Transformerless partial power converter (PPC) for the DC-DC stage of rapid-charging stations for electric vehicles (EV)

Abstract

Described is a new partial power converter (PPC) for the DC-DC stage of rapid-charging stations for electric vehicles (EV). The proposed converter manages only a fraction of the total power delivered from the grid to the battery, which increases the general efficiency of the system and the power density while potentially reducing the cost of the charger. The proposed topology is based on a switched capacitor between the AC terminals of a bridge converter H and does not require high-frequency isolation transformers in order to provide a source of controllable voltage between the CC link and the battery. The proposed concept can be implemented by using interposed power cells, which can improve energy quality, reduce the size of the inductor, and allow scalability for chargers of higher nominal power.

Claims

1. A transformerless partial power converter (PPC) for a DC-DC stage of rapid-charging stations of electric vehicles (EV), characterized in that it comprises: one or more switching H-bridge channels, wherein one or more each of said one or more switching H-bridge channels is formed by a bypass diode D, an output inductor L, and a switching H-bridge having forced commutation semiconductors S.sub.a1, S.sub.a2, S.sub.a3, and S.sub.a4, with a DC link floating capacitor C, and wherein said switching H-bridge channels are connected in parallel with each other at an input of the switching H-bridge, at an input of the diode D and at an output of the inductor L.

2. The transformerless partial power converter according to claim 1, characterized in that the forced commutation semiconductors S.sub.a1, S.sub.a2, S.sub.a3, and S.sub.a4, are insulated gate bipolar transistors IGBT.

3. The transformerless partial power converter according to claim 1, characterized in that the bypass diode D is replaced by an active semiconductor device of an IGBT bipolar transistor type or a metal-oxide-semiconductor field effect transistor (MOSFET).

4. The transformerless partial power converter according to claim 1, characterized in that the DC link floating capacitor C is connected to AC terminals (V.sub.p, i.sub.c) of the switching H-bridge while the DC terminals of the switching H-bridge are used to make a series connection between a positive terminal of an input voltage V.sub.d and the output inductor L, the output inductor L is connected between the output of the H-bridge and the positive terminal of the voltage of a battery to be charged, thereby regulating both a partial voltage in the AC terminal V.sub.p of the transformerless partial power converter and an output current i.sub.L that is injected into the battery to be charged.

5. The transformerless partial power converter according to claim 1, characterized in that the switching H-bridge channel is operated in such a way that a partial voltage V.sub.p is added or subtracted from an input voltage V.sub.d to establish a voltage of the output inductor L and, therefore, it allows to regulate current that is injected into a battery to be charged.

6. The transformerless partial power converter according to claim 1, characterized in that, in order to regulate a charging process, both a partial voltage V.sub.p of the floating capacitor C and an output current i.sub.L must be controlled, wherein these two variables are related to a sum and a difference in duty cycles of the switching H-bridge, which operates with a bipolar method of pulse width modulation (PWM).

7. The transformerless partial power converter according to claim 1, characterized in that, in order to reduce a relationship between a partial voltage V.sub.p and an output current i.sub.L, a control system is designed that imposes a slow dynamics for changes in the partial voltage V.sub.p for which a significantly smaller closed loop bandwidth is chosen compared to a current loop; thus, the slow dynamics is compensated by the partial voltage V.sub.p and an influence of V.sub.p in steady state of the output current i.sub.L is eliminated.

8. The transformerless partial power converter according to claim 1, characterized in that a control system consists of two closed loops, wherein a first loop is a conventional cascade control architecture, which regulates a voltage of a battery to be charged through an internal regulation of an output current i.sub.L of the output inductor L, where the voltage of the battery and the output current i.sub.L are regulated with proportional-integral (PI) controllers, and a voltage output of a first proportional-integral (PI) controller is d.sub.Δ, reference signals for the first control loop are defined by a battery management system, in addition to the where also a selected charge profile in the first control loop, is a constant current (CC)-constant voltage (CV) method; a second control loop is the one used to regulate a partial voltage V.sub.p, which is also controlled through a proportional-integral (PI) controller and a voltage output of a second proportional-integral (PI) controller is dΔ, and a reference for the partial voltage V.sub.p is the one that sets a partiality ratio of the control system and, therefore, defines an amount of power that the transformerless partial power converter operates; once a sum and a difference duty cycles have been obtained, a pulse width modulator (PWM) is electrically powered to generate activation signals.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 describes a two-stage conversion system for DCRC, of the rated power DC-DC converter type, which comprises the state of the art.

