PI2001 Datasheet, PDF (14/23 Pages) Vicor Corporation – Universal Active ORing Controller IC

during MOSFET turn-off after a reverse current fault
has been detected. In Active ORing applications when
one of the input power sources is shorted, a large
reverse current is sourced from the circuit output
through the MOSFET. Depending on the output
impedance of the system, the reverse current may
reach over 60A in some conditions before the
MOSFET is turned off. Such high current conditions
will store energy even in a small parasitic element.
For example, a 1nH parasitic inductance with 60A
reverse current will store 1.8µJ (½Li²). When the
MOSFET is turned off, the stored energy will be
released and will produce high negative voltage
ringing at the MOSFET source. This event will create
a high voltage difference across the drain and source
of the MOSFET.
The MOSFET current rating and maximum power
dissipation are closely related. Generally the lower the
MOSFET Rds(on), the higher the current capability
and the lower the resultant power dissipation. This
leads to reduced thermal management overhead, but
will ultimately be higher cost compared to higher
Rds(on) parts. It is important to understand the
primary design goal objectives for the application in
order to effectively trade off the performance of one
MOSFET versus another.
Power dissipation in active ORing circuits is derived
from the total source current and the on-state
resistance of the selected MOSFET.
MOSFET power dissipation:
Pd _MOSFET = Is ² ∗ Rds(on)
Where :
Is
: Source Current
Rds(on) : MOSFET on-state resistance
Note:
In the calculation use Rds(on) at maximum MOSFET
temperature because Rds(on) is temperature
dependent. Refer to the normalized Rds(on) curves in
the MOSFET manufacturers datasheet. Some
MOSFET Rds(on) values may increase by 50% at
125°C compared to values at 25°C.
The Junction Temperature rise is a function of power
dissipation and thermal resistance.
Trise_MOSFET = Rth_JA ∗ Pd_MOSFET = Rth_JA ∗ Is² ∗ Rds(on) ,
The PI2001 senses the MOSFET source-to-drain
voltage drop via the SP and SN pins to determine the
status of the current through the MOSFET. When the
MOSFET is fully enhanced, its source-to-drain voltage
is equal to the MOSFET on-state resistance multiplied
by the source current, VSD = Rds(on)*Is. The reverse
current threshold is set for -6mV and when the
differential voltage between the SP & SN pins is less
than -6mV, i.e. SP-SN≤-6mV, the PI2001 detects a
reverse current fault condition and pulls the MOSFET
gate pin low, thus turning off the MOSFET and
preventing further reverse current. The reverse
current fault protection disconnects the power source
fault condition from the redundant bus, and allows the
system to keep running.
The GATE pin output voltage is clamped to 10.5V
maximum with respect to the SP pin, which should be
tied to the MOSFET source pin, to support any
MOSFET with a Vgs rating of ±12V or greater. A Vgs
rating ≥12V is very common for industry standard N-
Channel MOSFETs.
OV/UV resistor selection:
The UV and OV comparator inputs are used to
monitor the input voltage and will indicate a fault
condition when this voltage is out of range. The UV
and OV pins can be configured in two different ways,
either with a divider on each pin, or with a three-
resistor divider to the same node, enabling the
elimination of one resistor. Under-Voltage is
monitored by the UV pin input and Over-Voltage is
monitored with the OV pin input.
The Fault pin ( FT ) will indicate a fault (active low)
when the UV pin is below the threshold or when the
OV pin is above the threshold. The UV and OV
thresholds are 0.50V typ with 25mV hysteresis and
their input current is less than ±1µA. It is important to
consider the maximum current that will flow in the
resistor divider and maximum error due to UV and OV
input current. Set the resistor current to 100µA or
higher to maintain 1% accuracy for UV and OV due to
the bias current.
The three-resistor voltage divider configuration for
both UV and OV to monitor the same voltage node is
shown in figure 13:
Ra = ^{V (OVTH )}
I _Ra
Where:
Rth_JA : Junction-to-Ambient thermal resistance
Rds(on) and PI2001 sensing:
Picor Corporation • picorpower.com
Figure 13: UV & OV three-resistor divider
configuration.
PI2001
Rev 1.0 Page 14 of 23