INTRODUCTION
Some electrical equipments and control systems with high security and reliability
requirements, use high reliability DC low-voltage power supply and generally
are in parallel with the load on the battery to ensure that in case of a system
failure or power off, the power can provide electrical energy of a certain time
for the control system or other safety system (Song, 2006).
Therefore, it is particularly critical that how to convert the DC high-voltage
power to a low-voltage DC power, and charge and manage the battery (Wei
and Zhou, 2010). This paper describes a high reliability DC converter. The
converter can convert 3-way isolation DC high voltage into a 24 V low-voltage
DC power supply which can charge to the 24 V battery connected to the converter
output, and also charge to the power consumptions equipment in parallel with
the battery which are connected with security requirements (such as a 24 V battery
fan) and the control equipments which require a higher reliability for its power
supply (such as operation and control system).
Main technical parameters and functional requirements of the DC converter:
The main technical parameters of the DC converter are shown in Table
1.
The DC converter changes 3-way 380 V DC power grid potential isolated into
the 24 V DC power supply isolated and ungrounded and monitors the asymmetric
insulation fault of the 24 V power supply and supplies power for the 24 V battery
fan and monitors its working condition and monitors the working condition of
the DC converter and protects for over current or overvoltage.
| Table 1: |
The main technical parameters of the DC converter |
 |
Modular structure of the DC converter: The DC converter takes slot type
modular structure to be convenient in repairing and maintenance (Wei
and Qi, 2011). The converter is made of nine switch power modules and a
back veneer of protection function; the back veneer has the protection function
of asymmetric insulation fault monitoring, and switching power supply module
working condition monitoring; the output wire takes copper bar which is exported
to the output interface connector by the cord (Wei et
al., 2012).
The block diagram for DC converter functions is shown in Fig.
1. Every 380 V power grid feeds to three switch power supply modules which
is connected to the high voltage fuse in the input terminal to ensure that the
damage of a single switch power supply module does not affect the normal work
of the 380 V power grid. The output of the nine switching power supply module
is in parallel to supply for 24 V battery and other load power. In normal work
case, three switch power supply modules can satisfy the system power supply
and the control system supplied by three isolated 380 V power grid and nine
switch power module, greatly improves the reliability of power supply.
|
| Fig. 1: |
Diagram for the relationship of the DC converter switching
power supply modules |
Even if two of the 380 V power grid break down or power off, the control system
can work normally and make protection control timely; The DC converter separately
provides 24 V power supply to 24 V battery fan and limits the maximum current
of the fan, so, when the fan is fault, it can also cut off the fan load in order
to protect the 24 V power grid to supply the power normally. The converter can
determine fan’s working condition according to the fan feedback signal
and feedback to grid diagnosis system; The converter detects the working condition
of the nine switching power supply module. The detection signal is connected
in series. When any one of the switching power supply module output is not normal,
the fault signal will be issued to the diagnosis system by the converter working
condition detection circuit which suggests the fault existed in some switching
power supply module of the converter (Wei et al.,
2011). At the same time, the symmetrical insulation condition to the ground
of the 24 V power grid can be detect and upload by each DC converter to ensure
24 V power grid load on the safety to human body.
Switching power supply module: The switching power supply module is
the core components of DC converter. The total output power of nine switching
power supply module is 1.8 kW and the rated output power of a single switch
power supply module is 200 W. The input terminal of the module takes two stage
LC filters to eliminate the conducted interference on 380 V power grid and uses
the strike circuit to restrain the maximum input impact current of the module
when it is power supplied. The main circuit of the module is realized by half
bridge topology. In this way, the switch tube which voltage stress is the single
times input voltage, can use 500 V mosfets (Wei and Hao,
2011) and the driving circuit that takes the transformer isolation drive
way to insulate the drive signal is simple (Wei and Jun,
2012). The loop control mode used voltage loop control is simple and efficient
(Wei and Ma, 2012). The switch frequency is a constant
for 100 kHz, using optical coupling isolation feedback; The output end uses
decoupling diode to ensure that the failure of a single switch power supply
module can not affect the normal work of the 24 V power grid. The working condition
detection of switching power supply module is to detect output voltage in the
positive pole of the decoupling diode and the output voltage module in the normal
range shows that the module working properly. At the same time, switching power
supply module has the protection function of input and output voltage overvoltage.
The overall module block diagram as shown in Fig. 2.
