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MC3423P1G

P1-P3P4-P6P7-P9P10-P10
MC3423
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4
APPLICATION INFORMATION
Basic Circuit Configuration
The basic circuit configuration of the MC3423 OVP is
shown in Figure 3 for supply voltages from 4.5 V to 36 V,
and in Figure 4 for trip voltages above 36 V. The threshold
or trip voltage at which the MC3423 will trigger and supply
gate drive to the crowbar SCR, Q1, is determined by the
selection of R1 and R2. Their values can be determined by
the equation given in Figures 3 and 4, or by the graph shown
in Figure 8. The minimum value of the gate current limiting
resistor, R
G
, is given in Figure 9. Using this value of R
G
, the
SCR, Q1, will receive the greatest gate current possible
without damaging the MC3423. If lower output currents
are required, R
G
can be increased in value. The switch, S1,
shown in Figure 3 may be used to reset the crowbar.
Otherwise, the power supply, across which the SCR is
connected, must be shut down to reset the crowbar. If a non
current−limited supply is used, a fuse or circuit breaker, F1,
should be used to protect the SCR and/or the load.
The circuit configurations shown in Figures 3 and 4 will
have a typical propagating delay of 1.0 s. If faster
operation is desired, Pin 3 may be connected to Pin 2 with
Pin 4 left floating. This will result in decreasing the
propagating delay to approximately 0.5 s at the expense
of a slightly increased TC for the trip voltage value.
Configuration for Programmable Minimum Duration
of Overvoltage Condition Before Tripping
In many instances, the MC3423 OVP will be used in a
noise environment. To prevent false tripping of the OVP
circuit by noise which would not normally harm the load,
MC3423 has a programmable delay feature. To implement
this feature, the circuit configuration of Figure 5 is used. In
this configuration, a capacitor is connected from Pin 3 to
V
EE
. The value of this capacitor determines the minimum
duration of the overvoltage condition which is necessary to
trip the OVP. The value of C can be found from Figure 10.
The circuit operates in the following manner: When V
CC
rises above the trip point set by R1 and R2, an internal
current source (Pin 4) begins charging the capacitor, C,
connected to Pin 3. If the overvoltage condition disappears
before this occurs, the capacitor is discharged at a rate ≅ 10
times faster than the charging rate, resetting the timing
feature until the next overvoltage condition occurs.
Occasionally, it is desired that immediate crowbarring of
the supply occur when a high overvoltage condition occurs,
while retaining the false tripping immunity of Figure 5. In
this case, the circuit of Figure 6 can be used. The circuit will
operate as previously described for small overvoltages, but
will immediately trip if the power supply voltage exceeds
V
Z1
+ 1.4 V.
Power
Supply
R1
R2
MC3423
R
G
(+ Sense
Lead)
(− Sense Lead)
−
+
1k
Z1
7
34
5
2
1
8
C
Figure 7. Configuration for Programmable
Duration of Overvoltage Condition Before
Trip/With Immediate Trip at
High Overvoltages
Additional Features
1. Activation Indication Output
An additional output for use as an indicator of OVP
activation is provided by the MC3423. This output is an
open collector transistor which saturates when the OVP
is activated. In addition, it can be used to clock an edge
triggered flip−flop whose output inhibits or shuts down
the power supply when the OVP trips. This reduces or
eliminates the heatsinking requirements for the crowbar
SCR.
2. Remote Activation Input
Another feature of the MC3423 is its remote
activation input, Pin 5. If the voltage on this CMOS/TTL
compatible input is held below 0.8 V, the MC3423
operates normally. However, if it is raised to a voltage
above 2.0 V, the OVP output is activated independent of
whether or not an overvoltage condition is present. It
should be noted that Pin 5 has an internal pullup current
source. This feature can be used to accomplish an
orderly and sequenced shutdown of system power
supplies during a system fault condition. In addition, the
activation indication output of one MC3423 can be used
