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1 IEEE PEDS 017, Honolulu, USA 1 December 017 High-Voltage Pulsed Power Supply for the Igniter Circuit of a Pulsed Plasma Thruster Bingyin Kang and Kay-Soon Low Satellite Technology And Research Centre (STAR) Department of Electrical and Computer Engineering National University of Singapore 1 Lower Kent Ridge Road, Singapore, elekang@nus.edu.sg, k.s.low@ieee.org Abstract In nano-satellites, the pulsed plasma thruster (PPT) is a promising electric propulsion system for orbit maintenance and attitude control. In this paper, a pulse power supply based on a flyback circuit is investigated for the PPT igniter. The proposed circuit uses the resonance among the magnetizing inductance of transformer and parasitic capacitances to generate a high pulse voltage. When compared with other topologies, the flyback circuit does not require a physical high voltage capacitor. Consequently, the reliability is improved. With a reduced number of components, the circuit suits well to the size constraint of a nano-satellite. Moreover, the output pulse voltage can be easily controlled by adjusting the peak charging current. To validate the concept, a prototype circuit has been developed. The experimental results show that the proposed circuit has a medium voltage conversion gain, and the plasma can be generated in vacuum environment successfully. I. INTRODUCTION In recent years, there is a significant development in the miniaturized satellites for both research and commercial applications. For satellites that weigh between 1 kg and 0 kg, they are known as nano-satellites [1, ]. For this class of satellites, they have found many new commercial applications in particular constellation of several tens or hundreds of such satellites. To provide more capability and flexibility in mission design, a propulsion system can be added such that the satellites can be moved in formation or orbit maintenance for precise constellation as well as lifespan extension. Among the various propulsion systems, the pulsed plasma thruster (PPT) is a suitable electric propulsion system that can be applied in nano-satellites for orbit maintenance and attitude control. The block diagram and operating principle of a conventional PPT have been presented in [3]. To generate a thrust using plasma, the plasma generation is triggered by an igniter circuit based on a high voltage to ablate the solid propellant. From existing studies [4-7], the high voltage required by a PPT igniter varies widely from.7 kv to 8 kv. However, the required power is low, e.g W for a pulse frequency of 1 Hz in [6]. The objective of this work is to investigate a reliable and compact pulse power supply to serve as a PPT igniter. One approach is to utilize non-isolated step-up DC-DC converters [8]. The output voltage is adjusted by the switching duty cycle. However, the output voltage gain cannot be increased further when the switching duty cycle approaches Fig. 1 Flyback circuit (a) without and (b) with parasitic capacitances. unity due to the non-ideality of components. In [9, 10], a high pulse voltage was achieved by switched capacitors. The voltage is adjusted by the stage number of switched capacitors. However, multiple semiconductors (diodes or solid switches) are required making the design bulky. To reduce the number of components, new circuit topologies have been proposed in [11, 1]. These topologies store energy in a choke first. Then a high pulse voltage is generated when the energy is transferred to a capacitor or a resonant tank. These topologies have reliability issue as the pulse load shares the same circuit ground with the low voltage terminal. To provide galvanic isolation, a transformer-based circuit is preferred in PPT applications [13-]. An extra circuit is introduced to pre-charge a capacitor to a high voltage, e.g. 1 kv [13]. Then, a high voltage pulse is generated by applying the capacitor to a step-up transformer. Though the output power can be adjusted by the pre-charged capacitor and voltage, the extra charging circuit increases the design complexity and volume. To remove the pre-charging circuit, some authors have proposed resonant topologies whose resonant tanks have the boost capability [3, 16]. From the literatures, a high