(2) FIG. 2 describes a two-stage converter for DCRC, of the partial power DC-DC converter type, which comprises the state of the art.

(3) FIG. 3 describes the invention of a partial power converter for DCRC based on H-bridge cell with interleaved switched capacitor.

(4) FIG. 4 describes the switching states and equivalent circuits of the invention.

(5) FIG. 5 describes a proposed control scheme for the partially classified charging station.

(6) FIG. 6 describes a table with the parameters of a battery charging operation.

(7) FIG. 7 describes the current output i.sub.L curve during DC mode.

(8) FIG. 8 describes the waveform of the result of the partial voltage V.sub.p, during operation.

(9) FIG. 9 describes the waveform of the result of the current in the capacitor i.sub.c during operation.

(10) FIG. 10 describes the behavior of the battery voltage during operation.

(11) FIG. 11 describes the behavior of the battery current during operation.

(12) FIG. 12 describes the result of the battery charge status during operation.

(13) FIG. 13 describes the total power delivered and the power processed by the converter of the invention.

(14) FIG. 14 describes the efficiency compared between a full power converter and a partial power converter.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(15) The proposed power circuit for the transformerless partial power converter shown in FIG. 3, comprising at least one switching H-bridge channel (cell), formed by the forced commutation semiconductors S.sub.a1, S.sub.a2, S.sub.a3, and S.sub.a4, such as an insulated gate bipolar transistor, IGBT, with a DC link floating capacitor C, a bypass diode D, which can also be replaced by an active semiconductor device such as an IGBT, a metal-oxide-semiconductor field-effect transistor (MOSFET) or similar, and an output inductor L. To achieve the desired power level through a modular and scalable approach, an interleaved or multi-channel configuration is used. FIG. 3 shows in detail the structure of the transformerless partial power converter with a detailed channel, which can be replicated to be connected in parallel and be able to divide the current handled by each of them.

(16) From FIG. 3, it is also observed how the connection of the switching H-bridge cells is carried out in an unconventional way, since the DC link floating capacitor C is connected to the AC terminals (V.sub.p, i.sub.c) while the DC terminals of the switching H-bridge are used to make the series connection between the positive terminal of the input voltage V.sub.d and the output inductor L. The latter is connected between the output of the H-bridge and the positive terminal of the voltage of the battery to be charged. This allows regulating both the partial voltage Vp of the transformerless partial power and the output current i.sub.L that is injected into the battery to be charged.

(17) With appropriate settings, the transformerless partial power converter presented herein can function as voltage reduction unit (buck), while providing higher efficiency and performance compared to conventional topology. In addition, the fact that the switches have to block reduced voltages, allows the use of switching devices with lower ratings, thus reducing the size and cost of the rapid-charging unit.

(18) The objective of the power circuit is to arbitrarily set the partial voltage V.sub.p to a value that allows to regulate the output current i.sub.L of the channel and, at the same time, maintain the conduction instants of the bypass diode D to a minimum. The result is a transformerless partial power converter that most of the time works with switches that block a partial voltage, and during the ON times of the bypass diode D these switches have to block half of the input DC voltage. This approach allows to effectively increase the energy conversion efficiency even though the structure has a greater number of devices when compared to a conventional buck converter, which only has one semiconductor.

