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ICInfA.2017.8079056

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Proceedings of the 2017 IEEE
International Conference on Information and Automation (ICIA)
Macau SAR, China, July 2017
Research on the Maximum Current Automatic CurrentSharing Control Based on DSP
Qingqing Li and Shulin Liu
Huisan Xu and Xiao Wang
Academy of Electrical and Control Engineering
Xi’an University of Science and Technology
Xi’an, Shaanxi Province, China
[email protected]
[email protected]
Academy of Electrical and Control Engineering
Xi’an University of Science and Technology
Xi’an, Shaanxi Province, China
[email protected]
[email protected]
Abstract - A maximum current automatic current-sharing
control method is implemented for high performance control of
paralleling Buck converters based on DSP controller. The system
samples the output current and sends it to a high precision
instrumentation amplifier INA128 to amplify. Then, the amplified
signal is sent to each parallel module by ADC module through
CAN bus, and transmitted back to DSP after decoded. The DSP
compares the received sampling current data and selects a main
module with the maximum output current. The main module
calculates the average current and sends it back to the DSP.
Meanwhile, the main module voltage is set as the output voltage
of current-sharing bus and compared with the reference voltage
to steady output voltage. Through comparing the current-sharing
value from each module and output current, DSP calculates and
sends the current error increment to advanced PI regulator and
changes the data in CMPR of ePWM module to control the duty
ratio of MOSFET. Then, a steady current-sharing output of
parallel Buck power supplies system could achieved. The parallel
Buck modules are used for digital current-sharing experiment
and relevant parameters are tested, which shows the prototype
can achieve bus current-sharing excellently.
sharing algorithm and DSC loop adjustment program module.
The function of CAN communication module is to exchange
data of every DC power modules in the distributed power
supply system and send modules current data to currentsharing algorithm module. And the algorithm selects a main
module with the maximum output current from divided
modules. DSC loop processing module sets the maximum
current as the reference current and regulates the module
current loop based on the difference between the reference
current and the module output. Through adjusting constantly,
the system would be able to achieve current-sharing [4].
Keywords-digital control; maximum current
current-sharing method; advanced PI algorithm
II. TECHNOLOGY SOLUTION
The current-sharing of multiple paralleling switch power
system supplies is shown in Fig. 1.
automatic
I. INTRODUCTION
Nowadays, power electronic technology is applied in wide
fields such as communication, spatial science and computer
science. And there has been a growing demand of power
source with higher power, higher power density and reliability.
In single power system, there would be difficulties in terms of
choosing power devices with achieving higher switching
frequency and higher power density. Furthermore, the whole
system would collapse if the power fails in single power
system [1]. Therefore, paralleling power is an important
direction of high power supply technology.
Similar to single power supply, paralleling power supplies
should be able to run reliably with constant output voltage
while input voltage and output load change [2]. Thus,
measures need to be taken to prevent electrical and heat stress
unbalance caused by the load difference of each paralleling
power supply [3]. Current-Sharing (CS) is a technique that
could provide equal current distribution of the load current
among the parallel-connected power supplies.
In this research, digital current-sharing is realized with the
method of maximum current automatic current-sharing. Digital
CS design comprises CAN communication module, current-
978-1-5386-3154-6/17/$31.00 ©2017 IEEE
Fig. 1 Parallel Current-sharing of multiple switching power supply.
A. The Principle of Automatic Maximum Current - Sharing
Method
The hardware structure block diagram of paralleling
power system based on the maximum current-sharing
technology is shown in Fig. 2. The experiment platform
consists of main power conversion circuit, driving circuit of
MOSFET, current sensing circuit, voltage sampling circuit,
auxiliary power supply circuit, over-current and over-voltage
protection circuit and control circuit based on TMS320F2803.
B. The Principle of Automatic Maximum Current - Sharing
Method
Maximum current CS method is also known as automatic
main-minor setting method or democratic CS method. In
paralleling power supply system, the output of every separated
parallel module is mounted on the current- sharing bus through
an external diode. The current-sharing connection method is
shown in Fig. 3 [5].
1044
CAN总总
•
•
CAN总总
均均均总线均
•
•
•
•
• • • •
Fig. 2 Hardware structure block diagram
achieved by automatic maximum current CS method. The
block diagram of main circuit current-sharing control is shown
in Fig. 5.
VˆAC
−
Fr
+
GCS
VˆA
Vref
+
Fig. 3 Method of connection between separated parallel module and currentsharing bus.
