A Switching Inductive-Capacitor Generator of Powerful Current Pulses

Abstract

A generator is studied that consists of a low-power source with an insufficient DC voltage level for the consumer, a semiconductor switch on an IGBT transistor, an inductive-capacitive link, a thyristor that is switched on by dynistors, and a pulse transformer. The generator is designed to power various consumers with high-power current pulses with a repetition frequency of up to 100 Hz or more. Experimental studies of a generator prototype model showed that in comparison with a low-power source (22 W, 13.8 V), the pulse power and voltage values for a consumer were 2.47 kW and 1122 V. At the same time, the current pulses had an amplitude of 2.2 A, a duration of 20 μs, and a repetition frequency of 50–167 Hz in the consumer’s resistance (510 Ω) at a source voltage of 13.8–22.8 V. As the DC voltage of the power supply increased, the number of capacitor-charging current pulses decreased, while the frequency of current pulses in the load increased. The maximum voltage at the capacitor during its charging is 3–5 times higher than the source voltage.

THE ELECTRIC CIRCUIT AND OPERATION OF THE GENERATOR

Generators with electric capacitors that are charged from a DC power source (a battery, a rectifier, and a power supply) and then discharged to the load (consumer) are used to supply power with powerful current pulses to many consumers, Such generators include a switching inductive-capacitor generator with a current-pulse repetition frequency in a load of up to 100 Hz or higher. The objective of this work is to study this generator when it is connected to a DC voltage source of limited power and with an insufficient voltage level for a consumer.

If the DC voltage level of a power supply is insufficient for a consumer, an intermediate AC link is often used, which consists of an inverter, a transformer, and a rectifier that charges capacitors [1, 2]. In [3], we considered inductive-capacitor DC voltage pulse converters with a semiconductor switching element, an IGBT transistor, which operate in the mode of supplying a consumer with a continuous oscillating voltage at the capacitor.

The electrical circuit of one of these converters was used in the generator as a link for increasing the voltage across the capacitor charged with a series of current pulses. A pulse transformer was used to further increase the voltage. A charged capacitor is connected to its primary winding through a thyristor, while the secondary winding is connected to a load. The thyristor is enabled by including one or more series-connected dynistors in the control circuit, and the capacitor is discharged, thus generating a current pulse in the load. The thyristor is disabled when its current decreases to zero.

The electrical circuit of the generator with the measuring resistors (1 Ω) is shown in Fig. 1. The Sw₁ switch (IGBT-transistor) periodically closes and opens the generator power supply circuit at a frequency f and an off-duty factor Q > 1. In this case, the switching period T and the time intervals t₁ (Sw₁ is closed), t₂ (Sw₁ is opened) are:

$$\begin{gathered} T = 1{\text{/}}f,\quad {{t}_{1}} = T{\text{/}}Q, \\ {{t}_{2}} = T - {{t}_{1}} = T(Q - 1){\text{/}}Q. \\ \end{gathered} $$

(1)

Fig. 1.

The electrical circuit of generation and recording of currents and voltages: (VS) DC voltage source (a channel of the Mastech HY3003D-2 power supply), (Osc) UNI-T UTD2025CL oscilloscope of the UTD-2000L series, 25 MHz; (Sw₁) 5P40.10GDA1-20-12-В88 DC solid-state relay,  f = 500 Hz; (D₁, D₂) VSHFA16TB120-N3 diodes (16 А, 1200 V, 76 А/μs), (D₃) Т242-80-7 thyristor (80 А, 700 V), (D₄) KN102А and KN102B; (Тr) transformer: two 2000NM E-shaped $20 \times 28$ ferrite cores, 0.1-mm gap, w₁ = 8 turns, w₂ = 250 turns (${{L}_{1}}$ ≈ 0.167 mH, ${{R}_{1}}$ < 0.1 Ω, ${{L}_{2}}$ ≈ 157 mH, ${{R}_{2}}$ ≈ 1.4 Ω, $M \approx$ 5 mH); (Ch) choke: two 2000NM E-shaped $20 \times 28$ ferrite cores, 2-mm gap, w = 194 turns (${{L}_{d}}$ ≈ 10.9 mH, ${{R}_{d}}$ ≈ 0.4 Ω); (C_e) 3 ECAP470 uF/63 V, 105°C electrolytic capacitors (1410 μF, 63 V), (С) 3 CBB-685-400V-685J-CL2 capacitors (20.9 μF, 400 V); (R_L) 5W510RJ-510 Ω resistor; and 5W1RJ (1 Ω) and 5W330RJ (330 Ω) resistors.

