14 CHAPTER 2 A SERIES PARALLEL RESONANT CONVERTER WITH OPEN LOOP CONTROL 2.1 INTRODUCTION Power electronics devices have many advantages over the traditional power devices in many aspects such as converting performance, size and weight and hence the cost. Nowadays the power electronics devices are widely used in drives, electro-heating, lighting control systems, automotive and power supply systems, telecommunication, aerospace and computing system. Over the years the fundamental approach to electronic power conversion in telecommunication and computing system has steadily moved toward high-frequency. This move is mainly motivated by the rapid progress in microelectronics technology. Facing these strict requirements due to dramatic improvement of microelectronic technology, power designers and researchers have proposed many solutions not only from single power module performance but from the power distribution architecture as well. There are two major types of power converters, the pulse width modulation mode converter and resonant mode converter.
15 2.2 CONVENTIONAL PWM CONVERTERS AND RESONANT MODE CONVERTERS Conventional PWM converters are the most common power supplies in present power electronic market. The block level description of traditional PWM isolated converter is shown in Figure 2.1. It consists of an inverter, isolation transformer, output rectifier and a low pass filter. The DC input voltage is chopped at a high frequency to create square wave AC voltage. This AC voltage can be raised and lowered with transformer, and then rectified and filtered to get a DC output voltage required. The duty cycle of square wave determines the amplitude of output voltage and it can be varied to regulate the voltage against input voltage variation. Figure 2.1 Block diagram of conventional PWM isolated converter In resonant mode converter the DC input voltage is applied to high frequency inverter, chopped into AC square wave and than the AC voltage is driven into a resonant tank which controls the energy flow to the output through a transformer. Then rectified and filtered to obtain a DC output voltage. The circuit diagram is illustrated in Figure 2.2.
16 Figure 2.2 Block diagram of Resonant Mode Converter The way the resonant converter regulates the output voltage can be considered as impedance divider between resonant tank and output stage (resistor). The impedance of resonant tank is controlled by the switching frequency of inverter, the input voltage and output load variations. When input voltage is higher, the switching frequency is increased to provide higher impedance of resonant tank so that the voltage on output resistance remains constant. While the load current is higher, output resistance is lower and hence the switching frequency can be reduced to keep impedance of resonant tank lower and then to regulate the output voltage. The resonant converter can operate both above and below the resonant frequency of resonant tank depending on the type of power switch components. If switching frequency is lower than resonant frequency, the converter can achieve zero current switching (ZCS). If switching frequency is higher than resonant frequency, the converter will operate at zero voltage switching (ZVS). When MOSFET is used as a switching component, the converter operation above resonant frequency is preferred to decrease switching loss. In PWM converter, the AC voltage created by chopper is a square shape waveform which is rich in harmonics of the fundamental. Both conducted and radiated Electromagnetic Interferences (EMI) is generated and needed to be attenuated carefully. As the switches are turned on and turned off with high currents, the switches dissipate power. This type of loss is
17 frequency dependent. Which increases the switching for high frequency and may even damage component. Resonant mode converter naturally has the characteristic of achieving ZVS. This makes resonant converter to generate less EMI noise than PWM converter does. 2.3 REVIEW OF RESONANT TOPOLOGIES In this section, series, parallel and series parallel resonant topologies for constant voltage and load independent operation of DC-DC converter application have been presented. 2.3.1 Series Resonant Topology Figure 2.3 shows the diagram of half bridge series resonant converter. Resonant inductor L s and resonant capacitor C s are connected in series to form a resonant tank. Q1, Q2, C1 and C2 provide bi-directional 50% duty cycle square waveforms and apply to the resonant tank. The resonant tank acts as a current source. The capacitive filter C o is required at secondary side to match the impedance of the resonant tank and output load R o which form a voltage divider network. When load or input voltage is changed, change in the impedance of resonant tank maintains the voltage across the load to be constant. By varying the switching frequency, the impedance of resonant tank can be controlled and thus the output voltage can be regulated. A DC gain characteristic of a SRC is shown in Figure 2.4. It can be observed that when output load decreases (Q value decreases), switching frequency is running much higher than the one with higher Q. For example, comparing the switching frequency difference between Q=1 and Q=10 with DC gain of 0.6, relative angular frequency is 1.1 rad/sec for Q=10 while 1.8 for Q=1. Considering the no-load condition, the converter could have trouble to control the output voltage by running frequency at infinitely high.
