Advanced Single-Stage Soft Switching PWM Power ...

Advanced Single-Stage Soft Switching PWM Power Conditioner with Coupled Inductor PWM Boost Chopper Cascaded PWM Inverter and Time-Sharing Sinusoidal Follow-Up Control Scheme Nabil A. Ahmed, Masafumi Miyatake Sophia University Dept. of Electrical & Electronics Eng. 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan Email: [email protected]

Abstract—This paper presents an advanced one-stage power conditioner circuit configuration suitable and acceptable for new energy generation systems specially low voltage large current distributed power supplies as solar photovoltaic and fuel cell or new storage applications as electric double layer capacitors and new type batteries. The circuit configuration depends mainly on eliminating the unreliable electrolytic capacitor DC link and using a bypass diode and coupled inductor assisted boost chopper to increase the boosting ratio and achieving high power conversion efficiency. This power conversion system is a modified version of the DC-DC converter previously proposed by the authors, which is composed of a time-sharing operated sinewave absolute modulated bypass diode and coupled inductor assisted boost chopper to produce a sinewave absolute value tracking AC voltage at the DC link and a sinewave PWM cascaded inverter to produce the sinusoidal AC voltage. This power conditioner is operated by selective time-sharing dual mode pulse pattern signal processing control scheme. The paper discusses also the soft switching operation of the coupled inductor boost chopper using only a passive auxiliary edge-resonant snubber circuit for further improvement in the total power conversion efficiency. The operating principle and the unique features of this new power conditioner are described and evaluated through a design example based on simulation results. Keywords—Boost chopper, inverter, one-stage power conversion, bypass diode, coupling inductor, passive and active soft switching snubbers, dual mode sinewave tracking, distributed power supply. INTRODUCTION

A. Technical Background Nowadays and with the use of new energy generation systems in many applications as telecommunication power plants, automotive power supplies, electric and hybrid electric vehicles (HEV), the power conditioners with power electronics semiconductor devices are became indispensable. DC-DC converters are necessary for boosting and conditioning the unregulated low DC voltage from solar photovoltaic (PV) modules or fuel cell (FC) stacks, or for storing this power by charging battery banks or super storage capacitors (SSC). Sinewave modulated inverters are necessary for the utility AC grid connection or for feeding stand-alone residential power applications. For practical applications and cost effective requirements such as higher efficiency, higher performances, smaller

Hyun Woo Lee and MutsuoNakaoka Kyungnam University Electric Energy Saving Research Center

449 Wolyoung-Dong , Masan, 631-701, Korea [email protected]

volumetric physical size and lighter weight of small scale stand-alone and utility interactive power generation systems, new concepts and cost effective power conditioners based on electronic circuit controller and signal processor should be developed and evaluated as compared with high frequency transformer linked converters. In general, the non-isolated single-phase sinewave power conditioning system topologies have practical advantages such as lower cost, smaller size, higher power density, higher response and higher efficiency [1]-[3]. Among these system topologies, the non-isolated power conditioner, which composed of two cascaded power conditioning and processing stages, is used for new energy generation and energy storage systems [4]. In the first stage, a high frequency PWM controlled boost chopper type DC-DC converter with an electrolytic capacitor DC link is required for boosting the low DC voltage of new energy systems. In the second stage, sinewave modulated inverter connected to commercial utility AC power grid or stand-alone power effective utilizations loads is widely used. However, this conventional PWM boost chopper and sinewave PWM inverter based system has some disadvantages, which must be solved as relatively poor power conversion efficiency especially in the low output power setting ranges due to switching and conduction power losses in boost chopper, electrolytic capacitor DC link and the cascaded sinewave full-bridge (FB) inverter system. Further, the bulky and temperature dependent unreliable electrolytic DC smoothing capacitor bank, which includes a short life time, impossible recycle easiness is actually required for constant voltage regulation at the DC link based on the PWM controlled boost chopper. B. Research Objectives The authors have previously proposed a novel one-stage power conversion system composes of selective time sharing dual mode sinewave absolute modulated boost chopper with a bypass diode assisted and PWM single-phase inverter is proposed, which could especially achieve high power conversion efficiency for wide range power setting requirements [5]. In addition, the electrolytic DC link capacitor in the intermediate link between the boost chopper and the inverter circuits has been largely reduced from practical point of view and a small ac film capacitor has been used.

