HV100/HV101
Short Circuit Protection
Application Information
The HV100/HV101 provide short circuit protection by shutting
down if the Miller Effect associated with hotswap does not occur.
Specifically, if the output is shorted then the gate will rise without
exhibiting a “flat response”. Due to the fact that we have normal-
ized the hotswap period for any pass element, a timer can be
used to detect if the gate voltage rises above a threshold within
that time, indicating that a short exists. The diagram below
shows a typical turn on sequence with the load shorted, resulting
in a peak current of 4A.
Turn On Clamp
Hotswap controllers using a MOSFET as the pass element all
include a capacitor divider from VPP to VNN through CLOAD, CRSS
and CGS. In most competitive solutions a large external capacitor
is added to the gate of the pass element to limit the voltage on
the gate resulting from this divider. In those instances if a gate
capacitor is not used the internal circuitry is not available to hold
off the gate and therefore a fast rising voltage input will cause the
pass element to turn on for a moment. This allows current spikes
to pass through the MOSFET.
The HV100/HV101 include a built-in clamp to ensure that this
spurious current glitch does not occur. The built-in clamp will
work for the time constants of most mechanical connectors.
There may be applications, however, that have rise times that
are much less than 1µs (100’s of ns). In these instances it may
be necessary to add a capacitor from the MOSFET gate to
source to clamp the gate and suppress this current spike. In
these cases the current spike generally contains very little
energy and does not cause damage even if a capacitor is not
used at the gate.
2A/div
Auto-Adapt Operation
The HV100/HV101 Auto-adapt mechanism provides an impor-
tant function. It normalizes the hotswap period regardless of
pass element or load capacitor for consistent hotswap results.
By doing this it allows the novel short circuit mechanism to work
because the mechanism requires a known time base.
The maximum current that may occur during this period can be
controlled by adding a resistor in series with the source of the
MOSFET. The lower graph shows the same circuit with a 100mΩ
resistor inserted between source and VNN. In this case the
maximum current is 25% smaller.
The above diagram illustrates the effectiveness of the auto-
adapt mechanism. In this example three MOSFETs with different
CISS and RDSON values are used. The top waveform is the hotswap
current, while the bottom waveform is the gate voltage. As can
be seen, the hotswap period is normalized, the initial slope of the
gatevoltageisapproximately2.5V/msregardlessoftheMOSFET,
and the total hotswap period and peak currents are a function of
For most applications and pass elements, the HV100/HV101
provides adequate limiting of the maximum current to prevent
damage without the need for any external components. The 2.5s
delay of the auto-retry circuit provides time for the pass element
to cool between attempts.
a MOSFET type dependent constant multiplied by CLOAD
.
Typically if MOSFETs of the same type are used, the hotswap
results will be extremely consistent. If different types are used
they will usually exhibit minimal variation.
NTE66 is a trademark of NTE Electronics
IRF530 is a trademark of International Rectifier Corporation
IRF120M is a trademark of International Rectifier Corporation
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SUPERTEX [ Supertex, Inc ]
