11
IXDD514 / IXDE514
Short Circuit di/dt Limit
A short circuit in a high-power MOSFET module such as the
VM0580-02F, (580A, 200V), as shown in Figure 28, can cause
the current through the module to flow in excess of 1500A for
10µs or more prior to self-destruction due to thermal runaway.
For this reason, some protection circuitry is needed to turn off
the MOSFET module. However, if the module is switched off
too fast, there is a danger of voltage transients occuring on the
drain due to Ldi/dt, (where L represents total inductance in
series with drain). If these voltage transients exceed the
MOSFET's voltage rating, this can cause an avalanche break-
down.
The IXDD514 and IXDE514 have the unique capability to softly
switch off the high-power MOSFET module, significantly
reducing these Ldi/dt transients.
Thus, the IXDD514/IXDE514 help to prevent device destruction
from both dangers; over-current, and avalanche breakdown
due to di/dt induced over-voltage transients.
The IXDD514/IXDE514 are designed to not only provide ±14A
under normal conditions, but also to allow their outputs to go
into a high impedance state. This permits the IXDD514/
IXDE514 output to control a separate weak pull-down circuit
during detected overcurrent shutdown conditions to limit and
separately control d
VGS
/dt gate turnoff. This circuit is shown in
Figure 29.
Referring to Figure 29, the protection circuitry should include
a comparator, whose positive input is connected to the source
of the VM0580-02. A low pass filter should be added to the input
of the comparator to eliminate any glitches in voltage caused
by the inductance of the wire connecting the source resistor to
ground. (Those glitches might cause false triggering of the
comparator).
The comparator's output should be connected to a SRFF(
Set
Reset Flip Flop). The flip-flop controls both the Enable signal,
and the low power MOSFET gate. Please note that CMOS 4000-
series devices operate with a V
CC
range from 3 to 15 VDC, (with
18 VDC being the maximum allowable limit).
A low power MOSFET, such as the 2N7000, in series with a
resistor, will enable the VMO580-02F gate voltage to drop
gradually. The resistor should be chosen so that the RC time
constant will be 100us, where "C" is the Miller capacitance of
the VMO580-02F.
For resuming normal operation, a Reset signal is needed at
the SRFF's input to enable the IXDD514/IXDE514 again. This
Reset can be generated by connecting a One Shot circuit
between the IXDD514/IXDE514 Input signal and the SRFF
restart input. The One Shot will create a pulse on the rise of the
IXDD514/IXDE514 input, and this pulse will reset the SRFF
outputs to normal operation.
When a short circuit occurs, the voltage drop across the low-
value, current-sensing resistor, (Rs=0.005 Ohm), connected
between the MOSFET Source and ground, increases. This
triggers the comparator at a preset level. The SRFF drives a low
input into the Enable pin disabling the IXDD514/IXDE514
output. The SRFF also turns on the low power MOSFET,
(2N7000).
In this way, the high-power MOSFET module is softly turned off
by the IXDD514/IXDE514, preventing its destruction.
APPLICATIONS INFORMATION
10uH
Ld
0.1ohm
Rd
Rs
20nH
Ls
1ohm
Rg
10kohm
R+
VMO580-02F
High_Power
5kohm
Rcomp
100pF
C+
+
-
V+
V-
Comp
LM339
1600ohm
Rsh
Ccomp
1pF
VCC
VCCA
IN
EN
GND
OUT
IXDD409
+
-
VIN
+
-
VCC
+
-
REF
+
-
VB
CD4001A
NOR2
1Mohm
Ros
NOT2
CD4049A
CD4011A
NAND
CD4049A
NOT1
CD4001A
NOR1
CD4049A
NOT3
Low_Power
2N7002/PLP
1pF
Cos
0
S
R
EN
Q
One Shot Circuit
SR Flip-Flop
GND
Figure 29 - Application Test Diagram
IXDD514/IXDE514