In this paper a modified version of the previously developed one-stage power conditioner is proposed, which is suitable and acceptable for low voltage large current power generation systems as PV and FC or new storage applications as electric double layer capacitors (EDLC). This power converter uses a bypass diode assisted coupled inductor PWM boost chopper-cascaded sinewave PWM modulated inverter to increase the voltage boosting ratio and achieving high power conversion efficiency. The diode assisted coupled inductor boost chopper is operated at a time-sharing to produce sinewave absolute value tracking AC voltage at the DC link. The sinewave PWM modulated inverter is operated at a complementary partially selective time sharing to produce the required sinusoidal AC voltage. Different configurations of soft switching techniques using active and passive resonant snubber circuits are introduced to obtain zero voltage switching (ZVS) commutation for the coupled inductor boost chopper for further improvement in the total system power conversion efficiency. The operating principle of this modified power conditioner with its dual mode PWM time sharing control scheme is evaluated and verified by simulation results in terms of its switching voltage and current waveforms as compared to the previously developed one.

inverter. As it is depicted in Fig. 1(c), when the sinewave instantaneous tracking coupled inductor PWM boost chopper operates under a condition of absolute sinewave PWM control, the single-phase FB inverter is designed as to have a non operation condition. When the voltage source FB inverter operates under a condition of sinewave PWM, the boost chopper is also designed as to be in non operative mode. Since the proposed sinewave PWM conditioner is not required to operate simultaneously its chopper and inverter converters as in the case of conventional power converter, which consists of boost chopper converter with electrolytic DC capacitor and voltage source inverter, the total number of switching operation times can be reduced. Therefore, the switching losses and conduction losses of this newly developed power converter, in addition to the power loss of electrolytic DC capacitor can be all substantially reduced.

Lf Db

C.

Circuit Description Fig. 1 illustrates the basic system configuration of the proposed time-sharing sinewave absolute value tracking selective dual mode controlled soft switching PWM single-phase power conditioner. Fig. 1(a) schematically shows the previously proposed power conditioner, while Fig. 1(b) shows the newly developed one. This power conditioner is composed of selective time-sharing absolute sinewave voltage tracking coupled inductor boost chopper with a bypass diode Db topology and time-sharing complementary sinewave full-bridge (FB) inverter with low pass filter in parallel to the load. This newly proposed power conditioner has the same components as the previously developed one except the coupled inductor boost chopper topology as shown in Fig. 1(b). The selective time-sharing sinewave voltage tracking PWM coupled inductor boost chopper with a bypass diode loop is used for boosting and converting the intermediate input side DC link voltage from PV modules or FC stacks to a constant quasi sinewave AC absolute. The partially sinewave PWM controlled full-bridge inverter with a low-pass filter is operated selectively by a time-sharing dual mode sinewave control processing scheme to produce the required AC output voltage for utility interface or for residential use. The unique operating principle of this single-phase sinewave PWM power conditioner with a bypass diode-assisted sinewave absolute voltage tracking coupled inductor boost PWM chopper is basically shown in Fig. 1(c). The operating principle is based on the selective dual mode time-sharing operation of the boost chopper and the FB

DC

SW3

CC

Lb Vin

NEWLY PROPOSED ONE STAGE POWER CONDITIONER

SW1

Cf SW2

SW C Boost Chopper with Bypass Diode

Ro

SW4

Full-bridgeInverter

Low-pass Filter

(a) Previously proposed configuration

(a) Newly proposed circuit configuration High-frequency switching

Sinewave PWM switching

vboost GND

Vin

GND

Non-switching Non-switching

Boost Chopper

Full-bridge Inverter

(c) Operating principle Fig. 1: One-stage single-phase time sharing power conditioner.

