Design and Simulation of 15 KV, 15 Stage Solid State Bipolar Marx Generator

Transcription

1 Design and Simulation of 15 KV, 15 Stage Solid State Bipolar Marx Generator 1 Rashmi V. Chaugule, 2 Ruchi Harchandani, 3 Bindu S. 1 chaugulerashmi0611@gmail.com, 2 ruchiharchandani@rediffmail.com, 3 bindubalu@rediffmail.com Abstract- Solid state Marx Generator is the primary source of pulse power. Pulse power is gaining considerable attention due to its various applications in several fields such as water purification, ion implantation, rock fracturing, air pollution control, food sterilization etc. This paper deals with design and simulation of solid state bipolar Marx Generator. The specifications of Marx Generator are 15 KV output voltage, 200 Hz repetitive frequency and a pulse width of 300 µs. Index Terms- Pulse Power, Power Semiconductor devices, Bipolar, High Voltage, Marx Generator. I. INTRODUCTION Marx Generator is the primary source in pulsed power applications. Conventional Marx generator [1] was first introduced by Erwin Ooto Marx in 1924 as shown in figure 1. Initially, all the capacitors were charged up to source voltage in parallel. Huge spark gaps were used to connect the capacitors into series for pulse generation. The spark gaps were triggered through an external triggering circuit which brings all the capacitors in series. The voltage of all the capacitors gets added up in series and is applied to the load. The output voltage achieved is therefore nv in where n is number of stages and V in is the voltage of each capacitor. These spark gaps had several disadvantages such as low life, high maintenance, huge size etc. Fig.1 Conventional Marx Generator The high voltage, high power pulse obtained just by the discharge of the capacitors are later replaced with PFN (Pulse Formation Network) through which we could get controlled pulse. Next modification made was that, these PFNs were used along with Thyratrons and pulse transformers to have more flat controlled pulse of high voltage and high power. PFN was limited to a fixed impedance load and a fixed pulse width [2,3,4]. Due to continues improvement in the power semi conductors in terms of switching speed, voltage and current ratings etc, solid state switches are recommended in modern Marx Generator. Taking the merits of solid state switches conventional Marx Generator has been modified by replacing the huge spark gaps by solid state switches and the resistive isolators by diodes. They have several advantages over the traditional switches such as maintenance free, easier triggering methods etc. After evolution of solid state switches, unipolar topologies using these switches were developed. The topology for unipolar pulses was used to generate either positive or negative pulses as per application. L. M. Redondo proposed an inductor based charging topology. Disadvantage of this circuit is that the source is not isolated during the pulse cycle [5]. Sakamoto et al. have proposed a unipolar configuration by series connection of half bridge switch capacitor cells (SCC) [6]. Here a switch is placed after the source which provides isolation to the source during pulse process. Disadvantage of this circuit is that it can produce only single polarity pulses. Next came the bipolar topology. In this topology, both positive and negative pulses can be generated at a time. Bae et al proposed a topology for the generation of bipolar pulses. This topology has a conventional H- bridge configuration that allows a flexible configuration in order to achieve both the unipolar and bipolar pulses [7]. A five switch topology was proposed by L. M. Redondo and H. Canacsinh [8]. In this topology only one charging path is available. Thus the stress on switches is increased as the same switches are again being used for discharging. The topology described in this paper uses six IGBT switches per stage. The advantage of six switch topology is that it can be used for all types of loads such as resistive, capacitive and inductive loads. In capacitive load it is compulsory to discharge the load capacitances after every positive and negative pulse. This can be obtained from six switch topology. 136

