A LLC RESONANT CONVERTER WITH ZERO CROSSING NOISE FILTER

A LLC RESONANT CONVERTER WITH ZERO CROSSING NOISE FILTER M. Mohamed Razeeth # and K. Kasirajan * # PG Research Scholar, Power Electronics and Drives, Einstein College of Engineering, Tirunelveli, India * Associate Professor, Department of Electrical and Electronics Engineering, Einstein College of Engineering, Tirunelveli, India mohamedrazeeth29@gmail.com k.kasirajan@rediffmail.com Abstract This paper proposes a LLC resonant converter with zero crossing noise filters. By adding a zero crossing noise filter (ZCNF) to the drain and source of the MOSFET, the false triggering can be effectively eliminated. Due to the intrinsic filtering and compensation ability, the ZCNF can be applied to all the other rectification applications such as quasi resonant mode and discontinuous current mode converter. Moreover, the resistor and capacitor in the filter can be used as a compensator to solve the duty cycle loss issue. Simulation results shows that, the proposed ZCNF can remove the impact of ringing caused by parasitic parameters and accurately control the turn-on and turn-off of the MOSFET and improve the reliability of the power circuit. Index Terms LLC dc-dc converter, resonant converter, zero crossing noise filter (ZCNF). I. INTRODUCTION The pursuit of higher efficiency and power density in DC- DC converter has pushed power electronic devices to their maximum capable switching frequency. Under such circumstances, LLC resonant converter becomes very popular due to its potential in low power loss with high switching frequency [1]. Resonant converters can achieve zero-voltage switching (ZVS) and enable power supplies to operate at high switching frequencies with high efficiency. In particular, LLC resonant converter can achieve ZVS for primary side switches and zero current switching (ZCS) for secondary side rectifiers. Therefore, the LLC resonant converter has very low switching loss and is highly efficient. LLC resonant converter can be optimized to achieve high voltage gain for holdup operation without sacrificing the nominal efficiency [2]. Many techniques have been proposed to optimize LLC resonant converters. However, most efforts are shown to improve the performance of LLC with diode rectifiers. Very little of the literature addresses the issues of LLC resonant converters with synchronous rectifiers (SR).For low output voltage dc-dc converters, conduction loss can be reduced tremendously by using SR instead of diode rectifier. To achieve high efficiency, the parameters of LLC resonant tank should be optimized [3].In addition the driving signal of the SR should be synchronised with the current through the rectifier [4]. Due to phase shift introduced by the resonant components, the secondary- side currents of the LLC resonant converter are not in phase with the switching actions of primary side MOSFETs [4]. There are two methods takes place for all of these schemes, they are current based method and voltage based method [5]. The current based method detects the current through the SR to generate the gate drive signal [6].The main problems of these methods are the large size of the current-sensing of transformer, the extra conduction loss of the winding, and the undesired delay that will cause duty-cycle loss of the SR and therefore more conduction loss. To avoid this problem, primary current sensing with magnetising current cancellation method is proposed in [7]. The voltage based method detects the voltage across the drain to the source of the SR to generate the driving signal [5]. Thus, even a very small zero-crossed ringing caused by the parasitic parameters of the circuit may result in the false gate driving signal which causes undesired circulating energy loss. Due to the phase shift mentioned previously, it will cause false triggering problem at light load condition. Thus in this proposed paper, by applying a zero crossing noise filter (ZCNF) between drain and source of MOSFET, the false triggering can be effectively eliminated. Moreover, the resistor and capacitor in the filter are also used to compensate the duty cycle loss. II. EXISTING CIRCUIT DIAGRAM Figure 1 shows the circuit diagram of the typical half-bridge LLC resonant converter with SRs. L LKP is the primary-side leakage inductance of the transformer. L LKS1 and L LKS2 are the secondary leakage inductances. C oss1 and C oss2 are the output capacitances of the SRs [13]. 176