18 Figure 2.3 Half bridge Series Resonant Converter Another disadvantage of series resonant converter is that the output capacitor has to carry high ripple current, about 48 percent of the output current. It is not suitable for the application of low voltage high current converter where it requires very small ripples. These small ripples can be maintained by high output capacitance. This may lead to the use of large number of capacitors and component cost may increase. Although series resonance offers several advantages such as inherently providing high impedance against short circuit and prevent it from using for low voltage high current applications. Figure 2.4 DC characteristics of series resonant converter
19 2.3.2 Parallel Resonant Topology Figure 2.5 shows parallel resonant converter. Where the capacitor is parallel with transformer. The resonant tank offers low impedance to the output circuit and is considered as a voltage source. An LC filter is placed in the output to match up the impedance. Compared to the current source type resonant converter, parallel resonant converter is found to achieve low output ripple using relative low cost L-C filter. Figure 2.5 Parallel resonant converter The DC characteristic gain curves for parallel resonant converter is shown in Figure 2.6. It is clearly seen that the series resonant converter can control the output voltage by running the frequency above resonance. The main disadvantage of the parallel resonant converter is high circulating current and it is relatively independent of load. It means that the conduction loss at light load is close to that at full load. The consequence of this characteristic is low efficiency of converter at light load. From the analysis above, it can be observed that parallel resonant converter is not a potential converter through it can provide low output ripple. High conduction loss at light load prevents the consideration.
20 Figure 2.6 DC characteristics of parallel resonant converter 2.3.3 Series-Parallel Resonant Converter Series-Parallel Resonant Converter (SPRC) duplicates the good characteristics of series and parallel resonant converter while eliminating their drawbacks, such as no-load regulation issue for series resonant converter and high circulating current at light load for parallel resonant converter. The circuit diagram of series-parallel resonant converter is shown in Figure 2.7. It consists of five sections, the high frequency inverter, resonant tank, high frequency transformer, output rectifier and output filter.
21 Figure 2.7 Series-Parallel Resonant Converter High frequency inverter provides bi-directional square waved signal that is fed into resonant tank. In resonant tank, there are three resonant components: L s, C s and C p. The resonant tank of series-parallel resonant converter can be considered as the combination of resonant tanks of series and parallel resonant converters. By adding a series capacitor, C s into parallel resonant tank, the circulating energy can be made smaller compared with parallel resonant converter. With the parallel capacitor C p, series-parallel resonant converter can regulate the output voltage at no load condition. Parallel capacitor C p also provides low impedance that can be matched up by L-C type output filter by which low output ripple can be easy to achieve.
22 2.4 GENERAL SERIES PARALLEL RESONANT CONVERTER The general block diagram of SPRC with FLC is shown in figure 2.8. The first stage converts a DC voltage to a high frequency AC voltage. The second stage of the converter converts the AC power to DC power by suitable high frequency rectifier and filter circuit. Power from the resonant circuit is taken either through a transformer in series with the resonant circuit or series in the capacitor comprising the resonant circuit. In both cases the high frequency feature of the link allows the use of a high frequency transformer to provide voltage transformation and ohmic isolation between the DC source and the load. The load voltage can be controlled by varying the switching frequency or by varying the phase difference between the inverters. The phase domain control scheme is suitable for wide variation of load condition because the output voltage is independent of load. The DC current is absent in the primary side of the transformer and hence there is no possibility of current balancing. Another advantage of this circuit is that the device currents are proportional to load current. This increases the efficiency of the converter at light loads to some extent because the device losses also decrease with the load current. If the load gets short at this condition, very large current would flow through the circuit. This may damage the switching devices. The resonant topologies are shown in figure 2.9 Figure 2.8 Block diagram of General Series Parallel Resonant Converter
23 Figure 2.9 Resonant topologies (a) CLL-T SPRC (b) LLC-T SPRC (c) LCL-T SPRC A schematic circuit diagram of General full-bridge SPRC is shown in figure 2.10. The resonant circuit consist of general series Impedance Z 1, Parallel Impedance Z 2 and series Impedance Z 3. The switching devices S 1 -S 4 are with base /gate turn-on and turn-off capability. The diodes D 1 - D 4 are anti-parallel diodes across these switching devices. The MOSFET (S 1 ) and its anti parallel diode (D 1 ) act as a bidirectional switch. The gate pulses for S 1 and S 2 are in phase but 180 degree out of phase with the gate pulses for S 3 and S 4. The positive portion of switch current flows through the MOSFET and negative portion flows through the anti-parallel diode. The RLE load is connected across bridge rectifier via L 0 and C 0. The voltage across the point AB is rectified and fed to RLE load through L 0 and C 0. For the analysis it is assumed that the converter operates in the continuous conduction mode and the semiconductors have ideal characteristics. The tank circuit may be LCL-T SPRC, CLL-T SPRC and LLC-T SPRC combination. The detailed analysis for LCL-T SPRC is presented in the following section.
