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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] REFERENCES [9] [1] [1] D.M. Bellur, M.K. 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