DN1036 - Using a Current Sharing Controller with Non-Redundant Supplies

Using a Current Sharing Controller with
Non-Redundant Supplies
Design Note 1036
Bob Smith
Introduction
The LTC®4370 is a 2-supply current sharing, diodeORing controller that uses MOSFETs to form ideal
diodes. In this way, the LTC4370 can actively balance
the output currents of two supplies, even those with
unequal output voltages. For two unequal voltage input
supplies, the forward voltage of the higher voltage supply
diode is servoed to balance the shared load current. The
maximum allowable voltage drop is programmed by a
resistor at the RANGE pin of the LTC4370.
A typical diode-OR system is a winner-take-all system
where the highest voltage supply sources the entire
load current. This one-supply-at-a-time scheme underutilizes the two supplies. The LTC4370’s current sharing
diode-OR solution, on the other hand, reaps the benefits
of sourcing and sharing current from both supplies:
• Supply lifetimes are extended if each takes on half
the load, spreading the supply heat and reducing
thermal stresses on supply components.
• Because the lower voltage supply is always operational, there is no surprise when transitioning
to a backup supply that may have already silently
failed—a possibility in a simple diode-OR system.
• The recovery dynamics on supply failure are
smoother and faster, since the supply changes are
on the order of less and more, not off and on.
• A DC/DC converter formed by two supplies running at half capacity has better overall conversion
efficiency than a single supply running near full
capacity.
11/15/1036
Normally the LTC4370 is used to current share the
outputs of two supplies, either of which is capable
of providing the entire load current when its mate
drops below the sharing voltage threshold set by the
RANGE pin of the LTC4370.
The current sharing function may also be used to
provide the total load current by two lower current
supplies with a combined current equal to, or in excess
of that required by the load.
A problem remains, however, that in normal operation, the LTC4370 allows the higher voltage supply
to source all of the load current. Since, in this case,
the remaining supply is incapable of providing the
full current, it is necessary to prevent such operation.
This design note describes a solution that disables
the downstream load when this situation occurs.
Principal of Operation
In normal operation, the LTC4370 monitors the current of both supplies. Normally with a perfect diode,
the supply with the higher voltage would source all of
the current to the load. The LTC4370 prevents this by
linearly controlling the MOSFET of the higher voltage
supply to provide current equal to that of the lower
voltage supply. The maximum voltage difference
allowed is determined by the resistor between the
RANGE pin and ground.
When the input supply voltage difference rises beyond
the programmed range, the LTC4370 disables the current sharing function. There are two alarm outputs,
L, LT, LTC, LTM, Linear Technology, the Linear logo and Over-The-Top are
registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
each monitoring the control voltage at the gate of each
MOSFET. In normal operation, when either MOSFET
is turned off (indicating a voltage difference beyond
the programmed range) its associated FETON signal
is set to a logic low.
In theory it seems that if these signals were passed
through a logic AND function, they could be used
to control the downstream load, disabling it when a
MOSFET is turned off (indicating the loss of current
sharing). These signals, however, both revert to
logic low when zero current is passing through the
MOSFET. In this situation, with the downstream load
disabled and drawing no current, the system would
remain in this state indefinitely.
The solution described here senses input voltage
differences and disables the downstream load when
a defined voltage difference between the supplies
is detected. This voltage difference is programmed
to be below the maximum voltage threshold of the
LTC4370. If an out of balance condition is detected, the
downstream power supply is disabled. To prevent an
oscillatory condition, the circuit enters hiccup mode,
where the supply is cycled on for 200ms every 3.2
seconds. The block diagram is shown in Figure 1.
As shown in the block diagram, two comparators are
used to sense when the absolute value of the difference
between power supply inputs VINA and VINB is beyond
that allowed for proper current sharing. When this
happens, the output of the negative true OR gate is a
logic high, enabling the hiccup circuit. Normally, the
output of the hiccup circuit is a logic high, enabling
the downstream load. When the out of range fault
condition is detected, the hiccup circuit is activated,
causing a logic low to disable the downstream load.
The hiccup circuit monitors the voltage differential
during the 200ms on period and is disabled when
the fault condition is cleared.
Circuit Description
Figure 2 shows the complete solution. In Figure 2,
U2 and U3 are LT1716 Over-The-Top ® voltage comparators used to detect voltage differences between
VINA and VINB.
