Feng Tian1, Frank CHEN1, Khalid Rustom2, Issa Batarseh1 1University of Central Florida

2 Petrasolar Co.,Ltd E-mail:Frankchen@eecs.ucf.edu

Abstract — A Pulse Frequency Modulation (PFM)

with Zero-Current-Switching (ZCS) flyback inverter is proposed in this paper. By using half-wave quasi-resonant ZCS flyback resonant converter and PFM control, this topology significantly alleviates switching losses. A detailed analysis revealed nonlinearity in the power stage when the secondary side inductance gets smaller. A modified structure with a secondary side MOSFET is also proposed to solve this issue. Finally, the experimental results of 250W converter are provided.

I. INTRODUCTION

Renewable energy sources play an important role in energy systems for communications, commercial, and residential applications. The traditional solar inverter system consists of two-stages. The first stage is a DC-DC converter that performs input voltage regulation through Maximum Power Point Tracking (MPPT), the second stage consists of Sinusoidal Pulse Width Modulation (SPWM) inverter. This structure has simple control scheme, but with the disadvantage of the having two series stages that results in lower efficiency and higher cost. In recent years, a single-stage inverter structure started to attract more attention due to the potentially higher efficiency and lower component count [1-4]. The single stage structure, utilizes the power stage to create rectified sinusoidal signal that gets directly folded into the grid. Variable frequency Pulse Width Modulation (PWM) converter operating at the Boundary Conduction Mode (BCM) is typically preferred for this application due to the zero current turn-on, but due to the nature of the single stage inverter operation the power level changes during the line cycle and BCM might lead to a very high switching frequency near zero crossings. On the other hand, Pulse Frequency Modulation (PFM) converter uses constant on-time and variable frequency to control the energy transfer from the input to output [5]. The switching frequency decreases at low power levels leading to higher overall efficiency. For this reason PFM converters are commonly used in portable devices to maximize the battery life, especially when devices stay idle for long duration. This paper proposes PFM quasi-resonant ZCS flyback single-stage inverter that combines the low component cost of flyback and the single stage structure, and the efficiency improvement of the PFM and ZCS in a unique solution that might encourage further adaption of the growing renewable energy solutions by the communication sector.

II. FLY BACK ZCS CONVERTER OPERATION ANALYSIS A half-wave quasi-resonant ZCS flyback converter is shown in Figure 1, which uses a quasi resonant technique to achieve zero current switching for the switch of a flyback converter.

iLres iCres

VCres

iDrec

VCo

VSfly

iLo

iSfly

Fig. 1 Flyback ZCS Single-Stage Inverter with PFM Control Figure 2 shows the simplified equivalent circuit of the flyback ZCS single-stage inverter and the detailed operation stages. The timing diagram is illustrated in Figure 3. The one-switching cycle may be divided into the following four stages: Stage 1 [t0, t1] This stage starts at t=t0 by turning on of the flyback switch Sfly, and the voltage across the resonant capacitor Cres is clamped to the output voltage of the converter. The current of the transformer secondary winding is freewheeling through the rectifier diode Drec. On the primary side, the current of the primary winding is increased from zero, which means the switch is turning on at zero current without switching loss. Stage 2 [t1, t2] As the current of the rectifier diode Drec decreases linearly to zero, Drec is turned off at t=t2 with zero-current. Then, the resonance between Lres and Cres starts at zero initial capacitor current, and initial resonant inductor current, iLres equals to iSfly. Stage 3 [t2, t3] Stage 3 starts at t=t2 when the primary current, iLres(t) = iSfly reaches zero, and the resonance between Lres and Cres stops due to the blocking of Dfly. In this stage, the resonant capacitor Cres is discharging through the the reflected transformer primary side magnetizing inductance. As the discharging continues, the voltage across the resonant capacitor Cres changes its polarity.

