IXDN430 / IXDI430 / IXDD430 / IXDS430
APPLICATIONS INFORMATION
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 27, 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.
In this way, the high-power MOSFET module is softly turned off
by the IXDD430, preventing its destruction.
Supply Bypassing and Grounding Practices,
Output Lead inductance
When designing a circuit to drive a high speed MOSFET
utilizing the IXDD430/IXDI430/IXDN430, it is very important to
keep certain design criteria in mind, in order to optimize
performance of the driver. Particular attention needs to be paid
to Supply Bypassing, Grounding, and minimizing the Output
Lead Inductance.
TheIXDD430hastheuniquecapabilitytosoftlyswitchoffthe
high-power MOSFET module, significantly reducing these
Ldi/dttransients.
Say, for example, we are using the IXDD430 to charge a 15nF
capacitive load from 0 to 25 volts in 25ns.
Thus, the IXDD430 helps to prevent device destruction from
both dangers; over-current, and avalanche breakdown due to
di/dt induced over-voltage transients.
Using the formula: I= C ∆V / ∆t, where ∆V=25V C=15nF &
∆t=25ns we can determine that to charge 15nF to 25 volts in
25ns will takeaconstantcurrent of 15A. (Inreality,thecharging
current won’t be constant, and will peak somewhere around
30A).
The IXDD430 is designed to not only provide ±30A under
normal conditions, but also to allow it's output to go into a high
impedance state. This permits the IXDD430 output to control
a separate weak pull-down circuit during detected overcurrent
shutdown conditions to limit and separately control dVGS/dt gate
turnoff. This circuit is shown in Figure 28.
SUPPLYBYPASSING
In order for our design to turn the load on properly, the IXDD430
must be able to draw this 5A of current from the power supply
in the 25ns. This means that there must be very low impedance
between the driver and the power supply. The most common
method of achieving this low impedance is to bypass the power
supply at the driver with a capacitance value that is a magnitude
larger than the load capacitance. Usually, this would be
achievedbyplacingtwodifferenttypesofbypassingcapacitors,
with complementary impedance curves, very close to the driver
itself. (These capacitors should be carefully selected, low
inductance, low resistance, high-pulse current-service
capacitors). Lead lengths may radiate at high frequency due
to inductance, so care should be taken to keep the lengths of
the leads between these bypass capacitors and the IXDD430
to an absolute minimum.
Referring to Figure 28, 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 VCC range from 3 to 15 VDC,
(with 18 VDC being the maximum allowable limit).
GROUNDING
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
theVMO580-02F.
In order for the design to turn the load off properly, the IXDD430
must be able to drain this 5A of current into an adequate
grounding system. There are three paths for returning current
that need to be considered: Path #1 is between the IXDD430
and it’s load. Path #2 is between the IXDD430 and it’s power
supply. Path #3 is between the IXDD430 and whatever logic is
driving it. All three of these paths should be as low in resistance
and inductance as possible, and thus as short as practical. In
addition, every effort should be made to keep these three
ground paths distinctly separate. Otherwise, (for instance), the
returning ground current from the load may develop a voltage
that would have a detrimental effect on the logic line driving the
IXDD430.
For resuming normal operation, a Reset signal is needed at
the SRFF's input to enable the IXDD430 again. This Reset can
be generated by connecting a One Shot circuit between the
IXDD430InputsignalandtheSRFFrestartinput. TheOneShot
will create a pulse on the rise of the IXDD430 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 IXDD430 output. The
SRFF also turns on the low power MOSFET, (2N7000).
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