ISL6144 Datasheet, PDF (8/29 Pages) Intersil Corporation – High Voltage ORing MOSFET Controller

ISL6144
High Voltage Pass and Clamp
A high voltage pass and clamping circuit prevents the high
output voltage from damaging the comparators in case of
quick drop in V_IN. The comparators are running from the 5V
supply between HVREF and V_IN. These devices are rated
for 5V and will be damaged if V_OUT is allowed to be present
(as the output is powered from other parallel supplies), and
does not fall when V_IN is falling. For example, if V_IN falls to
30V, V_OUT remains at 48V and the differential Voltage
between the “-” and “+” terminals of the comparator would be
18V, exceeding the rating of the devices and causing
permanent damage to the IC.
Fault Detection Block
The fault detection block has two monitoring circuits (refer to
Figure 2):
1. Gate monitoring detects when the GATE < V_IN + 0.37V
2. V_OUT monitoring detects when V_IN - 0.41V > V_OUT
These two outputs are ORed, inverted, level shifted, and
delayed using an internal filter (T_FLT)
The following failures can be detected by the fault detection
circuitry:
1. ORing FET off due to dead short in the sourcing supply,
leading to V_IN < V_OUT
2. Shorted terminals of the ORing FET
3. Blown fuse in the power path of the sourcing supply
4. Open Gate terminal
5. HVREF UV
The FAULT pin is not latched off and the pull down will shut
off as soon as the fault is removed and the pin becomes high
impedance. Typically, an external pull-up resistor is
connected to an external voltage source (for example 5V,
3.3V) to pull the pin high, an LED can be used to indicate the
presence of a fault.
GATE
FAULT
DELAY
120µs
LEVEL SHIFT
0.37V
+
-
VIN
0.41V
+
-
VOUT
FIGURE 2. FAULT DETECTION BLOCK
Application Considerations
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 use of ORing
MOSFETs becomes more important at higher current levels,
as power loss across the traditionally used ORing diode is
very high. The high power dissipation across these diodes
requires special thermal design precautions such as heat
sinks and forced airflow.
For example, in a 48V, 40A (1+1) redundant system with
current sharing, using a Schottky diode as the ORing
(auctioneering) device (see Figure 3), the forward voltage
drop is in the 0.4V to 0.7V range. Let us assume it is 0.5V,
power loss across each diode is as shown in Equation 4:
Ploss_(D1)
=
Ploss_(D2)
=
I_{--O------U----T--}
2
⋅
VF
=
20A ⋅ 0.5V
=
10 W
(EQ. 4)
Total power loss across the two ORing diodes is 20W.
INPUT BUS 1
36VDC TO 75 VDC
CIN1
100µF
C_d1
220nF
C_cs1
1nF
DC/DC
#1
+IN +OUT
PC
+S
SC
PR
-S
+OUT1 = 48V
R_pb1
10
(Note 11)
Figure 14
-IN -OUT
D₁
0.5V@ 20A
VOUT
(40A)
INPUT BUS 2
36VDC TO 75 VDC
C_IN2
100µF
C_d2
220nF
C_cs2
1nF
SECONDARY
DC/DC
GROUND
#2
+IN +OUT
PC
+S
+OUT2 = 48V
R_pb2
10
SC ^{(Note 11)}
Figure 14
PR
-S
-IN -OUT
PRIMARY GROUND
D₂
0.5V@ 20A
FIGURE 3. 1 + 1 REDUNDANT SYSTEM WITH DIODE ORing
If a 5mΩ single MOSFET per feed is used, the power loss
across each MOSFET is as shown in Equation 5:
Ploss_(M1)
=
Ploss_(M2)
=
⎛
⎝
I_{--O-----2-U----T--⎠⎞}
2
⋅
r_D
S
(ON)
(EQ. 5)
Ploss_(M1) = (20A)² ⋅ 5mΩ = 2W
Total power loss across the two ORing MOSFETs is 4W.
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
8
FN9131.5
October 8, 2010