Download Current Sharing in Multiphase Interleaved Converters by Means of

Current Sharing in Multiphase Interleaved Converters by Means of
One Current Sensor
Jens C. Schroeder, Marinus Petersen, Friedrich W. Fuchs
Institute of Power Electronics and Electrical Drives
Christian-Albrechts-University of Kiel
Kaiserstr. 2, 24143 Kiel, Germany
Email: [email protected], [email protected], [email protected]
Abstract
To improve the efficiency and to reduce the volume in dc-dc converters and their filters, interleaved converters are used. In these multiphase converters, appropriate active current sharing
is necessary due to the inherent current inequality caused by the parameter deviations between the components and the dutycycles. The common method to realize current sharing is
to use a single current sensor for each phase whereas the current in each phase is controlled
independently. Due to the costs of the current sensors, a reduction is mandatory. In this contribution, a method is presented which allows the current sharing by means of only one current
sensor without extra circuitry and computation effort. This method is analyzed and the success
is verified in practice.
1
Introduction
The main issues in electric vehicles are the maximization of range and the lifetime of the costintensive battery. In a common electric propulsion system, the battery is connected to the
voltage source inverter (VSI) which feeds the machine. So the battery has to deliver or absorb
all the power which is demanded or recuperated by the propulsion system in all operating
points. Due to the internal battery resistance, the battery voltage varies significantly depending
on the battery current. Operating under high currents, the lifetime decreases in an accelerated
way and in general the recuperation efficiency is poor. To optimize battery lifetime, efficiency
and voltage stabilization, a battery buffer system (BBS) can be implemented [1, 2]. The block
diagram of an exemplary system can be seen in fig. 1. To improve the efficiency and to reduce
the volume, a threephase interleaved converter is used here, which is shown in fig. 2.
Interleaved converters show several benefits compared with conventional converters. Especially in automotive applications, the resulting volume reduction and the efficiency improvement
by interleaved operation are fundamental [3, 4, 5]. Interleaved converters consist of N parallel
arranged identical topologies whose pulse width modulation (PWM) is phase shifted 360◦ /N
against one another. Interleaved operation results in the benefit of current ripple reduction and
inductor volume reduction [4].
The components in an N-phase interleaved converter should be rated for 1/N part of the current.
Due to possible component and dutycycle deviations between the different phases, the current
might be not divided equally without an active current sharing. For a safe operation, an active
current sharing is mandatory to load all phases with the same current.
The common way to realize current sharing is to use N single current sensors, i.e. one for each
phase, whereas the current in each phase is controlled independently. To minimize costs and
CK
+
-
Ibat
Vlink
Buffer System
Ld
Inverter
Ud
AC
DC
M
3~
Ud =
Induction
Motor
IDCDC
IL2 L2
DC
DC
EDLC
IEDLC
Ud
IL1 L1
IEDLC
VEDLC
Id
Power
Management
Figure 1: Lift truck propulsion system
IDCDC
IL3 L3
VEDLC
EDLC
Battery
Vlink
Figure 2: Three phase interleaved converter
the complexity of the converter, the goal is the reduction to only one single current sensor. In
this work, a threephase interleaved converter is optimized, which is used in an electric lift truck
to connect a 24 V lead acid battery and electric double layer capacitors (EDLC) in a battery
buffer system. The system power is 5 kW, the EDLC voltage level is in the range of 26V -38V.
Several authors have designed methods of current sharing in the past. In [6], a method is
presented in which a current sensor is switched between the different phases. Due to the
additional switches, the component number increases considerably. In [7], a master slave
method is developed. All slave phases have to be operated in discontinuous current mode
there, thus it cannot be implemented in continuous current mode. The author in [8] measures
the voltage at the output capacitor and can detect a phase unbalance via a fourier analysis, [9]
adapts the method. This method demands high calculation capability and very fast sampling. In
[10] a current sharing method for a two phase interleaved converter is presented. The current
through the highside MOSFETs is measured by one sensor. By symmetrical sampling during
a precalculated timestep, the phase currents can be detected and controlled independently for
a limited range of dutycycles.
This method is extended here by further investigations to realize a current sharing in a three
phase interleaved converter with the given margins of the battery buffer system.
In section 2, the demand for current sharing is shown. In section 3 the new current sharing
method is introduced and analyzed, simulation results are presented. In section 4, measurement results verify the benefit of the new method and in section 5 a conclusion is given.
