SIC BASED Z-SOURCE RESONANT CONVERTER WITHCONSTANT FREQUENCY AND LOAD REGULATION FOR EV WIRELESS CHARGER
Abstract
Traditional load regulation methods for resonant converter mainly rely on frequency modulation. It is always atrade-off between the design of the resonant network and therange of load. Especially for wireless power transfer (WPT)systems, the resonant network usually has a high quality factor.Small variation on frequency leads to huge drop on gain andefficiency. Due to this problem, many WPT systems areunregulated and they need one or two more front-end stages toregulate the DC bus voltage and perform power factor correction(PFC). In order to lower the cost and complexity of two or threestages structure, a single-stage solution with SiC based Z-sourceresonant converter (ZSRC) was recently proposed. The Z-sourcenetwork provides high reliability as being immune to shootthrough problems. Additionally, Z-source resonant converter canboost the DC bus voltage while the traditional voltage sourceinverter can only produce a lower voltage. However, the loadregulation of this new topology has not been addressed. Twoeffective load regulation methods with constant frequency ispresented for this SiC based ZSRC specifically. Operationprinciple of the two load regulation methods are described in thispaper. Experimental results based on a 200-W scale-downprototype with full-bridge series resonant DC-DC converter(SRC) are presented to illustrate the mechanism of these twomethods.
EXISTING SYSTEM:
In WPT application, the SRC is widely adopted as theDC-DC converter because of its simplicity and high efficiency. SRC, like other resonant converters, can realize loadregulation by frequency modulation method (PFM). However,owing to the large ratio between the leakage inductance andmagnetizing inductance (greater than 10:1) in WPT application,SRC has a high quality factor. Small drift away from resonantfrequency leads to huge drop on gain and huge increment oncirculating current. In from Oak Ridge NationalLaboratory (ORNL), SRC is investigated and verified that thepeak power range is broader for a given frequency band andthen drops quickly outside the band. In their later work,they change to series-parallel configuration that has a widerplateau in the power versus frequency curve, which gives themmore freedom on PFM control. Moreover, the resonant converter is left unregulated andfreewheeling with different air gap or load. Thus, onemore front-end stage is needed to regulate the DC bus voltage, and this front-end stage usually also performs powerfactor correction . In, a 7-kW charger system isdivided into three stages—a front end PFC converter, a buckconverter for voltage regulation, and a SRC, such that the SRCcan always operate at resonant frequency with high efficiency.
PROPOSED SYSTEM:
Two or more stage solutions are high cost and complicated,even though the design for each stage is simple. The overallperformance may not be the best. Z-source inverter, wellknown for its boost feature and being immune to shoot-throughproblem, can be applied to any kind of power conversionbetween DC and AC. A combination of Z-source network(ZSN) and SRC has been studied in. It can improve theefficiency over a wide input voltage and load variation.Furthermore, Z-source resonant converter was proposed in and proved its advantage over conventional boost PFC with acascaded DC-DC. Fig. 4 shows the overall schematic for ZSRC.The input diode is a SiC device with almost zero reverserecovery loss which is a previous headache for Z-sourceinverter in many applications. A Z-source network is placedbetween the input diode and the H-bridge inverter of theconventional SRC system. The original Z-source topology ischosen as it can share the input diode with the diode bridgewhen connecting to AC source. The SRC converts the DCpower to high frequency AC power and pass it to the secondaryside. This is a single stage solution that saves the cost andcomplicity. However, the control scheme is challenged, as it shouldaccomplish PFC and load regulation by controlling only fourswitches. And the load regulation method for ZSRC has notbeen addressed. In terms of Z-source, plenty of research on thecontrol has been done for DC-DC converter. In oneswitching period, the load current that the inverter draws ismodeled as a constant current source. However, in resonantconverter, the load current is sinusoidal over one switchingperiod which makes everything interesting. Those conventionalcontrol schemes pay attention to the duty cycle of shoot throughstate only, while the position of shoot through state in oneswitching period is also crucial for the resonant converter. In, the shoot through state is evenly distributed over one cycleand it results in less voltage ripple on DC-link capacitor. Acontrol method [30] with shoot through state right after activestate can provide soft switching at turn-on action. In this paper,for ZSRC specifically, two constant frequency controlmethods—phase shift method and pulse notch method arediscussed.
CONCLUSION
This paper focus on the load regulation methods in SiC based Z-source resonant converter for wireless power transferapplication. Two constant frequency control methods—phaseshift control and pulse notch control are presented withcomparison on load regulation performance. Phase shift controlis easier in implementation but it suffers from high distortion atlight load in discontinuous mode. On the other hand, pulsenotch control is more predictable than phase shift control overthe entire load range. However, both control methods are doinghard switching, which is a drawback for ZSRC. Fortunately, itdoes not have much switching loss since the switchingfrequency is less than 20 kHz in this IPT application.Experimental results based on a 200-W scale-down prototypewith ZSRC are presented. For future work, these two controlmethods can be applied for PFC and load regulation at the sametime in ZSRC that leads to a single stage solution for EVcharger. Pulse notch control is a better candidate for PFCfunction to achieve high power factor. Also, soft switchingtechniques will be examined on ZSRC.
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