ISL6144_07 Datasheet, PDF (21/29 Pages) Intersil Corporation – High Voltage ORing MOSFET Controller

ISL6144
Fault 6: ORing FET Body Diode Conduction
(V_IN - 0.41V > V_OUT). If the voltage drop across the
MOSFET approaches 410mV, a fault will be indicated. Make
sure the selection of the ORing MOSFET takes this fact into
account.
Application Considerations and
Component Selection
“ISL6144 + ORing FET” vs “ORing Diode” Solution
“ISL6144 + ORing FET“ solution is more efficient than the
“ORing Diode” Solution, which will result in simplified PCB
and thermal design. It will also eliminate the need for a heat
sink for the ORing diode. This will result in cost savings. In
addition is the fact that the ISL6144 solution provides a more
flexible, reliable and controllable ORing functionality and
protects against system fault scenarios (Refer to the fault
detection block description.)
On the other hand the most common failures caused by
diode ORing include open circuit and short circuit failures. If
one of these diodes (Feed A) has failed open, then the other
Feed B will provide all of the power demand. The system will
continue to operate without any notification of this failure,
reducing the system to a single point of failure. A much more
dangerous failure is where the diode has failed short. The
system will continue to operate without notification that the
short has occurred. With this failure, transients and failures
on Feed B propagate to Feed A. Also, this silent short failure
could pose a significant safety hazard for technical
personnel servicing these feeds.
“ISL6144 + ORing FET” vs “Discrete ORing FET”
Solution
If we compare the ISL6144 integrated solution to discrete
ORing MOSFET solutions (with similar performance
parameters), the ISL6144 wins in all aspects, the main ones
being simplicity of an integrated solution, PCB real estate
saving, cost savings, and reduction in the MTBF of this section
of the circuit as the overall number of components is reduced.
In brief, the solution offered by this IC enhances power
system performance and protection while not adding any
considerable cost, on the contrary saving PCB board real
estate and providing a simple to implement integrated
solution.
ORing MOSFET Selection
Using an ORing MOSFET instead of an ORing diode results
in increased overall power system efficiency as losses across
the ORing elements are reduced. The benefit of using ORing
MOSFETs becomes even more significant at higher load
currents as power loss and forward voltage drop across the
traditionally used ORing diode is increased. The high power
dissipation across these diodes requires paralleling of many
diodes as well as special thermal design precautions such as
heat sinks (heat dissipating pads) and forced airflow.
For example, in a 48V, 32A (1 + 1) redundant system with
current sharing, using a Schottky diode as the ORing device
(Refer to Figure 31), the forward voltage drop is in the
0.4V to 0.7V range, (let us assume it is 0.5V). The power loss
across each diode is shown in Equation 10:
Ploss_(D1)
=
Ploss_(D2)
=
I_{--O------U----T--}
2
⋅
VF
=
16A ⋅ 0.5V
=
8W
(EQ. 10)
The total power loss across the two ORing diodes is 16W.
INPUT BUS 1
DC/DC
1
+IN1 = 48V
CS
INPUT BUS 2
DC/DC
2
+IN2 = 48V
D₁
0.5V@ 16A
V_OUT
(32A)
D₂
0.5V @ 16A
FIGURE 30. 1 + 1 REDUNDANT SYSTEM WITH DIODE ORING
If we use a 4.5mΩ MOSFET (refer to Figure 32), the nominal
Power loss across each MOSFET is:
Ploss_(M1)
=
Ploss_(M2)
=
⎛
⎝
I_{--O-----2-U----T--⎠⎞}
2
⋅
r_D
S
(ON)
Ploss_NOM(M1) = (16A)² ⋅ 4.5mΩ = 1.152W
(EQ. 11)
The total power loss across the two ORing MOSFETs is
2.304W.
In case of failure of current sharing scheme, or failure of
DC/DC 1, the full load will be supplied by DC/DC 2. ORing
MOSFET M2 or ORing Diode D₂ will be conducting the full
load current. Power lost across the ORing devices are:
Ploss_MAX(D2) = IOUT ⋅ VF = 32A ⋅ 0.5V = 16W
(EQ. 12)
Ploss_MAX(M2) = (IOUT)² ⋅ r_DS(ON) = (32A)² ⋅ 4.5mΩ = 4.6W
(EQ. 13)
INPUT BUS 1
INPUT BUS 2
DC/DC
1
CS
DC/DC
2
+IN1 = 48V
M1
4.5mΩ
0.072V @ 16A
+IN2 = 48V
M2
4.5mΩ
0.072V@ 16A
V_OUT
(32A)
FIGURE 31. 1+1 REDUNDANT SYSTEM WITH MOSFET ORING
21
FN9131.3
February 15, 2007