PULSE FREQUENCY MODULATION WITH SOFT-SWITCHING FLYBACK

SINGLE-STAGE INVERTER

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978-1-4244-3384-1/10/$25.00 ©2010 IEEE

Stage 4 [t3, t4] Stage 4 starts when the resonant capacitor voltage reaches to the voltage across CO at t=t3 then the rectifier diode turns on at zero voltage, and the transformer secondary side winding starts to charge output capacitor CO. Stage 4 stops at t=t4=Ts when the driving signal is applied again to Sfly switch.

iLres iDrec

VCo

iLo

VSfly

VCres

a) [t0, t1]

iCres

VCres

iLres

VCo

iLo

VSfly

b) [t1, t2]

iCres

VCres VCo

iLo

c) [t2, t3]

iDrec

VCoVCres

d) [t3, t4]

Fig. 2 Detailed Analysis of Flyback ZCS Single-Stage Inverter

a) Typical theoretical waveforms

b) Simulation Results Fig. 3 Steady-State timing Diagram of the Flyback

Converter

Figure 3 shows the detailed timing diagram for the four stages shown in Figure 2. As Figure 3 shows, the flyback switch Sfly is turning on and off at zero current and the rectifier diode at the secondary side is turning off at zero current also. The given flyback ZCS single-stage inverter has zero switching loss both on the primary and the secondary side of the transformer. Only conduction losses exist. Meanwhile, the simulation and theoretical waveforms of the flyback ZCS single-stage inverter agree.

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III. OPTIMIZATION OF FLYBACK ZCS SINGLE-STAGE INVERTER

1) Control Algorithm of Flyback ZCS Single-Stage Inverter

Based on the flyback ZCS converter analysis above, a flyback ZCS single-stage inverter is proposed. The control algorithm is pulse frequency modulation as illustrated in Figure 4.

Fig. 4 Control Algorithm of Flyback ZCS Single-Stage Inverter As Figure 4 shown, the ON time of flyback switch is constant. But the OFF time is varied accordingly to generate rectified sinusoidal current. A closed loop control is formed to adjust the frequency of the constant ON pulses. A Psim simulation performed as shown in Figure 5 and simulation results in Figure 6.

Fig. 5 Proposed Flyback ZCS Single-Stage Inverter

Fig. 6 Simulation Results of Flyback ZCS Single-Stage Inverter

2) Nonlinear Transfer Function Behavior To demonstrate the nonlinear transfer function behavior between the switching frequency and output power of the inverter, a simulation was performed that scanned the switching frequency from zero Hz to 500 kHz as plotted in Figure 7 illustrates the nonlinear transfer function of PFM control for the flyback ZCS converter due to the inductance of the transformer secondary winding. As the inductance decreases, the nonlinearity becomes worse, and the nonlinearity occurs at a higher switching frequency. This distortion is caused by the nonlinearity of PFM control and cannot be solved by any feedback control alone. This nonlinearity will cause the distortion of the inverter output voltage. It can only be relieved by using a larger inductance of transformer.

Lsec = 30µ

Lsec = 50µ

Lsec = 30μ

Lsec = 50μ

Lsec = 100μ

Lsec = 1000μ

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Lsec = 100µ

Lsec = 1000µ

Fig. 7 Distortion Caused by Nonlinearity of Flyback ZCS DC/DC Converter

3) Optimized Flyback ZCS Single-stage Inverter

Fig. 8 Optimized Flyback ZCS Single-Stage Inverter

without Nonlinearity

Fig. 8 proposed an optimized flyback ZCS single-stage inverter by using the synchronous rectification technique, which will eliminate completely the nonlinearity. The simulation results of the power stage transfer function of the conventional PWM converter is illustrated in Figure 9. Transfer Function of ZCS PFM Flyback is shown in Figure 10. The conventional PWM flyback has a typical –40dB/decade two-pole system, but the PFM ZCS flyback has about -20dB/decade with the same load. Also, the phase delay of the PFM ZCS flyback converter as a function of frequency changes much smoother, which makes the compensator design much easier. From 100kHz to 500kHz, the PFM ZCS flyback also demonstrates power stage transfer functions that have much less dependence on the

effective duty cycle variation, which also make the compensator design easier [6]. All these features of the PFM ZCS flyback converter results in improvement to the conventional PWM flyback converter on the application of a single-stage inverter in terms of efficiency, control ability, voltage and current stress.