Fig.1 LLC resonant converter with SR. False-triggering at the turn on of SRs MOSFET: As shown in Fig. 2, voltage has many high-frequency spikes when QS1 turns OFF. If the voltage spikes reach the SRs turn-on threshold, the SRs will be false-triggered. Figure 3 shows the false-turn-on of the SR at light-load condition. This will result in the energy reverse from the output capacitor to the input source, or even worse breakdown of the power circuit. One possible solution to prevent false-triggering at turn-on of the SRs is to add an RC filter to absorb the voltage spikes as shown in Fig.3 [13]. V filter is used as a substitute for V DS. DC POWER SUPPLY Fig.3. False-turn-on of SR under-light load condition III. PROPOSED SYSTEM INVERTER TRANSFOEMER RECTIFIER WITH ZCNF DC OUTPUT Fig.4. Block diagram of proposed system Fig.2. Parasitic ringing when the SRs turned OFF Fig.5 Circuit diagram of the half bridge LLC resonant converter with ZCNF The block diagram of proposed system is shown in fig.4. It consists of inverter, center tapped transformer and rectifier with zero crossing noise filter. Where DC supply is given to the inverter and the function of inverter is to convert DC voltage in to AC voltage. Then converted AC voltage is fed in to the rectifier circuit and it converts AC voltage in to variable 177

DC voltage with high voltage spikes. To reduce that voltage spikes, a zero crossing noise filter is added with the rectifier circuit. A Zero Crossing Noise Filter (ZCNF) is used in LLC resonant converter to eliminate the false triggering of MOSFET. During turn OFF process of the QS1, QS2, the primary side and secondary side of the leakage inductance of transformer resonate with output capacitance. In this turn OFF process voltage has high frequency spikes. If the voltage spikes reach the turn ON time, then false triggered will takes place and this problem occur at light load condition. To solve this turn on delay problem, an anti parallel diode is added to discharge the RC filter capacitor. To overcome these problem zero crossing noise filter method are used. Figure 5 shows the circuit diagram of the half bridge LLC resonant converter with zero crossing noise filters. L LKP is the primary-side leakage inductance of the transformer. L LKS1 and L LKS2 are the secondary leakage inductances. C oss1 and C oss2 are the output capacitance. Let us discuss the detailed operation of LLC resonant converter as given below. The DC characteristic of LLC resonant converter could be divided into ZVS region and ZCS region. For this converter, there are two resonant frequencies. One is determined by the resonant components L LKP and C s. The other one is determined by L m, C s and load condition. As load getting heavier, the resonant frequency will shift to higher frequency. With this characteristic, for 400V operation, it could be placed at the resonant frequency of f r1, which is a resonant frequency of series resonant tank of C s and L LKP. While input voltage drops, more gain can be achieved with lower There are some interesting aspects of this DC characteristic. On the right side of f r1, this converter has same characteristic of SRC (Series resonant converter). On the left side of f r1, the image of PRC (Parallel resonant converter) and SRC are fighting to be the dominant. At heavy load, SRC will dominant. When load get lighter, characteristic of PRC will floating to the top. With these interesting characteristics, we could design the converter working at the resonant frequency of SRC to achieve high efficiency. Then we are able to operate the converter at lower than resonant frequency of SRC still get ZVS because of the characteristic of PRC will dominant in that frequency range. From above discussion, the DC characteristic of LLC resonant converter could be also divided into three regions according to different mode of operation as shown in Fig.6. In this region 1, Lm never resonates with resonant capacitor C s ; it is clamped by output voltage and acts as the load of the series resonant tank. With this passive load, LLC resonant converter is able to operate at no load condition without the penalty of very high switching frequency. Also, with passive load L m, ZVS could be ensured for any load condition. Here the operation is not discussed in detail. There are several other modes of operation for light load condition. In region 2, the operation of LLC resonant converter is could be divided into two time intervals. In first time interval, L lkp resonant with C s. Lm is clamped by output voltage. When L lkp current resonant back to same level as L m current, the resonant of L lkp and C s is stopped, instead, now L m will participate into the resonant and the second time interval begins. During this time interval, the resonant components will change to C s and L m in series with L lkp. In fact, that is a Fig.6 Three operating region of LLC resonant converter switching frequency. With proper choose of resonant tank, the converter could operate within ZVS region for load and line variation. Fig.7 Waveform of LLC resonant converter part of the resonant process between L m + L lkp with C s. From this aspect, LLC resonant converter is a multi resonant converter since the resonant frequency at different time 178