voltage capacitor is always required for PPT. Though the dielectric stability has been investigated widely, the capacitor remains the key component that affects the circuit reliability in high voltage system [17]. In this paper, a topology based on the flyback circuit shown in Fig. 1 is proposed for a PPT igniter. A metal-oxide-semiconductor field-effect transistor (MOSFET) with a freewheeling diode D is employed as the semiconductor switch S. This topology uses the parasitic components including the transformer winding capacitance C w, magnetizing inductance L m, MOSFET output capacitance C oss, diode junction capacitance C D, and capacitance formed by igniter spark plugs C ig. The high pulse /17/$ IEEE 94

2 voltage can be generated by the resonance occurring among parasitic components when the MOSFET is turned off. Hence, the physical high voltage capacitors are not needed, and the circuit reliability is improved. The output voltage can be adjusted by the peak charging current, easing the control implementation. Moreover, the current of magnetizing inductance reverses the flow direction due to the resonance. This would reset the flux in the magnetic core. Hence, a clamp circuit discussed in [18] is also not required. II. OPERATION OF THE PROPOSED TOPOLOGY In this analysis, the leakage inductance of transformer is neglected to simplify the theoretical analysis. All the parasitic capacitances are lumped to C r determined by C C DCig r = C w C n oss (1) C C D ig where the notation n is the transformer turns ratio of the secondary winding to the primary winding. The nominal waveforms are shown in Fig.. The operation can be subdivided into five modes: 1) Mode 1 [t 0 t 1]: The MOSFET is turned on at time t 0. The transformer primary side is applied with the input voltage V in. The current i L in the magnetizing inductance L m increases linearly to the peak current I pk. The input current i in equals to i L, and the primary side voltage v p equals to V in. ) Mode [t 1 t ]: Once the MOSFET is turned off, the resonance occurs among L m and C r. i L and v p in this mode can be written as Vin il() t = Ipk cos( ωo( t t1 )) sin ( ωo( t t1 )) Zr () vp() t = Vin cos( ωo( t t1 )) ZrIpk sin ( ωo( t t1 )) where ω o =1 L m C r and Z r =L m C r. From (), the output pulse voltage V o,max can be determined as ( ) r V = n V Z I (3) o,max in pk. The pulse voltage can be adjusted by I pk to adapt to different voltage requirements, and applied to a PPT igniter. In Mode, i in remains zero. This mode ends when i L drops to zero at time t. 3) Mode 3 [t t 3]: Due to the resonance, the current i L reverses and the amplitude of v p drops. The diode D s in the secondary side is blocked. As the freewheeling diode D is still blocked, the input current i in remains zero in this mode. 4) Mode 4 [t 3 t 4]: When v p = V in at time t 3, the freewheeling diode D is conducting. Hence, i in equals to i L, and the input voltage V in is applied to the primary side again. As i in reverses, the flux in magnetic core is reset. Hence, the ferrite core saturation can be prevented, and a reset clamp circuit is not required. ) Mode [t 4 t ]: When v p < V in after time t 4, the freewheeling diode is blocked. As the parasitic capacitances have been charged to V in in Mode 4, there is a slight oscillation between L m and C r. In practice, the oscillation will terminate shortly due to the damping by the circuit DC resistance. Fig. Nominal waveforms of flyback circuit. v Gate: MOSFET gating signal, i in: input current, i L: current of magnetizing inductance, v p: primary side voltage, v o: output voltage. Table I Comparison with other PPT igniter circuit Pre-charging Number of high Maximum output Circuit circuit voltage capacitor voltage [13] Vchg [14] 1 nvchg [] m n m V chg Flyback circuit Output voltage (kv) 10 0 n V 0 ( ) Circuit [13] Circuit [14] Circuit [] Flyback circuit in Z I r pk Fig. 3 Output voltage results of PPT igniter circuits if an energy of 0.43 mj is transferred

