A Modified Boost Topology with Simultaneous AC and DC Load

Olive Ray and Santanu Mishra Department of Electrical Engineering

Indian Institute of Technology, Kanpur, U.P., India olive@iitk.ac.in, santanum@iitk.ac.in

AbstractThis paper proposes a modified Boost converter topology which can supply DC as well as AC loads simultaneously. This topology is realized by replacing the controlled switch of a conventional Boost converter with a voltage source inverter (VSI) bridge network. The duty cycle of the Boost converter, in this case, is controlled by the shoot-through period of any particular inverter leg. The proposed topology is immune to EMI induced misgating-on of complementary switches of each VSI leg. A PWM control strategy for this converter is presented. The advantages and limitations of the converter are also discussed. The proposed approach can be easily extended to higher order boost converters (e.g., quadratic boost) in order to achieve higher gains. Experimental and simulation results are provided to verify the operation of the converter.

I. INTRODUCTION Small-scale Distributed Generation Systems (DGSs) involve different types of localized energy sources like Diesel Generators, Solar panels, Wind turbines, etc., serving different types of loads, which can be of either DC or AC type. Power electronic converters efficiently interface the varied source types to the loads. One way of implementing step-up DC-AC power conversion with higher efficiency and power density is by using a Boost converter stage cascaded to a Voltage Source Inverter (VSI) stage. [1] As shown in Figure 1, the single stage Boost converter output (vdcout) is inverted using the VSI to obtain an AC voltage output (vacout). Depending upon the application requirements, boost converter topologies with higher gains, such as higher order Boost converters or transformer-based isolated designs, can also be used for the DC-DC step-up conversion. The resulting topology thus obtained provides inversion operation in two stages. The converter can be used to feed both AC as well as the DC loads; the DC loads being supplied by the output of the DC-DC converter stage. In traditional Voltage Source Inverters, the two switches of a single leg of the bridge network are complementary in operation. During switching transition, sufficient dead-time has to be provided to the switches to prevent shorting of the input source. In addition, due to EMI or other spurious noise,

misgating-turn-on of the inverter leg switches may take place, resulting in damage to the switches. Z-Source Inverter, proposed in [2], can mitigate the problem of misgating. The use of a unique impedance network at the input of the Z-source inverter allows a shoot-through state in which both the switches of an inverter leg can be turned-on simultaneously. Extended Boost Z-Source Inverter has been proposed where a higher gain is achieved using this Z-Source topology [3]. However, Z-Source converter cant supply both DC as well as AC loads simultaneously. This is due to the fact that it has two capacitors which have to be matched with equal load across them. Unmatched loads on the capacitors might lead to dynamic instability [4]. The Switched Boost Inverter (SBI) was proposed in [5], which can achieve similar advantages as a Z-Source converter with lesser number of passive components and supply simultaneous DC as well as AC loads. This paper proposes a modification of a conventional Boost converter which can achieve advantages similar to a Z-Source Inverter as well as supply both AC and DC loads simultaneously. This single converter topology, supplying both AC as well as AC loads, can be well utilized in Microgrids as well as Nanogrids, where different types of power conversions need to be realized. It will be shown that this topology can be easily extended to achieve higher

Figure 1. Schematic of Conventional Transformer-less Boost Converter Cascaded VSI

978-1-4673-0803-8/12/$31.00 2012 IEEE 2454

conversion ratio. The proposed modification is illustrated using the conventional Boost converter and its operating behavior has been described next in Section II. The PWM control strategy for the converter is described in this section followed by the advantages and disadvantages of the proposed topology. The converter and its control strategy have been validated using an experimental prototype in Section III. Extension of this scheme to a higher order boost is validated using PSPICE simulation.

II. PROPOSED MODIFIED BOOST CONVERTER TOPOLOGY Fig. 1 shows the schematic of a single Boost converter stage which is cascaded to a single-phase Voltage Source Inverter stage. Note that the inverter can also be three phase in this case to produce a three phase output. A conventional Boost converter, shown in Fig. 2(a), is operated using the single controlled switch Sa between nodes s and n. Fig. 2(b) shows the schematic of the proposed extension, which is referred to as Hybrid Boost Converter (HBC) in this paper. The circuit is primarily a Boost DC-DC converter (Fig. 2(a)), where the controllable switch Sa is replaced by the bridge network to obtain the additional DC-AC operation. The proposed extension can thus provide both DC output across Vdcout as well as AC output across Vacout, simultaneously, in a single stage conversion. Therefore, the topology is referred to as the Hybrid Boost Converter.