II.

DUAL MODE CONTROL STRATEGY

v out

The selective time-sharing dual mode control strategy of the proposed power conditioner is clearly summarized in the following; A. Operation mode of coupled inductor boost chopper with bypass diode When the input DC voltage Vin is less than the absolute value of the required sinewave output voltage vout, the chopper switch SWC in the coupled inductor boost chopper with a bypass diode operates partially at high frequency switching mode for boosting the input voltage and producing quasi sinusoidal pulse modulated waveform at the intermediate film capacitor Cc. On the other hand, the switches (SW1~SW4) in the voltage source type full-bridge inverter operate under commercial frequency-based synchronous polarity switching; For example, when the positive half sinewave of output voltage is required, the switches SW1 and SW4 only are designed as to be in on state and when the negative half sinewave of output voltage is required, the switches SW2 and SW3 only are conducting. When the input DC voltage Vin is greater than or equal the absolute value of the required sinewave output voltage vout, the switch SWC in the coupled inductor boost chopper with the bypass diode is always in off state. The switches (SW1~SW4) of the voltage source full-bridge inverter operate under a condition of high frequency switching sinewave carrier-based PWM switching mode. In this case, the input current from DC supply does not flow through the boost coupled inductors L11, L22 and free wheeling diode Dc. Consequently, it continuously flows through the bypass diode Db of the boost chopper. Therefore, the conduction losses of boost coupled inductors L11, L22 and freewheeling diode Dc in the power delivery stage do not occur in this operating mode. B.

Time-Sharing Sinusoidal Follow-up Control

Considering the well Known steady-state voltage conversion characteristic of the typical PWM boost converter, which generally represented by

vout =

Vin 1− D

(1)

Rearranging (1), the duty cycle D of the coupled inductor boost PWM chopper switch SWC as a function of the input voltage Vin and absolute value of the desired sinusoidal output voltage vout, can be represented by

D=

1 v out / Vin

(2)

Equation (2) is used to determine the desired duty cycle for gating the chopper switch SWC to obtain a quasi sinusoidal voltage at the intermediate capacitor Cc tracking the absolute vale of the desired output voltage. The gate voltage pulse timing sequences of proposed single-phase PWM power converter are depicted in Fig. 2. More details of the control circuit can be found in [5]-[7].

Reference waveform

Vin - Vin

SWC

SW1

SW2

SW3

SW4

Fig. 2: Gate pulse sequences of proposed power conditioner. C. Features of Proposed Time-Sharing Power Conditioner The attractive features and excellent advantageous of the time-sharing sinewave single-phase power conditioner with sinewave voltage tracking boost PWM chopper and electrolytic DC capacitor less link can be summarized in the following points: In the conventional single-phase power modulated converter, the boost PWM chopper with the electrolytic capacitor DC link as well as the single-phase full-bridge sinewave inverter always operates during all periods under a high frequency carrier sinewave PWM switching conditions. However, in the proposed converter when the inverter-side bridge arm power switches operate under a condition of time sharing controlled sinewave modulation, the boost chopper-side power switch does not operate. In this case, the bypass diode in the boost chopper works independently. On the other hand, when the boost chopper-side power switch operates, the inverter-side power switches operate only under a condition of commercial frequency-based synchronous polarity switching. Therefore, the number of switching times of the proposed type power conditioner is substantially decreased as compared with the conventional one. As a result, the newly proposed power conditioner can suppress the switching power losses and conduction power losses. In addition, at the operation mode of the inverter-side power switches, the input current does not flow through the boost inductor Lb and diode Dc, it flows through the bypass diode Db. Therefore, the conduction power losses of boost PWM chopper can be reduced. Furthermore, the time-sharing sinewave PWM operated full-bridge inverter operates around zero or low current value. Therefore, the switching power losses and conduction power losses of voltage source FB inverter stage are kept to be low as compared with the conventional one.

