2013 IEEE INTERNATIONAL CONFERENCE ON CIRCUITS AND SYSTEMS
A Fixed Frequency Continuous Conduction LCCL Series Resonant Inverter Fed High Voltage DC-DC Converter Asim Amir, Soib Taib and Shahid Iqbal School of Electrical and Electronics Engineering Engineering Campus, University Sains Malaysia 14300, Nibong Tebal, Pulau Pinang, Malaysia Email:
[email protected] Abstract— This paper presents a continuous conduction mode LCCL series resonant inverter fed high voltage dc-dc converter. The proposed converter is derived by adding an inductor in parallel with a capacitor and another capacitor in series with the leakage inductance of the transformer forming a resonant tank. The proposed converter is capable to reduce the parasitic oscillations where all of the parasitic capacitance and leakage inductances are absorbed by the resonant tank circuit. The proposed topology was simulated using PSPICE software. The obtained results show that proposed converter provide a wide range of output voltage control, low output voltage ripple over the entire range and low current stress at light load compare to the conventional circuit.
load, the series resonant converter (SRC) becomes virtually uncontrollable. Moreover, parasitic capacitance is not integrated into the resonant tank in the SRC [5]-[7]. Among all the components, high voltage transformer is most challenging that may disturb the performance of the power supply. Its high turn ratio and insulation requirements intensify transformer non-idealities for example leakage inductance and parasitic capacitance [8]- [10]. These non-idealities cause voltage and current spikes and increase loss and noise. In order to engage these non-idealities as useful elements, several types of resonant converters are employed such as series resonant converter (SRC), parallel resonant converter (PRC) and series-parallel resonant converter (SPRC) that have been proposed by researchers worldwide [11,12]. Each of these converters has its own merits and demerits. The SRC is free from possible saturation of HV transformer and it absorbs the leakage inductance of high voltage transformer into power topology. Additionally, it behaves as a current source which provides inherent over-load protection. It allows capacitive filter at the output and gives high efficiency over a wide range of load. Though the transformer winding capacitance is not absorbed in the tank circuit, SRC has been used in the high voltage converters due to several other advantages [13]. The most significant feature of SRC is the ability to provide high output load current [14]. To remove the above limitations, load resonant dc-dc converter topology is chosen with three resonant elements connected in series with the load. In this converter, control and soft switching are maintained over wide load and voltage conversion ratio ranges by circulating an additional amount of reactive energy through a fourth order resonant tank. Due to the increased complexity of the resonant circuit, multi element resonant converters exhibit complex dynamic behavior. This characteristic often precludes fast and robust transient response. Hence, the inclusion of a parallel LC tank with the conventional network results in higher efficiency increased switching control and reduction amongst power losses. This paper proposes a novel design for the ZVS converters, which not only claims to control the complex dynamic behavior of the circuit but also proves to have overcome the limitations confronted by the conventional SRC, higher efficiency, in terms of providing the required output at the load unit in reference to the input. Increased switching control, as the
Keywords - Series resonant (SR), high voltage inverter, dc-dc converter, Pulse width modulation (PWM). I. INTRODUCTION DC-DC converter topologies are widely used as power supplies in various types of electronic equipment’s such as battery chargers and dischargers, uninterruptible power systems, hybrid electric vehicles, industrial and medical X-ray imaging, traveling wave tube, RF generation, etc [1]. The Design of high-voltage dc-dc converter is problematic because the large turn’s ratio of the transformer exacerbates the transformer non-idealities. In particular, the leakage inductance and the winding capacitance can significantly change converter behavior. In switched-mode converters, the output transformer leakage inductance causes undesirable voltage spikes that may damage circuit components and the winding capacitance may result in current spikes and slow rise times. These non-idealities can lead to greatly increased switching and snubbed losses and reduced converter efficiency and reliability [2]. The choice of a converter topology for high-frequency high-voltage applications is severely limited by the characteristics of the high-voltage transformer, which is the central component of any highvoltage converter. Output filter inductors at high voltage side, normally, cannot be used due to the high voltage drop on the inductors and reverse over-voltage across diodes caused by ringing of parasitic parameters. This suggests the proper selection of low reverse-recovery time diodes to mitigate current spikes. Based on the above considerations, many power converters have been proposed in the past as a means of supplying high output voltages [3, 4]. While operating at light
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overlapping of the modes offer an interactive pattern so as to device the mechanism with soft switching control and finally the eradication of offshoot peak currents and voltages render the circuit with heavily reduced power losses. II.
