MC3425 Datasheet, PDF (8/12 Pages) ON Semiconductor – POWER SUPPLY SUPERVISORY/ OVER AND UNDERVOLTAGE PROTECTION CIRCUIT

MC3425
Figure 17. Crowbar SCR Surge Current Waveform
l
lpk
di
dt
Surge Due to
Output Capacitor
Current Limited
Supply Output
t
2. 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 18) to a
safe level which is consistent with the system’s 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 16A, 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.
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 16B.
CROWBAR SCR SELECTION GUIDE
As an aid in selecting an SCR for crowbar use, the
following selection guide is presented.
Figure 18. Circuit Elements Affecting
SCR Surge & di/dt
RLead LLead
ESR
R
Output
ESL
Cap
L
To
MC3423
Device
MCR310 Series
MCR16 Series
MCR25 Series
2N6501 Series
MCR69 Series
MCR264 Series
MCR265 Series
IRMS
10 A
16 A
25 A
25 A
25 A
40 A
55 A
ITSM
100 A
150 A
300 A
300 A
750 A
400 A
550 A
R & L EMPIRICALLY DETERMINED!
UNDERVOLTAGE SENSING
An undervoltage sense circuit with hysteresis may be
designed, as shown in Figure 11, using the following
equations:
+ ^* R1
VCCU VCC1
12.5 mA
+ _* R2
2.5 R1
VCC1 2.5
where: VCCU is the designed upper trip point
(output indicator goes off)
VCC1 is the lower trip point
(output indicator goes on)
8
MOTOROLA ANALOG IC DEVICE DATA