Adaptive Control for ZVS Three Phase Full Active
Bridge Converter with ARCN
J.M. Molina, O. García, R. Asensi, P. Alou, J. A. Oliver, J.A. Cobos
Centro de Electrónica Industrial
Universidad Politécnica de Madrid
Madrid, Spain
Abstract—This paper presents an adaptive control for the
auxiliary circuit, called ARCN (Auxiliary Resonant
Commutating Network), used to achieve ZVS in full active
bridge converters under a wide load range. Depending on the
load conditions, the proposed control adapts the timing of the
ARCN to minimize the losses. The principle of operation and
implementation considerations are presented for a three phase
full active bridge converter, proposing different methods to
implement the control according to the specifications. The
experimental results shown verify the proposed methodology.
I.
INTRODUCTION
DC/DC converters used for high power and high voltage
applications usually have some constraints regarding weight,
volume and low electromagnetic interferences [1]. Especially
the volume is a critical parameter within on board
applications. Increasing the switching frequency helps
reducing it, as well as the volume of the magnetic
components. However switching losses and electromagnetic
interferences are increased.
For the presented converter, the technique proposed in [4]
is used. This technique consists of adding an additional leg of
switches in parallel to the principal switches, connected
through an inductor. This circuit is presented in Figure 1.
This paper presents the study of an ARCN (Auxiliary
Resonant Commutating Network) for a three-phase active
bridge converter and its adaptive control as a function of the
load. The adaptive control strategy proposed in this paper
calculates the accurate value for each state of the converter in
order to increase the efficiency. Two implementations have
been analyzed. One option achieves Full ZVS for the whole
range of load, and the other option achieves partial ZVS in
some operating points, but reduces the losses added by the
ARCN.
The application of soft switching techniques [2] allows to
decrease these switching losses and the electromagnetic noise.
This paper studies the losses reduction using soft switching
techniques, concretely, ZVS (Zero Voltage Switching).
Soft switching techniques, such as ZVS, are useful to
decrease the losses in the turn-on transient in the switches.
These losses are mainly caused by an abrupt change of the
energy stored in the parasitic capacitances of the devices. In
order to reduce the conduction losses, placing several
switches in parallel is a widespread technique applied in high
power converters. However, the value of the parasitic
capacitance is increased.
The ZVS techniques used in Full Bridge converters
usually include some extra storage elements. Furthermore,
additional switches to control the energy storage in these
elements are necessary [3]. Typically, the control of these
switches is independent of the circuit state; therefore, at some
operating point, the energy storage in the auxiliary elements is
larger than the necessary energy, producing circulating energy
around the circuit which causes extra losses.
This research is funded by Ministerio de Ciencia e Innovación of Spain
through the project TECMUSA (PSE-370000-2009-5)
Figure 1. ARCN connected to a main leg of MOSFETS
II.
ZVS IN THE THREE-PHASE FULL ACTIVE
BRIDGE CONVERTER
This section analyzes the ZVS operation for the threephase full bridge converter shown in Figure 2 [5]-[7]. The
ZVS range for this converter is limited to a specific range of
load [8]. This range is defined by the parasitic elements, the
parasitic capacitance of the switches and the leakage
inductance of the transformer.
Figure 2.Conventional Three-Phase Full Active Bridge Converter topology.
ZVS is achieved because during the dead time, a
resonance is produced in each leg due to a leakage inductance
and the parasitic capacitors of both switches of the
corresponding leg. Taking into account the leakage
inductance energy (1) and the parasitic capacitance energy (2)
the ZVS condition (3) can be obtained as:
The equivalents modes are presented in Figure 4 and the
circuit behavior is described below.
Figure 4.a shows the initial state of the converter before the
ARCN is active. While M1 is conducting M2 is blocked, so
the parasitic capacitance is charged to the input voltage.
In Figure 4.b the switch M2arcn is turned on and
consequently the current through Lr increases linearly during
the interval b (Tarcn) until its maximum value (Iarcn). At that
moment M1 and M2arcn are both switched off.
In Figure 4.c the parasitic diode of M1arcn is conducting.
In this interval, C2, the parasitic capacitance of M2 is
discharged, while C1, the parasitic capacitance of M1 is
charged. Lr and Llk resonate with the capacitance until the
voltage in C1 reaches Vin, and the voltage in C2 reaches 0V.
(2)
In Figure 4.d the flux of the current through M1
disappears. However, the energy storage in Lr is discharged
through the parasitic diode of M1arcn, and the parasitic diode
of M2before M2 is switched on with ZVS conditions.
(3)
In Figure 4.e M2 is conducting and the ARCN circuit is
deactivated until the next commutation of M1.
(1)
Where Llk is the leakage inductance, Ilk is the current through
the leakage inductance, Ceq is the equivalent capacitance of
the MOSFETs and Vin is the input voltage.
III.
THREE-PHASE FULL ACTIVE BRIDGE
CONVERTER WITH ARCN
Three-phase full active bridge converter cannot achieves
ZVS at light load conditions. However, a three-phase full
active bridge with ARCN solves the problem of the ZVS
range for light loads. Figure 3 shows the three-phase full
bridge converter with an ARCN for each leg [9].