A. Operating Principle For the following explanation CCM operation of the Boost converter is assumed (the Boost inductor current never goes to zero). The Hybrid Boost Converter is controlled using the four switches (S1-S4) of the bridge network (Fig. 2(b)). The bidirectional switches, formed by the combination of Si and Di (i=1 to 4), replace the single controllable switch Sa of the Boost converter, shown in Fig. 2(a). The Boost operation of the proposed converter can now be realized by turning on both switches of any particular leg of the bridge network (either S1-S4 or S3-S2), simultaneously. This is shoot-through switching as far as the VSI is concerned and it is strictly forbidden in the case of a conventional VSI. However, for the proposed modification, this operation is equivalent to switching on of the switch Sa of the conventional Boost converter (Fig. 2(a)). The inverter operation is realized by applying appropriate sinusoidal PWM switching technique to the inverter leg switches S1-S4. Note that during inverter mode of operation, the DC output of the boost converter (vdcout) acts as the input voltage level for the inverter. It is important to note that the inverter input current can be either positive or negative during a particular cycle. When this input current is positive, the power flows through the Boost inductors (L1-L2). However, when the input current is negative, the current flows into the DC load through the Boost diode (D). This operating principle is illustrated using PSPICE simulation in Figure 3. The major difference between this inverter and a conventional inverter is that the input voltage

of this inverter is a switched signal varying between Vdcout and zero. That effectively means that the inverter output can only be modulated when its input is non-zero. This is the fundamental motivation behind the switching strategy for this topology. The inverter output voltage (Vab) is positive, negative or zero in the case of a Unipolar Sine-PWM control. In the proposed switching scheme, the inverter output voltage is non-zero only when the Boost diode (D) conducts. Figure 3 shows that the inverter output voltage (Vab) is positive (Fig. 3(a)) as well as negative (Fig. 3(b)) only when the Boost diode D conducts (Vsw 0). The conduction of the diode is indicated by a positive voltage at the switch node (Vsw). The gate signals for the bridge network are also shown in this figure. The inverter output voltage (Vab), on the other hand, can be zero either during shoot-through condition (Vsw = 0) or during non shoot-through (Vsw > 0) condition. Based on the above switching conditions, an appropriate PWM switching strategy for the proposed converter has been described in the next section.

B. Switching Strategy The switching states of a conventional single phase Voltage Source Inverter (Fig. 1) are shown in TABLE 1. Assuming the switch state is denoted by 1 when it is turned on and 0 when it is turned off, TABLE 1 shows the various

(a)

(b) Figure 2. Proposed Hybrid Boost Converter obtained by modification of a conventional Boost converter switch Sa of (a) with bridge network (b).

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switching conditions for this topology. It identifies several undesirable states, denoted by u, which represent the shoot-through conditions and measures have to be taken to protect the topology against these conditions. The

undesirable states, shown in TABLE 1a, can be eliminated by using the proposed circuit, shown in Fig. 2(b). In the proposed circuit, both the boost operation as well as inverter operation can be performed using four switches (S1-S4). Boost operation occurs when both the switches in a single leg are turned on at the same time. This takes care of the undesirable states u, mentioned in TABLE 1a. TABLE 1b shows the switching states of the proposed converter. Various switching schemes for control of Z-Source converters have been reported in literature [6-8]. The shoot-through states define the boost interval for the DC output. The PWM control circuit to generate the gate signals for this topology is shown in Fig. 4 (a-b). The reference signals to the PWM generation circuit are vm(t) and vST(t). The signals S1-S4 of Fig. 4(b) are provided to the gates of the controlled switches. vST(t), a DC signal, controls the shoot-through period and hence the duty ratio for the DC output of the Boost converter and vm(t) controls modulation index for the inverter. The PWM switching waveforms for the proposed converter during positive reference signal are shown in Figure 5. The figure indicates that the boost current alternates between the two inverter legs, enabling high frequency operation and improves the converter dynamics. As the same set of switches controls both the DC and AC output, there is limitation to the maximum duty cycle or modulation index that can be achieved for this topology. The switching strategy must satisfy the following constraint

1, (1) Where, ma = modulation index (=max(vm(t))) and D = Duty ratio of the Boost converter.

(a) (b)

Figure 3. Steady state switching waveforms for the HBC showing that when the switch node voltage (shown in purple) is non zero (equivalent to the diode of Boost converter operating), the VSI operation takes place as shown by positive value of Vsw in (a) and negative value of Vsw in (b). The corresponding gate signals to the VSI switches are also shown.

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The maximum gain between DC input and AC output (peak) that can be achieved for HBC is unity. TABLE 2 shows a comparison of outputs of various topologies using the conditions ma = 0.5 and D = 0.4. For a HBC, the maximum output AC voltage (pk) can be equal to the input voltage and this holds true when equation (1) holds true with unity relationship. If higher gain is desired, converters with higher order gains can be used. This is verified in the experimental section. TABLE 2 provides comparative figures for a Hybrid Boost Converter (Boost converter based topology), Quadratic Boost based converter as well as Switched Boost Inverter [5]. In all the analyses, continuous conduction mode of operation was assumed. The use of Quadratic Boost based converter to achieve higher gains has been validated using simulation as shown in Section III.