Fig. 9 Transfer Function of Conventional PWM

Fig. 10 Transfer Function of ZCS PFM Flyback

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IV. EXPERIMENTAL RESULTS Preliminary experimental results were obtained for a 250W prototype built for the given PFM ZCS flyback single-stage inverter includes an unfolding circuit as the power stage to produce 110VAC, 60Hz output voltage as shown in the top and bottom views in Figure 11. The controller circuit has a PFM circuit, a driver circuit, an error compensator and an auxiliary power supply and was built on a four-layer PCB board with proper thermal pad design for the surface mounted MOSFETs and diodes. No heat-sink was needed in this prototype. With further planar transformer design, this prototype can be designed with higher power density and integration.

Fig. 11 Prototype of PFM ZCS Flyback Inverter

Figure 12 illustrates the preliminary experimental results of the PFM ZCS flyback inverter. The yellow waveform is the gate driving signal, the blue waveform is current flow through the MOSFET Sfly, the green waveform is the voltage across the MOSFET Sfly and the magenta waveform is the voltage across the blocking diode Dfly.

Fig. 12 Preliminary Experimental Results for switching and

output waveforms The experimental results verify closely the theoretical analysis and simulation results obtained in the above sections. The thermal images shown in Figure 13 gives the hot spots of the PFM ZCS flyback single-stage inverter prototype as located on the resonant capacitors and resonant inductor. The Mosfet, blocking diode and rectifier diodes are not hot spots, which illustrates that this topology has less switching loss than a conventional flyback converter. The peak efficiency is achieved at 92.5% during full load [7].

Fig. 13 Thermal Images of PFM ZCS Flyback Single-Stage Inverter Prototype

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V. CONCLUSIONS To increase the efficiency of the single-stage inverter system, a new ZCS flyback inverter with Pulse Frequency Modulation (PFM) control is given in this paper. With further optimization, the non-linearity of the PFM ZCS flyback inverter is completely removed. Detailed steady-state theoretical analysis and simulation results are given to verify the operation of the new topology as a good design for single-stage low power inverter circuit. Experimental results show that ZCS flyback topology can achieve good performance because of very simple control strategy, less components and higher efficiency.

VI. REFERENCE [1] R.Chaffai, K Ai-Haddad, V. Rajagopalan, “A 5kW Utility-Interactive Inverter Operating At High Frequency and Using Zero Current Turn Off Comfet Switches”. [2] Sivakumar, S., Parsons, T. and Sivakumar, S. C., “Modeling, analysis and control of bidirectional power flow in grid connected inverter systems,” Power Conversion Conference, PCC Osaka 2002, Volume 3, pp.1015-1019, April 2002 [3] Beristain, J., Bordonau, J., Gilabert, A. and Velasco, G., “Synthesis and modulation of a single phase DC/AC converter with high-frequency isolation in photovoltaic energy applications,” Power Electronics Specialist Conference, 2003. PESC '03. 2003 IEEE 34th Annual, Volume 3, pp.1191-1196, June 2003. [4] Songquan Deng, Hong Mao, Mazumdar, J., Batarseh, I. and Islam, K.K., “A new control scheme for high-frequency link inverter design,” Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE, Volume 1, pp.512-517, Feb. 2003. [5] “Practical considerations in current mode power supplies”, Unitrode Application Notes, U-111. [6] “A novel control for two-stage DC/DC converter with fast dynamic response”, Xiangcheng Wang Shangyang Xiao Batarseh, I [7] Feng Tian, “single stage grid connected inverter system with PFM ZCS flyback DC/DC converter,” Ph.D. dissertation, Department of Electrical and Computer Science in the College of Engineering at the University of Central Florida Orlando, 2009.

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