For the closed Sw₁ switch when a source (VS) of a DC voltage U_e is connected, energy is stored in the magnetic field of a choke (Ch); for the opened Sw₁ when the source VS is disconnected, this energy is transferred to the electric field of the capacitor С, which is charged to the maximum voltage U_mC within several Sw₁ switching periods T.

This voltage is determined by the voltage of switching D₄; when the capacitor voltage reaches U_mC this enables the D₃ thyristor, the capacitor discharges through the primary winding of the transformer Tr, and a current pulse i_L is generated in the load R_L. The capacitor charging and discharging and the generation of current pulses i_L are periodically repeated at the frequency f_L< f. In order to separate the energy storage processes in the choke and capacitor in time, a D₁ semiconductor diode is installed; the D₂ semiconductor diode is connected to the capacitor to prevent the possible occurrence of a negative voltage at the capacitor during its discharge.

Two modes are possible during the capacitor charging: the continuous current i_d mode of the choke, when this current does not reach zero during most periods Т of switching Sw₁ and the pulse current mode of the choke, when this current during most Sw₁ switching periods reaches zero. Based on the necessity to periodically disable the D₃ thyristor after the capacitor discharges, the pulse current mode of the choke appears to be most acceptable.

If the quality factor of the choke is q = 2πfτ_d ≫ 1 with its inductance L_d, resistance R_d, and time constant τ_d=L_d/R_d, then, without allowance for the measuring resistors (1 Ω), the pulse current mode of the choke can be provided if the capacitance С of the capacitor satisfies the condition:

$${{C}_{m}} = \frac{{4t_{2}^{2}}}{{{{\pi }^{2}}{{L}_{d}}}} > C.$$

(2)

THE PROTOTYPE GENERATOR MODEL

The prototype generator model consists of the following elements (Fig. 1).

One channel of a controlled two-channel Mastech HY3003D-2 power supply unit (MASTECH, Hong Kong, China) was used as the source VS of a DC voltage U_e. The maximum output values of one channel are 30 V and 3 А. Three parallel-connected ECAP470 uF/63 V, 105°C electrolytic capacitors (Taiwan) with a total capacitance of C_e ≈ 1410 μF and a voltage of up to 63 V (analogs of a K50-35 capacitor) are connected to the voltage-source output to reduce the voltage amplitudes at the Sw₁ switch and at the inner elements of the source when opening the Sw₁ switch.

A 5P40.10GDA1-20-12-В88 solid-state DC relay (ZAO Proton-Impul’s, Orel, Russia) based on an IGBT transistor serves as the controlled Sw₁ switch and has the following parameters: the interrupted current is up to 20 А; the maximum voltage in the disconnected state is up to 1200 V; the switching frequency is up to 100 kHz; the switching time is 5 μs; the residual voltage in the closed state is U_res< 2.5 V, and its measured average value is U_res ≈ 1.4 V.

For protection, the output power terminals of the Sw₁ relay are shunted by an SN2-1А varistor for 1200 V (AS Energiya, Korolev, Russia) and a VS-HFA16TB120-N3 diode (www.vishay.com) with parameters of 16 А, 1200 V, and 76 A/ μs, which is oppositely connected in parallel to the relay (both are not shown in Fig. 1).

