Easy Balanced Load Sharing for Three or Four Supplies, Even with Unequal Supply Voltages

Easy Balanced Load Sharing for Three or Four Supplies,
Even with Unequal Supply Voltages
Vladimir Ostrerov and Chris Umminger
Using multiple small power supplies is often more
economical and more reliable than using a single large
power supply. For instance, separate batteries can be
used for higher reliability. In a multi-supply system, it
is important that the load is equally shared; otherwise,
one supply may attempt to carry the entire load.
This article shows how to easily load balance three
or four supplies by cascading LTC4370 circuits.
The LTC4370 controller enables current
sharing between two supplies with a
modest difference between the output
voltages, as shown in Figure 1. To perfectly
balance the current in both sides, the
controller regulates the gate-source voltage
of an N-channel MOSFET in whichever
side has the higher voltage. This creates
a voltage drop across the MOSFET’s
RDS(ON) plus the current sense resistor.
The LTC4370 can compensate for a voltage
difference between two rails of up to
0.5V. If the voltage difference of the two
supplies is somewhat less than 0.5V, the
Figure 1. The LTC4370 currentbalancing controller enables balanced
load sharing between two supplies,
even when their voltage outputs are
different.
of the total load current equally. The
output voltage at the load is less than
the minimum of the supply voltages V1,
V2 and V3. Because there are two stages
of cascading, it is possible to have as
much as 1V difference between V3 and
V1 or V2, if the difference between V1
and V2 is already at the 0.5V limit.
LTC4370 can regulate its output to match
the lower value rail, set by adding an
appropriate resistor on the RANGE pin.
BALANCING THE LOAD BETWEEN
THREE SUPPLIES WITH TWO
CASCADED LTC4370s
Figure 2 shows a 3-input, 12V system
delivering 10A. Notice that one LTC4370
(U1) performs equal current sharing
between supplies V1 and V2, while the
second LTC4370 (U2) implements a 2:1
relation between the output current of
U1 and the current of a third supply, V3.
Thus, each supply contributes one third
39nF*
0.1µF
NC
VIN1
GATE1
OUT1
VCC
OUT2
EN2 CPO2
VIN2
*OPTIONAL, FOR FAST TURN-ON
2mΩ
11.875V
0.18µF
SUM85N03-06P
+ 25mV –
5A
26 | May 2016 : LT Journal of Analog Innovation
10A
GATE2 COMP
39nF*
11.9V
2mΩ
FETON2
RANGE
•Sense resistor tolerance, worst-case
for 1% resistors is 2% overall.
11.875V
FETON1
LTC4370
GND
LIMITATIONS
•LTC4370 error amplifier input
offset, ±2mV (maximum)
+ 325mV –
SUM85N03-06P
EN1 CPO1
Cascading three LTC4370 controllers
(Figure 2) allows four supplies to share
the load. In the first stage, U1 and U2
force equal sharing between a pair of
supplies, where the output current of U1 is
I12 = I1 + I2 , and the output current of U2
is I34 = I3 + I4 . A third LTC4370, the second
stage, keeps I12 = I34 . Thus, each supply
contributes one fourth of the total load
current. The two stages, as above, allow
the possibility of as much as 1V difference between the four supply voltages.
The main error sources that affect
perfect current sharing are:
5A
12.2V
BALANCING THE LOAD BETWEEN
FOUR SUPPLIES
Sharing error attributed to the error
amplifier input offset decreases with
increasing sense voltage, but power
dissipation increases. For the simple
LTC4370 circuit with two supplies, this
error is expressed as an imbalance in
the supplies’ sharing of current:
design ideas
12.4V
EN1
SUM85N03-06P
∆I = I1 − I2
39nF 50V
EN1 CPO1
VIN1
10k
Using the worst-case errors,
above, the error is:
GATE1
OUT1
VCC
VCC
0.1µF
U1
LTC4370
GND
NC
2mΩ
FETON2
2mΩ
RANGE
10k
VIN2
GATE2 COMP
39nF 50V
12.0V
Figure 2. Two LTC4370s
can be cascaded to
enable current sharing
of three supplies.
 2mV

∆I ≤ 
+ 0.01• ILOAD  [A]
 RSENSE

I12 = I1 + I2 = ILOAD
I2 = ILOAD
OUT2
EN2 CPO2
EN2
I1 = ILOAD
FETON1
For the circuit of Figure 2, where ideal
load sharing means the load is distributed into 1⁄3ILOAD and 2⁄3ILOAD , it is easier
to estimate the worst-case imbalance
via an expression of the maximum and
minimum current of each supply:
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
SUM85N03-06P
39nF 50V
EN1
EN1 CPO1
VIN1
10k


2mV
IMAX =  0.672 • ILOAD +
[A]
3.01• RSENSE 

GATE1
OUT1
VCC
VCC
0.1µF
U2
LTC4370
GND
NC
2mΩ
FETON2
4mΩ
VIN2
10k
GATE2 COMP
39nF 50V
12.0V
V1
EN1
SUM85N03-06P
VIN1
GATE1
OUT1
VCC
U1
LTC4370
GND
NC
2mΩ
FETON2
2mΩ
OUT2
VIN2
EN2 CPO2
GATE2 COMP
39nF 50V
Figure 3. Four supplies
can each support an equal
share of a load by using
three LTC4370s in a 2-stage
cascade.
V2
V3
I1 = ¼ILOAD
FETON1
RANGE
10k
By cascading the shared output of one
LTC4370 with another LTC4370, three or
more supplies can be efficiently controlled
to provide equal current to the load. With
errors on the order of the sense resistor
tolerance, the voltage drop is minimal. n
39nF 50V
EN1 CPO1
EN2
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
SUM85N03-06P
EN1
EN1 CPO1
OUT1
VCC
VCC
0.1µF
U2
LTC4370
GND
NC
10k
2mΩ
FETON2
2mΩ
VIN2
GATE2 COMP
39nF 50V
V4
GND
I12 = ½ILOAD
FETON1
2mΩ
FETON2
2mΩ
RANGE
VIN2
GATE2 COMP
39nF 50V
ILOAD
I34 = ½ILOAD
OUT2
I3 = ¼ILOAD
FETON1
OUT2
RANGE
EN2 CPO2
EN2
GATE1
U3
LTC4370
EN2 CPO2
10k
GATE1
OUT1
VCC
NC
VIN1
VIN1
10k
39nF 50V
EN1 CPO1
39nF 50V
VCC
0.1µF
SUM85N03-06P
10k
I12 = I1 + I2 = ½ILOAD
I1 = ¼ILOAD
EN2
EN1
CONCLUSION
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
10k
VCC
0.1µF
ILOAD
I3 = ILOAD
OUT2
EN2 CPO2
EN2
FETON1
RANGE


2mV
IMIN =  0.328 • ILOAD +
[A]
3.01• RSENSE 

I12 = I1 + I2 = ILOAD
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
I4 = ¼ILOAD
I34 = I3 + I4 = ½ILOAD
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
May 2016 : LT Journal of Analog Innovation | 27
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