24 Figure 2.10 Circuit of General Series Parallel Resonant Converter 2.4.1 Operation of LCL-T Series-Parallel Resonant Converter To simplify the analysis of the basic operation of SPRC topology, the following assumptions have been made: 1. The MOSFETs are ideal with no conduction voltage drops, no switching loss and no switching time. 2. The output filter inductor, L 0 is large enough so that the ripple current is neglected. L 0 is represented by a current source. 3. The output filter capacitance, C o is large enough so that the output voltage is constant 4. There is no dead-time between the MOSFET on-off state transitions 5. The transformer leakage inductance can be neglected 6. The current through resonant inductor is sinusoidal.
25 Using the assumptions above, the operation of the converter circuits (conducting and non-conducting branches) are considered for the time intervals t 0 -t 5.The mode of operation can be described below. Period -1: Operation (t 0 -t 1 ) During this time interval switches S 1 and S 4 are turned on and the power from the input voltage source V i is set to a positive value and applied to LCL-T resonant tank. The capacitor C is being charged through the inductor L 1, the period-1 ends when the inductor current equals zero. At t 0, the resonant current is crossing zero. The voltage on parallel capacitor, C is at negative value. The topological circuit for period 1 operation is depicted in figure 2.11. Figure 2.11 Period 1: Operation (t 0 -t 1 ) Period- 2: Operation (t 1 -t 2 ) At t 1, voltage on C crosses to positive side. At this instant t 2 the switches S 1 and S 4 are non-conducting and the inductor current flows through diodes D 1 and D 4. The anti-parallel diode D 1 and D 4 starts to conduct. There is no current through capacitor C. The capacitor C is being discharged. The
26 topological circuit for period-2 operation is shown in figure 2.12. The voltage across V AB is positive. Figure 2.12 Period 2: Operation (t 1 -t 2 ) Period-3: Operation (t 2 -t 3 ) During this time interval the switches S 2 and S 3 are turned on, the diode D 1 and D 4 are turned off and the power is supplied to the load from the voltage source V i. The negative voltage is applied to resonant tank. Period-3 ends when the voltage across C equals V o. The topological circuit for period-3 operation is depicted in figure 2.12. Figure 2.13 Period 3: Operation (t 2 -t 3 )
27 Period -4: Operation (t 3 -t 4 ) The topological circuit for period-4 operation is shown in figure 2.12. The switches S 2 and S 3 are in non-conducting state. The diodes D 2 and D 3 are conducting and the capacitor discharges through L 1.At t 4 voltage on C reaches zero towards to negative side. The current on transformer thus changes its direction. Figure 2.14 Period 4: Operation (t 3 -t 4 ) Period 5: Operation (t 4 -t 5 ) The period 5 operation is same as that of the period 1 operation. At the instant t 5 the current changes again its direction, the diodes D 2 and D 3 are turned off. The resonant current reaches zero and new cycle starts. During the continuous condition mode the voltage on parallel capacitor although has some distortion on the up slope side, the voltage is continuous without any zero period. This is the case when output load is relatively small. Figure 2.15 illustrates the operation waveforms of series parallel resonant converter in continuous conduction mode. The converter operation can be described through a few intervals from t 0 to t 5.
