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A P P L I C AT I O N S N O T E
Dual, High and Low Side ORing Application
Using PICOR’s PI2003 and PI2007
Bob Kent • Picor Corporation
Introduction
The Active ORing circuit in Figure 2 uses both the PI2003, to
control the low-side MOSFET, and the PI2007, to control the
high-side MOSFET. Since Active ORing components are not
designed to enhance current sharing between the supplies,
one of the supplies will dominate and provide the current to the
load. The PI2003 has a low-current fault feature that is used to
either enable or disable the PI2007 associated with it. If there is
little or no current through the PI2003 it issues a fault condition
that pulls its FT pin low. The MOSFET that is used to provide a
return path for the PI2007 bias supply is then disabled, which in
turn disables the PI2007. If the input supply is still in-circuit, then
the PI2003 will remain powered up and will continue to monitor
the supply. If the supply were to be removed or disabled, then
the supply to the PI2003 would be discharged and the PI2003
would disable its MOSFET. The resulting circuit would be just the
two parasitic body diodes of the high and low side MOSFETs.
When designing redundant power systems, the ability to isolate
a failed power source from the working power source is critical
in maintaining uninterrupted operation of the load circuitry.
To achieve this ability, the power sources are “OR’ed” together
through fast-acting diodes (Figure 1), which will quickly isolate
a shorted or disabled source from the OR’ed output supply by
simply reverse biasing the diode. This method works very well
and is relatively inexpensive to implement. The downside of
using diodes is that the inherent voltage drop they have will
reduce overall power efficiency when used at higher currents.
To significantly improve efficiency and still retain the isolation
protection of the ORing diodes, a MOSFET can be used, in
conjunction with a controller, to replace the ORing diodes (Figure
2). When current flows from the input source to the output
redundant bus, the controller senses the direction of the current
flow and will turn the MOSFET “on” and drive it into
its minimum on-state resistance (RDS(on)) value. The power
dissipated in the MOSFET is the value of the current squared,
multiplied by the MOSFET’s RDS(on) resistance. Since this
resistance is very low, the power loss is much less than the loss
across a diode. When the controller senses a reversal of current
flow from the OR’ed output supply back to the input power
source, the controller responds quickly to turn-off the MOSFET
and the MOSFET’s parasitic body diode will then isolate the
OR’ed output redundant bus from the failed input supply.
OR'ed Supply
IRF7853PbF
RVC
11K
SP
GATE
IRF7853PbF
SN
SN
GATE
VC
1uF
VINA
RVC
11K
SP
VC
PI2007
SGND
PI2007
PGND
SGND
1uF
VINB
PGND
Ch1
OR'ed Supply
1uF
VC
10K
GND
PI2003
SN
GATE
VINB
47uF
RPG
11K
FT
10K
FT
Si7922DN
Si7922DN
Ch2
Figure 2 – PI2003 / PI2007 FET OR’ed supplies
GND
Figure 1 – Diode OR’ed Supplies
Dual, High and Low Side ORing Application
Using PICOR’s PI2003 and PI2007 • REV 1.0
1
PI2003
GATE
SN
Ch4
GND
Ch1
VC
SP
Ch2
IRF7853PbF
1uF
GND
47uF
SP
Ch3
VINA
RPG
11K
v i c o r p o w e r. c o m
IRF7853PbF
PI2003 / PI2007 ORing Performance
The waveforms in Figure 3 represent the performance of the
diode ORing circuit shown in Figure 1. Channel 1 (blue) is the
voltage seen on the input supply “A” rail; channel 2 (red) is the
OR’ed output voltage. Both waveforms are referenced to the
OR’ed output GND and are DC measurements with a 30 V offset.
Prior to shorting the “VINA” supply at 200 us, the voltage
measured on “VINA” is approximately a diode higher than the
OR’ed output voltage. The voltage of “VINB” is purposely
set to be 700 mV lower that “VINA” so that “VINA” is the
dominate supply.
Once the short occurs, the voltage on “VINA” drops off the
screen and the OR’ed output voltage will start to decay to the
value of “VINB”, minus the diode drop. When compared to the
original value of “VINA”, this difference is about 1.6 V. The drop
in the OR’ed output voltage is the difference in the set voltage
of the two input supplies, about 700 mV.