3 III. BENCHMARKING WITH OTHER PPT IGNITER CIRCUITS Table I summarizes the benchmarking of the proposed flyback circuit with other PPT igniter circuits. A. Design Complexity The circuit topologies in [13-] require an extra capacitor in the charging circuit for the PPT. This increases the design complexity and becomes more bulky. Since the flyback circuit utilizes the least number of components, it can be built easily and compactly. B. Number of High Voltage Capacitors The PPT igniter circuit in [13] uses a resonant tank for high voltage generation. Two high voltage capacitors are required. The PPT igniter circuit in [14] needs one high voltage capacitor for pre-charging. For an m-stage magnetic pulse compression circuit in [], m high voltage capacitors are required to yield the high voltage. Instead of using a physical high voltage capacitor, the flyback circuit in this paper uses the parasitic capacitances to generate a high pulse voltage. Hence, a physical high voltage capacitor is removed. C. Maximum Output Voltage Table I lists the output voltage calculations, and Fig. 3 shows an example when an energy of 0.43 mj is transferred. From Table II, the lumped capacitance C r in the flyback circuit can be determined as.08 nf. If the circuits in [13-] pre-charge a capacitor of.08 nf, the voltage V pre is 643 V. From Fig. 3, the PPT igniter circuit with a resonant tank in [13] has a poor voltage boost capability. Benefitting from a step-up transformer and the stages of magnetic pulse compression circuit, the circuit in [] generates the highest output voltage. For the PPT igniter circuit in [14] and the proposed flyback circuit using parasitic components, they have a comparable output voltage. Compared to other topologies, the proposed flyback circuit has a medium voltage boost capability. D. Reliability From the statistics in [19], the failure rate in a year for capacitor, MOSFET, diode and transformer are 1/0, 1/114, 1/0 and 1/00, respectively. The capacitor contributes 13.4% 1 0 ( ) to the failure probability of a PPT igniter circuit. Hence, the proposed flyback circuit can improve the reliability without a physical high voltage capacitor. IV. SIMULATION AND EXPERIMENTAL RESULTS To validate the flyback circuit, a prototype has been developed in the laboratory. Since the winding capacitance of a transformer dominates the parasitic capacitances, the transformer design determines the output voltage from (3). In the prototype, a single layer with turns is coiled in the primary winding. From the datasheet of selected ferrite core B6643U03K187, its inductance factor is 3 nh, and the calculated magnetizing inductance is μh. For a step-up transformer, the winding capacitance is dominated by the secondary winding. Though it is difficult to estimate the transformer winding capacitance precisely, there are some models for estimation. From [0], the winding capacitance referred to primary side is Table II Components used in flyback circuit Transformer Ferrite core B6643U03K187 Winding capacitance C w 1.86 nf Magnetizing inductance L m μh Turns of primary winding C w n p Turns of secondary 0 winding n s MOSFET Part number STB8NM0N Drain-source breakdown 00 V voltage V BD Output capacitance C oss 418 pf Diode Part number SF1600-TR Parasitic capacitance C D 40 pf Fig. 4 Experimental setup of flyback circuit. Fig. Experimental waveforms of the flyback circuit. v Gate: gating signal of MOSFET, i in: input current, v o: the output voltage. ( Ns ) l ( N r ) 4 1 = n (4),layer turn s 0 εε 0 e N 3d s,layer eff where N s,layer is the layer number of secondary winding. N s is the turn number of secondary winding. r 0 is the radius of wire. l turn is the length of one turn. ε 0 is the vacuum permittivity. ε e and d eff are the effective permittivity and distance, respectively. They are estimated by εdεf ( δ h) εe =, () εfδ εdh d = 0.r h.3 δ, (6) eff 0 947

4 Maximum output voltage V o,max (kv) 4 3 Experiment Calculation Simulation Peak current I pk (A) Fig. 6 Maximum output voltage V o,max versus peak current limit I pk. Fig. 7 PPT integration test in vacuum environment. Fig. 8 Discharging waveforms. v Cap: voltage of PPT capacitor bank, v IG: voltage of PPT igniter. where ε D and ε F are the dielectric constants of wire and foil, respectively. Moreover, δ and h are the thicknesses of wire insulation and foil, respectively. The custom design transformer utilizes wire RRW-A-10 Fig. 9 Plasma generation in vacuum. and two layers of Kapton tape as the foil between wire layers. From (4) - (6), the calculated winding capacitance is.33 nf when three wire