As it is shown in section II this converter achieves ZVS
for high loads states using the energy stored in the leakage
inductance.
Within the ARCN it is possible to obtain ZVS in the full
load range because this circuit adds an extra element to store
energy as shown in (4).
(4)
Figure 3.Three Phase Active Bridge Converter with ARCN.
Figure 4. Equivalent circuits in the different modes
Figure 6. Energy in the storage elements Llk, Lr and Cequ
Figure 6 represents the equation (4) in function of the
load. To achieve ZVS the energy stored in the inductors has
to be higher than the energy in the parasitic capacitors. The
energy stored in the capacitors is constant for the full range of
load, but the energy stored in the leakage inductance is
incremented with the load and the adaptive control has to
compensate the balance until the energy in the leakage
inductance is equal to the energy in the parasitic capacitors, in
that operating point the adaptive control is disconnected.
Equation (5) represents the energies balance and (6) the
relation between the ARCN time connected and the current
through the ARCN inductor. From these equations can be
extracted the equation (7), where Tarcn is the optimum time
to achieve ZVS and Ip is the current in the primary.
Figure 5. Waveform of the different modes
To obtain ZVS, one important parameter is the dead time
between switches in the same leg (in Figure 5 dead time is
divided in the intervals c and d). Analyzing Figure 5, during
interval b, the ARCN is activated to store energy in the
auxiliary inductor, and this energy is delivered during
intervals c and d. During the dead time, the energy stored in
both inductances (auxiliary inductor and leakage inductance
of the transformer) is discharged. Besides, the parasitic
capacitance of M2 is discharged, although this action does not
last for the whole dead time. Analyzing this event, for this
specific case, the conclusion is that the energy stored in the
inductors is larger than the necessary energy because the
discharge of the parasitic capacitance is not using the whole
dead time. How the adaptive control achieves an optimum
solution for each case is analyzed in the following sections.
IV.
DESIGN CONSIDERATIONS FOR THE ARCN
CIRCUIT
The ARCN design is analyzed in this section. The design
of the relevant magnetic components as well as the ARCN
semiconductor selection is also discussed.
A. ADAPTIVE CONTROL
The Adaptive control has two functions:
-For light load: calculate the appropriate timing.
-For heavy load: disable the ARCN, because the main
power stage may achieve ZVS without the external help of
the ARCN.
(5)
(6)
T
L C
I L
V
L
(7)
Figure 7 shows an example of curves for the specific case
that is presented in the experimental results section. Figure 7.a
shows the curve of the optimum ARCN time to obtain ZVS
for each operating point of load. It is shown that after
determinate point the ARCN time is zero because the ARCN
is not necessary to obtain ZVS.
If the ARCN time is the optimum, ZVS is achieved in the
whole range of load and no extra losses are added by the
ARCN to the circuit, but if it is higher, extra losses are added.
If the ARCN time is lower than the optimum, the losses are
lower than the rest of the cases, however, ZVS is not
achieved.
Losses in the ARCN using a classical control strategy are
represented in Figure 7.b in comparison with the losses for the
adaptive control. It is represented that the losses in the
classical control are higher in the whole range of load.
Figure 8. Example of the current through the body diode
Figure 7. a) Curve for the optimum Tarcn for each load b) Losses with
adaptive control and without adaptive control
B. DESIGN OF THE ARCN INDUCTOR
The selection of the value for the extra inductor included
in the ARCN is a very important parameter, which depends
on the leakage inductance of the transformer, the parasitic
capacitance and on the current through the inductance.
Once the transformer is made, the value of the leakage
inductance is estimate and measured. The value of the ARCN
inductance has to compensate the energy delivered by the
leakage inductance for light loads to discharge the parasitic
capacitance. The energy delivered by the leakage inductance
is almost zero for small loads.
The equation (8) is used to calculate the ARCN
inductance value for the worst case to obtain ZVS. This case
is when the load is zero, and the energy in the leakage
inductance is zero.
(8)
The value of this inductance also depends on the
maximum time that it is connected, because the current Ilr
depends directly on Tarcn. Equation (6) shows this relation.
The current through the semiconductors of the ARCN
also depends on this maximum time that they are connected;
therefore, a trade-off between the inductance value and the
time connected is necessary.
Figure 9. Solution to protect the MOSFET
The evolution of the current in the body diode is as
shown in Figure 8. To protect the device, this body diode has
been prevented from conductions, using the typical
arrangement shown in Figure 9.
D. ARCN LOSSES
The ARCN adds to the original circuit two MOSFET and
one inductor for each leg of the principal circuit. These
components have losses, and also include additional losses in
the principal circuit.
The losses in the ARCN circuit are principally conduction
losses. The current decreases inversely with the load until
zero. In Figure 10.a is observed a curve with the losses in the
ARCN depending of the load.