Figure 4 – Active ORing, Using MOSFET’s
Design/Safety Concerns
In the event that a power feed is not shorted, but is instead
disabled or removed, the response of the FET ORing circuit must
be to completely isolate the input terminals to the OR’ed output
supply. If the circuit in Figure 2 were designed without the
MOSFET’s that enable the high side PI2007 controller (Figure 5),
there can be conditions where the OR’ed output voltage can
appear on the input terminals even without a supply being
connected. This condition could pose a potential safety hazard.
OR'ed Supply
IRF7853PbF
RVC
11K
SP
GATE
IRF7853PbF
SN
SN
GATE
1uF
VINA
SGND
Figure 3 – Diode ORing
Dual, High and Low Side ORing Application
Using PICOR’s PI2003 and PI2007 • REV 1.0
VC
PI2007
PI2007
SGND
1uF
VINB
PGND
PGND
The waveforms in Figure 4 represent the active ORing circuit
shown in Figure 2. Prior to the short, the OR’ed output voltage
and the voltage of “VINA” are the same due to the very low
impedance of the MOSFET in its “on” state. Once the “VINA”
supply (blue) is shorted, the OR’ed output voltage (red) begins to
drop to the value of “VINB”, minus the ORing FET's body-diode
voltage drop. The FT pin of the PI2003 (pink) associated with the
“VINB” supply begins to turn “on” once the OR’ed output
voltage has decayed to the level where the load current in not
being supplied by the OR’ed output capacitor. Once the FT pin is
fully “on” it enables the PI2007 high-side ORing controller and
less than 2 ms later the OR’ed output voltage is seen rising due
to the high-side MOSFET being turned “on”. Now the OR’ed
output voltage is equal to the “VINB” voltage, which is still set to
be ~700 mV lower than “VINA”. The FT pin of the PI2003 associated with the “VINA” supply (green) shuts off shortly after the
short-circuit event due to the fact that the PI2007 was first to
react to the short circuit and has shut itself “off”, isolating the
short from the OR’ed output supply. With no voltage source
available to power the PI2003, the PI2003 shuts off its MOSFET
once its supply (VC) pin passes below the under-voltage fault level.
RVC
11K
SP
VC
Ch4
10K
1uF
VC
10K
GND
PI2003
SN
GATE
SP
1uF
VC
GND
RPG
11K
FT
Ch2
FT
47uF
Ch1
Ch3
RPG
11K
IRF7853PbF
PI2003
SP
GATE
SN
IRF7853PbF
GND
Figure 5 – Active ORing circuit with PI2003 and PI2007 enabled by
OR’ed output.
With the high- and low-side MOSFETs in Figure 5 fully “on” for
either input supply, the removal of that supply creates a highimpedance path made up of the 11 k RPG resistor and the
voltage drop across the PI2007. The reversed current in this path
is not enough to trigger a “reverse current fault condition”,
where the controllers would turn “off” their respective MOSFETs.
Effectively, the OR’ed output voltage is present on the supply
input nodes without the supply being connected, as shown in
Figure 6.
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MOSFET that provides the power return path of the PI2007.
With its power return path “open”, the PI2007’s supply (VC) rail
starts to decay and will turn off its MOSFET drive once an undervoltage fault level is reached (pink). The high-side MOSFET gate
drive is charged-pumped by the PI2007 to be about 10 V higher
than VIN. The gate voltage can be seen to go from about 10 V
greater than VIN to about 10 V lower, which is the voltage seen
on the PI2007’s power return pin. As the voltage across the
PI2007’s VC bypass capacitor decays, the difference in voltage
between the high-side gate and VIN decays as well until they are
equal. As VIN continues to decay the supply to the PI2003 also
decays until it too reaches an under-voltage fault level and turns
off its MOSFET (red).
Figure 6 – VINA Supply unplugged, ORing MOSFETs remained in RDS(on)
minimum state.
The circuit in Figure 7 illustrates the use of the FT signal of the
PI2003 to disable the PI2007; by opening the power return path
from ground and forcing the high-side ORing MOSFET to shut
off. With the high-side MOSFET “off”, the positive path of the
OR’ed output voltage is isolated from the positive input voltage
node. With no positive input voltage present, and no
connection to the OR’ed output voltage supply, the low-side
controller’s supply (VC) decays until it reaches the under-voltage
fault level of the PI2003 and shuts off the MOSFET. With both
the high and low side MOSFET “off’, the inputs are completely
isolated from the OR’ed output supply.
Figure 8 – VINA Supply disabled, but left in-circuit.