C. Advantages of the proposed topology

The proposed topology (Fig. 2(b)) can be used to generate simultaneous DC (at vdcout) as well as single phase AC outputs (at vacout) through a single stage conversion from a single DC input (vdcin). The proposed Hybrid Boost Converter has the following advantages:

The problems associated with misgating-on of the two complementary switches of each inverter leg due to EMI or other spurious noise have been eliminated by the proposed topology. Shoot-through condition does not cause problems in the operation of the circuit. On the contrary, having a Shoot-through is necessary for Boost converter operation.

Implementation of dead-time is not essential for this topology. This improves the nature of the inverter output with respect to its harmonic content [9].

TABLE 2 COMPARISON OF THEORETICAL DC AND AC (PEAK) OUTPUT VOLTAGES FOR AN INPUT DC VOLTAGE OF VIN FOR D = 0.4 AND MA = 0.5

For D=0.4 and ma=0.5 Gain Vdcout Vacout

HBC

1

1.67 Vin 0.835 Vin

Hybrid Q-Boost

Converter

1

2.78 Vin 1.39 Vin

SBI [5] 1 .

1 2 3.0 Vin 1.5 Vin

Figure 5. PWM switching waveforms for the proposed converter

(a)

(b) Figure 4. (a) and (b) shows the circuit for generation of gate signals for the proposed converter.

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The number of controllable switches is reduced when compared to a Boost cascaded inverter topology; both the VSI and Boost Converter are controlled using the same bridge configuration, thus reducing control circuit.

In this topology, the duty ratio and modulation index of the DC and AC structure can be independently controlled. In contrast to a Z-Source or SBI [5], the maximum duty cycle for DC-DC conversion is not limited to 0.5. This has been demonstrated using simulation results in Figure 6. When the HBC is not used for DC-AC operation, the converter can be solely used for Boost operation. The figure shows the gate signals when the modulation index is zero and the duty cycle is 0.7.

The current during boost interval of the Boost converter alternates between the two legs of the inverter. This enables use of higher switching frequency for the boost converter thus reducing magnetic size and improving the dynamics of the system.

The converter can supply both AC as well as DC loads. There is single stage conversion for DC-DC as well as DC-AC operation.

III. VERIFICATION The modified converter proposed in this paper has been verified used PSPICE simulation and experimental

prototype. The schematic of implementation for a modified Boost converter is given in Fig. 2(b) and parameters for the circuit are provided in the caption of Fig. 7. The PWM signals have been generated using TMS320F28335 DSP kit and its operational schematic is shown in Fig. 4 (a-b). Fig. 7(a) shows the actual control signals provided to the switches. It shows that when switches S1 and S4 or S2 and S3 are on at the same time, Vsn=0. These two intervals, controlled by vST(t), are the boost intervals for the DC output. The AC output is obtained using Unipolar Sine PWM modulation technique, by controlling the value of the reference signal vm(t). The AC and DC outputs are shown in Fig. 7 (b). For a 48 V DC input, with D = 0.4 and ma = 0.5, the DC and AC outputs obtained are 78.3 V and 28 V AC (rms) respectively.

Figure 6. PWM switching waveforms for the proposed converter when ma = 0 and D = 0.7.

(a)

(b) Figure 7. (a) shows the Gate Signals for controlling the proposed converter. The input-output behavior is seen in Fig 7(b). Prototype parameters: L (=L1+L2) =2.4 mH, C=100 uF, Lac (=Lac1+Lac2) =1 mH, Cac=20 uF, Rac=Rdc=100 Ohms.

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If a higher gain is desired high gain Boost stages [10-12] can be used for the basic DC-DC converter. Fig. 8 (a) shows the implementation of a Quadratic-Boost topology based modified Quadratic-Boost converter. The input to this converter 48 V and it produces a 133 V DC and a 133 VAC (pk-pk). Note that using a Q-Boost based converter changes the stresses of the switching devices and the converter has to be designed accordingly.

IV. CONCLUSION In this paper a modified single stage Boost converter for providing both DC as well as AC outputs is proposed. The advantages of the converter over conventional VSIs are described. Simulation results show that this concept can be extended to higher order Boost converters and this is shown for a Quadratic Boost based converter. Experimental results verify the operation of a single stage Boost based converter in open loop.

ACKNOWLEDGEMENT The authors would like to thank Ravindranath Adda, Senior Research Scholar, IIT Kanpur for his suggestions and comments for improving this paper.

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(a)

(b)

Figure 8. (a) Schematic of proposed modified Quadratic Boost Converter (b) Simulation of the behavior of the converter with input voltage of 48 V DC (deep blue). The output DC voltage (green) is 133 V DC at D = 0.4. The AC output (blue) (peak value of 66.5 V at ma = 0.5) and inverter output (pink) is also shown. The load ratings are same as those used for experimentation with HBC..

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