Fault tolerant active current sharing

Details Fault tolerant active current sharing Fault tolerant active current sharing

2000-04-10

2001-11-20

Wong, Peter S. (Department: 2838)

Electric power conversion systems

Current conversion

Having plural converters for single conversion

C323S272000

Reexamination Certificate

active

06320771

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to regulating power supplies, and in particular to a system and method for load-balancing within a parallel power supply. Still more particularly, the present invention relates to an open-loop system in which an external voltage source provides an independent reference for balancing a current load within a parallel power supply.
2. Description of Related Art
Conventional power supply systems often utilize multiple power supply circuits connected in parallel for improving load variation adaptability and increased reliability. Examples of such parallel power supply systems can be found in Malik, U.S. Pat. No. 5,319,536, Gerner, U.S. Pat. No. 5,266,838, and Line; U.S. Pat. No. 5,745,670.
Referring now to
FIGS. 1
,
2
, and
3
, conventional parallel power supply systems
101
,
201
, and
301
are illustrated. As shown in
FIG. 1
, parallel power supply system
101
includes two parallel power supply modules, PS
1
102
and PS
2
104
that provide independent sources of power to a system load
106
. PS
1
102
and PS
2
104
generate output currents I
1
108
and I
2
110
, respectively. Included among the advantages provided by the parallel configuration of power supply system
101
over a unitary power supply system are reduced power supply component costs, enhanced operational flexibility, and improved system reliability.
In the interest of uniformity of device stress and corresponding reduced device failure rates, it is desirable to maintain a balanced power output provided by parallel power sources such as PS
1
102
and PS
2
104
.
FIGS. 2 and 3
illustrate conventional systems for achieving such a balance under varying electrical loads.
As depicted in
FIG. 2
, power supply system
201
includes parallel power supply modules PS
1
202
and PS
2
204
that operate in a “droop” current sharing mode in supplying power to a system load
206
. PS
1
202
and PS
2
204
maintain a required system voltage level, V
OUT
at a common output node
215
. Decoupling diodes D
1
212
and D
2
214
serve as decoupling barriers between supply-side nodes
217
and
219
and common output node
215
. In order to achieve the desired operating balance between PS
1
202
and PS
2
204
, the current, I
1
, through node
217
should be approximately equal to the current, I
2
, flowing through node
219
(one half of the I
T
flowing through common output node
215
).
As further illustrated in
FIG. 2
, power supply system
201
includes circuitry within each of the power supply modules
202
and
204
for maintaining a balance in the relative current load borne by each supply. The technique by which such current sharing is achieved in power supply system
201
is commonly referred to in the art as droop current sharing. As depicted in
FIG. 2
, PS
1
202
and PS
2
204
each include a variable resistor (R
1
220
and R
2
222
respectively) which serves as a setting and adjustment mechanism for the respective output voltages at nodes
217
and
219
. Error amplifiers U
1
224
and U
2
226
are configured as negative feedback devices such that as the current drawn by system load
206
from PS
1
202
and PS
2
204
increases, the output voltage level at output node
215
decreases.
The current balancing achieved by the closed-loop voltage “droop” effect relies on very precise and symmetric device mirroring between the individual power supply modules PS
1
202
and PS
2
204
. Such precision is costly and difficult to achieve. Dynamic variations in system load
206
further magnify imbalances between I
1
and I
2
and result in a wider tolerance required for current imbalance within power supply system
201
.
FIG. 3
depicts an alternate closed-loop current sharing technique known in the art as “active current sharing.” As illustrated in
FIG. 3
, power supply system
301
includes a pair of parallel power supply modules PS
1
302
and PS
2
304
each having a current sense resistor (R
1
306
and R
2
308
). Active current sharing at current sharing feedback node
310
is achieved by utilizing the voltage references developed across R
1
306
and R
2
308
. For example, if the voltage output of PS
2
304
at supply-side node
314
is higher than that at supply-side node
312
within PS
1
302
, the voltage differential across current sense resistors R
1
306
and R
2
308
indicates that PS
2
304
is providing the majority of the current to system load
306
.
The imbalance causes the voltage drop across current sense resistor
308
to be higher than the voltage drop across current sense resistor
306
. A higher voltage drop across R
2
308
results in an increase in the voltage level at current share node
310
. This increase in current share node
310
voltage will increase the driving voltages or amplifiers
316
and
317
and will force a higher current to flow through resistor
320
and transistor
318
. An increased voltage drop across resistor
320
will result in a higher voltage at node
312
. This process continues until both the power supplies PS
1
302
and PS
2
304
start current sharing. In this manner, whichever of PS
1
302
or PS
2
304
has a higher voltage becomes the master and the other become a slave.
The load balancing within power supply system
301
thus relies on closed-loop feedback provided between PS
1
302
and PS
2
304
at current sharing feedback node
310
. A varying current through activated transistor
318
or transistor
319
develops an offset voltage across resistors
320
or
321
which causes the output voltage of PS
1
302
or PS
2
304
to increase until both PS
1
302
and PS
2
304
have equal output voltage levels and thus provide equal current to system load
306
.
By providing feedback between each of its constituent power supply modules, the active current sharing system depicted in
FIG. 3
provides a more accurately balanced current load between its respective power supply modules than the droop current sharing system depicted in FIG.
2
. However, this active sharing method requires expensive overhead components and devices such as very precise and low resistance current sense resistors (R
1
306
and R
2
308
), and low-offset amplifiers (U
1
322
and U
2
324
) within each power supply module. This closed-loop feedback approach relies on interdependencies among parallel power supplies to ensure accurate load balancing. Low tolerance devices, such as amplifiers with very low offsets are required to achieve a reliable interdependence.
It would therefore be desirable to provide an system and method for maintaining an accurate load balance for parallel power supplies without relying on interdependent operating parameters among individual power supply modules.
SUMMARY OF THE INVENTION
A system and method are disclosed for maintaining a balanced current level among multiple power supply modules within a parallel power supply. An external voltage is applied to an internal reference translator circuit within each of the parallel power supply modules, such that an internal current-sharing voltage reference is generated within each power supply module. An external reference translator circuit is biased by an independent voltage level, such that the external reference translator circuit generates an external current-sharing voltage reference that is maintained at a higher voltage level than the internal current-sharing voltage references generated within the parallel power supply modules. The external current-sharing voltage reference acts as a master current share reference with respect to which each of the internal current-sharing voltage references. A current adjustment circuit within each of the power supply modules compares the internal current-sharing voltage reference with the master current share reference and generates a current adjustment signal in response to detecting a voltage differential between the master current share reference and the internal current-sharing voltage reference, such that load-sharing is provided among the parallel power supply modules.


REFERENCES:
patent: 4924170 (1990-

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