2 This paper is organized as follows. Section II describes the working principle of the topology. Section III gives the design of the circuit. Section IV shows the simulation results obtained and section V concludes the paper with brief description. II. CIRCUIT TOPOLOGY A circuit topology having six switches which can be used to generate repetitive high voltage bipolar pulses is shown in figure 2 [9]. T dc - D dc - T ai - anti parallel diode of T bi - D fi - T ei T dc - D dc - D hi - T gi - anti parallel diode of T di - T ci In above three paths i = 1, 2, 3, n. These various paths have been provided so that the switches are relieved from the over stress which may in turn increase the required power ratings of the switches. For reference the second path T dc - D dc - T ai - anti parallel diode of T bi - D fi - T ei is shown in figure 3. The capacitors C i are being charged from the DC source. To protect the switches from initial transients, the supply is being increased slowly. Fig. 2 Solid state Marx type Topology for bipolar pulse generation with six switches In this circuit U dc is the source voltage, r dc is the equivalent internal resistance of source. D dc and T dc are input diode and a switch respectively which are useful for isolating the source during pulse process. This topology consists of several stages. Each stage is independent of the other stages. Even during fault conditions, the stages remains unaffected as the fault current can pass through the anti parallel diode of the IGBTs. A single stage contains six solid state switches denoted as T ai, T bi, T ci, T di, T ei and T gi, two diodes namely D fi and D hi and one capacitor C i. Since this topology gives bipolar pulse, an additional capacitor is required at the end. This additional capacitor will be useful in generation of negative pulse. Thus, for n stage topology, the number of capacitors required is (n+1). The six switch topology in figure 2 works on the same principle as that of the conventional Marx Generator. The capacitors are being charged in parallel to the source voltage and are discharged in series by triggering the appropriate switches to get the required pulse. The switches in two different legs are triggered alternately to get the positive and negative pulse. A certain dead time has been provided between turning ON of the switches on the same leg to avoid shoot-through problem i.e. two switches on the same leg getting switched ON at the same time. This may create a direct short across the source. The operating mode depends on the type of application, type of pulse required (positive or negative) and the required output in the load Charging While charging, the switches can be triggered in three different manners which will give us three different paths of charging. They are as follows. T dc - D dc - D hi - T gi - D fi - T ei Fig.3 Charging mode for the bipolar topology 2.2. Negative Voltage Pulse Once all the capacitors are charged, they are discharged by triggering the required switches. For negative pulse, the switch T bi and switch T ci are to be triggered. As said earlier, in negative pulse process last capacitor C (n+1) is involved. But the first capacitor does not contribute for the negative pulse. Figure 4 shows the circuit for the negative pulse mode. The voltage available across the load is given by, V o = - nu dc (1) Fig. 4 Negative Voltage Pulse mode 2.3. Discharge of negatively charged load capacitances As this topology is applicable for all types of load, the discharge of load is mandatory especially for capacitive and inductive load. For the discharge of load capacitances, the switch T di is triggered. Now, the load capacitance can discharge through anti parallel diode of T ci and the switch T di. This is shown in figure 5. Fig. 5 Discharge of negative charged load capacitances 137

3 For inductive load, the load inductances can freewheel through the anti parallel diodes of switches T bi and T ci respectively for negative pulse Positive Voltage Pulse After obtaining the negative pulse, a dead band is provided within which the capacitors are charged which are again to be discharged to produce positive pulse. For this purpose switches T ai and T di are triggered. Fig. 6 Positive Voltage Pulse mode In positive pulse process first capacitor participates but the n+1 capacitor do not contribute for this process. Figure 6 shows the circuit for positive pulse mode. The voltage applied across the load is given by, V o = nu dc (2) 2.5. Discharge of positively charged load capacitance As explained in negative pulse mode even after positive pulse, the capacitive and inductive loads need to be discharged. Figure 7 shows the discharge circuit for positively charged load capacitances. Fig. 7 Discharge of positive charged load capacitances Once the positive pulse is obtained, a current of same magnitude but opposite polarity is imposed on load which freewheels through the anti parallel diodes of the switches T bi, T ci and then through the capacitors C i. For positive pulse, the load inductance freewheels through the anti parallel diodes of switches T di and T ai respectively. III. CIRCUIT DESIGN A 15 stage, 15 KV six switch solid state Marx Generator has been designed and simulated. The name six switch topology is based on the number of switches per stage. An extra switch next to the voltage source is placed in order to isolate the source from the stages during the pulse process. The specifications decided for the output pulse of the Marx Generator is 15 KV, maximum pulse width of 300 µs, PRF of 200 Hz, 5-10% droop Selection of Switches The application for which Marx Generator is being designed has high power ratings. Thus, the IGBT power low frequency operation. Its selection is based on the high current which flows through the load and the switches during the pulse process. The load current should be less than the maximum pulse current rating of the switch. The input voltage source is of 1 KV. The output voltage to be obtained is 15 KV. Thus number of stages considered is 15. Hence, No. of switches required = n (3) Where n is number of stages. Here n = 15. Therefore, No. of switches required = 15 * = 91. Hence 91 IGBTs are used for generation of 15 KV bipolar output pulses Selection of Capacitor The value of the capacitor is selected based on the discharge time constant. C e = τ V o V Z switches are selected instead of MOSFETs for high 138 C e = C e = 3 µf 300µ (4) In equation 4, C e is the equivalent value of the capacitances. V o is the output voltage, V is the voltage droop in the output voltage. It can be taken between 5 to 20% of the output voltage. Here we have considered it as 10% of V o, τ is the maximum pulse width, Z is the load considered. In simulation, a resistive load of 1000 ohms have been considered. The value of individual capacitor can be calculated as C i = C e n (5) C i = 3 µf 15 C i = 45 µf Thus the value of each stage capacitor is 45µF. The number of capacitors required for this topology is n+1. Here n = 15. Thus, No. of capacitors required for the designed Marx Generator = 15+1 = 16. Hence a total of 16 capacitors will be used in the designed topology for simulation of 15 stages Selection of Diodes The peak inverse voltage or V RRM of diode should be more than the maximum input DC voltage. The peak current rating or forward current (I F ) of diode is greater than the maximum source current which is input DC voltage divided by the input resistance (R in ). IV. SIMULATION RESULTS Initially simulation of single stage was carried out in MATLAB. Figure 8 shows the single stage of Bipolar Marx Generator using IGBT switch. The results