The current demanded by the ARCN to charge the
auxiliary inductance, causes losses in the principal circuit, in
Figure 10.b is shown the current in the principal MOSFET
M1, and emphasize the current caused by the ARCN.
The losses caused in the principal circuit by the ARCN are
directly related with the maximum value of Tarcn. The value
of Tarcn has been studied in section IV.A.
V.
C. SELECTION OF THE ARCN SEMICONDUCTORS
The semiconductors of the ARCN handle a small power
compared with the main switches, though their voltage is the
same. The di/dt of this current in their turn off process is so
high that potential problems with the trr of their body diodes
may arise.
ADAPTIVE CONTROL IMPLEMENTATION
In order to implement the adaptive control, the
compromise between the losses added by the auxiliary circuit
and ZVS achievement should be found. Full ZVS provides
lower switching losses in the converter but higher conduction
losses in the circuit, because the auxiliary circuit should be
active during a longer period. On the other hand, partial ZVS
(partial ZVS occurs when the voltage across the MOSFET is
lower than the initial voltage, but is not zero) increases
switching losses, but decreases conduction losses.
VI.
EXPERIMENTAL RESULTS
A scaled prototype of the three-phase full active bridge
converter was simulated for different load levels and built
(Figure 13), in order to demonstrate the adaptive control. The
converter topology is shown in Figure 12 and the
specification is shown in TABLE 1. The value of leakage
inductance shown in the specification is the average value of
the three phases. The proposed control strategy has been
implemented only in the primary side of the transformer to
validate the theoretical calculations, but it also can be applied
on the secondary. The adaptive control has been implemented
with a FPGA (Spartan 3 from Xilinx) with 20 ns of accuracy.
Figure 10. a) Losses in the ARCN in function of the load b) Current through
the principal MOSFET
The implementation OPTION 1 has been selected for this
prototype, because analyzing both options for this converter,
the OPTION 1 has lower losses at light load. To verify the
adaptive concept, three different tests have been done for the
same load point (6,4%), but for different Tarcn values. The
optimum value for this load is Tarcn =600 ns, but the
implementation of OPTION 1 has a margin to ensure the soft
switching, then the value is Tarcn =640 ns.
TABLE 1. SPECIFICATIONS OF THE PROTOTIPE
Vin
Vo
Np/Ns
Llk
250 V
125 V
2
3,6 uH
Po(6,4%)
fc
Cn
Lr
775 W
40 kHz
8 nF
5 uF
a)
Figure 12. Schematic circuit of the implemented converter.
b)
Figure 11. Adaptive control implementation. a) Implementation to obtain
Full ZVS for the whole range of load. b) Implementation to obtain Partial
ZVS
Regarding this trade-off, two options of discretization of
the curve in Figure 7.a are presented in Figure 11. In order to
simplify the control, the curve is divided into three different
ranges. OPTION 1 shows the case where full ZVS for the
whole load range is achieved, keeping the ARCN active
during a longer time period than the theoretically predicted
and adding losses in the ARCN. OPTION 2, pretends to
reduce Tarcn, causing lower losses in the ARCN but having
partial ZVS for a certain load range. The selection of the best
option depends of the specification of the circuit.
Figure 13.Prototype implemented and tested
Figure 14 shows the discretized curve of the ARCN
optimum time connected. Three tests have been done in order
to demonstrate the adaptive control. The three tests are
represented with circles in Figure 14. The three tests are done
at the same load point, but changing the Tarcn.
The results for the test are explained bellow.
The first test has a Tarcn=160 ns, theoretically with this
value, cannot achieve ZVS. The practical result is shown in
Figure 15.
The second test has a Tarcn=320 ns, theoretically with this
value, cannot achieve ZVS. The practical result is shown in
Figure 16.
The last test has a Tarcn=640 ns, theoretically with this
value, can achieve ZVS. The practical result is shown in
Figure 17.
Figure 16. Waveform in M2 for Tarcn=320 ns
Figure 14. Tests for the adaptive control
Figure 17. Waveform in M2 for Tarcn=640 ns
TABLE 2. LOSSES MEASUREMENTS IN THE DIFFERENT TESTS
Figure 15. Waveform in M2 for Tarcn=160 ns
TABLE 2 described the results obtained in terms of
losses, this results are also shown in Figure 18. The cost of
the ARCN is very low in comparison with the power
converter circuit, and according to the measurements of Table
2, the converter losses are reduced a 17 % for a light load.
[2]
[3]
[4]
Figure 18. Losses in the converter in function of Tarcn
[5]
VII. CONCLUSION
In order to improve the performance of the full active
bridge converters for all load ranges, a novel adaptive control
for its ARCN is presented. Using this control strategy, the
auxiliary circuit will be active only during the required time
interval to achieve ZVS, allowing minimizing the losses of
the converter at low load. The implementation has been
discussed, and two options are proposed. The behavior for
light load is validated with a scaled prototype, using one of
the proposed implementations to achieve ZVS. In this
particular case, the adaptive control is a 17,6 % better in terms
of losses at very low load than the classical control strategy.
[6]
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