MC3423 데이터 시트보기 (PDF) - Motorola => Freescale
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MC3423 Datasheet PDF : 8 Pages
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MC3423
Figure 11. Typical Crowbar OVP Circuit
Configurations
(11A)
Vin
Vout
DC
Power
Supply
+
OV
Cout Sense
Vin
*
(11B)
Vout
DC
Power
Supply
+
Cout
OV
Sense
*Needed if supply not current limited
Figure 12. Crowbar SCR Surge Current
Waveform
l
lpk
di
dt
Surge Due to
Output Capacitor
Current Limited
Supply Output
t
Figure 13. Circuit Elements Affecting
SCR Surge and di/dt
RLead LLead
ESR
R
Output
ESL
Cap
L
To
MC3423
R & L EMPIRICALLY DETERMINED!
The usual design compromise then is to use a garden
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
gate drive signal. A center–gate–fire SCR has more di/dt
capability than a corner–gate–fire type, and heavily
overdriving (3 to 5 times IGT) the SCR gate with a fast < 1.0 µs
rise time signal will maximize its di/dt capability. A typical
maximum number in phase control SCRs of less than 50
A(RMS) rating might be 200 A/µs, assuming a gate current of
five times IGT and < 1.0 µs 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 made between
speedy voltage reduction and di/dt.
Surge Current
If the peak current and/or the duration of the surge is
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. 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
possess an I2t rating less than that of the SCR and yet have
a high enough continuous current rating to survive normal
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.
Device
2N6400 Series
2N6504 Series
2N1842 Series
2N2573 Series
2N681 Series
MCR3935–1 Series
MCR81–5 Series
IRMS
16 A
25 A
16 A
25 A
25 A
35 A
80 A
IFSM
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
6
MOTOROLA ANALOG IC DEVICE DATA
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