The high voltage power supply mentioned above is fed by constant power source, to the resonant inverter that converts DC input voltage into AC output voltage. The inverter output voltage is fed to the LCCL tank circuit which offers resonance to the circuit. The LCCL design is opted for the removal of the offshoot peak currents and voltages. Hence, a third order resonant tank circuit is provided, with a combination of an inductor LP in parallel with capacitor CP, both connected in series with capacitor CS with the leakage inductance of the transformer in series, as shown in Figure 3 equivalent circuit of the proposed converter. These parasitic components of the resonant tank are used as the main circuit elements to provide resonant transitions during the switching time intervals. High-voltage transformer which performs isolation between the inverter and rectifier circuits, delivers a stepped up inverter output voltage. A full bridge high voltage rectifier is fed with the increased AC voltage from the isolation transformer, which then converts the AC voltage to regulated DC voltage as required by the resistance (load unit), capacitive filter C0 is chosen here to limit the output voltage ripple and spikes and a smooth DC voltage is obtained at the output of the converter. Fixed frequency pulse width modulation is employed to control the converters switching frequency, while simultaneously keeping a dead time between the pulses. The maximum output voltage (power) is achieved when the switching frequency is more than half of the resonant frequency and the converter is operating in continuous conduction mode. For this frequency, the converter operates in the optimal operation point while its transistors are switched at zero voltage conditions, inverter operates at a fixed frequency and fixed duty ratio. The duty ratio is set to approximately 50%, switching frequency is given by:
PROPOSED TOPOLOGY
A. Proposed System Architecture and Description Continuous conduction mode LCCL series resonant inverter fed high voltage dc-dc converter, is constructed using four main components. A full-bridge series resonant inverter, full bridge diode rectifier, both separated by an isolation transformer and the LCCL tank circuit as shown in Figure1.
Figure 1: Block diagram of the proposed converter.
The full-bridge inverter section involves four gate controlled semiconductor switches (S1, S2, S3, and S4) such as IGBTs, with diodes (D1, D2, D3, and D4) connected in antiparallel fashion. ZVS condition is chosen as the technicality for switching of the power transistors, as the pattern of overlapping mode is presented by the operation of the circuit as shown in Figure 4. The terminology of Continuous conduction mode further explains switching at zero voltage with continuous current through the transistors which elevates the switching control of the circuit reducing the switching losses. The schematic diagram is presented in Figure 2.
1
Figure 2: Schematic diagram of the LCCL SR inverter fed high voltage dc-dc converter.
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(1)
8 Figure 3: Equivalent circuit of the proposed topology.
Figure 3a equivalent circuit of the LCCL-SR converter, shows an inductor LP in parallel with the resonant capacitor CP with an
impedance of
Z Z
Z
Mode II (t1-t2) Mode II operates from (t1-t2) as shown in Figure 5, energy stored in the capacitors Cp and Cs during Mode I travel in reverse direction back to the dc source and during this all the switches remain OFF but the power is supplied to the load from time (t1-t2) through the diodes D1 and D2, bidirectional energy flow cannot occur because diodes connected in antiparallel only allow unidirectional energy transfer, during this mode input voltage is transferred to the load until time t2 and switches S1 and S4 remain OFF under ZVS conditions.
represented in Figure 3b and a
resonant capacitor CS in series with the leakage inductance LS of the transformer, represented in Figure 3b with an whereas the total impedance of impedance of the circuit ZT can be expressed as: 2 The proposed system operates in a continuous conduction mode where the inverter switching frequency is equal to resonant frequency for the parallel half of the resonant tank and is set more than half of for the series half of the resonant tank, enabling the switches to turn On-Off at zero voltage switching, the relationship between the switching frequency and the resonant frequency is given by: 0.5 3 0.5 4 and 5 Where .
√
.
B. Steady-state analysis The analysis of LCCL SR inverter fed high voltage dc-dc converter is carried out with an emphasis placed on fixed frequency PWM method. The switches (S1, S2, S3, and S4) employed in the full bridge inverter are driven in a diagonal fashion. Power is only transferred to the output section during the ON times of the switches which corresponds to a specific duty cycle when operated at fixed frequency; all switches operate with a duty cycle of 50%, ideally. Additionally, the complete range of required duty cycles is unique to the application. The behavior of the circuit shown in Figure 2 can be described according to switches ON-OFF conditions and inductor currents. Period analyzed is (t0-t4=T), as shown in Figure 4. Mode I (t0-t1) As shown in Figure 4 Mode I starts from (t0-t1), during the positive cycle of operation the switches S1 and S4 are switched ON under ZVS condition, allowing a path for the input voltage to flow to the output load through the LCC tank circuit, such that the stored energy in the parallel inductor Lp and the leakage inductance of the transformer Ls is transferred to the capacitors Cp and Cs. the switches turn OFF at the time (t1).
Figure 4: Steady-state waveform of the proposed system.
Mode III (t2-t3) Mode III starts from time (t2-t4) as shown in Figure 6 during the negative cycle here switches S2 and S3 are turned ON under ZVS and input voltage is supplied to the output load flowing through the LCC resonant tank allowing induction in Lp and Ls, this mode ends at time t4 when the energy from the inductors Lp and Ls is sufficiently stored in capacitors Cp and Cs. Mode IV (t3-t4) Mode IV works in similar fashion to Mode II as shown in Figure 7 from time (t3-t4). Energy stored in the capacitors Cp and Cs during Mode-III in the resonant circuit now flows in the reverse direction through the anti-parallel diodes D2 and D3 and the switches S2 and S3 remain OFF under ZVS condition until time t4 and the source voltage keeps the path to carry voltage to the load.