(19) Operational Description

(20) Depending on the selected value of the partial voltage V.sub.p, the transformerless partial power converter works as a voltage reduction unit (buck) with improved efficiency. The switching H-bridge cell is operated in such a way that the partial voltage V.sub.p is added or subtracted from V.sub.d to establish the voltage of the output inductor L and, therefore, it allows to regulate the current that is injected into the battery to be charged. To determine the input/output ratio of the transformerless partial power converter, a volt-second balance analysis is performed based on the defined quantities. Considering that the transformerless partial power converter is operating in a steady state, the variations in the output current i.sub.L during its charging and discharging processes must be equal throughout a switching period, leading to:

(21) $\begin{array}{cc}\frac{\left(V_d+V_p-V_b\right)}{L}{t}_1=\frac{\left(V_p+V_b-V_d\right)}{L}{t}_2+\frac{V_b}{L}{t}_d& \left(1\right)\\ V_b{T}_s=V_d\left(t_1+t_2\right)+V_p\left(t_1-t_2\right)& \left(2\right)\end{array}$

(22) Where t.sub.1 represents the time when the partial voltage is added to the input voltage, a switching state that can be seen in FIG. 4a, t.sub.2 is the time when the partial voltage is subtracted from V.sub.d as shown in FIG. 4b, and t.sub.d is the time when bypass diode D conducts as illustrated in FIG. 4c. By defining the duty cycles (that is, d.sub.1=t.sub.1/T.sub.s, d.sub.2=t.sub.2/T.sub.s, d.sub.d=t.sub.dT.sub.s), it is possible to determine the input-output ratio of the transformerless partial power converter.
V.sub.b=V.sub.d(d.sub.1+d.sub.2)+V.sub.p(d.sub.1−d.sub.2)   (3)

(23) Having defined the steady state transfer function of the proposed transformerless partial power converter, and introducing the sum and difference duty cycles, the equations that model the dynamics of the transformerless partial power converter are as follows:

(24) $\begin{array}{cc}L\frac{{\mathrm{di}}_L}{\mathrm{dt}}+R_L{i}_L=V_d{d}_{.Math.}+V_p{d}_{\Delta }-V_b& \left(4\right)\\ C_p\frac{{\mathrm{dV}}_p}{\mathrm{dt}}=i_L{d}_{\Delta }& \left(5\right)\\ d_{.Math.}=d_1+d_2& \left(6\right)\\ d_{.Math.}=d_1-d_2& \left(7\right)\end{array}$

(25) The duty cycles alternate between the different switching states of the transformerless partial power converter shown in FIG. 3. There are basically two states in which the partial voltage V.sub.p of the floating capacitor C alternates between the positive and negative connection and a derivative state, in which the current stored in the output inductor L circulates freely and is discharged through the bypass diode D. The proposed transformerless partial power converter behaves largely like a regular converter in which the active switch is replaced by a controlled capacitor switch. The fact that the partial voltage V.sub.p is the difference between the input and output voltage, produces the partial power operation in the transformerless partial power converter. It should be noted that when operating in the free circulation state, with the rated current flowing through the output inductor L and the bypass diode D, the transformerless partial power converter behaves exactly like a classical full power converter.

(26) Control scheme

(27) To properly regulate the charging process, both the partial voltage V.sub.p of the floating capacitor C and the output current i.sub.L must be controlled. These two variables are related to the sum and difference of the switching H-bridge duty cycles, and their introduction simplifies the controller design. By defining the duty cycles of the sum-delta domain and suitably adjusting the dynamics of the closed loop, the control scheme presented in FIG. 4(a, b, c) is designed. The switching H-bridge is operated with a bipolar PWM method.

(28) Considering that there is a dependence between the output current i.sub.L and the partial voltage V.sub.p, this is appropriately addressed to maintain the stability of the transformerless partial power converter. To reduce this coupling, the partial voltage V.sub.p regulator is designed in such a way that it imposes slow dynamics for changes in the partial voltage V.sub.p for which a significantly smaller closed loop bandwidth is chosen compared to the current loop. The slow dynamics will be compensated by the partial voltage V.sub.p regulator and its influence in steady state is eliminated.

(29) As shown in FIG. 5, a proposed control system consists of two closed loops. The first loop is the conventional cascade control architecture, which regulates the voltage of the battery to be charged through the internal regulation of the output current i.sub.L of the output inductor L. Each of the above-mentioned quantities are regulated with proportional-integral (PI) controllers, and their output is d.sub.Σ. As usual, the reference signals for this control loop are defined by the battery management system, in addition to the selected charge profile, which in this case is the constant current (CC)-constant voltage (CV) method, which stands for constant current-constant voltage (CC-CV).