The relay operates under the control of unipolar rectangular voltage pulses u_con with an amplitude of 8 V, an off-duty factor of Q = 2, and a frequency of  f = 500 Hz, which are fed through a current-limiting 426.6-Ω resistor to the relay input, thus providing an input pulse current of 15 mA. The conditional power contacts of the Sw₁ relay are opened at the control voltage u_con = 0 and closed at u_con = 8 V.

A DC voltage source of 12–15 V with a supply current of at most 30 mA is connected to the relay supply terminals via a current-limiting 50-Ω resistor. Regulated sources of the ED-1 laboratory complex (OOO Uchebnaya Tekhnika, Chelyabinsk, Russia) are used as a source of rectangular control-voltage pulses and a relay power source, which are not shown in Fig. 1. As a result, at a frequency of f = 500 Hz and an off-duty ratio of Q = 2, the Sw₁ switch, according to (1), provides T= 2 ms and t₁ = t₂ = 1 ms.

To provide pulsed periodic energy storage in the magnetic field, a choke Ch with a magnetic conductor of two E-shaped $20 \times 28$ 2000НМ ferrite cores (Ferropribor, St. Petersburg, Russia) was manufactured. A 2-mm air gap is provided between the cores of the magnetic conductor. The choke winding with a number of turns of w = 194 is wound with PET-155 wire with a copper diameter of 1.18 mm. A 0.1-mm-thick Mylar film is placed between the wire layers and provides insulation. The measured parameters of the choke at 25°С are as follows: ${{L}_{d}}$ ≈ 10.9 mH, ${{R}_{d}}$ ≈ 0.4 Ω, and the mass is 0.7 kg; in this case, τ_d ≈ 27 ms ≫ t₁ and q ≈ 85.6 ≫ 1.

The D₁ and D₂ are both of the VS-HFA16TB120-N3 type (www.vishay.com) with parameters of 16 A, 1200 V, and 76 A/μs.

According to condition (2), when C_m ≈ 37.2 μF, we choose a capacitor in the form of three CBB-685-400V-685J-CL2 parallel-connected film capacitors (ELECYINGFO, China) with a total measured capacitance of C = 20.9 μF and a maximum voltage of 400 V.

The charged capacitor is connected to the primary winding of the transformer Tr using a Т242-80-7 low-frequency thyristor (D₃) (Elektrovypryamitel, Saransk, Russia) with these parameters: the average current is 80 A and the repetitive voltage in the disabled state is 700 V. This thyristor is enabled using a series connection of a current-limiting 5W330RJ resistor (330 Ω) and two, KN102А and KN102B (www.katod-anod.ru/rd/kn102), dynistors (D₄) in its control circuit. The certified switching-on voltages of the dynistors are 20 and 28 V, respectively. Experimentally, their switching-on voltage was approximately the same and equal to 36 V; when they were connected in series, the switch-on voltage was 72 V. As a result, the maximum voltage at the capacitor must be equal to U_mC ≈ 72 V.

The thyristor is turned off when its current i₁ decreases to zero. Therefore, the duration of the current pulse i₁ must be much shorter than the time interval t₂ = 1 ms (1); this is provided by the capacitance value of the capacitor (C = 20.9 μF) and the parameters of the transformer (L₁, L₂, M, R₁, R₂) and load (R_L).

The pulse transformer Tr with a magnetic conductor of two E-shaped $20 \times 28$ 2000NM ferrite cores, which are identical to those of the choke, was manufactured in order to increase the voltage across the load R_L in comparison with the maximum voltage U_mC across the capacitor. An air gap of 0.1 mm is provided between the cores of the magnetic conductor for their demagnetization by unipolar current pulses i₁.

The primary (external) and secondary (internal) windings are located on a common plastic frame with a wall thickness of 1.25 mm. The primary winding has a single layer and consists of three parallel windings, each of which is wound with a PET-155 wire with a copper diameter of 1.18 mm and a number of turns of w₁ = 8. The secondary winding has a number of turns of w₂ = 250 and is wound with a PET-155 wire with a copper-wire diameter of 0.8 mm; a 0.1-mm-thick Mylar film is placed between the layers of this wire.