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Figure 4: Equivalent circuit of Mode I.
Figure 5: Equivalent circuit of Mode II.
Figure 6: Equivalent circuit of Mode III.
III.
Figure 7: Equivalent circuit of Mode IV.
the output voltage of converter approximately at 2.32 kV in red curve and peak inverter current 3.9 A. It can observed that the current through the inverter is in continuous conduction mode and on the other hand inverter switches operate under ZVS conditions sustaining soft switching between the inverter switches. Now the load resistance is again varied to 1 MΩ. The obtained waveforms from the simulation results in Figure 11, show the output voltage of converter for this case is approximately at 2.75 kV, while the peak inductor current is 4 A. This proves that proposed converter is able to control its output voltage over a wide range with minimum switching losses.
SIMULATION RESULTS AND EVALUATION
In order to confirm that the LCCL SR dc-dc converter can be practically implemented, simulation verification is performed to verify the results, simulation is performed by using PSPICE software. Now the proposed system is simulated with an input voltage of Vdc =30 V, resonant capacitance CP =120 nF, CS =120 nF, resonant inductance LP =80 uF, Load resistance is varied over, RL= 200 kΩ, 500 kΩ & 1 MΩ, the turns ratio of high voltage transformer is set a ratio of 40. The proposed circuit is simulated for wide range of output voltage and load settings, with CP, CS =120nF capacitance of resonant tank circuit for which the converter operates in continuous conduction mode up to a switching frequency of 40 kHz. Figure 8 shows the gating signals of the power switches and the resonant current flowing through circuit, in inductor LP at 40 kHz switching frequency for a load resistance of RL=500 KΩ. As can be seen in Figure 8, all IGBTs are turned ON and OFF under ZVS condition. Hence, the switching losses are negligible. For load resistance of 200KΩ it can be seen from Figure 9 that the output voltage in the red curve of the converter is approximately 1.80 kV and the peak resonant current in the green curve is 3.8 A. If compared to the conventional SRC topology an increased output voltage is observed. Furthermore, the current though inverter sustains a full sinusoidal wave for the complete period of operation, thus resonant inverter is operating in continuous conduction mode where the switching loses do not occur. The simulated waveforms for load resistance 500 KΩ in Figure 10 shows
Figure 8: Resonant current and gating signals
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Figure 9: Simulation result of the output voltages and inductor currents of the (a) proposed converter (b) conventional SRC for Ro= 200 kΩ.
Figure 10: Simulation result of the output voltages and inductor currents of the (a) proposed converter (b) conventional SRC for Ro= 500 kΩ. 2.76KV
1.480KV
2.75KV
1.475KV
SEL>> 2.74KV 4.0A
SEL>> 1.470KV
Vo
4.0A
0A
-4.0A 7.90ms 7.93ms Ls
Vo
0A
7.95ms 7.97ms Time
7.99ms
-4.0A 4.90ms 4.93ms Ls
8.00ms
(a)
4.95ms
4.97ms
4.99ms
5.00ms
Time
(b)
Figure 11: Simulation result of the output voltages and inductor currents of the (a) proposed converter (b) conventional SRC for Ro= 1MΩ.
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IV.
[6] Y. J.Kim (1995) Comparative performance evaluations of high-voltage transformer parasitic parameter resonant inverter-linked high-power dc-dc converter with phaseshifted PWM scheme. PESC’95, pp.120—127.
CONCLUSION
In this paper a continuous conduction mode LCCL-SR inverter fed high voltage dc-dc converter is presented. A simple design procedure is illustrated using PSPICE simulation software and results have been presented to evaluate the performance of the converter. The simulated results show that the proposed converter can be used for high power applications which give constant dc output voltage with fewer losses. Furthermore, it has been shown by simulation results that the proposed system operates in continuous conduction mode over the entire range of output voltage. It is observed also that all the switches are operating under soft switching conditions.
[7] B. S. Jacobson and R. A Dipema,. (1993) Fixed frequency resonant converter for high voltage high density applications. PESC’93, pp.357—363. [8] V. Garcia (1994) An optimized dc-to-dc converter topology for high-voltage pulse-load applications. PESC’94, pp.1413—1421. [9] V. Vlatkovic (1996) Auxiliary series resonant converter: A new converter for high-voltage, high-power applications.IEEE APEC’96, pp.493—499. [10] J. Sun, X. Ding, M. Nakaoka and H. Takano, “Series resonant ZCSPFM DC-DC converter with multistage rectified voltage multiplier and dual-mode PFM control scheme for medical-use high-voltage X-ray power generator,” Proc. Inst. Elect. Eng., Elect. Power Application., vol. 147, no. 6, pp. 527-534, Nov. 2000.
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