(30) The second control loop is the one used to regulate the partial voltage V.sub.p, which is also controlled through a PI, and its output is d.sub.66 . It should be noted that the reference for this partial voltage V.sub.p is the one that establishes the partiality relationship of the proposed control system and, therefore, defines the amount of power that the transformerless partial power converter processes. Once the sum and difference duty cycles have been obtained, d.sub.1 and d.sub.2 are reconstructed and fed to a pulse width modulator to generate the activation signals.

(31) Operation Results

(32) To validate the proposed configuration, a test has been developed that considers a rapid-charging station that provides a charging power of 70 kW, while the power converter only processes approximately 40 kW, with the CC-CV charging profile, and the transition between modes will be done at SOC=94%. After validating the conversion method, the same conversion is simulated using a conventional full power converter, in order to establish a comparison in terms of current fluctuation and conversion efficiency. Table 6 presents the rest of the test parameters.

(33) A. Steady State Performance

(34) Since the transformerless partial power converter initially operates in CC mode, the rapid-charging unit feeds the battery with its rated current, which in one case is 200A, as shown in FIG. 7. The current through the battery, in CC mode, follows the 200A reference, this current exhibits ripple of 7.527% of its average value.

(35) Then the transformerless partial power converter can satisfactorily regulate the charging process, while keeping the partial voltage regulated at 200V as seen in FIG. 8. It can be seen how the current i.sub.L and the capacitor voltage V.sub.p are tightly regulated. In addition, it can be seen that the charge and discharge cycles are well balanced, allowing the partial voltage V.sub.p to be kept in check, as shown in FIG. 8. The transformerless partial power converter voltage has a ripple of 3.487 V, equivalent to 1.74% of its average value. Consequently, the net current flowing through the floating capacitor C is zero according to FIG. 9.

(36) A broader look at rapid-charging operation is presented in FIGS. 10 and 11, which present the battery quantities throughout the charging process. The CC-CV charging profile is clearly observed, which means that the battery charging process has two modes of operation. First, during CC mode a constant current is provided to the battery until the controller switches to CV mode at t=342.6 s, when it reaches a particular state of charge. In this mode, the current begins to decrease exponentially until the current through the battery reaches 10% of this initial value and the State of Charge (SOC) reaches 94% as shown in FIG. 12.

(37) As mentioned above, the main characteristic of the proposed charge topology is the reduction in power that the power electronics must handle. In the present results, it is observed that the total power delivered to the battery reaches approximately 68.4 kW during CC mode, according to the measurements in FIG. 12. However, the power actually handled by the transformerless partial power converter is simply 40.1 kW. Therefore, the transformerless partial power converter operates with a partial power ratio of 58.62% during the entire charging process as shown in FIG. 13. As mentioned above, the partial power ratio depends on the partial voltage V.sub.p, and considering that this voltage remains constant throughout the test, the converter also maintains its partiality.

(38) B. Loss Assessment

(39) Once the operation of the transformerless partial power converter has been validated, an efficiency analysis is required. In order to evaluate the efficiency performance of the transformerless partial power converter, the following conduction and switching losses are considered; for this purpose, the thermal modeling tool of the PLECS software is used. The thermal description required for this test is taken from the device data sheets. IXYS IXGN200N60B3 IGBT and GP2D060A120B silicon carbide diode are considered.

(40) In order to make a comparison of the proposed configuration, tests are carried out with a conventional full power converter under the same operating conditions and providing the same amount of power to the charge. FIG. 14 shows the efficiency comparison between the two converters, the proposed transformerless partial power converter has an efficiency of 98.77% relative to a 94.87% efficiency of the full power converter in CC mode. The efficiency in the two converters increases slightly when the controller switches to CV mode, reaching at the end of the charging process values of 99.41% for the transformerless partial power converter and 96.73% for the full power converter.

(41) Conclusion

(42) The invention proposes a transformerless partial power converter based on a high-frequency transformer topology for an electric vehicle rapid-charging station.

(43) The transformerless partial power converter takes advantage of the voltage characteristic of battery packs and only processes part of the total power provided by the charger. The result of the proposed conversion method is a significant improvement in the efficiency of the converter. The transformerless partial power converter only handles a fraction of the DC link voltage.