−
In the process of the automatic maximum current-sharing,
Vb (the voltage of current-sharing bus) reflects the voltage of
the module with the highest current output. In steady state,
parallel power modules distribute balance current to load. If
the output current of one module increased and became the
maximum current, the voltage of this branch Vi would also
increased, and it will be set as bus voltage. In other words, Vb
is set equal to Vimax. Meanwhile, this module would be set as
the main module, and others become minor modules. Then,
those voltage data of minor modules are compared with the Vb
of current-sharing bus, and the output increments make effect
on the reference voltage Vr of voltage closed loop to regulate
the reference voltage Vr' of voltage amplifier. In this way,
automation maximum current CS can be achieved [6]. The
circuit control principle of automatic maximum current CS is
shown in Fig. 4.
Gv ( s )
1
Um
Gvd(s)
Vout
Fig. 5 Main circuit current sharing control block diagram.
In Fig. 5, VAC is current sharing signal, VA is current
sharing error signal, H is the gain of output current, Gcs is the
gain of current sharing, Fr is the transfer function of module
load current convert into the voltage process [7].
III. HARDWARE DESIGN
TMS320F28035 is adopted to improve the performance of
experimental prototype, which has higher efficiency CPU that
frequency can reach 60 MHz per second, compared to
traditional DSP chip. The block diagram of experimental
prototype circuit is shown in Fig. 6.
Fig. 4 Circuit control principle of maximum current-sharing.
C. Small Signal Control Model of Maximum Current-sharing
Considering the high current sharing requirement of
designed system, the built model with voltage closed loop is
Fig. 6 Block diagram of experimental prototype circuit based on DSP.
1045
In this research, PCA82C250 is implemented as the CAN
bus driver. It serves as the interface between CAN protocol
controller and physical bus, and enables CAN bus with the
capability of differential transmission and receiving [8]. The
eCAN interface circuit is shown in Fig. 7.
CPU clock source TIMES0/1/2. In this paper, EPWM cycle
interrupt is adopted to start A/D conversion. The A/D interrupt
service routine will be triggered by the generated A/D interrupt
signal after the conversion is completed. In the process of A/D
interrupt service routine, CAN communication service
program, current-sharing regulation and DSC loop adjustment
program, which affect the performance of the whole system.
And these three sub-programs are fundamental for the digital
current-sharing adjustment algorithm. The interrupt service
program flowchart is shown in Fig. 9.
Fig. 7 eCAN interface circuit.
IV. SOFTWARE RESULTS
The design of system software consists of main program
and interrupt response service program. The main program is
responsible for system initialization and settings. And
functions constant setting, variable setting, I/O, interrupt
vector, A/D module and key register of HRPWM are involved
in these two items. Due to the control system is mainly realized
by A/D interrupt service routine, the initialized system goes
into circle until receiving interrupt signal is received. Then, the
CAN communication service program, the current-sharing
regulation and the DSC loop adjustment program would be
called by the main program in A/D interrupt procedure. Then,
the controlled quantity calculation and the output updates can
be accomplished. The main program flowchart is shown in Fig.
8.
Fig. 9 A/D interrupt service program flowchart.
B. eCAN Communication Service Program
In this design, TMS320F28035 sends current-sharing data
to each divided module through CAN bus. Each module
calculates the fine adjustment quantity by comparing the
branch and CAN current, the final output current of each
module can be determined by loop regulation, and the output
current of all sub-modules of the system can be realized.
Finally, system could achieve current-sharing with loop
adjustment. The program flowchart of eCAN communication
service is shown in Fig. 10.
The main purpose of CAN communication services
program is to send ID from DSP controller to sub-modules.
After ID is received, the outputs of each divided modules are
compared and the main module with the maximum output
current is selected. This main module determines the voltage
increment by comparing the feedback voltage and reference
voltage. Then DSP sends the voltage regulation command to
Fig. 8 Software main program flowchart.
A. Interrupt Service Routine of A/D
A/D interrupt subroutine is the core of the whole digital
control, as it performs tasks such as calculating real-time PID
parameters in every cycle. A/D conversion can be triggered by
S/W, EPWM, external interrupt trigger, ADCINT1 1/2 and
1046
regulate output voltage. Meanwhile, minor-module calculates
the number of online modules and receives the maximum
current to prepare current-sharing. The CAN communication
module rechecks the order of divided modules per 500ms to
update the main module and ensure the number of online
modules is same as the initial situation. Through applying the
above method, the reliability of the system can be enhanced,
and power sub-modules can be achieved data interaction with
the assist of CAN bus [9].
sharing not only performs digital current-sharing adjustment
algorithm but also ensures the system minimum flow accuracy
indicators and achieves higher stability and reliability [10].
Digital current-sharing adjustment algorithm flowchart is
shown in Fig. 11.
Fig. 11 Digital current-sharing adjustment algorithm flowchart.
D. DSP Loop Adjustment Program
The DSC loop procedure regulates the data in compareregister of HRPWM module based on the current-sharing
increment. Through changing duty ratio of switching tube in
DC/DC circuit, the final output current converges to expected
value. The DSC loop adjustment program flowchart is shown
in Fig. 12.
Fig. 10 eCAN communication service program flowchart.