28 Figure 2.15 Key operating waveforms of series parallel resonant converter
29 2.5 RESULT AND DISCUSSION The performance of open loop control response for different resonant converter topologies LCL-T SPRC, CLL-T SPRC and LLC-T SPRC have been estimated with resistive load. The LCL-T, CLL-T and LLC-T series parallel resonant converter have been simulated using MATLAB/Simulink. The entire system is simulated with a switching frequency of 100 KHz. The elements and the parameters used for analysis is presented in Table I. Table 2.1 : Parameters used for Analysis Symbol Parameter Value V i Minimum input voltage (V) 100V V o Minimum output voltage (V) 100V I L Maximum load current (A) 1.33A I Lo Maximum overload current (A) 4A K L Transformer Turns ratio 1 f s Switching frequency (KHz) 100 L 1 Series Inductance (µh) 15.5 C Capacitance (µf) 0.14 L 2 Parallel Inductance (µh) 15.5 L 0 Load Inductance (µh) 150 C 0 Load Capacitance (µf) 1000
30 The resonant current and resonant voltage for 50% and 100% load condition were estimated for CLL-T SPRC and presented in figure 2.16 and 2.17 respectively. It has been seen that the oscillation and harmonics are high. The slight droop in the characteristics is due to the increase in conduction losses in the bridge inverter and resonant network. Figure 2.16 Resonant current and resonant voltage at 50% of load for V r =100V (CLL-T SPRC with open loop operation) The output voltages of the open loop CLL-T SPRC are shown in figure 2.18. It is seen that the settling time of 0.5 Sec. and 0.46 Sec is obtained for 50% and 100% load respectively. The steady state error for 50% and 100% is found to be 0.05 and 0.062 respectively. The Harmonic spectrum and ripple component present in output voltage are high. The output voltage response is flexible and sensitive.
31 Figure 2.17 Resonant current and resonant voltage at 100% of load for V r =100V (CLL-T SPRC with open loop operation) Figure 2.18 Output voltage and Harmonic Spectrum at 50% and 100% of load for open loop CLL-T SPRC
32 The LLC-T SPRC has been simulated and the response is shown figures 2.19-2.22. The LLC-T SPRC performance was also compared with the converters performance. The plots of resonant voltage, resonant current, output voltage across load and measured THD values are shown in figures 2.19-2.22. From the results it can be justified that the settling time of output voltage in open loop controller is more. Figure 2.19 Resonant current and resonant voltage at 50% of load for V r =100V (LLC-T SPRC with open loop operation) It can be clearly seen from the characteristics that the resonant voltage have small harmonics when operated in half load condition. It is due to the increase in conduction losses in the bridge inverter and resonant network. The power loss and switch loss is more for the reason that of the resonant voltage and resonant current are in phase. The voltage and current waveforms at the output of the inverter bridge are shown in figure 2.20 for 100 % load.
33 Figure 2.20 Resonant current and resonant voltage at 100% of load for V r =100V (LLC-T SPRC with open loop operation) Figure 2.21 Output voltage and Harmonic Spectrum at 50% and 100% of load for open loop LLC-T SPRC
34 Figure 2.21 shows the harmonics spectrum of the output voltage. It can be seen that the harmonic content is more when the converter is operated at full load condition and the percentage of the fundamental voltage is also more. Here the settling time is 0.9 for 50% of load and 1.01 for 100% of load, the steady state error for 50% of load is 0.052 and 100% of load is 0.06. Figures 2.22-2.24 shows the MATLAB/Simulink simulation results of the converter at half and full load with the switching frequency of 100 KHz. The anti parallel diodes D 1, D 2, D 3 and D 4 connected across the switches are not needed because they have inherent anti-parallel body diodes. The forward current and the reverse voltage ratings of the diode are the same as the current and voltage ratings of the MOSFET. The internal diode is characterized by forward voltage drop and reverse recovery parameters like a discrete diode. Figure 2.22 Resonant current and resonant voltage at 50% of load for V r =100V (LCL-T SPRC with open loop operation)
35 The resonant current and resonant voltage for different load condition were estimated and shown in figure 2.22 and 2.23. It is seen that the converter are operated at resonance frequency the inverter output voltage and inverter current are in same phase. Due to this, switching loss and conduction loss are less compared to other resonant topologies. The parallel capacitor voltage wave form is totally distorted due to the effect of the leakage inductance at operated in full load condition. It can be seen from the figure 2.22 that the current contains low harmonics and it presents a good sinusoidal shape. Figure 2.23 Resonant current and resonant voltage at 100% of load for V r =100V (LCL-T SPRC with open loop operation) The above figure 2.23 show the resonant voltage and resonant current for the LCL-T SPRC are operated at full load. The inverter voltage has harmonics at the resonance frequency and the inverter current contains a slight droop in phase, due to the leakage inductance effect and switching losses in the MOSFET.