4 obtained for single stage is shown in figure 9. In figure 8 IGBT switches are triggered using pulse generators. Different pulse generators are used to generate different pulses for various modes of operation of the Bipolar Marx Generator. In figure 8, pulse generator 1 generates pulses for charging mode, pulse generator 2 generates pulses for positive discharge mode and pulse generator 3 generates pulses for negative discharge mode. Resistive load of 1000 ohms is used for simulation. Input source is 1 KV. Result for this single stage is obtained as 1 KV bipolar pulses (figure 9). T e1 D g1 Fig. 10 Repetitive Bipolar Output Pulses of 15 KV Figure 11 shows a single output voltage pulse of 14.3 KV. The erection coefficient (ƞ ) of Marx Generator can be calculated as output voltage (V o ) divided by the product of number of stages (n) and input voltage (V in ). T dc T a1 T b1 ƞ= V o n V in (6) C 1 T c1 T f1 D h1 T d1 C 2 = = 95.33% Hence the erection coefficient obtained is around 95.33%. LOAD Fig. 8 Single stage of Bipolar Marx Generator in MATLAB Fig 9 Output for a single stage of Bipolar Marx Generator By connecting such 15 stages in series simulation of 15 stages, 15 KV Bipolar Marx Generator is done. Maximum pulse width is 300 µs and pulse repetitive frequency is 200 Hz. The input source is 1 KV. Sixteen capacitors of 45µF are chosen as per calculations in equation 5. The output voltage obtained in simulation is 14.3 KV. The repetitive output pulses of 14.3 KV are shown in figure 10. Fig. 10 Single output voltage pulse Figure 12 shows the triggering pulses of the switches with particular delay time. In figure, the first waveform is the pulses to the switches T dc, T gi and T ei, the second waveform is the pulses given to the switches T bi and T ci and the third waveform are the pulses given to switches T ai and T di. Fig.12 Pulses given to the switches The total time period for the first pulses i.e. charging pulses is 5 ms, pulse width is 4.7 ms with a zero delay. The pulses for positive and negative pulse process have same time period of 10 ms and a maximum pulse width of 300 µs. the delay time is different for these two pulses. Delay for positive discharge pulse is 4.77 ms and delay for negative discharge mode is ms. 139

5 V.CONCLUSION A generalized solid state Bipolar Marx Topology with 15 stages for the application of water purification has been designed and simulated in MATLAB. The design and selection of switches is done based on the input voltage source. IGBTs have been selected for high power application. 45µF capacitors are selected. The diodes are selected based on peak inverse voltage and forward current. The designed peak value of output voltage is 15 KV and the output voltage is obtained as 14.3 KV. The pulse repetitive frequency is 200 Hz as per design. The efficiency of the Generator obtained is 95.33%. This topology is suitable for all types of load i.e. Resistive, Capacitive and Inductive Load as it provides the discharge path after every pulse process. Also, this topology can be used to obtain both Unipolar and Bipolar pulses. REFERENCE [1] C.L. Wadhawa, Prof. and head of department of Electrical Engineering Delhi Coe; Delhi , India. New Age International (P) ltd [2] H. Akiyama, T. Sakugawa, T. Namihira, K. Takaki, Y. Minamitani, and N. Shimomura, Industrial applications of pulsed power technology, IEEE Trans. Dielectr. Electr. Insul., vol. 14, no. 5, pp , Oct [3] Dongdong Wang, Jian Qiu, and Kefu Liu, Allsolid-state repetitive pulsed-power generator using IGBT and magnetic compression switches, IEEE Trans. on Plasma Sci. vol. 38, no. 10, Oct [4] T. Sakugawa and H. Akiyama, An all-solid-state pulsed power generator using a high-speed gateturn-off thyristor and a saturable transformer, Elect. Eng. Jpn., vol. 140, no. 4, pp , [5] Harshada C. Bhosale, Bindu S., Sincy G., P.C. Saroj, Archana S., Design and Simulation of 50 kv, 50 A SolidState Marx Generator [6] T. Sakamoto, A. Nami, M. Akiyama and H. Akiyama, A repetitive Solid state Marx-type Pulsed Power Generator using Multistage Switch capacitor cells, IEEE Trans. Plasma Sci., vol.40, no. 10, pp. [7] Sungwoo Bae, Alexis Kwasinski, Mark M. Flynn, Robert E. Hebner, High-power pulse Generator with flexible output pattern, IEEE Trans. on power elect. vol. 25, no. 7, July [8] L. M. Redondo, H. Canacsinh, Bipolar solidstate arbitrary waveform Marx generator for capacitive loads. [9] L. M. Redondo, H. Canacsinh, J. Fernando Silva, Generalized solid-state Marx modulator topology, IEEE Trans. on Dielectr. and Electric. Insul. Vol. 16, No. 4; August