Between the primary and secondary windings an 0.6-mm-thick insulation made of electrically insulating PVC tape is placed, which has a thickness of 0.15 mm and is declared by the manufacturer for a breakdown voltage of 6 kV, presumably rated for 1 mm (Safeline, China). The following parameters of the transformer were measured at 25°С: ${{L}_{1}}$ ≈ 0.167 mH, ${{R}_{1}}$ < 0.1 Ω, ${{L}_{2}}$ ≈ 157 mH, ${{R}_{2}}$ ≈ 1.4 Ω, $M$ ≈ 5 mH, ${{w}_{2}}{\text{/}}{{w}_{1}} \approx 31$, and the mass is 0.7 kg.

A 5W510RJ resistor with a resistance of R_L = 510 Ω is used as the load. A UNI-T UTD2025CL-model digital oscilloscope (Osc) of the UTD-2000L series (Uni-Trend Technology Limited, Dongguan, China) is used to record voltages. In this case, the voltages that are picked off the 1-Ω measuring resistors (5W1RJ) are used to register currents. The same resistor is placed in the circuit of the D₂ diode to convert the remaining energy of the capacitor into heat if the polarity of its voltage changes during its discharge. The 5W1RJ, 5W330RJ, and 5W510RJ resistors are manufactured in Russia (AS Energiya, Korolev).

THE EXPERIMENTAL RESULTS

Figure 2 shows waveforms of the voltage u_C across the capacitor at different output voltages U_e of the power supply. These curves allow us to find the following values: U_mC ≈ 72 V, the period T_c of the voltage u_C, f_L, the number of capacitor-charging current pulses N, K_UC ≈ U_mC/U_e: (Fig. 2а) U_e ≈ 13.8 V, N ≈ 10, T_c ≈ 0.02 s, f_L ≈ 50 Hz, K_UC ≈ 5.2; (Fig. 2b) U_e ≈ 19.5 V, N ≈ 5, T_c ≈ 0.01 s, f_L ≈ 100 Hz, K_UC ≈ 3.7; and (Fig. 2c) U_e ≈ 22.8 V, N ≈ 3, T_c ≈ 0.006 s, f_L ≈ 167 Hz, K_UC ≈ 3.2.

Fig. 2.

Oscillograms of the voltage u_C at the capacitor: (а) for a source voltage of U_e ≈ 13.8 V, the sweep is 5 ms/division; (b) for U_e ≈ 19.5 V, 2 ms/division; and (c) for U_e ≈ 22.8 V, 2 ms/division. The vertical scale is 20 V/division. The left mark is the zero level.

Figure 3 shows a waveform that corresponds to the current i_L in the load R_L = 510 Ω. The following can be found on this basis: the current i_L pulse duration (FWHM) is 20 μs, the amplitude of this current is I_mL ≈ 2.2 А, the voltage amplitude across the load is U_mL = R_LI_mL ≈ 1122 V, the amplitude of the load power is P_mL = U_mLI_mL ≈ 2.47 kW, and for U_e ≈ 13.8–22.8 V, the voltage step-up ratio is K_U = U_mL/U_e ≈ 81–49. The amplitude of the current i_L in the primary winding of the transformer can be estimated as I_m1 ≈ (w₂/w₁)I_mL ≈ 69 A.

Fig. 3.

An oscillogram of the voltage across the 1-Ω resistor that corresponds to the current i_L in the load R_L = 510 Ω. The vertical scale is 500 mV/division, the horizontal scale is 50 μs/division. The left mark is the zero level.

The operation of the generator is also illustrated by oscillograms in Figs. 4 and 5, which correspond to the voltage u_C in Fig. 2a for U_e ≈ 13.8 V and N ≈ 10.

Fig. 4.

Oscillograms of the voltages across the 1-Ω resistors corresponding to (а) the current ${{i}_{{S{{w}_{1}}}}}$ of the Sw₁ switch, the sweep is 500 μs/division and (b) the current i_d in the choke. The sweep is 1 ms/division. The source voltage is U_e ≈ 13.8 V. The vertical scale is 500 mV/division. The left mark is the zero level.