Equalizing current control and charging management: When more than one
module output is in parallel to supply power for the load, it's best to share
the load current for all parallel module and is not the case that some module
output larger current, or some modules output smaller current, even the extreme
situation that some module output maximum current, or some module output no
current. Load sharing can lead to the thermal equilibrium in the box and can
equalize the loss in whole module (Lin et al., 2009)
and can avoid that the load imbalance leads to the electrical stress of some
module device so as to increase the damage probability (Bo
and Zhang, 2009). The switching power supply module equalizing current control
and charging management control circuit diagram as shown in Fig.
3.
The switching power supply module outputs current through the mΩ resistance
detection module and then sets a maximum output current value through the op-amp
to ensure that the module will not output over-current.
|
| Fig. 2: |
Block Diagram for the switching power supply module function |
|
| Fig. 3: |
Diagram for the equalizing current charging circuit |
The maximum output current of each module is 11A and the output current restriction
of a single module also determines the total output current of the DC converter.
The switching power supply module takes the decoupling diode cathode as its
output voltage test point, that is to test battery voltage. In the wiring of
PCB layout, all modules set a voltage telemetry end in order to remove the influence
of wire pressure drop and then connect to the point of common coupling of the
parallel module through the backplane, and have this point as the battery voltage
test point to ensure that the voltage passed on to the point by each module
output is the same. At the same time, the output current signal of the switching
power supply module affects battery voltage feedback signal inverse ratio, i.e.,
if the output current increases, the output voltage expected value of module
will be adjusted small and vice versa.
The output current of the battery and the voltage of the switching power supply
module determine the three work mode of converter. Three kinds of mode which
are constant current charging, equalizing current charging and constant voltage
floating charging, as shown in Fig. 4. When the battery voltage
is too low to reach the minimum adjusting voltage Vmin of module
internal, the converter goes into constant current charging work mode. Until
the module output current signal takes part in the duty ratio adjustment of
module switch tube, and the battery voltage feedback signal is shielded by current
signal, all module output the largest setting current constantly to supply power
for the battery and the other load, and the battery voltage rises. When the
battery voltage rises to the minimum adjusting voltage Vmin, the
converter goes into equalizing current charging work mode. At this point, the
battery voltage feedback signal starts to participate in the duty ratio adjustment
of module switch tube and the module output current signal participates in the
duty ratio adjustment of module switch tube through fine adjusting the battery
voltage feedback signal and the battery voltage rises, the module output current
drops.
|
| Fig. 4: |
Diagram for the output curve of switching power supply module |
When the battery voltage rises to the highest adjusting voltage Vmax
set in module, the converter goes into constant voltage floating charging
work mode. At this point, the output current signal of the module exits loop
adjustment, the duty ratio of module switch tube depends on the battery voltage
feedback signal absolutely and the battery voltage keep constant, the battery
charging current decreases gradually and the module specific output current
value is determined by the other load.
CONCLUSION
The loads on 24 V power grid are all the key control equipment of the control
system, so they have higher requirements for the reliability of DC converter.
The DC converter considers the possibility of reliability design for structure
and protection function and other aspects to improve its reliability. One is
to make full use of the redundant system, for example , there are 3-way 380
V power input, 9 switch power supply modules output in parallel to form a 24
V power grid. The second is the protection function of the system. The 24 V
power grid parallels with battery and battery fan with protection function and
the switching power supply module is equipped with charging management control
and equalizing current charging control and the module output end is cascaded
with a decoupling diode and it has working condition monitoring and protection
functions. These measures greatly improve the reliability of DC converter.
This kind of DC converter can be applied to all kinds of control ystems requiring
for high reliability to ensure the safe power supply of control system and has
a high application value.
ACKNOWLEDGMENTS
We would like to thank the anonymous reviewers for their valuable comments.
This program is supported by Scientific Research Program Funded by Shaanxi Provincial
Education Department (Program No. 2013 JK1139). and Supported by China Postdoctoral
Science Foundation (No. 2013M542370). The study is partially supported by Natural
Science Basic Research plan in Shaanxi province of China (Program No. 2012JM8047).
And this project is also supported by NSFC Grant (Program No.11226173). This
work is also supported by Scientific Researeh Program Funded by Xiam University
of Science and Technology (Pragram No. 201139). This job is also supported by
Science and Technology Project of Xian (CX1262
).
Subscribe Today
