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Pulsed Power Technology

Design and Construction of 30 kV Capacitor Charger Using of Series Resonant Converter1 H.R. Hafezi, S.J. Mousavi, M. Barati, and M.H. Rahdan Electronic and Communication Research Center, Majdzadeh, Tehran, 1355874541, I.R. IRAN Phone: +98 (021) 55765748, E-mail:[email protected] Abstract – A power supply specifically designed for capacitor-charging applications that uses a seriesresonant circuit topology, a constant ontime/variable frequency control scheme, and zerocurrent switching techniques has been developed. The CCPS is based on a series load resonant topology and uses IGTBs as switching devices. The CCPS differs from conventional power supplies in that capacitor charging requires operation over a wide range of load conditions (varying from nearly short-circuit when the capacitor has no charge to nearly open-circuit when the capacitor is almost fully charged). This kind of performance can be best achieved by supplying a constant charging current: the instantaneous output power then is minimal at the charge beginning and maximum at the charge end. The performance of this capacitorcharging power supply (CCPS) has been evaluated in the laboratory by charging several values of load capacitance at various repetition rates. The CCPS has charged a 40 nF capacitor from 0 to 30 kV DC in 9 ms, exhibiting a charging power of 2000 J/s. This operation has been repeated at a rate of 100 charges per second. These results indicate that this design is feasible for use in capacitor-charging applications.

resonant inverters are based on resonant current oscillation. The capacitor-charging power supply utilizes a series resonant “H” bridge topology. [1–3] The operating frequency is typically in the range of 40 to 60 kHz. This topology is inherently short circuit proof because of the high source impedance it presents to the output circuit. The CCPS differs from conventional power supplies in that capacitor charging requires operation over a wide range of load conditions (varying from nearly short-circuit when the capacitor has no charge to nearly open-circuit when the capacitor is almost fully charged). This kind of performance can be best achieved by supplying a constant charging current: the instantaneous output power then is minimal at the charge beginning and maximum at the charge end. Three types of resonant inverter topologies are usually used for CCPS; series-resonant circuit, parallel resonant circuit and series-parallel resonant circuit. Due to its simple circuit topology and control system, excellent short circuit proof capability and constantcurrent characteristic is obtained. The series resonant circuit was adopted in CCPS [1–3]. This paper describes the design and experimental results of the CCPS. 2. System description

1. Introduction Interest in rep-rate operation of pulsed power systems has led to an increase in power requirements of high voltage (HV) capacitor charging power supplies (CCPS). The requirements for a power supply charging the capacitor are severe The main requirements for these applications should be as follow: • Charging voltage regulated up to few kV. • Charging time as low as few ms. • The capability of withstanding short circuit conditions. The turn-on and the turn-off losses of the switching devices are significantly high in converters with PWM (Pulse-Width Modulation) control. The disadvantages of PWM control can be eliminated or minimized if the switching devices are turned “on” and “off” when the voltage across the devices and/or their current is zero. The voltage and the current are forced to pass through zero by an LC-resonant circuit. The series 1

The whole system is realized by a simple combination of the desired number of HV modules that may be connected both in series or parallel. The difficulty to reliably control a single 30 kV/2 KJ/s module in addition to the associated high voltage insulation problems forced us to use two modules of 15 kV/1 kJ/s in series. The simple schematic of a CCPS is presented in Fig. 1. As seen in the Fig. 1 the CCPS includes four basic modules: input power module, inverter module, highvoltage output module, and control module. The AC input section includes an EMI filter, an inrush current-limit circuit, a rectifier, and filter capacitors. When the AC power is energized, a high inrush surge during the charging period of the input capacitors is prevented by the inrush current-limiting circuit. The rectifier and the filter provide a stable DC voltage to the inverter section. The AC/DC converter includes Series of Resonance Converters. Also, the highvoltage tank consists of a high-frequency transformer, multiple full-wave bridges, and voltage and current monitoring circuits.

The work was supported by Electronic and Communication Research Center under Contract No. 34Etr76J.

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Fig. 1. Simplified block diagram of the CCPS 2

3. Principle of series resonance converter (SRC)

⎛N ⎞ ĆL  ⎜ 2 ⎟ CL . ⎝ N1 ⎠

Figure 2 shows the circuit diagram of SRC.

Usually in high-voltage applications ĆL is at least one order of magnitude greater than Cr. Therefore, it can be neglected: 1 Ceq   Cr . 1 1  ' Cr CL

Fig. 2. Simplified block diagram of SRC

The characteristic impedance Z0 of the bridge load is given by:

L , Z0  Ceq and its resonance frequency f0 can be considered as

f0 

1 2 LCeq

,

where Ceq is the series combination of capacitors ĆL and Cr, in which ĆL is the equivalent value of CL transferred to the primary:

V11 31.1 Vdc

V1 = 0 V2 = 5 V6 TD = 0 TR = 1n TF = 1n PW = 5u PER = 20u

S5 GND1

+ + - -

Sbreak

V1 = 0 V2 = 5 V7 TD = 10u D9 TR = 1n Dbreak TF = 1n PW = 5u PER = 20u

Therefore Z0 and f0 are extracted by Cr and Lr. Operating the circuit at the frequency fs < f0/2, all switches and anti-parallel diodes turn on and off at zero current. Therefore switching losses are reduced to a minimum and no snubbers are required. 4. Simulation of SRC with commercial circuit analysis programs Operation of SRC can be accurately simulated through the commercial circuit analysis programs (Fig. 3). As it can be seen from the simulation results (Fig. 4) the current flowing in the series- resonant load Cr and Lr is a series of sinusoids and switches are commutated when the currents are zero. The charging voltage of capacitor increases at the constant rate (Fig. 5) while the pulsed charging current average value is constant during all cycles.