interval is different. Because of the resonant between Lm and C s, a peak on the gain appears at resonant frequency of L m + L LKP and C s. Next the operating of LLC resonant converter in region 2 will be discussed in detail. It is divided into three modes. Mode 1 (t0): This mode begins when QL is turned off at t0.at this moment, resonant inductor L LKP current is negative; it will flow through body diode of QH, which creates a ZVS condition for QH. Gate signal of QH should be applied during this mode. When resonant inductor L LKP current flow through body diode of QH, IL LKP begins to rise, this will force secondary switch QS1 conduct and Io begin to increase. Also, from this moment, transformer sees output voltage on the secondary side. L m is charged with constant voltage. Mode 2 (t1 to t2): This mode begins when resonant inductor current IL LKP becomes positive. Since QH is turned on during mode 1, current will flow through MOSFET QH. During this mode, switch QS1 conduct. The transformer voltage is clamped at Vo. L m is linearly charged with output voltage, so it doesn't participate in the resonant during this period. In this mode, the circuit works like a SRC with resonant inductor L LKP and resonant capacitor C s. This mode ends when L LKP current is the same as L m current and Output current reach zero. Mode 3 (t2 to t3): At t2, the two inductor s currents are equal and Output current reach zero. Both output rectifier switch QS1 and QS2 is reverse biased. Transformer secondary voltage is lower than output voltage. Output is separated from transformer. During this period, since output is separated from primary, L m is freed to participate resonant. It will form a resonant tank of L m in series with L LKP resonant with C s. This mode ends when QH is turned off. For next half cycle, the operation is same as discussed above. IV. SIMULATION RESULT The simulink model is given on the basis of a LLC resonant converter with zero crossing noise filters as shown in fig.8. Figure 9 shows the input voltage of LLC resonant converter (400v). Figure 10 shows the waveforms of gate pulse of switch QH and QL. Figure 11 shows the waveforms of gate pulse of switch QS1 and QS1. Figure 12 Shows the drain source voltage across switch QH (V ab ), Inductor current I LS (A), Gate voltage of switch QS1 (V gqs1 ), Drain source voltage (voltage spike occurrence) across QS1 (V gqs1 ). Figure 13, 14 shows the Output current and voltage of resonant converter (50A, 12V). Figure 15 shows the Output power of resonant converter (600W). Fig.8 Simulink model of LLC resonant converter with zero crossing noise filter Fig.9 Input voltage of resonant converter Fig.10. Gate pulses of switch QH and QL 179

Fig.13 Output current of LLC resonant converter Fig.11 Gate pulses of switch QS1 and QS2 Fig.14 Output voltage of LLC resonant converter Fig.12 The drain source voltage across switch QH (V ab), Inductor current I LS (A), Gate voltage of switch QS1(V gqs1),drain source voltage across QS1(V gqs1). Fig.15 Output power of LLC resonant converter VI. CONCLUSION Thus in this project, Zero crossing noise filter is proposed, that can remove impact of ringing caused by parasitic parameter and can accurately control the turn on and off of the MOSFET in rectifier circuit and reduce switching losses. Due to intrinsic filtering and compensation ability, the Zero Crossing Noise Filter can be applied to all other rectification application with zero-current-off characteristics such as quasi resonant mode and discontinuous current mode converter. Thus Simulation results shows that the zero crossing noise filter can significantly reduce the false triggering of MOSFET and improve the reliability of the power circuit. 180

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