layers are coiled in the secondary winding, and each layer has 0 turns. The parameters of transformer are extracted by an LCR meter E4098A, and they are listed in Table II. The calculated winding capacitance has an absolute error of.1% with respect to the measurement. Nevertheless, the model of transformer winding capacitance discussed in [0] can be used for the initial estimate, and guide the transformer design. In this study, a MOSFET STB8NM0N has been selected whose drain-source breakdown voltage is 00 V. As the custom made transformer has a turns ratio of 10, the output voltage is set to smaller than 4. kv to provide a safety margin of the MOSFET. The experimental setup in laboratory is shown in Fig. 4. The input DC voltage is V. A current probe CP30 and a high voltage differential probe CT3681 are used to measure the input current i in and output voltage v o, respectively. To measure the potential maximum output voltage, the experiment is performed without connecting to the spark plugs of PPT igniter. As the input capacitance of high voltage differential probe is 10 pf, it can be considered as C ig in Fig. 1(b). Hence, the lumped capacitance C r is.98 nf from (1) and Table II. Fig. shows the experimental waveforms when the peak current I pk is.1 A. The charging time of Mode 1 is 1.3 μs. Once the MOSFET is turned off at time t 1, the circuit starts to resonate to generate a high pulse voltage. As the resonant angular frequency is as high as rad/s according to transformer parameters listed in Table II, the time duration for Modes and 3 is less than μs. Hence, t 1, t and t 3 are labeled in the zoomed-in screenshot in the upper right corner. The measured pulse voltage is 3.7 kv. It is also noted that the input current i in reverses in Mode 4 (t 3 t 4) since the resonance cannot transfer all the stored energy to C ig. Consequently, the flux in the transformer core is reset. To validate the output voltage regulation, an experiment has been performed by adjusting I pk. Fig. 6 shows the output voltage versus I pk. For comparison, theoretical and simulation results are also included. From Fig. 6, it is observed that the output voltage can be regulated by I pk. The theoretical calculation based on (3) assumes that all the components are ideal. Hence the theoretical results give the highest output voltage. In the 948

5 experiment, the voltage across the MOSFET decreases with the increase in I pk. This is because more voltage drops on the current sensing resistor which is used to detect the peak charging current. This increases the MOSFET output capacitance C oss, and enlarges the voltage error between the simulation and experiment according to (3). The largest error is.3%, occurring at the peak current of 3.7 A. To validate that the prototype circuit can ignite the plasma generation in space, an experiment has been conducted with the in-house PPT placed in a vacuum chamber. The test setup and experimental waveforms are shown in Figs. 7 and 8, respectively. The capacitor bank is charged to 1. kv, and the pulse frequency is set to 1 Hz. The energy stored in the capacitor bank is released to form the main discharging when the prototype circuit supplies a high voltage of around 3 kv to the pulsed plasma thruster (PPT) igniter. The discharging pulse current flowing through the surface of the Teflon propellant produces the plasma. The successful plasma generation in the vacuum environment is shown in Fig. 9. V. CONCLUSION This paper presents a flyback circuit to generate a high pulse voltage for the igniter of a pulsed plasma thruster. The objective is to make use of the parasitic capacitances of components. In this way, a physical high voltage capacitor can be eliminated to improve the system reliability by 13.4%. Moreover, the circuit has the advantage of small form factor and ease of control the switch. The simulation and experimental results have validated that the flyback circuit can serve well as the PPT igniter circuit. IEEE Trans. Dielectr. Electr. Insul., vol. 1, pp , Apr [10] L. Müller and J. W. Kimball, "High gain DC-DC converter based on the Cockcroft-Walton multiplier," IEEE Trans. Power Electron., vol. 31, pp , Sep [11] S. Zabihi, F. Zare, G. Ledwich, A. Ghosh, and H. 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