OR'ed Supply
RVC
11K
SP
GATE
SN
SN
GATE
SGND
VC
PI2007
PI2007
PGND
Ch4
PI2003
GATE
SP
VINB
1uF
GND
SN
1uF
RPG
11K 10K
RPG
11K
1uF
SGND
PGND
10K
VC
RVC
11K
SP
VC
1uF
VINA
In Figure 9, the supply was completely removed from the circuit
with essentially the same results as the disabled supply. The time
for the VIN discharge was greater with the removed supply since
the impedance of the “disabled” versus the “opened” supply is
much lower and will discharge the input quicker. The addition of
some “bleeder” resistors can decrease this time to more closely
resemble the “disabled” supply waveforms.
IRF7853PbF
IRF7853PbF
VC
GND
FT
FT
47uF
Ch2
Ch1
Si7922DN
Ch3
Si7922DN
IRF7853PbF
PI2003
SP
GATE
SN
IRF7853PbF
GND
Figure 7 – Active ORing circuit with PI2003 and PI2007 enabled by
OR’ed output.
The waveforms in Figure 8 show the response of the circuit in
Figure 7 to an input supply being disabled, but still in-circuit.
When the input supply is disabled, the current to the OR’ed
output load is now provided by the alternate input supply,
making the PI2003 of the “disabled” supply issue a low-current
fault on its FT pin (blue). Pulling the FT pin low shuts off the
Dual, High and Low Side ORing Application
Using PICOR’s PI2003 and PI2007 • REV 1.0
Figure 9 – VINA Supply removed from circuit.
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v i c o r p o w e r. c o m
PI2003 eventually losses its supply voltage and turns off its
MOSFET.
Active ORing Timing Diagrams
VINA
REMOVED/
DISABLED
VINA
VINB
PI2007 UV Fault
VINA
Preventing leakage current from one input supply to another is a
feature of ORing diodes that also needs to be replicated when
using ORing MOSFET’s. The ability of the PI2003 to disable the
PI2007’s high-side ORing MOSFET makes this possible.
VINA
SHORTED
VINB
INPUT
SUPPLYS
Once the input voltage is greater than the UVLO of the PI2003 it
will turn on its MOSFET shorting the input voltage return to the
OR’ed output voltage return. With both input supplies, VINA and
VINB, at this voltage level or greater, both PI2003s will turn on
their MOSFETs. This will effectively short the returns of VINA, VINB
and the OR’ed supply together. Whichever supply is the first to
supply the load current will enable its PI2007 and associated
high side MOSFET. The supply not sourcing the load current will
have a low current fault that will keep its PI2007 controller and
high-side MOSFET disabled.
PI2003 UV Fault
VINA RTN
PI2007 Gate
PI2007
Gate Pin
VINA
PI2007 PG/SG
PI2003 VC
PI2003
FT Pin
GND
PI2003 VC
PI2003
Gate Pin
GND
PI2003 Enabled
PI2007 - Gate Turn-On Delay
PI2003 - Gate Turn-Off Delay
VINA/VOUT
VINB + VDIODE
Figure 10 – Typical Timing Waveforms of the PI2007 and PI2003.
VINA
VINB/VOUT
VINB
INPUT
SUPPLYS
In Figure 10, the VINB supply is shown reaching its peak value
first to illustrate that the PI2003 and PI2007 that are associated
with the VINA supply are not active until the VINA supply starts to
ramp up. Once the VINA supply surpasses the value of the VINB
supply, the system current will be provided by the VINA supply
and the PI2003 will release its FT pin (blue) to enable the supply
of the PI2007 and will turn “on” its ORing MOSFET (red). A
short time later, the PI2007 will turn “on” the gate of the high
side MOSFET (pink) and both ORing MOSFETs will be at their
RDS(on) minimum value. The gate voltage of the PI2007 (pink) is
about 10 V higher than VINA and typically turns on in under 1 ms.
LOAD Current
VINB
Current
0A
LOAD Current
VINA
Current
0A
Figure 11 – Load current transitioning between two input supplies.