S6 GND1

D10

+ + - -

Sbreak

Dbreak C6

D18 Dbreak

L3 34uH 1

2

TX3

D20 Dbreak

70nf C5

V1 = 0 V2 = 5 V8 TD = 10u TF = 1n TR = 1n PW = 5u PER = 20u

S7 GND1

+ + - -

Sbreak

V1 = 0 V2 = 5 V9 D11 TD = 0 TR = 1n Dbreak TF = 1n PW = 5u PER = 20u

R3

S8 GND1

40n

D12

+ + - -

Sbreak

Dbreak

1m

D19 Dbreak

K528T500_3C8

L1_TURNS = 10 GND1

0

Fig. 3. Circuit simulation of SRC

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L2_TURNS = 400

0

D17 Dbreak

0

Pulsed Power Technology

The characteristic impedance Z0 of the bridge load is:

Z0 

I0  

Lr 34uH   22, Ceq 70nF

Vin V 400   in   18 A. Z0 22 Lr Ceq

It should be noted that in the beginning of charge process the current flow through the IGBTs is I0.On the contrary, at the end of the charge process due to the energy accumulated in the resonant load (i.e. Cr and Lr) load current increases up to 2I0: Time

Iswitch  2 I0  2 18  36 A.

Fig. 4. Simulated waveform of the SRC

The full-bridge inverter is composed of four IGBTs (IRG4PH50UD, 1200 V-42 A-200 kHz) manufactured by International rectifier. 5.1. High voltage transformer In the series resonant circuit, the stray inductance of the high voltage transformer is the resonant inductance (Lr). High frequency requires a small, high permeability cores (e.g. ferrite cores) and twisted winding topology. Wounding of primary and secondary on two different legs of the core lets us to make a better insulation capability between high voltage and low voltage sections. According to above statements the parameters of interested transformer are listed in Table 2. Table 2. Transformer requirements Time

Symbol V1 V2 I1 I2 f L1

Fig. 5. Simulation waveform of series resonance

5. SRC design The required parameters of the CCPS are summarized in Table 1. Table 1. Parameters of the CCPS DC input voltage Peak power Output voltage Capacitance Charging time Switching frequency

Parameter Primary Voltage Secondary Voltage Primary RMS Current Secondary RMS Current Switching frequency Primary leakage inductance

The primary is composed of three windings of 11 turns each connected in parallel to minimize skin effect losses. The secondary is composed of a single winding of 400 turns. The core of the transformer is two U-shape ferrite cores (Type U103). In Fig. 6 the high voltage transformer is shown.

400 Vdc 2 kW 30 kV 40 nF 9 ms 50 kHz

The working frequency of the inverter of the serial resonant circuit is set at 50 kHz. The resonant inductance of 34 μH is the stray inductance of the high voltage transformer. The capacitance of the resonant capacitor can be calculated by: f0 = 2fs ⇒ f0 = 2  50 kHz = 100 kHz.

f0  

1 2 Lr Cr

⇒ Cr 

1 4  34uH  100kHz  2

2

Value 400 V 15 kV 4A 200 mA 50 kHz 34 uH

 70nf . Fig. 6. High voltage transformer

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Oral Session C. High voltage full-bridge rectifier

The full-bridge rectifier is composed of 60 fast recovery diodes (BYT43M) rated at 1000 V/1 A. Each bridge arm is composed of 15 diodes in series. The rectifier is merged in oil beside the transformer. Fig. 7 shows the full-bridge rectifier.

voltage of 400 V. The obtained peak-charging rate is 2 kJ/s. Figure 10 shows the resonant current waveform in details at the beginning of the charge process. The resonant frequency is 50 kHz and the peak resonant current is 26 A.

Fig. 7. Full-bridge rectifier

6. Experimental results Experiments were carried out to test the performance of the CCPS (Fig. 8). The experimental setup is shown in Fig. 9.

Fig. 10. Resonant circuit current waveforms

7. Conclusion The series resonant inverter power supply is designed and constructed for the capacitor charging applications. The performance of this capacitor-charging power supply (CCPS) has been evaluated in the laboratory by charging several values of capacitance load at various repetition rates. The CCPS has charged a 40 nF capacitor up to 30 kV DC in 9 ms, exhibiting a charging power of 2000 J/s. Future works will needed to improve the system efficiency to reduce the charging time, burst mode charging , and higher charging voltages.

Fig. 8. Charging waveform for a 40 nF capacitor at 30 kV and 9 ms charging time

Fig. 9. Experimental setup

The charging experiment was carried out on 18 J by a 40 nF capacitor bank. The prototype inverter power supply linearly proves its performance by charging a 40 nf capacitor bank up to 30 kV in a 9 ms. The average output current is 200 mA with original

References [1] Heqing Zhong, Zhixin Xu, Xudong Zou et al., “Current Characteristic of High Voltage Capacitor Charging Power Supply Using a Series Resonant Topology”, in Proc. of the 29th Annual Conference of Industrial Electronics Societythe, V. 1, 2–6 Nov, 2003, pp. 373–377. [2] A.C. Lippincott, R.M. Nelms, M. Garbi, and E. Strickland, “A series resonant converter with constant on-time control for capacitor charging applications”, in Proc. of 5th Annual Conf. and Exposition APEC '90, 1990, pp. 147–154. 1990, Fifth Annual 11–16 March 1990, pp. 147–154. [3] M.M. McQuage, V.P. McDowell, F.E. Peterkin, and J.A. Pasour, “High Power Density Capacitor Charging Power Supply Development for Repetitive Pulsed Power”, presented at the Power Modulator Symp., 2006.

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