2
Verification of the demand for current sharing
At first, the demand for a current sharing control is shown. Therefore, the current characteristics
are presented for a system without current sharing. In this system, the target dutycycle in all
three phases is the same. An open loop control or a closed loop control with a feedback of the
resulting current can be used. To validate the consequences of such a control in practice, some
deviations in the different phases are set. The phase resistances, for instance in the inductor,
can deviate as well as the inductance and the dutycycle which is applied at the gates of the
MOSFETs. In fig. 3 the current characteristics are shown in case of phase inequalities. In (a)
a 1 % dutycycle increase in phase two is set, in (b) a 20 % increase of the phase resistance. It
is obvious that especially an inequality in the dutycycles, which is rather a problem in analogue
control circuits, results in high deviations between the phase currents. Phase resistance inequalities cause problems as well. Inductance deviations do not influence the average current,
they only result in different current ripples.
Inequal current distribution causes higher losses in the converter, and components which are
not rated for currents higher than the maximum expected average could be destroyed in the
worst case. Thus, a current sharing is needed which guarantees equal current distribution
between the phases.
0.6
0.6
Phase 1
Phase 2
Phase 3
0.55
0.5
0.45
0.45
0.4
0.4
I [p.U.]
I [p.U.]
0.5
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
0.15
0.15
0.1
160
162
164
t/T
(a)
166
168
Phase 1
Phase 2
Phase 3
0.55
0.1
160
170
162
164
t/T
(b)
166
168
170
Figure 3: Simulated current characteristics without current sharing: (a) 1 % dutycycle increase
in phase 2 (b) 20 % resistance increase in phase 2
To get a reference on how the current can be shared under perfect circumstances, fig. 4 is
shown. The currents of the inductors are measured by three sensors and controlled independently in each phase, thus the different phases carry the same current despite deviations in the
dutycycle or the phase resistance. The goal is to achieve this behavior by means of only one
current sensor.
0.6
0.6
Phase 1
Phase 2
Phase 3
0.55
0.5
0.45
0.45
0.4
0.4
I [p.U.]
I [p.U.]
0.5
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
0.15
0.15
0.1
160
162
164
t/T
(a)
166
168
Phase 1
Phase 2
Phase 3
0.55
170
0.1
160
162
164
t/T
(b)
166
168
170
Figure 4: Simulated current characteristics with current sharing (3 sensors): (a) 1 % dutycycle
increase in phase 2 (b) 20 % resistance increase in phase 2
3
Reducing the number of current sensors
The challenge is to realize current sharing with only one current sensor. In [10] a method to
realize current sharing in a twophase interleaved converter is presented in which the resulting
current through the highside MOSFETs is measured. The topology is shown in fig. 5a. Due to
the interleaving of the currents, each current can be measured independently during the time
when only one current is passing the highside MOSFETs, which is during the freewheeling
time interval in boost mode and during the switch-on time interval in buck mode. The limitations
concerning the allowed operating dutycycles are presented as well: The minimum dutycycle
in boost operation is dependent on the number of phases and the ratio between the minimum
time Tmin,AD which is needed by the AD-converter to measure the current and the period time
TP . dmin can be calculated in (1). The maximum dutycycle in boost operation can be calculated
with (2) [10].
dmin,boost =
N − 2 Tmin,AD
+
N
TP W M
dmax,boost = 1 −
(1)
Tmin,AD
TP W M
(2)
In a threephase interleaved converter which is used in the lift truck buffer system, operated at
a switching frequency of 16 kHz and an estimated Tmin,AD of 5 µs, it results in dmin,boost =0.413
and dmax,boost =0.92. The EDLC voltage level, which is within the range of 25V -38V, is always
higher than the battery voltage, which is around 24V. The operated dutycycle can be calculated
via (3).
Vlink
d=1−
(3)
VEDLC
It results in 0.08 < d < 0.37. Thus, this method cannot be used to realize current sharing in this
system.
Now the idea is to change the location of the current sensor to the common branch below the
lowside MOSFETs like it is shown in fig. 5b.
A
IEDLC
IEDLC
IDCDC
VEDLC
IL3 L3
EDLC
IL2 L2
IL2 L2
IDCDC
VEDLC
IL3 L3
EDLC
IL1 L1
IL1 L1
Vlink
Vlink
A
(a)
ILS
(b)
Figure 5: Topology of converter with current measurement (a) above highside MOSFETs (b)
below lowside MOSFETs
In fig. 6, the current characteristics of the converter are shown for one period both for boost and
for buck mode. ILS exceeds zero only during conducting of the lowside switches, which is during
the switch-on time in boost mode and during switch-off time in buck mode. If the sampling of
the current is done in the middle of the respective current slope, the average currents of the
three single phases can be detected by only one current sensor. The minimum and maximum
dutycycles can be calculated by (4) and (5).