Fig. 5.

Oscillograms of (а) the voltage ${{u}_{{S{{w}_{1}}}}}$ at the Sw₁ switch and (b) the voltage ${{u}_{D}}_{{_{1}}}$ across the D₁ diode. The source voltage is U_e ≈ 13.8 V. The vertical scale is 20 V/division, the horizontal scale is 2 ms/division. The left mark is the zero level.

Figure 4а shows an oscillogram of the voltage across the 1-Ω resistor, which is similar to the current ${{i}_{{S{{w}_{1}}}}}$ of the Sw₁ switch. The pulse current ${{i}_{{S{{w}_{1}}}}}$ with an amplitude of 1.6 А corresponds to the capacitor charging and discharging, while pulses with an amplitude of 1.05 А correspond only to the capacitor charging. As a result, for the maximum generated power of the power supply P_me= 13.8 × 1.6 ≈ 22 W, the power gain was K_P= P_mL/P_me ≈ 112.

Figure 4b shows an oscillogram of the voltage across the 1-Ω resistor that corresponds to the choke current i_d. The dual current pulse i_d with an amplitude of 1.4 A corresponds to the capacitor charging and discharging, while pulses with an amplitude of 0.95 A correspond only to the capacitor charging.

An approximate largest maximum value of the voltage ${{u}_{{S{{w}_{1}}}}}$ can be calculated as follows: ${{U}_{{mS{{w}_{1}}}}}$ ≈ U_e + U_mC = 85.8 V, which approximately corresponds to an experimental value of 80 V.

Figure 5a shows an oscillogram of the voltage ${{u}_{{S{{w}_{1}}}}}$ across the Sw₁ switch that reflects the operation of the generator: nine voltage pulses with increasing amplitudes correspond to the capacitor charging, while the tenth voltage pulse obviously corresponds to the capacitor charging and discharging. Figure 5b shows the voltage waveform ${{u}_{D}}_{{_{1}}}$ across the D₁ diode: D₁ is disabled at negative voltage values and enabled for ${{u}_{{{{D}_{1}}}}}$ ≥ 0. Nine voltage pulses with increasing amplitudes ${{u}_{{{{D}_{1}}}}}$ correspond to the capacitor charging, while the tenth pulse with the smallest voltage amplitude ${{u}_{{{{D}_{1}}}}}$ obviously corresponds to the capacitor charging and discharging. The lowest voltage ${{u}_{{{{D}_{1}}}}}$ that disables the D₁ diode can be approximately estimated as ${{U}_{{m{{D}_{1}}}}}$ ≈ $- {{U}_{{mS{{w}_{1}}}}}$ = –85.8 V, which is close to the experimental value of –76 V.

CONCLUSIONS

A switching inductive-capacitor generator with a semiconductor switching element—an IGBT transistor, whose prototype model generated pulse power and voltage values P_mL ≈ 2.47 kW and U_mL ≈ 1122 V at an ohmic load of R_L = 510 Ω, was investigated. When the power-supply voltage changed in the range U_e = 13.8–22.8 V, the obtained voltage step-up ratio was K_U = U_mL/U_e ≈ 81–49; for a power-supply voltage of U_e = 13.8 V and its power of P_me ≈ 22 W, the pulse-power gain was K_P = P_mL/P_me ≈ 112. The generator prototype operated reliably at these parameters without adjustments and shutdowns, providing current pulses with an amplitude of 2.2 А, a duration of 20 μs, and a repetition rate of 50–167 Hz, when the source voltage changed in the range U_e = 13.8–22.8 V. As the power-supply voltage U_e increased, the number of pulses N of the capacitor-charging circuit decreased, while the frequency f_L of current pulses in the load increased. The maximum voltage at the capacitor (U_mC ≈ 72 V) resulting from its charging is K_UC ≈ 5–3 times higher than the power-supply voltage (U_e = 13.8–22.8 V).