36 Figure 2.24 Output voltage and Harmonic Spectrum at 50% and 100% of load (LCL-T SPRC with open loop operation) Figure 2.24 shows the output voltage of the LCL-T SPRC. The Harmonic spectrum and ripple component present in output voltage are very less compared to other resonant topologies. It is observed that the settling time is 0.4 sec. for 50% and 0.52 sec. for 100% load. The steady state error for 50% load is 0.06 and 100% load is 0.079. The result justifies that settling time, percentage overshoot and steady state error of output voltage in other resonant topologies is more than that of the settling time in LCL-T SPRC.
37 The performance of open loop controller response for resonant topologies such as LCL-T, LLC-T and CLL-T have been estimated and provided in Table 2.2 and Table 2.3. It is seen that the open loop controller provides less settling time, high steady state error and more percentage overshoot voltage are appear in output voltage. The SPRC is not operated in resonant frequency with variation of load and the load independent operation is not possible. Table 2.2: Comparative evaluation of transient and steady state performances of SPRC fed with Resistive Load by using Open Loop Operation Resonant Topologies Rise time in Sec. Settling time in Sec R L R L R L R L Load Load Load Load (50%) (100%) (50%) (100%) % Over Shoot In Volts R L R L Load Load (50%) (100%) LLC-T 0.48 0.58 0.9 1.01 0.9 1 CLL-T 0.5 0.46 0.72 0.96 1.2 1.2 LCL-T 0.4 0.52 0.66 0.8 1.02 1.05 From Table 2.2 and Table 2.3, it can be seen that the rise time and settling time of open loop controller has been compared for different resonant topologies. The overshoot, steady state error, THD and the settling time are high and the response is having more oscillation. It can be concluded that the LCL-T SPRC has got better performance, but the converter performance need to be improved. Due to this performance is not enough for higher power application. Here due to the wide load range operations, the output current
38 attain the dangerous value. Its affects the inverter performance and high circulating in the load side as well as inverter and rectifier and may lead possible short circuit in the inverter and rectifier leg. To recovery the above specifics, the closed loop system is proposed. Table 2.3: Comparative evaluation of steady state error and THD in % of SPRC fed with Resistive Load by using Open Loop Operation Steady state error THD in % Resonant Topologies R L Load (50%) R L Load (50%) R L Load (100%) R L Load (100%) LLC-T 0.052 36 38.3 0.06 CLL-T 0.05 32 30.15 0.062 LCL-T 0.06 29.98 26.05 0.079 The LCL-T SPRC has been fabricated and tested. A prototype LCL- T SPRC has been designed to operate at 300 W, 100 khz. The IRF840 MOSFETs used as the switch in the bridge converter. The diodes MUR 4100 has been used in the output bridge rectifier. The component values and their computed rating are summarized in Table 2.1. Figures 2.25-2.28 shows the open loop characteristics of the LCL-T SPRC. The converter is set to operate at a 100V DC input voltage and the load is varied from 50 % to 100 % load and the characteristics are obtained and plotted.