Threshold offset voltage to the comparators is
provided by current sink transistors Q5 and Q6 in
combination with R8 and R9. Current at the collectors
of Q5 and Q6 is stabilized at 100µA by transistors
Q1, Q2, Q3, and U6, an LT6650 voltage reference.
In this case, R8 and R9 are set to 3.01k, resulting
in an offset of 300mV. These resistor values can be
changed to provide a different offset to match that
of the LTC4370.
VINA
LTC4370
VINB
–
–
VTH
VTH
+
–
+
+
–
+
A > (B + TH)
FEED
TO LOAD
DIODE-OR
CURRENT SHARING
CONTROLLER
(ON 200ms, OFF 3.2s)
HICCUP
CIRCUIT
ENABLE
B > (A + TH)
Figure 1. Block Diagram of Current Summing Circuitry
TO LOAD
dn1036 F01
VINB
EN2
GND
EN1
2
IN
1
U6 FB
LT6650
5
OUT
GND
4
C5
1µF
R10
10M
C6
1µF
Q3
2N2907(2)
2N2907
Q1
R6
1k
R11
40.2k
Q4
Q2
R7
1k
C1
0.1µF
R12
40.2k
Q5
R8
3.01k
R9
3.01k
13
5
1
3
4
U3
LT1716
3
4
U2
LT1716
C7
0.47µF
2
5
2
OUT2
FETON2
FETON1
R14
499k
5
4
2
1
10
9
C8
0.1µF
5
3
7
8
9
OFF1
D
C
A
6
3
8
13
12
U4
74HC132
2
7
10
B
C4
0.18µF
OFF2
R2
4.7k
M2
SUM85N03-06P
COMP
GATE2
Figure 2
1
4
6
C3
0.039µF
VIN2
12
M1
SUM85N03-06P
GATE1
10
OUT1
U1
LTC4370CDE
VIN1
CPO2
EN2
RANGE
EP
GND
VCC
EN1
R13
40.2k
2N2222X3
Q6
R1
30.1k 1
2
17
15
14
16
CPO1
11
C2
0.039µF
+
–
VINA
+
–
9
CP
PE
CEP
CET
11
R3
4.7k
3
4
P1
5
P2
13
Q1
12
Q2
U5
74HC163
14
Q0
P0
C9
0.1µF
R5
2mΩ
1%
R4
2mΩ
1%
6
MR
TC
11
8
dn1036 F02
1
15
16
VCC
Q3 GND
P3
VIN
FAULT
ENABLE
(TO LOAD)
OUTPUT
(TO LOAD)
GND
When either comparator U2 or U3 reaches the threshold determined by the offset, their output becomes
logic low, enabling the hiccup circuit.
U4 is a 74HC132 quad CMOS NAND gate with hysteresis on each input. U5 is a 74HC163 4-bit programmable CMOS counter.
The output of U4A is logic low when VINA and VINB
is within the threshold determined by R8 and R9.
When VINA and VINB is beyond this threshold, the
corresponding comparator output becomes logic low,
causing the output of U4A to become a logic high.
A logic high output of U4A is inverted by U4B, producing a logic low at one input of NOR gate U4C. The
resulting logic high output of U4C causes counter U5
to begin counting. The first count is zero, causing the
TC (terminal count) pin to become logic low. This
output remains low for the next 15 counts regardless
of the input from U11B, due to the feedback from its
output to the other input of NOR gate U4C. On count
16, the TC becomes high for a period of 200ms. During this period, the downstream load is enabled. If
the comparators determine the voltage difference is
within limits, the counter stops with the TC output
Data Sheet Download
www.linear.com/LTC4370
Linear Technology Corporation
remaining logic high, enabling the load. If the voltage
difference is not within limits, the counter begins
again, counting to 15 with the TC output a logic low.
In this way, the load is enabled for 200ms every 3.2
seconds until the fault condition is cleared.
The clock is provided by U4D, a hysteretic relaxation
oscillator with a period of 200ms determined by R14
and C7.
U1 is the LTC4370, which provides the current sharing function. The threshold is set to 300mV by R1.
Operation of this device is described in the data sheet.
Power for the additional circuitry is derived from VCC
of the LTC4370.
Summary
The LTC4370 is designed primarily as a current sharing, diode-OR controller for two redundant supplies.
With a few additional components, it can be easily
be used in a non-redundant supply environment as
a robust load sharing controller, where both of the
supplies are necessary to support the entire load.
The solution described here provides that function.
For applications help,
call (408) 432-1900
dn1036f LT 1115 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2015
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