Moreover, the smoothing DC link capacitor stage between this time sharing sinewave voltage tracking boost PWM chopper and time sharing sinewave controlled voltage source FB is not requiring a large-volumetric electrolytic capacitor DC link. Therefore, the total solar PV or FC power generation systems can achieve system downsizing and lighter weight in addition to longer lifetime and higher reliability operating due to the possibility of using the film capacitor as a DC link capacitor instead of unreliable and temperature dependent bulky type electrolytic DC smoothing capacitor link.

Fig. 3: Generic topologies of coupled inductor one-stage soft switching power conditioner.

III.

SOFT SWITCHING BOOST PWM CHOPPER

Fig. 4 shows possible generic alternative circuit topologies of two winding and three winding coupled inductor assisted boost chopper one-stage power conditioner with different soft switching topologies of trapped energy losses capacitor snubber and auxiliary resonant snubber circuit to achieve zero voltage switching (ZVS) operation of the boost chopper. These topologies are suitable for low voltage/large current generation systems to achieve high voltage boosting ratio. These circuits will be discussed and evaluated from simulation and practical points of view in future work. Figs. 3(a) and (b) show passive circuit, where Fig. 3(a) uses a simple lossless capacitor and Fig. 3(b) uses a passive auxiliary edge resonant snubber circuit. The topologies of Figs. 3(a) and (b) enables soft switching operation only at turn-off switching transition of the coupled inductor boost chopper. Figs. 4(c) and (d) show an active auxiliary edge resonant snubber circuit and a lossless capacitor snubber integrated switch, respectively. Active snubber circuits enable complete soft switching operation at turn-on and turn-off switching transitions of the coupled inductor boost chopper. Fig. 4(e) illustrates a three winding coupled inductor with tapped energy lossless capacitor snubber which is suitable for a very low DC input voltage applications. The different topologies shown in Fig. 3 are suitable for low voltage large current generation systems to achieve high voltage boosting ratio. These circuits will be discussed and evaluated from simulation and practical points of view in future works. Based on the sinewave voltage tracking power conditioner shown in Fig. 1(c) and using a passive auxiliary resonant snubber circuit, a soft switching operation of the boost chopper can be achieved using a passive auxiliary snubber circuit shown in Fig. 3(b), which is composed of a resonant inductor Lr, a resonant capacitor Cr, a lossless snubber capacitor Cs, and auxiliary diode D1~D3. This soft switching coupled inductor boost PWM chopper, which can operate at ZVS commutation in turn-off switching transition of the boost chopper switch SWC is shown in Fig. 4. The detailed mode transitions and the relevant operating voltage and current waveforms of the sinewave voltage tracking soft switching boost PWM chopper during one switching period can be find in [5]-[7].

Fig. 4: Proposed time-sharing soft switching power conditioner with passive auxiliary edge resonant snubber.

IV.

SIMULATION RESULTS AND EVALUATIONS

The simulation specifications and circuit parameter constants of the previously proposed and newly developed single-phase sinewave power conditioner with time-sharing sinewave absolute modulated boost chopper are listed in Table I. TABLE I. SIMULATION SPECIFICATIONS AND CIRCUIT CONSTANTS.