39 Figure 2.25 CH1: Resonant Voltage [Volt. Scale: 40 V/div.], CH2: Resonant Current [Amp. Scale: 0.5A/div.] Operated at 50 % of load with open loop operation The voltage and current waveforms at the output of the inverter bridge are shown in figure 2.25. The converter is operated at 100 KHz with resonance frequency fed 50 % of resistive (100 Ω) load. It is seen in the above waveform the inverter output voltage contains the harmonic due to the increase in conduction losses in the bridge inverter. The inverter output current also carries the harmonic. The voltage and current are not operated in phase due to some switching loss in the inverter leg, leakage loss form the transformer and diode conducting loss in the rectifier side.
40 Figure 2.26 Output voltage (CH1: Output Voltage [Volt. Scale: 50 V/div.] Operated at 50 % of load with open loop operation The output voltage across the load is shown in figure 2.26. It is clearly seen from the above figure that the open loop control, the load independent operation and constant out put voltage are not possible. It can be clearly seen in figure 2.27 that the full load (resistive load - 200 Ω) inverter voltage is seriously disturbed, as the inverter output current is not very high. In figure 2.27 the resonant voltage and resonant current is distorted at full load condition due to the effect of leakage inductance of the transformer. The frequency oscillation can also be viewed due to the resonance between the transformer leakage inductance and the capacitor of the MOSFET switches. The voltage and current of the inverter contains more harmonics with increase in load.
41 Figure 2.27 CH1: Resonant Voltage [Volt. Scale: 40 V/div.], CH2: Resonant Current [Amp. Scale: 0.5A/div.] Operated at 100 % of load with open loop operation Figure 2.28 shows the output voltage of the resistive load at full load condition. The plot clearly shows the output voltage is not constant with variable load. The steady state error and settling time are very high in the open loop control. The ripple is also high in the output voltage. The steady state error voltage is nearly 5 V. Due to the above reason the efficiency of the converter is affected seriously. In the open loop control MOSFETs switching losses, diode conduction losses, snubber circuit loss and leakage inductance losses can not be controlled and hence to prevent these losses the feed back control is necessary.
42 Figure 2.28 Output voltage (CH1: Output Voltage [Volt. Scale: 50 V/div.] Operated at 100 % of load with open loop operation Table 2.4: Comparative analysis of open loop operation for LCL-T SPRC fed with Resistive Load Rise time in Sec. Settling time in Sec % Over Shoot in Volts R L Load (50%) R L Load (100% ) R L Load (50%) R L Load (100% ) R L Load (50%) R L Load (100% ) 1.2 2.1 0.9 1.3 4.5 5.8 The rise time, settling time of the converter and the percentage overshoot in the output side are measured and tabulated. It is clear from the above table that the peak overshoot is not able to eliminate and the settling time is higher in the open loop control in the LCL-T SPRC.
43 Table 2.5: Comparative evaluation of steady state performances and Harmonic Spectrum performance for LCL-T SPRC fed with Resistive Load in Open loop Steady state error THD in % R L Load (50%) R L Load (100% ) R L Load (50%) R L Load (100% ) 4 5 28.8 37.5 It is clear from the above Table 2.45 that the THD is value is high under the open loop control for LCL-T SPRC. The stead state error for open loop is shown in the above Table 2.5. It can be seen that the steady state error increases when the load is increased. It is also observed from the above table that the open loop controller is applicable for the load independent operation and high power application. 2.6 CONCLUSIONS In this chapter, series parallel resonant converter with open loop condition has been investigated. The different resonant topologies such as CLL-T SPRC, LLC-T SPRC and LCL-T SPRC are simulated and estimated with open loop operation. The LCL-T SPRC are implemented and verified from the simulation results. Their detailed operation principal, transient and dynamic analyses of the series parallel resonant converter were presented. It s clearly seen the above analysis the variation of load condition the output voltage is not constant and the load current increase to attain the maximum value. The load independent operation was not possible in the open loop system. The higher application, telecommunication and aero space application require less power loss and load independent operation. It has been found that these converters experience high switching losses, reduced reliability, Electromagnetic Interference (EMI) and acoustic noise at high frequencies. It is concluded that the feed back control are necessary to accomplish the conduction loss, switching loss, load independent operation, transient and dynamic performance of the converter.