C. Current-sharing Adjustment Algorithm
DSP receive the current value from the CAN bus in the
implementation of current-sharing control algorithm. DSP
determines the current adjustment increment based on the
maximum current. If the real module current is smaller than
maximum current, the output current would increased; if the
real current is same as the average current or there is only a
fine difference, DSP would find the difference between real
output current and maximum current and compare this
difference with system accuracy requirement of currentsharing. Among this comparing, if the difference will less than
the requirements, it indicates that the digital current-sharing
has been completed. Otherwise, system would be fine adjusted
to ensure system accuracy. The strategy of digital current-
Fig. 12 DSC loop adjustment program flowchart.
It can be seen from Fig. 12 that the principle of currentsharing output based on current increment in the various submodules of the parallel current-sharing system. DSC loop
1047
adjustment strategy has strong stability and robustness. The
control quality is insensitive to the changing of controlled
object [11]. An advanced digital PI algorithm is adopted in
increment processing to restrain integral saturation problem
and improve the stability of parallel system.
Integral method with extinction limiting is adopted in this
research to restrain integral saturation phenomenon. The basic
principle is that system weakens the integral accumulation and
ignores the accumulating command while the control volume
entering saturated region. In other words, the system judges
whether the last u(n-1) has beyond the limit before calculating
u(n). If u(n-1) is larger than the umax, system would only
performs negative deviation accumulation; if u(n-1) is smaller
than umin, system would only performs positive deviation
accumulation [13]. This can be represented as follows.
E. The Advanced Digital PI Algorithm
Digital controller is based on discrete sampling control
method. It calculates increments only with the deviation data
derived from sampling times. Thus, we have to discretize the
analog PID algorithm [12]. Discretization is achieved by using
bilinear transformation that sampling period is 10 μs. The
difference equation is given by
u ( n) = u ( n − 1) + k1e( n) + k 2 [e( n) − e(n − 1)]
(1)
(3)
u (n − 1) < umin
In this formula, umax is the biggest value for the period
register; umin is the minimum value for period register.
In the above formula, u(n) is the PI regulator output of the
Nth sampling. Because of the regulation of output voltage and
output current is achieved by the duty ratio of DC/DC system.
And DSP28035 changes the duty ratio through changing the
CMPR data of HRPWM module. Thus, the u(n) here is the
value of compare register CMPR; е(n) is the error between the
voltage and reference voltage of the Nth sampling. The
expression is given as follows.
e( n) = V f − Vref
u (n − 1) > umax
u
u (n) =  max
 umin
V. ANALYSIS OF CURRENT-SHARING EXPERIMENTAL RESULTS
A current-sharing performance test of power supplies
which in parallel in steady state is made based on the prototype
which is under 24V rated input voltage. 9 sets of data was
measured and listed in table I, including bus output current,
output current of each modules, the relative error of currentsharing.
(2)
TABLE I
The Result of Current-Sharing Performance Test
In order to avoid the integral saturation, the improved PI
algorithm is used in the program, and the control algorithm
flowchart is shown in Fig. 13.
7.91
1#
output
current
/A
0.20
2#
output
current
/A
0.21
7.97
0.60
7.98
0.80
8.01
7.92
system
output
current /A
relative
error
/%
0.41
4.8
0.61
1.21
3.3
0.82
1.62
2.5
0.99
1.01
2.00
2.0
1.19
1.21
2.40
1.7
8.06
1.38
1.40
2.78
1.4
7.95
1.59
1.61
3.20
1.3
7.92
1.78
1.80
3.58
1.1
7.96
2.00
2.02
4.02
0.9
system
output
voltage /V
From the table I, the relative error of current-sharing can
be expressed by
CSerror =
N ⋅ ΔI o max
Io
(4)
The voltage adjustment rate of paralleling power supplies
is calculated based on table I. The result is followed.
SV =
8 − 7.91
× 100% = 1.125% ≤ 3%
8
(5)
According to the analysis results, these two paralleling
power modules have realized current-sharing with the
proportion nearly 1:1. And the relative error of current-sharing
is less than 5 percent of the expected relative error, which
meets the expected requirements of technical indicators.
Fig. 13 Advanced PI control algorithm flowchart.
1048
[5]
VІ. CONCLUSION
A Buck converter experimental prototype based on the
maximum current automatic current-sharing method is made in
this paper, and the practical application of digital currentsharing technology is analyzed. An advanced digital PI
algorithm is used for restrains integral saturation problem, and
TMS320F28035 is implemented in this prototype to improve
the stability, real-time and reduce the cost of parallel system.
According to the current-sharing performance test of paralleled
power converter, the prototype is able to realize currentsharing steady and reliably. The measured relative error of
current sharing is less than 5% which means the prototype can
achieve bus current-sharing excellently.
[6]
[7]
[8]
[9]
[10]
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