to activate another MC3423 if a single transistor inverter
is used to interface the former’s indication output to the
latter’s remote activation input, as shown in Figure 7. In
this circuit, the indication output (Pin 6) of the MC3423
on power supply 1 is used to activate the MC3423
associated with power supply 2. Q1 is any small PNP
with adequate voltage rating.
MC3423
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5
Power
Supply
#1
R1
Power
Supply
#2
+
−
+
−
10k
Q1
1.0k
5
7
1
7
6
1
Figure 8. Circuit Configuration for
Activating One MC3423 from Another
Note that both supplies have their negative output
leads tied together (i.e., both are positive supplies). If
their positive leads are common (two negative supplies)
the emitter of Q1 would be moved to the positive lead of
supply 1 and R1 would therefore have to be resized to
deliver the appropriate drive to Q1.
Crowbar SCR Considerations
Referring to Figure 11, it can be seen that the crowbar
SCR, when activated, is subject to a large current surge
from the output capacitance, C
out
. This capacitance
consists of the power supply output caps, the load’s
decoupling caps, and in the case of Figure 11A, the supply’s
input filter caps. This surge current is illustrated in Figure
12, and can cause SCR failure or degradation by any one
of three mechanisms: di/dt, absolute peak surge, or I
2
t. The
interrelationship of these failure methods and the breadth
of the applications make specification of the SCR by the
semiconductor manufacturer difficult and expensive.
Therefore, the designer must empirically determine the
SCR and circuit elements which result in reliable and
effective OVP operation. However, an understanding of the
factors which influence the SCR’s di/dt and surge
capabilities simplifies this task.
di/dt
As the gate region of the SCR is driven on, its area of
conduction takes a finite amount of time to grow, starting
as a very small region and gradually spreading. Since the
anode current flows through this turned−on gate region,
very high current densities can occur in the gate region if
high anode currents appear quickly (di/dt). This can result
in immediate destruction of the SCR or gradual
degradation of its forward blocking voltage capabilities −
depending on the severity of the occasion.
V
T
, TRIP VOLTAGE (V)
0 5.0 10 15 20 25 30
Min
30
20
10
0
R1, RESISTANCE (k)Ω
Typ
R2 = 2.7 k
Max
Figure 9. R1 versus Trip Voltage
V
CC
, SUPPLY VOLTAGE (V)
35
30
25
20
15
10
01020304050 607080
RG, GATE CURRENT LIMITING RESISTOR ()
R
G(min)
= 0
if V
CC
< 11 V
Figure 10. Minimum R
G
versus Supply Voltage
t
d
, DELAY TIME (ms)
C, CAPACITANCE (F)
µ
123571
1
5
2
10.0001
0.001
0.01
0.1
1.0
0.001 0.01 0.1 1.0 10
Figure 11. Capacitance versus
Minimum Overvoltage Duration
MC3423
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6
OV
Sense
OV
Sense
*Needed if supply not current limited
V
in
V
in
V
out
V
out
DC
Power
Supply
DC
Power
Supply
C
out
C
out
+
+
*
(11A)
(11B)
Figure 12. Typical Crowbar OVP Circuit
Configurations
l
pk
Surge Due to
Output Capacitor
Current Limited
Supply Output
di
dt
l
t
Figure 13. Crowbar SCR Surge Current
Waveform
R & L EMPIRICALLY DETERMINED!
ESR
ESL
R
Lead
L
Lead
Output
Cap
To
MC3423
R
L
Figure 14. Circuit Elements Affecting
SCR Surge and di/dt
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 I
GT
) the SCR gate with a fast
<1.0 s 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/s, assuming a gate
current of five times I
GT
and < 1.0 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
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 I
2
t 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 I
RMS
I
FSM
Package
2N6400 Series 16 A 160 A TO−220 Plastic
2N6504 Series 25 A 160 A TO−220 Plastic
2N1842 Series 16 A 125 A Metal Stud
2N2573 Series 25 A 260 A Metal TO−3 Type
2N681 Series 25 A 200 A Metal Stud
MCR3935−1 Series 35 A 350 A Metal Stud
MCR81−5 Series 80 A 1000 A Metal Stud
P1-P3P4-P6P7-P9P10-P10

MC3423P1G

Mfr. #:
Buy MC3423P1G
Manufacturer:
ON Semiconductor
Description:
Supervisory Circuits 6.5V OverVoltage Crowbar Sensing
Lifecycle:
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