When the VINA supply is disabled or removed from the system,
the PI2003 senses the loss of current flow (light load current
fault) and will disable the supply of the PI2007 by asserting its FT
pin “low”. The gate pin of the PI2007 will keep the high-side
MOSFET “on” and maintain a gate voltage greater than the
VINA voltage until the voltage across its bypass capacitor,
connected between the PI2007’s VC and PG/SG pins, decays to
the point where it causes an under-voltage fault within the
PI2007. The PI2007 will then turn “off” its gate drive, which
drops to the value of PG/SG. As the bypass capacitor decays the
gate voltage eventually follows the value of VINA. After the
PI2007 has shut off and as the VINA voltage continues to decay,
the PI2003’s gate voltage will start to drop due to the decaying
of the voltage across its bypass capacitor. Once the voltage has
passed below the under-voltage fault level of the PI2003 the
gate of the low-side MOSFET is turned “off” and now both
ORing MOSFETs are disabled, leaving their parasitic body diodes
to provide the electrical isolation between the OR’ed output
voltage and the input leads.
The powering up sequence of the supplies is not critical; they
can be started together or one after the other. Their voltages are
not critical; they can be the same or vastly different. If the
voltages are the same, then one OR’ed side will turn on first and
the other will stay off due to differing circuit parasitics, even
when powered from the same supply. If the voltages are
different, then the higher voltage will dominate, except when
the voltage difference is less than the body diode voltage drop
of the disabled ORing MOSFET. For example, if VINA were 48 V
and was applied to the circuit first, VINB being 48.4 V and
applied afterwards would not dominate and shut off the ORing
circuit of VINA. VINB’s ORing circuit would remain disabled. For VINB
to dominate, its voltage would have to be increased to a level
where it is greater than the body diode voltage of VINB’s high side
MOSFET. Once this voltage is exceeded, the VINB supply will start
to supply some portion of the load current to the OR’ed supply
through the MOSFET’s body diode. As VINB’s voltage is increased,
so will the amount of current it sources to the load. Once it
reaches a point where the current it is sourcing is enough to
clear the low current fault, the high-side MOSFET of VINB will
turn on and VINB will now provide the total load current. Since
VINB’s voltage is now greater than VINA’s, the OR’ed output voltage
will follow VINB and VINA will go into a low current fault and its
high side MOSFET will be disabled.
Reestablishment of the VINA supply re-enables the PI2003 and
PI2007 ORing devices and their associated MOSFETs. If a short
occurs across the inputs of the VINA supply, the PI2007 responds
first and disables its MOSFET, isolating the VINA supply from the
OR’ed output voltage. With the loss of the VINA supply, the
Dual, High and Low Side ORing Application
Using PICOR’s PI2003 and PI2007 • REV 1.0
VINA
4
v i c o r p o w e r. c o m
The waveforms in Figure 11 depict the transitioning of the load
current when the starting dominate supply (VINB) is surpassed in
voltage by an alternate supply (VINA) and is disabled.
Final Design Conclusion
By using the PI2003 controller’s built-in “low-current-fault”
monitor to enable or disable its paired PI2007 controller, the
same system protection that is provided by ORing diodes can be
achieved under any fault condition; while the lower power
disipation of the MOSFET approach will provide a substantial
power savings.
VINB is the first supply sequenced and supplies the load current
and determines the value of the OR’ed output voltage (VOUT).
Shortly after, the second supply (VINA) is powered up, but it is not
of a sufficiently greater voltage to assume the dominate position
and provide the load current to VOUT. As the voltage of VINA is
increased it eventually reaches a point where the body diode of
VINA’s high-side FET starts to conduct and delivers a small portion
of the load current. The increasing VINA voltage and current
reaches the level where the low current fault of VINA’s PI2003 is
cleared and VINA’s PI2007 is enabled, turning on the high-side
MOSFET. With VINA’s MOSFET “on”, the output supply (VOUT)
now follows VINA and it sources all of the current to the load.
VINB, being at a lower voltage potential and no longer being the
source of the load current, gets disabled due to a low current
fault. The only current that the non-dominate ORing supplies
source is the current to power the PI2003.
Picor PI2007D3-EVAL1, Dual High-side/ Low-side Evaluation Board
OR'ed Supply
IRF7853PbF
IRF7853PbF
RVC
11K
SP
GATE
SN
SN
GATE
VC
1uF
VINA
SGND
VC
PI2007
PI2007
1uF
VC
PI2003
SN
GATE
SGND
1uF
VINB
PGND
PGND
10K
RVC
11K
SP
RPG
11K
RPG
11K
10K
1uF
GND
GND
FT
FT
SP
VC
PI2003
SP
Si7922DN
Si7922DN
IRF7853PbF
GATE
SN
IRF7853PbF
GND
Figure 12 –– PI2003 biased to VIN, used to enable/disable the PI2007.
Dual, High and Low Side ORing Application
Using PICOR’s PI2003 and PI2007 • REV 1.0
5
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