VG, M2
VG, M1
VG, M4
VG, M3
VG, M6
VG, M5
IL1
-IL1
IL2
-IL2
IL3
-IL3
ILS
ILS
topt,1
1
3
T topt,2
(a)
2
3T
topt,3
Tt
topt,2
1
3
T
topt,3
(b)
2
3T
topt,1 T t
Figure 6: Current characteristics in a three phase interleaved converter and optimum sample
time (a) boost mode (b) buck mode
dmin,boost
Tmin,AD
=
TP W M
(4)
dmax,boost = 1 −
N − 2 Tmin,AD
+
N
TP W M
(5)
For the margins of the three phase converter in the lift truck propulsion system, it results in
0.08 < d < 0.587, so it can be applied here.
The optimal sampling times topt,boost and topt,buck for continuous current operation can be calculated by (6) and (7), whereas i is the phase number:
topt,boost = T ·
d i−1
+
2
N
(6)
topt,buck = T ·
d 1 i−1
+ +
2 2
N
(7)
In fig. 7, the simulation results are presented, again for an inequality in the dutycycle and the
phase resistance. It is shown that the current sharing control works well and that the characteristics are nearly the same as in the three sensor current sharing. This nearly perfect behavior
is caused by the fact that each current can be controlled seperately. The only drawback of this
kind of current sharing method is the location of the current sensor. It has to be implemented
between the lowside MOSFETs and the ground plane, which is within the commutation path of
the semiconductors. Thus, on-circuit sensors have to be used to keep the stray inductance as
low as possible.
0.6
0.6
Phase 1
Phase 2
Phase 3
0.55
0.5
0.45
0.45
0.4
0.4
I [p.U.]
I [p.U.]
0.5
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
0.15
0.15
0.1
160
162
164
t/T
(a)
166
168
Phase 1
Phase 2
Phase 3
0.55
170
0.1
160
162
164
t/T
(b)
166
168
170
Figure 7: Simulated current characteristics with current sharing by means of one sensor below
the lowside MOSFETs: (a) 1 % duty cycle increase in phase 2 (b) 20 % resistance increase in
phase 2
4
Laboratory setup and measurement results
The developed current sharing method is implemented in the laboratory test bench in an FPGA.
The FPGA generates the threephase interleaved PWM and realizes the symmetric sampling of
the three currents. IRFS3006-7PPbF MOSFETs (60V /240A ) are chosen. The printed circuit
board (PCB) of the converter with the Allegro ACS758xCB current sensor is shown in fig. 8.
The symmetrical sampling in boost mode is shown in fig. 9. The AD-converter sampling is
started at the falling edge of the trigger signal of CH3. It is shown that there are three symmetrical samplings in every period and that the output signal of the current sensor displays the
rising current slopes of the different phases.
Figure 8: PCB of the three phase interleaved converter; red circle marks the current sensor
between lowside MOSFETs and power ground
In the measurement results in fig. 10 it can be seen that the currents are not equally divided for
both unbalanced cases without current sharing. By means of the single sensor current sharing
operation, the currents can be controlled independently and so they can be divided equally.
Figure 9: Symmetrical current sampling in boost mode for d=0.4; CH1: Phase current
(5 A/div); CH2: Current sensor signal (200 mV/div); CH3: AD-converter SPI chipselect signal
(500 mV/div); time: 10 µs/div
Thus, the success of the method is shown.
(a)
(b)
(c)
(d)
Figure 10: Measured current characteristics (20 A/div, 4 ms/div) (a) unbalanced equivalent serial resistances without current sharing (b) unbalanced equivalent serial resistances with current sharing (c) unbalanced equivalent serial resistances without current sharing (d) unbalanced equivalent serial resistances with current sharing
To analyze the influence of the current sensor in the commutation path to the MOSFETs overvoltage during switch-off, further measurements are performed. First, a maximum voltage of
66.8V occurs during maximum EDLC voltage operation of 36V. The voltage can be reduced
by an additional 10 nF capacitor, which is applied between the drain source connectors of the
MOSFETs. The maximum voltage can be reduced to 54.4V, which is acceptable.
5
Conclusion
In this contribution, the demand for current sharing in a threephase interleaved converters has
been shown and a method has been investigated to realize current sharing in a threephase
interleaved converter by means of only one current sensor and without further effort. Simulation
results show the ability of this method to replace the three sensor current sharing without further
complexity or drawbacks. A three phase interleaved converter with an on-circuit current sensor
between the lowside MOSFETs and power ground has been built up in the laboratory and its
functionality has been successfully approved. The occuring overvoltage during switching due
to the current sensor in the commutation path has been analyzed and the operation could be
satisfactorily improved by an additional capacitor. Thus, the single sensor current sharing has
been succesfully designed and tested and can be implemented in the lift truck’s battery buffer
system.
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