gate
current of five times I GT and < 1.0 m s rise time. If having
done this, a di/dt problem is seen to still exist, the designer
can also decrease the di/dt of the current waveform by
adding inductance in series with the SCR, as shown in
Figure 13. Of course, this reduces the circuit’s ability to
rapidly reduce the DC bus voltage and a tradeoff must be
*Needed if supply not current limited
Figure 12. Typical Crowbar OVP Circuit
Configurations
made between speedy voltage reduction and di/dt.
Surge Current
If the peak current and/or the duration of the surge is
l
di
dt
l pk
Surge Due to
Output Capacitor
Current Limited
Supply Output
Figure 13. Crowbar SCR Surge Current
Waveform
t
excessive, immediate destruction due to device
overheating will result. The surge capability of the SCR is
directly proportional to its die area. If the surge current
cannot be reduced (by adding series resistance ? see
Figure 13) to a safe level which is consistent with the
systems requirements for speedy bus voltage reduction, the
designer must use a higher current SCR. This may result in
the average current capability of the SCR exceeding the
steady state current requirements imposed by the DC
power supply.
A WORD ABOUT FUSING
Before leaving the subject of the crowbar SCR, a few
words about fuse protection are in order. Referring back to
Figure 11A, it will be seen that a fuse is necessary if the
power supply to be protected is not output current limited.
ESR
R Lead
L Lead
R
This fuse is not meant to prevent SCR failure but rather to
prevent a fire!
In order to protect the SCR, the fuse would have to
ESL
Output
Cap
L
possess an I 2 t rating less than that of the SCR and yet have
a high enough continuous current rating to survive normal
To
MC3423
R & L EMPIRICALLY DETERMINED!
Figure 14. Circuit Elements Affecting
SCR Surge and di/dt
The usual design compromise then is to use a garden
supply output currents. In addition, it must be capable of
successfully clearing the high short circuit currents from
the supply. Such a fuse as this is quite expensive, and may
not even be available.
CROWBAR SCR SELECTION GUIDE
As an aid in selecting an SCR for crowbar use, the
following selection guide is presented.
variety fuse (3AG or 3AB style) which cannot be relied on
to blow before the thyristor does, and trust that if the SCR
does fail, it will fail short circuit. In the majority of the
designs, this will be the case, though this is difficult to
guarantee. Of course, a sufficiently high surge will cause an
open. These comments also apply to the fuse in Figure 11B.
The value of di/dt that an SCR can safely handle is
influenced by its construction and the characteristics of the
Device
2N6400 Series
2N6504 Series
2N1842 Series
2N2573 Series
2N681 Series
MCR3935?1 Series
MCR81?5 Series
I RMS
16 A
25 A
16 A
25 A
25 A
35 A
80 A
I FSM
160 A
160 A
125 A
260 A
200 A
350 A
1000 A
Package
TO?220 Plastic
TO?220 Plastic
Metal Stud
Metal TO?3 Type
Metal Stud
Metal Stud
Metal Stud
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