(a) DC link voltage

Item

Symbol

DC Input voltage

Vin

AC output voltage

vout

Switching frequency

fs

Boost inductor Intermediate capacitor Snubber capacitor

Value

L11 L12 L22

Lb

Previously

Newly

160 V 200 V 20 kHz

40 V 200 V 20 kHz 0.05 mH 0.045 mH 0.05 mH

1 mH

Cc

2.2 µF

2.2 µF

Cs

Filter Capacitor

Lr Cr Cf

12 nF 7 µH 12 nF 10 µF

12 nF 7 µH 12 nF 10 µF

Filter inductor

Lf

1 mH

1 mH

Resonant inductor Resonant capacitor

Fig. 5 shows the current and voltage operating waveforms of the previously proposed non-coupled one stage power converter at a working DC input voltage of 160 V. Fig. 5(a) illustrates the intermediate DC link voltage. Fig. 5(b) shows the boost inductor current waveform. Fig. 5(c) illustrates the time sharing dual mode sinewave modulated voltage waveforms across the inverter bridge of proposed power conditioner before the low pass filter. The output voltage and current waveforms are shown in Fig. 5(d). The harmonic spectra of the output current are shown in Fig. 5(e). A high quality sinusoidal voltage and current waveforms are produces with a total harmonic current distortion (THD) of 2.82%. The same operating voltage and current waveforms are depicted in Fig. 6 at a low DC input voltage of 40 V, which is similar to the low voltage/large current applications as the output of the proton exchange membrane fuel cell (PEMFC) stacks or new storage applications as electric double layer capacitors and new type batteries. Although this previously proposed power conditioner behaves well at a considerably high input voltage, it is output voltage and current waveforms are remarkably distorted at a very low input DC voltage as shown in Fig. 6. Fig. 6(e) shows that the total harmonic distortion factor of the output current is about 19.63%, which is not acceptable by the standard regulations which limit the total harmonic distortion to be less than 5%.

(b) Boost inductor current, ILb.

(c) Inverter voltage

(c) Load voltage and current

(d) Load current harmonic spectra

Fig. 5: Simulated voltage and current waveforms of non-coupled inductor power conditioner, Vin =160 V.

(a) DC link voltage

(a) DC link voltage

(b) Boost inductor current, ILb.

(b) Boost inductor current, IL11.

(c) Inverter voltage

(c) Inverter voltage

(c) Load voltage and current

(d) Load voltage and current

(d) Load current harmonic spectra

(e) Load current harmonic spectra

Fig. 6: Simulated voltage and current waveforms of non-coupled inductor power conditioner, Vin =40 V.

Fig. 7: Simulated voltage and current waveforms of coupled inductor power conditioner, Vin =40V.

The operating voltage and current waveforms of the newly developed power conditioner with coupled inductor boost chopper at a low DC input voltage of 40 V are illustrated in Fig. 7. The boost inductor current waveform in Fig. 7(b) is the current through the self inductance L11 of the coupled inductor. Observing these voltage and current waveforms, it is proven that the coupled inductor boost chopper enables high boosting ratio and the newly proposed single-stage power conditioner can provide high quality sinusoidal voltage and current waveforms even at a very low input DC voltage applications. The total harmonic distortion factor of the output current is 4.57% which is less than the standard regulations.

V.

CONCLUSIONS

In this paper, an advanced novel circuit topology of coupled inductor one-stage single-phase power conditioner suitable for low voltage/large current new energy generation systems as fuel cell applications or new storage applications as electric double layer capacitors and new type batteries. The unique operating principle of the proposed power conditioner system with selective time-sharing dual mode partial sinewave modulation scheme is described and discussed with a simulation results. Unique features as high power conversion, low total harmonic distortion, more reduction in the intermediate capacitor size and elimination of the electrolytic capacitor has been achieved. The paper also has introduced a front stage soft switching sinewave-tracking voltage controlled soft switching PWM coupled inductor boost chopper with a bypass diode loop using a passive auxiliary edge-resonant snubber circuit to achieve further high efficiency power conversion. Other generic topologies of coupled inductor assisted boost chopper one-stage power conditioner are suggested for the proposed one-stage power conditioner to increasing the voltage boosting ratio and achieving high power conversion efficiency. These topologies will be investigated and evaluated in the future work.

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